V. L. MAL'EV,
mechanical engineer, professor and head of the Laboratory of Heat Engines
of the Tomsk Technological Institute of EMPEROR NICHOLAS II.

TESTING

OF STEAM TURBINES.

INSTRUMENTS, DEVICES, AND METHODS USED
IN TESTING AND IN CONTINUOUS OBSERVATION
OF THE OPERATION OF STEAM TURBINES.

With 385 drawings in the text.

Tomsk.
Typo-lithography of the Siberian Society for Printing, corner of Dvoryanskaya St. and Yamskoy Lane, own building
1914.

Printed by order of the Director of the Tomsk Technological Institute of
Emperor Nicholas II.

All rights, in particular translation into other languages, are reserved by the author.

Preface.

At the present time, when steam turbines are becoming ever more widespread; when they have won for themselves a firm position as prime movers for large power plants and especially electric stations, and are beginning to find wide application on seagoing vessels as well; when cases of tests—acceptance, control, and others—are becoming more and more frequent, and the necessity of tests is perfectly obvious, there is no need to prove the need for a manual devoted specifically to the testing of steam turbines. Rather, one may express surprise and regret that until now there has been no such manual either in Russian or even in foreign literature.

The author hopes to fill this gap and has endeavored to set forth the most important methods and to describe the corresponding instruments and devices, partly relying on his own many years of experience in testing prime movers in general and steam turbines in particular, partly making use of material kindly communicated to him by the respective firms and works, and also, of course, using the communications of others scattered through the technical literature, although they are far from being as numerous as are, in general, reports on tests performed on steam turbines. It should be noted that the majority of reports on the numerous tests that appear in the literature, unfortunately, are most often limited to a brief summary of the figures obtained, less often give a more detailed picture of the operation of the turbine, and still more rarely report on the special devices used in the tests.

As regards the scope of the exposition, although the operation of a steam turbine is most closely connected with the operation of the boiler installation, and although their testing is almost always carried out simultaneously, the author has decided to confine himself only to questions concerning the testing of the turbines themselves. He does this, on the one hand, so as not to obscure the main question, and, on the other, bearing in mind that for tests of boiler installations there exists a sufficient number of manuals, some of them very detailed, and finally because the tests of steam boilers themselves are comparatively simple and the corresponding measurements few in number.

Preface

In describing more complex instruments, the author first provides either a schematic drawing of the instrument or briefly states the principle of its operation—so to speak, its theoretical basis—since an instrument can be used correctly only when one has a clear idea of its construction; and only under this condition is it possible to choose correctly the instrument most suitable for the task at hand.

Apart from cases of the actual testing of turbines, the proposed book may also serve as a guide for checking the correctness of turbine operation in their everyday work at stations.

The point is that, contrary to the statements of many turbine-building works, the steam consumption of turbines increases with the passage of time. The cause of this increase in consumption lies in the inevitable wear of the blades. Although, in different systems and depending on different conditions, the service life of blades after which a noticeable increase in steam consumption occurs—making it desirable to replace the blades with new ones—varies considerably, on the average it may be taken as 25,000 hours*). In any case, every fairly large turbine should be tested for steam consumption at least once a year.

In expressing the hope that the proposed book will prove useful both to the practicing engineer dealing with turbines at works and stations**), and likewise to mechanical-engineering students in their work in engineering laboratories, the author regards it only as a first attempt at systematizing the corresponding information and therefore will gratefully accept any indications of possible oversights, omissions, and ambiguities.

In concluding the present work, the author permits himself to express the wish that it may render all possible assistance to everyone who will use it, and that it may contribute to the speediest accumulation of more detailed and reliable data on the operation of turbines—data which, in turn, will facilitate the further improvement of steam turbines, one of the youngest, yet also one of the most viable and most promising fields of mechanical engineering.

Tomsk, March 1911.
V. Malʹtsev.

) See, for example, Glückauf, 1912, No. 15, or Z. Dampfk. Masch.*, 1912, p. 296.

**) Incidentally, the author thinks that the present book may also be useful for persons dealing with reciprocating steam engines, since many instruments and methods of testing are the same for both kinds of machines.

Contents

	Page
Preface	I
Contents	III
Noted misprints	XI
Adopted abbreviations and designations	X

CHAPTER I.

Measurement of Speed.

	Page
1. Revolution counters	1
Connection of counters and putting them into operation	3
2. Tachometers. General instructions	5
3. Stationary tachometers	6
Tachometers with weights	6
Tachometers with liquids	9
Magneto-electric tachometers	12
Electric tachometers	13
Resonance tachometers	13
Conclusion. Comparison of the various systems	16
Tachometer based on air friction	297
4. Handling tachometers	18
Putting stationary tachometers into operation	18
Checking tachometers	18
5. Hand tachometers	19
Tachometers with pendulums	19
Special attachment (electro-magnetic)	22
Other hand tachometers	22
6. Tachographs	23
Recording tachometers	24
Horn tachograph	25
Checking the Horn tachograph	28
Instructions for use	28
Strength of the instrument	29
Specimens of tachograms	29
Tachogram of the action of the safety governor	297

V. L. Mal’ev.

CHAPTER II.

Measurement of Work.

		Page
7.	Work of turbines and its measurement	34
8.	Measurement and absorption of electrical energy	35
	Load resistances	37
	Metallic resistances	38
	Water resistances	39
	Determination of electrode dimensions	40
	Water resistance for small loads	300
9.	Brakes with shoes	43
	Simple brakes. Their calculation	43
	Band brake for small power	46
	Alden brake	300
10.	Water brakes	47
	Stumpf brake	47 and 301
	Leavitt brake	51
	Heermann brake	52
	Froude brake	54
	Rateau brakes	56
	Fettinger brake	301
	Westinghouse brake	60
11.	Torsion indicators. General remarks	62
12.	Mechanical indicators	64
	Koli indicator	64
	Fettinger indicator	65 and 303
	Denny-Johnson indicator	69
13.	Optical indicators	71
	Bevis-Gibson indicator (axial)	71
	Bevis-Gibson indicator (radial)	73
	Amsler-Laffon indicators	74
	Hopkinson-Spring indicator	76
	Frahm indicator	79
	Saiehro indicator	80
14.	Electrical indicators	82
	Denny-Johnson indicator	82
	Rambala indicator	84
	Friedrich Lux indicator	86
	Johnson indicator	89
15.	Calibration and testing of torsion indicators	93
	Fettinger apparatus	93
	Gibson apparatus	94
	Adjustment of the zero of the scale	95
	General testing of torsion indicators	96
16.	Self-recording load indicator	96

Testing of Turbines

CHAPTER III.

Measurement of Temperatures.

		Page
17.	Mercury thermometers. Types of thermometers	98
	Handling thermometers	100
	Rupture of the mercury column	101
	Corrections. Protruding mercury column	102
	Influence of pressure	103
	Displacement of the 0° point of the scale. Checking 0° and 100°	104
18.	Thermoelements. General instructions	105
	Choice of thermocouple	106
	Methods of measurement	107
	Corrections for wire resistance	108
	Influence of cold junctions	109
	Influence of room temperature	110
19.	Design and accessories of thermoelements	110
	Connecting the ends. Mounting and insulation of thermoelements	111
	Series connection of several elements	113
	Cold junctions	113
	Bristol compensator. Schwarz compensation device	114
	Millivoltmeters	115
	Self-recording instrument	118
	Switches and commutators	118
20.	Resistance thermometers. General instructions	119
	Methods of measurement	120
	Sources of errors and their elimination	122
21.	Designs of resistance thermometers	124
	Schultz instrument for method 2	124
	Braun instruments for method 3	124
	Hartmann & Braun instruments	125
	Heraeus instruments made of quartz glass	126
	Cambridge K-and-E instrument for method 4	127
	Uehling set	127
22.	Measurement of temperatures in turbines	128
	Steam temperatures	128
	Influence of steam velocity	130
	Water temperatures	132

CHAPTER IV.

Measurement of Pressures.

		Page
23.	Spring pressure gauges	133
	Ducommun pressure gauge (Schaeffer)	134
	Schinz pressure gauge (Bourdon)	135
	Standard pressure gauges	136

VI

V. L. Maleyev.

Self-recording manometers ........................................ 138
Connection of manometers ........................................ 139
24. Mercury manometers ........................................... 141
    Differential manometer ..................................... 143
    Self-recording mercury manometer ............................ 143
25. Vacuum gauges ................................................ 144
    Metal vacuum gauges ......................................... 144
    Baro-vacuum gauges .......................................... 146
    Mercury vacuum gauges ....................................... 147
    Connection of vacuum gauges ................................. 150
26. Verification of manometers and vacuum gauges .................. 150
    Verification by comparison .................................. 151
    Verification by water pressure. Verification by steam pressure ... 152
    Verification by loading ..................................... 154
    Verification of vacuum gauges ............................... 155
27. Indicators ................................................... 156
    Dreyer, Rosenkranz, and Droop indicator ...................... 156
    Maikha indicator ............................................ 158
    Wagener indicator ........................................... 159
    Mato apparatus .............................................. 160
28. Measurement of pressures at turbines. Places of measurement ... 161
    Valve for switching the manometer ........................... 162
    Valve for switching the indicator ........................... 163
    Connection of manometers .................................... 164
    Recording pressure. Specimens of diagrams .................... 165
29. Barometers ................................................... 167
    Requirements imposed on barometers .......................... 167
    Barometer-hypsometer ........................................ 168

CHAPTER V.

Determination of the Moisture of Steam.

1. General instructions .......................................... 168
       Calorimeter-condensers ...................................... 169
       Other instruments: with heating, mixing, and separation of water ... 170
       Taking a steam sample ....................................... 170
       Taking a sample of low-pressure steam ....................... 304
2. Throttling calorimeters ....................................... 172
       Carpenter calorimeter ....................................... 172
       Barrus calorimeter .......................................... 173
       Stott and Ligott calorimeter for low-pressure steam .......... 304
       Zentner calorimeter ......................................... 174
3. Water-separator calorimeter ................................... 175
       Corrections during operation ................................ 176

CHAPTER VI.

Measurement of the consumption of steam and water.

		Page
33.	Steam meters. Principle of operation	177
	Steam meter of the Beyer & Co. works	179
	Steam meter of the Renania works	180
	Gehre steam meter	181
	Eckardt steam meter	184
	General instructions	185
	Measurement of steam by thin diaphragms	305
34.	Water meters	187
	Velocity water meters: water meter with Woltmann rotor	188
	Siemens and Halske water meter	189
	Dreyer, Rosenkranz and Droop water meter	190
	Ludwig Grefe apparatus	191
	Venturi water meter	306
	Köttgers “Rota” measuring apparatus	192
	Piston water meters: Kennedy water meter	192
	Schmidt water meter	192
	Thomson disk water meter	192
	Open water meters: apparatus with fixed tanks:
	Reichling water meter	195
	Schilde water meter	196
	Torent water meter	196
	Eckardt water meter	197
	Apparatus with rocking tanks:
	Eckardt water meter	198
	Leinert water meter. Steinmüller water meter	198
	Siemens Brothers water meter	201
	General remarks on open water meters	202
	Verification of water meters	202
35.	Other methods of measuring water	202
	Weighing apparatus	203
	Volumetric methods	204
	Danaids	207
	Calibration of nozzles. Measurement of flow at variable $h$	209
	Allowance for changes in water temperature	211
	Zülchert danaid water meter	212
	Danaid for measuring condensate per minute	307
	Selection of the appropriate method	213
36.	Measurement of the consumption of steam and water in turbines	213
	Consumption of working steam. Determination of the quantity of feed water	213
	Steam consumption by volume entering the turbine	215
	Leakage through leaks in the steam boiler	309

Measurement of the amount of condensate . . . . . . . . . . . . . . . . . . . . 216
Checking the tightness of the surface condenser . . . . . . . . . . . . . . 217
Steam consumption according to the heat balance of the condenser . . . . 310
Measurement of the cooling-water flow rate . . . . . . . . . . . . . . . . . 218

CHAPTER VII.

Lubrication.

1. Investigation of the principal properties . . . . . . . . . . . . . . . . . . 218
       Requirements imposed on turbine oils . . . . . . . . . . . . . . . . . . 218
       Choice of oil grade in practice. Specific gravity . . . . . . . . . . . . 219
       Acid content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
       Absence of resinous impurities. Ash content . . . . . . . . . . . . . . 221
       Flash temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
       Oil viscosity. Engler viscometer . . . . . . . . . . . . . . . . . . . . . 226

2. Determination of the coefficient of friction of oil. Wilkens apparatus . . . 227
       Dettmar apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

3. Observations of lubrication. Pressure . . . . . . . . . . . . . . . . . . . . . 232
       Temperature. Lubricant consumption . . . . . . . . . . . . . . . . . . 233
       Incidental measurements . . . . . . . . . . . . . . . . . . . . . . . . . 233

CHAPTER VIII.

Special measurements on turbines.

1. Measurement of steam losses . . . . . . . . . . . . . . . . . . . . . . . . . 234
       Measurement of leakage of sealing steam . . . . . . . . . . . . . . . 235
       Steam leakage in an Ehrmann turbine . . . . . . . . . . . . . . . . . 238
       Steam leakage through Parsons unloading pistons . . . . . . . . . . 239

2. Measurement of heat loss by radiation. Arrangement of the experiment . . 242
       Relative magnitude of the loss . . . . . . . . . . . . . . . . . . . . . 244

3. Measurement of mechanical losses . . . . . . . . . . . . . . . . . . . . . . 245
       Tests with retarded rotation . . . . . . . . . . . . . . . . . . . . . . . 246
       Breaking down the resisting moment into component parts . . . . . 247
       Mechanical output of the turbine . . . . . . . . . . . . . . . . . . . . 250
       Tests with rotation of the turbine by a dynamo . . . . . . . . . . . . 251

4. Additional remarks. Construction of the curve \(n\) . . . . . . . . . . . . . 251
       Construction of the differential curve \(\frac{dn}{dt}\) . . . . . . . . . . 252
       Mirror ruler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
       Wagener mirror derivator . . . . . . . . . . . . . . . . . . . . . . . . 254
       Determination of the moment of inertia of rotating parts by a dynamo . . 255
       Determination of the moment of inertia of one turbine . . . . . . . . 257

5. Minor measurements. Friction moment at rest . . . . . . . . . . . . . . . 259
       Axial pressure. Tozi apparatus for measuring axial pressure . . . . . 260
       Apparatus for measuring axial pressure on a shaft . . . . . . . . . . 310
       Measurement of clearance magnitudes . . . . . . . . . . . . . . . . . 261

Testing of Turbines

	Page
Curtis device for measuring axial clearance	262
Measurement of vibrations. Seismograph	263
Instrument of the firm Siemens & Halske	264
Instrument of Academician B. V. Golitsyn	265
Instrument of Professor Grunmach	266
Investigation of noise. Detectophone. Bolt instrument	268
Simplified “stethoscope”	269
Instrument for determining oscillations transmitted by air	269
45. Investigations of a scientific and technical nature	269
Motion of steam through nozzles	270
Motion of steam through the guide apparatus	271
Motion of steam through the working blades	271
Influence of shrouding and of the distance between blades	271
Magnitude of ventilation resistances	271
Experiments with diffusers	271
Leakage through gland labyrinths	272
Testing of materials and of individual parts	272
Testing of the entire turbine	272
Factory laboratories	273

CHAPTER IX

Conduct of Tests

	Page
46. Preparation of reports. Beginning of reports	273
Intervals between reports. Duration of the test	274
Specimens of logs	274
47. Processing of experimental material. Finding mean quantities	279
Accuracy and its determination. Criticism and correction of entries	280
Corrections from instruments	282
48. Steam consumption	282
Reduction of quantities, $D$	283
Influence of the initial steam pressure and of the superheat temperature	283
Influence of steam moisture and of pressure in the condenser	284
Summary of the influence of different operating conditions	286
Finding the correction $\Delta D$ by experimental means	287
Normal conditions	287
Influence of load	288
Allowance for the influence of load	291
Magnitude of consumption $D$	291
49. Obtaining general conclusions. Calculation of efficiency	292
Determination of the efficiency of individual stages	293
Influence of incomplete observance of the established state	293
Graphical representation of results. Examples	295
Summary of results	297
Addenda (distributed among the corresponding §§)	297

Appendix 1: rules for testing steam turbines ................ 312
Appendix 2: Table 17—list of firms supplying various measuring instruments for testing turbines ........ 320
Table 18—list of instruments used in testing turbines, their costs with indication of suppliers ................................ 322
Appendix 3: subject and name index ................ 326

Noticed Misprints

Page	Line	Printed:	Should read:
12	1 from top	e	c
13	19 from top	electrically	empirically
19	13 from top	shaft with	shaft, with
22	8 from top	too much	too much
22	13 from bottom	creator	end
24	14 from bottom	by the opening	by the opening
25	7 from top	ordinary	ordinary
29	6 from bottom	that	that
40	19 from bottom	electric leads	electrodes
41	4 from top	sheet	sheets
43	9 from bottom	constructions	construction
43	13 from bottom	other	others
45	19 from bottom	μ possible	μ, possible
45	20 from bottom	section	cross-section
49	7 from bottom	ball-type	ball-type
49	9 from bottom	perpendicularity of the axis	perpendicularity to the axis
51	3 from bottom	Olden	Livitt
54	—	the caption under the drawings should be turned by 180° and moved to the outer edge of the page.
56	11 from bottom	not by two	by two
56	7 from bottom	lining	lining
58	5 from top	“Furt”	“Firth”
60	22 from bottom	conde-	conde-
64	1 from bottom	Eng.	Engng.
66	13 from bottom	concepts	comprehensible
70	10 from bottom	hammered in	driven in
71	10 from bottom	lantern with	lantern with
71	1 from bottom	Engng. 1908, p. 197;	Engng. 1908, LXXXV, p., 197;
71	6 from top	contrasts	contacts
75	7 from bottom	operation of disks	operation of air friction of disks
76	—
80	9 from bottom	to the tank	to the shaft
86	9 from bottom	adjustment pieces	adjustment piece
89	5 from top	γ²	?²
105	13 from bottom	excessive	excessively
128	18 from bottom	dimensions, clear	dimensions clear

Page	Line	Printed:	Should read:
135	7 from bottom	ellipse.	ellipse f, Fig. 187.
136	8 from bottom	a definite	a definite
140	—	after Fig. 202 turn through 180°.	after Fig. 202 turn through 180°.
142	7 from bottom	in manometer	in the manometer
144	18 from bottom	210	211
144	16 from bottom	211	210
146	7 from bottom	in accordance	with accordance
147	14 from top	communication with	communication of both knees with
148	15 from bottom	lowered-	lowered-
150	25 from bottom	insignificant	insignificant
150	20 from bottom	pressure.	pressure.
155	6 from top	destroys	diminishes
156	15 from bottom	necessary	necessary
166	—	Fig. 482.	Fig. 248.
168	12 from top	determine	determine
174	3 from bottom	Konstantan iron	Konstantan iron
176	14 from top	barometer deviations.	barometer deviations
179	14 from top	Fig. 258.	Fig. 258
184	6 from top	hanging	hanging
186	10 from bottom	adjustable	adjustable
197	12 from top	jumping	jumping
198	—	after Figs. 292 and 293 interchange them.	after Figs. 292 and 293 interchange them.
203	6 from top	bath	baths
203	7 from top	desired	desired
207	7 from bottom	Fig. 310,	Fig. 309,
215	2 from bottom	Wasseidampf.	Wasserdampf.
217	3 from bottom	$\pm 1{,}7\%$	$\pm 0{,}7\%$
219	3 from bottom	tube $d$	tube $b$
220	11 from bottom	distilled	distilled
225	15 from bottom	diameter;	diameter;
225	10 from bottom	into her	into it
230	1 from top	glass	glass
230	1 from bottom	3 kg/cm$^2$	3 kg/cm$^2$
238	13 from bottom	$p_2 = B - h_2 \, / \, 737{,}4,$	$p_2 = (B - h_2) / 737{,}4,$
265	9 from bottom	mass $a$	spring $b$
271	18 from bottom	Parsons’	Parsons’
272	17 from bottom	Parsons’	Parsons’
282	1 from bottom	$D_e = 0{,}7367\eta_i D_k;$	$D_e = 0{,}736\eta_e D_k;$
288	in Fig. 355	interchange the designations $D_1$, $D_2$ and $D_3$ instead of $G_1$, $G$ and $G_3$, and vice versa.	interchange the designations $D_1$, $D_2$ and $D_3$ instead of $G_1$, $G$ and $G_3$, and vice versa.
291	13 from top	7.0 kg/kWh.	7.0 kg/kWh, and at 2000 kW with superheated steam only 5.6 kg/kWh.
301	10 from top	Fig. 366,	Fig. 365,
301	16 from top	$^{168}$)	$^{168}$)
304	14 from top	specific weights	specific weights
304	5 from bottom	combined	combined,

Adopted Abbreviations and Symbols

Abbreviations of terms:

abs.—absolute,
mm., mm.\(^2\), mm.\(^3\)—millimeters, square mm, cubic mm,
cm. (cm.\(^2\), cm.\(^3\))—centimeters (square, cubic),
m. (m.\(^2\), m.\(^3\))—meters (square, cubic),
l.—liters,
g.—grams,
kg.—kilograms,
sec. (s.)—seconds,
min. (min.)—minutes,
h.—hours,
rev./min.—number of revolutions per minute,
h.p.—horsepower;
e.h.p.—effective h.p. (brake or useful),
kW—kilowatts,
V—volts,
A—amperes,
atm.—atmosphere \(= 1\) kg/cm.\(^2 = 735.4\) mm Hg,
mm Hg—height of a mercury column (mm or cm),
mm water—height of a water column (mm or cm),
h.u.—heat units (kg-calories).
\(-\), \(;\), \(/\)—division signs.

Symbols:

\(t^0\)—temperature in \(^{\circ}\)C,
\(T^0 = t^0 + 273\)—temperature in absolute degrees,
\(\vartheta^0\)—temperature of wet steam,
\(c\)—specific heat,
\(Q\)—heat in h.u.,
\(p\)—pressure in kg/cm.\(^2\),
\(B\)—barometric pressure in mm Hg,
\(B_n\)—normal barometric pressure \(= 737.4\) mm,
\(l, L\)—length (mm or m),
\(D\)—diameter in mm or m, and also steam consumption in kg per 1 unit of power per 1 hour,
\(f, F\)—area (mm.\(^2\) or m.\(^2\)),
\(v\)—specific volume in m.\(^3\)/kg,
\(V\)—volume in m.\(^3\),
\(N\)—work in h.p.,
\(N_e\)—work in e.h.p.,
\(N_k = (KW)\)—work in kW,
\(n\)—rev./min.,
\(\eta\)—efficiency (in %).

Abbreviations of Literature

C. r.—Comptes rendus de l’Académie des sciences (Paris).
Eng.—The Engineer.
Engng.—Engineering.
Forsch.—Mitteilungen über Forschungsarbeiten aus dem Gebiete des Ingenieurwesens.
Proc. Am. Inst. El. Eng.—Proceedings of the American Institute of Electrical Engineers.
Rev. Méc.—Revue de Mécanique.
Techn. u. Wirtsch.—Technik u. Wirtschaft, herausgegeben vom Verein deutscher Ingenieure.
T. Nav Arch.—Transactions of the Institution of Naval Architects.
Turb.—Die Turbine.
Z. Turb.—Zeitschrift für das gesamte Turbinenwesen.
Z. Dampf. Masch.—Zeitschrift für Dampfkessel und Maschinenbetrieb.
Z. V. d. I.—Zeitschrift des Vereines deutscher Ingenieure.

Chapter I.

Measurement of Speed.

1. Revolution counters.—Permanent revolution counters, the so-called summating counters, are almost never used with steam turbines. This is explained by the following circumstances:

1) unlike piston engines, in the majority of cases exact measurement of the number of revolutions of a turbine, when the turbine is directly connected to an electric-power generator or to a centrifugal air compressor, is not necessary, since one revolution is not any completed working period;

2) in view of the considerably high usual number of turbine revolutions, from 1000 to 3000 per minute, a summating counter would have to have a considerable number of figures on the scale, since it would have, for example, in 1 hour to count from 60,000 to 180,000 revolutions; such counters are rather bulky and absorb comparatively much work;

3) only very few designs of counters are suitable for such considerable numbers of revolutions, and even they are subject to excessively rapid wear.

For the exact measurement of the number of revolutions, use is usually made of hand counters, either consisting of a worm with a pair of worm wheels, or of a system of toothed, or rather, lantern pinions.

The operation of a counter of the first type, Fig. 1, is based on the relative

Fig. 1.

motion of two worm wheels having a common axis, of which the upper one, with the dial, has 100 teeth, and the lower one—101. The upper dial is indicated by the large, fixed pointer,

Chapter I. Measurement of Speed. § 1

on the inner one—a small pointer connected with the hub of the lower wheel. Both wheels are coupled with a worm. At each full revolution of the upper wheel, corresponding to 100 revolutions of the worm, the lower wheel lags by \(1/100\) of a revolution, and its pointer thus indicates hundreds and thousands of revolutions.

So that, when the three-sided end is pressed into the recess at the center of a shaft making a large number of revolutions, it will not slip and give diminished readings, it is useful to make several notches in the recess with a chisel.

To avoid rapid wear of the worm and the teeth of the wheels, as well as of all the axes of rotation of the instrument, they must be well lubricated with vaseline or wood oil.

Counters of more complex construction, but with a more convenient reading by means of figures appearing in small windows, consist of a system of gear wheels, of which each subsequent one makes \(1/10\) of a revolution for 1 revolution of the preceding one. In hand counters there are usually 4 successive wheels, not counting the one mounted on the shaft of the counter, so that the counter indicates up to 9999, and then 0000 is displayed, and the counting starts over again. The figures are placed directly on the cylindrical surface of the wheels.

When using the indicated hand counters, one must at the same time keep watch on the seconds hand of a clock, in order to press the counter for a definite, precisely noted interval of time—usually for 60 seconds—and by timely and correct pressing of the counter during the count. The work requires great attention and skill, since an error of 1 second in 60 seconds amounts to almost 2%, and the measured number of revolutions will be wrong by the same amount.

The indicated inconvenience is eliminated when using the so-called tachoscope, consisting of a combination of a revolution counter with a chronoscope, or, more simply, with a seconds meter. The mechanisms of both instruments are connected with one another in such a way that when the end of the axis \(d\), Fig. 2, is pressed against the center of the rotating shaft, both the counter and the seconds meter, located behind the counter, are set in motion. When the axis of the counter

Fig. 2.

§ 1. Counting Revolutions. Tachoscope.

They are taken away from the shaft; together with the counter the stopwatch is stopped. By pressing the pawl, not shown in Fig. 2, the hands of the stopwatch are returned to 0. Button $m$ serves to wind the mechanism of the stopwatch. At the left are shown various tips: $a$, a three-sided steel one; $b$, also steel, but with a recess, if the end of the shaft has a projection at its center; $c$—a rubber tip, suitable, however, only for moderate numbers of revolutions, not above 200–300 per min.; at higher numbers it slips and itself wears out quickly.

The tachoscope is a fairly compact instrument; in Fig. 2 it is shown at about $3/5$ natural size, and is usable up to 6000 rev./min. The clock mechanism is made insensitive to electromagnetic action, so that it can be used even in the vicinity of large direct-current machines.

Tachoscopes are also made in such a way that both instruments—the revolution counter and the stopwatch—are arranged side by side, i.e. both dials are visible at once. In use this affords a certain convenience, but the instrument is more bulky. In addition, the mechanism of the revolution counter is different, with hands, which does not permit use of the instrument above 2000 rev./min.

If the turbine has a shaft making a smaller number of revolutions, but connected with it by means of a rigid gear transmission—for example, the shaft of the lubricating pump—then it is sometimes more convenient to attach the revolution counter to this shaft, multiplying its readings by the precisely established gear ratio between the shafts.

Sometimes the number of revolutions of a turbine has to be, and can be, measured even without a counter at all, if the turbine has some part that makes relatively slow oscillatory motions at 1 for some strictly definite number of turbine revolutions, for example, a piston air or oil pump connected with the turbine shaft by a rigid transmission, or a lever in the regulating apparatus, rocking from an eccentric or cam, etc. In this case one can, by counting by eye, for example, 100 strokes and measuring the corresponding time with a stopwatch to an accuracy of $1/5$ sec., calculate the number of rev./min. of the turbine.

In especially precise tests it is useful to measure the number of revolutions not periodically, as is ordinarily done by means of hand counters, but throughout the entire test, which usually lasts about 1 hour. In such a case the following arrangement may be used, Figs. 3 and 4, p. 4: the tip $f$ of the counter $a$ enters a hole drilled in the center of the turbine shaft $d$; the pin $e$, screwed into the end face of shaft $d$, catches on the bend of the tip in the form of a hook and brings the counter into rotation. The counter is attached to the board $b$, which can be moved along the support $c$ with raised edges on three sides, bringing the counter closer to the machine or

Chapter I. Measurement of Speed.

§ 1.

removing it. If the counter is moved to the right, the pin \(e\) will cease to touch the ring \(f\), and the counter will stop. The spring \(g\) holds the counter, notwithstanding its inevitable vibration, in the position in which it has been set.

Drawings 3 and 4.

Another device for a larger counter, shown in drawings 5 and 6¹: the counter \(a\) is fastened immovably to the bracket \(c\); beneath the counter a rack \(b\) can move, while its left bent end moves the coupling \(f\); the end face of the latter is furnished with a rubber facing, pressed against the surface of the flange \(e\), which is ribbed with radial grooves and screwed into the center of the turbine shaft. The coupling \(f\) is connected to the axis of the counter by a cotter pin and a slot, allowing the coupling to move along the axis of the counter. The wing nut \(g\) serves to secure the rack \(b\) when the counter is switched on for a longer period of time.

Drawings 5 and 6.

§ 1. Putting Counters into Operation

Then one may also mention another method of putting revolution counters into operation—by means of a belt drive, in which the counter is usually combined in one mechanism with a tachometer, Fig. 7. The merit of this method is its simplicity, but it, of course, cannot be considered entirely reliable in precise tests.

Fig. 7.

Finally, some factories equip their turbines with revolution counters combined in one mechanism with a tachometer, but driven from the turbine shaft by means of a gear drive. Against this only the following objection can be made: if the instrument is kept constantly in operation, it wears out comparatively quickly; if it is switched on only from time to time, and for switching it off one of the gears is removed, then it will not achieve its purpose as a control instrument during normal operation of the turbine, since it can be switched on again only while the turbine is stopped. During a test, however, such an instrument is one of the most convenient and accurate.

2. Tachometers. General remarks.—The revolution counters described above make it possible to determine the average number of revolutions of the turbine per minute over a known interval of time. So-called tachometers, i.e. speed meters, directly indicate the number of rev./min. that the turbine is making at a given moment. Owing to various causes, partly dependent on the instruments themselves, partly on the method of putting them into operation, and partly also on the incomplete uniformity of rotation of the turbine, the tachometer pointer usually oscillates somewhat and cannot indicate the average number of revolutions of the turbine as accurately as calculation from the readings of the counter.

Thus, the tachometer, which is a necessary accessory of every steam turbine, must be regarded rather as an auxiliary instrument for supervising the operation of the turbine. However, in those cases when a turbine coupled to an electric-current generator is being tested, the number of revolutions is not included in the calculation of the work, and the tachometer is usually the only instrument for determining the number of revolutions. When calculating the work of a turbine from the torque, the tachometer should not be relied upon.

Nevertheless, a tachometer is also necessary in precise tests of a turbine, namely for determining the degree of controllability of the turbine, i.e. the change in the number of revolutions when the magnitude of the load fluctuates. For this purpose, however, it is better to use a tachograph, or a tachometer with a self-recording device.

According to the principle of action, tachometers used and suitable for use in work with steam turbines may be divided into three principal groups: those based on the action of centrifugal force, on electromagnetic phenomena, and on the phenomenon of resonance. The first two groups are further subdivided into several smaller ones: the first includes instruments with weighted pendulums, with liquid displacement, and with liquid pressure; the second—those with eddy currents, with direct-current dynamo machines, and, finally, with alternating current.

There are also other instruments, for example those based on the viscosity of a liquid, purely kinematic ones, etc. We shall not discuss them, since for turbines they are either unsuitable or not adapted.

According to their purpose, tachometers may be distinguished as stationary and hand-held; the latter are the same in principle, differing only in size and constructional details.

Finally, tachometers may also be distinguished as having remote transmission of readings—such are instruments with a dynamo machine, as well as tachometers based on resonance in conjunction with electrical transmission.

We shall make a comparative appraisal of the numerous types of tachometers after describing them.

3. Stationary tachometers.—Until comparatively recently, these instruments were built chiefly on the principle of centrifugal weighted pendulums.

Tachometers with weights. The design of a good tachometer with pendulums is one of the difficult problems. With every change of speed, the pendulum must first overcome the inertia of the masses and the friction in the hinges. The latter depends to a considerable extent on the lubrication, its abundance and viscosity. In some designs the influence of inertia is reduced by making the mass of the moving parts small and the acting centrifugal force, on the contrary, as large as possible. This can be achieved by increasing the number of revolutions of the pend—

nevertheless, is undesirable from the point of view of wear of the rubbing parts.

The influence of friction in the joints can be reduced by connecting the weights directly with springs and by reducing the number of joints, as, for example, in the instrument in Figs. 8 and 9, in which the rotation of pendulum \(a\)

Figs. 8 and 9.

is transmitted to pointer \(h\) by means of levers \(b\) and \(d\), with the intermediate slide \(c\), toothed sector \(e\), and pinion \(f\); the centrifugal force is balanced by twisting the helical spring \(g\). In order for the mechanism to be in stable equilibrium, the tension of spring \(g\) must, of course, increase more rapidly than the increase of the centrifugal force.

In order to eliminate the influence of accidental shocks, vibration, or an incorrect position of the instrument on its readings, the mechanism must be designed so that the center of gravity of the system of moving parts remains stationary. Such, for example, is the instrument in Figs. 10 and 11, p. 8, in which this condition is achieved by employing a pair of pendulums \(B\) and \(B_1\) of a peculiar form, more readily understood from the perspective view, Fig. 12. The divergence of pendulums \(B\) and \(B_1\) moves rod \(f\), and by means of a rack and pinion is transmitted to pointer \(s\). The balancing of the centrifugal force is effected by twisting the ribbon spiral spring \(c\). The numerous lubrication points indicate careful attention to this matter.

Finally, one may point to the Amsler mechanism, Fig. 13, in which, in order to reduce friction, sliding friction is replaced by friction of the second kind: the pendulum weights \(a,a\) (there are 4 of them in all, but, so as not to obscure the drawing, the front one is shown removed), under the action of centrifugal force, roll along the rails \(b,b\);

Figure 10–12.

Figure 13.

in doing so the rollers \(c,c\) of the pendulums move to the right the square rack \(d\) situated between them, to which is attached the spindle \(e\), transmitting motion to the pointer in the usual manner. The spindle \(e\) also moves with very little friction, being guided by 6 toothed rollers \(f,f\).

§ 3. Tachometers with Loads.

The balancing of the centrifugal force is effected by compression of a cylindrical spring enclosed in the casing \(g\).

In addition to the sensitivity of the readings, this mechanism is distinguished by very little wear, and can even dispense with lubrication.

In concluding the discussion of tachometers with pendulums, it may also be mentioned that they are sometimes made combined together with a revolution counter, as, for example, the one indicated above, Fig. 7.

Liquid tachometers. If a vertical cylindrical vessel, half filled with liquid, is rotated about its axis, then, as is known from hydraulics, the surface of the liquid assumes the form of a paraboloid of revolution, its vertex descending below the level in the state of rest by an amount proportional to the square of the angular velocity, and which must be measured.

The further improvement of a design based on this same principle is the instrument with two liquids, the so-called “bichromate tachometer,” shown schematically in Figs. 14 and 15. In the glass vessel \(a\), of peculiar shape, a little mercury \(b\) is poured onto the bottom, and over it is poured colored spirit \(c\). When the vessel is rotated, the mercury rises into the lateral vertical knees \(d\) and drives upward the spirit in the middle tube \(e\), the height of the level in which indicates the number of revolutions. The limits of measurement and the mean number of revolutions are determined by the width of the lower part of the vessel \(a\), the diameter of the tube \(e\), and the quantity of mercury. The transitions at the knees \(d\) play the role of a cataract, damping the oscillations of the liquids; the middle channel \(o\) connects the tube \(f\) with the middle of the vessel \(a\).

Figs. 14 and 15.

In Figs. 16 and 17, p. 10, two sections and the scale of such an instrument are shown in its constructive embodiment\(^2\); the vessel itself, owing to the comparative complexity of its outlines and for the sake of strength, is made of cast iron, while only the tubes \(e\) and \(f\) are of glass; the remaining designations are the same as in Figs. 14 and 15; the pulley \(s\) serves for transmitting rotation by a belt and may be replaced by a conical wheel for positive transmission. To reduce friction and wear, the end of the axis \(g\) rotates in oil,

\(^2\) Rig. Industrie-Ztng. 1904, S. 182.

which is poured into the lower part of the stand and, through channel \(h\), reaches the axis. The scale consists of four racks \(i\), forming a cross and surrounded by a glass casing—tube \(k\); the scale divisions are applied on both sides of the racks.

Drawing 16.          Drawing 17.

These instruments are very convenient; in view of the small magnitude of the liquid friction, they are very sensitive; their readings do not change with time. To the same group of tachometers must also be assigned the device used in the newest Zoelly turbines². Out of a desire, as far as poss—

§ 3. Tachometers with liquid.

it is possible to avoid a gear transmission for bringing the speed indicator into action; the pressure of oil is used, this oil serving in the first instance to regulate the number of revolutions of the turbine. For this purpose the oil is supplied by a special centrifugal pump, separate from the oil used for lubricating all the rubbing parts.

Because the pump takes the oil from a vessel with an overflow, so that the initial pressure of the oil, i.e. its admission, is always strictly the same, the pressure in the delivery pipe is proportional to the square of the number of revolutions of the pump. The latter is mounted on a shaft connected by a worm gear with the turbine shaft. Thus every change in the number of revolutions of the turbine is appreciably reflected in the oil pressure. Instead of measuring this pressure with an ordinary tubular manometer, the works uses for this the instrument shown in Figs. 18 and 19.

Figs. 18 and 19.

A tube with oil is connected to the opening \(a\); the oil presses upon the tightly fitted piston \(b\), which compresses the helical spring \(c\) proportionally to the pressure, i.e. to the number of revolutions. The displacement of the piston (upward when the number of revolutions decreases) turns, by means of a toothed sector and a pinion, the shaft \(d\) with the pointers \(e, e\), one at each end. As the number of revolutions increases, the pointers \(e, e\) follow the lowering of the piston \(b\) under the influence of the spiral spring \(f\). The tube \(g\) serves for continuous lubrication of all moving parts. To the branch pipe \(h\) is attached a tube for draining the oil that seeps through back to the pump. The bolt \(i\) serves for precise

Chapter I. Measurement of Speed. § 3.

setting the tension of the spring \(c\) when graduating the scales and during subsequent verification.

The instrument works very well, is very sensitive, its readings vary hardly at all with time, and they can again be corrected by means of the bolt \(i\). The two dials, on two sides, are very convenient when attending to the turbine.

Magneto-electric tachometers. One of the simplest is the instrument with eddy currents, shown schematically in Fig. 20. To the shaft of the instrument \(a\), rotating in ball bearings, is fastened a curved steel magnet \(b\), the lines of force of which are closed by the iron armature \(c\), situated between its poles and rotating together with \(b\). In the annular gap between the poles of the magnet and the armature there is a bell \(d\) of aluminum, which can turn very easily about an axis coinciding with the axis of the shaft. To the bell \(d\) are fastened the spiral spring \(f\) and the pointer \(e\). When the magnet rotates, eddy currents are formed in the bell, turning it until the twisting of the spring \(f\) balances the turning moment, which is the greater the higher the speed of rotation.

Fig. 20.

Figs. 21 and 22.

Owing to the very small mass of the bell and the absence of gearing, the readings of the instrument are very sensitive, although not very accurate, and are also subject to the influence of temperature. The instrument almost does not wear out; it is true that the strength of the magnet changes with time, but only very little. In addition, in some designs devices are made for changing the magnitude of the air gap between the magnet and the aluminum bell, or in general the part carried along by the eddy currents, whereby the change in the strength of the magnet can be corrected.

§ 3.

Magneto-Electric Tachometers

A change in temperature has a rather strong effect on the readings of the instrument; this, however, can be combated in two ways: either by making the part turned by the currents not of aluminum, but of an alloy whose electrical resistance does not vary with temperature—constantan, manganin, etc.—or by providing the instrument with a table of correction factors for increases in temperature.

It is, however, inconvenient to use such an instrument with marine turbines: it has no cataract to damp the swings of the pointer when the speed of rotation fluctuates strongly, and its pointer turns in different directions depending on the direction of rotation, as a result of which the divisions on each half of the scale are too fine, especially since in this case it is impossible to make a shortened scale with readings not starting from 0.

Electric tachometers, which at the same time are instruments with indications at a distance, are made either in the form of a small direct-current machine or of a small alternating-current generator. In both cases the voltage of the current is proportional to the speed of rotation. The indicator is a voltmeter with a scale graduated electrically directly in numbers of revolutions.

For direct-current machines a d’Arsonval-type voltmeter is used. The direction of deflection of the pointer depends on the direction of rotation, so that the same observation applies here as in the case of eddy-current tachometers.

For alternating-current machines a Ferraris-type voltmeter is used. The direction of rotation does not affect the direction of deflection of the pointer. Since the voltage increases in proportion to the square of the speed, the divisions of the scale become larger as the number of revolutions increases. The generator itself is made with a rotating magnet and a stationary winding, so that there are neither brushes nor contact rings; wear of the instrument is negligible, its maintenance is very simple; its readings change scarcely at all with time, but they also depend somewhat on temperature.

Resonance tachometers. The phenomenon of resonance consists in the fact that, for a body of a certain shape, under the influence of periodic impulses a strong oscillatory motion arises if the number of impulses per unit time nearly coincides with the so-called natural number of oscillations of the body.

Fram’s instrument, based on this phenomenon, consists of a row of narrow steel springs, Figs. 21–22, p. 12, fastened at one end to a common bridge. The thickness of the plates is usually about 0.25 mm, the width 3–4 mm, and the length 40–60 mm; the free end of the plate is bent, Fig. 22, and a piece of tin, \(e\), is soldered into the corner. The number of oscillations of such a plate depends, for a homogeneous material and identical cross-section, on its free length and

from the weight of a small load at the free end. With the dimensions indicated above, one can obtain plates with any number of vibrations from 15 to 120 per sec., i.e. from 900 to 7200 per min.

A series of plates, 13 to 50 in number, fastened to a common bar, form a comb, Figs. 23 and 24, which is attached to the bedplate

Figs. 23 and 24.

by means of two flat springs $a$, allowing the whole comb to make oscillatory motions perpendicular to its length. The plates are “tuned” so that the number of vibrations of each succeeding one is 5 to 20 vibrations per min. greater than that of the preceding one.

If periodic, even weak, impulses are imparted to such a comb of plates, then that plate whose natural number of vibrations is equal to the number of these impulses, or close to it, will enter into a fairly strong vibration with an amplitude of 5 to 30 mm, and even greater; the adjacent plates will make a smaller amplitude, the next ones still smaller, and the vibration of the rest will be quite imperceptible.

True, not only the plate whose number of vibrations is equal to the number of impulses comes into noticeable vibration, but also those whose number of vibrations is 2 or 3 times greater. However, these vibrations are still weaker, and, moreover, it is difficult to make a mistake in counting the number of revolutions by a factor of 2 or 3.

As for the methods of exciting the vibrations, the simplest is to place the instrument directly on the bedplate of the machine. For stronger transmission of the shaking, the instrument may be attached to a bracket of strip iron 6 × 20 mm in section, 60–80 mm high, Fig. 25. This method is very convenient for Laval and other small turbines. It is inapplicable for quieter-running large turbines with better-balanced masses, and often also standing on a massive foundation.

For these machines one may use electrical excitation. Its idea is explained by Fig. 26: the lever $b$, attached to the bridge with the plates, is periodically attracted by electromagnets $c$ when an induced alternating current passes through their winding; the latter is obtained by means of a small dynamo, Fig. 27

§ 3. Electric tachometers. Resonance tachometers.

and 28, consisting of a toothed disk \(d\) of soft iron, the teeth of which pass by the ends of the horseshoe-shaped magnet \(e\) and induce

Fig. 25.

an induction current in its windings, transmitted to the windings of the electromagnets mentioned above. In Fig. 29 the corresponding receiver is shown; in external appearance it differs almost in no way from the instrument

Fig. 27.

Fig. 28.

Fig. 26.

Fig. 29.

which is placed directly on the turbine bedplate. Finally, in Fig. 30, p. 16, such an instrument is presented, in which the dynamo machine and the electromagnetic vibrator, together with the Frahm comb, are combined in one common casing. Such an instrument can serve two purposes: either it can be used for observing the running of the machine near it, assuming that the vibration of the machine is sufficient for ...

excitation of a Framm comb or, in addition, one or several receivers, by means of two wires, can be connected to it according to Fig. 29 for transmitting readings over a distance.

A certain shortcoming of Framm instruments is the stepped scale, in steps of 5–10 revolutions, which usually corresponds to from 1 to 2% of the mean number of revolutions.

Fig. 30.

Fig. 31.

To eliminate this shortcoming, the Hartmann and Braun works build combined tachometers, connecting to the Framm instrument, driven by a small alternating-current generator, Fig. 31, 1:3 natural size, a voltmeter with a scale likewise graduated, as indicated above, in numbers of revolutions, Fig. 32, likewise 1:3 natural size. The Framm instrument, whose readings are very constant and do not change even after long use, checks the voltmeter, which, however, has finer divisions.

Conclusion. Above we have indicated tachometers which are used with steam turbines, or at least can be used with sufficient success. There exist many other tachometers, based on other principles, about which we shall not speak, in view of the fact that for steam turbines they are less suitable than those indicated above.

As regards the comparative merits and shortcomings of the enumerated instruments, from this point of view they may be arranged in the following order: the most reliable, almost not requiring subsequent checking, are resonance tachometers. Their

§ 3. Comparison of Various Tachometers

the readings may change only because of rusting of the tongues, to which attention must be paid. The degree of accuracy of their readings depends on the interval, in revolutions, between adjacent tongues and can be brought to \(\pm 0.25\%\). The effect of temperature is no more than \(0.2\%\) per \(10^\circ\).

Fig. 32.

Next come instruments with liquids—bifilar tachometers and the Zöllner instrument, in which, however, the readings may change with time owing to changes in the stiffness of the spring. The degree of accuracy of their readings is from \(\pm 0.5\) to \(\pm 1.0\%\). The effect of temperature is very small.

Electrical instruments require calibration and, in general, are less reliable than the preceding ones, although they do not wear appreciably from use. The degree of accuracy of the readings of instruments with eddy currents is about \(\pm 0.5\%\), but the effect of temperature is rather strong and may increase the error by another \(1\%\) per \(10^\circ\). Approximately the same degree of accuracy is found in electrical instruments with voltmeters.

Tachometers with centrifugal pendulums are the least sensitive and accurate. The errors even in good instruments range from \(\pm 2\) to \(\pm 5\%\). The effect of temperature, however, is small: per \(10^\circ\), about \(-0.4\%\) at the beginning of the scale, and at higher numbers of revolutions two to three times less. Their chief drawback is rather noticeable wear and a comparatively large expenditure of energy to put them into operation. Nevertheless, up to the present they have been the most widely used tachometers for steam turbines. This is explained by a certain conservatism, since they were invented and improved long before the other tachometers.

4. Handling Tachometers

When handling tachometers, one must always remember that they are fairly delicate instruments, subject in most designs to unavoidable wear. In order to reduce the latter as far as possible, it is necessary to ensure that all places requiring lubrication are always sufficiently lubricated. This applies both to stationary instruments, which usually have several oil cups, and to the hand instruments described below, in which several openings are provided into which a drop of oil must be introduced periodically. The oil used should be the thinnest, not thickening, so as not to increase friction: vaseline oil, bone oil, or the highest grade of mineral spindle oil; good wood oil may also be used.

It is further necessary to ensure that, as far as possible, dust does not get into the tachometer, since it increases friction. To remove dust, which sometimes forms a sticky mass together with the oil, all bearings and lubricating points should from time to time be washed with kerosene. Disassembly of tachometers should be avoided and entrusted only to an experienced mechanic accustomed to delicate work.

Bringing stationary tachometers into operation in steam turbines is almost never done by means of a belt, in view of the large number of revolutions. Even on small turbines the tachometer usually constitutes an indispensable accessory, forming, as it were, a single whole with the turbine. For turbines of medium power, of 150–200 h.p., and for large turbines, this is true without exception. In this case the tachometer is driven from the turbine shaft by means of gear, screw, or worm transmission. Sometimes the tachometer is driven not from the main shaft, but from some auxiliary shaft, for example, the governor shaft. The arrangement of the tachometer may then be quite varied—with a horizontal axis of rotation and with a vertical one; in the latter case, with the dial upward or downward. Sometimes, though rarely, the tachometer shaft is connected to the turbine by means of an engaging clutch, allowing the instrument to be switched on while the turbine is running. The rare provision of the instrument with such a clutch is explained by the importance of the tachometer in the operation of the turbine.

Testing of tachometers.—The readings of tachometers, except perhaps for bifilar and resonance tachometers, require testing from time to time, since they are subject to changes owing to changes in the stiffness of springs, the strength of magnets, and so forth.

The actual testing of tachometers of all systems is possible only directly by experimental means; the tachometer is, by one means or another, successively given exactly known numbers of revolutions, and it is observed whether the readings of the tachometer pointer coincide with them. The numbers of revolutions are measured in this case by summing counters or by means of an accurate, calibrated tachometer.

If the tachometer is “built into” a steam turbine, its testing is carried out on the turbine itself.

§ 4. Checking Tachometers.

Of course, when being checked, the tachometer must be in completely proper order: well cleaned, washed with kerosene, lubricated, and all parts properly tightened.

If the check reveals incorrect readings, then one may either find a correction coefficient for the range close to the normal number of revolutions of the given turbine, or compile a table of corrections, or, finally, install a new dial, divided according to the observed deviations of the pointer.

In some engineering laboratories of German polytechnic institutes, and also at larger factories that build tachometers, special devices are available for their calibration. The principal requirement imposed on such devices is to obtain strictly uniform rotation of a shaft, to which, in one way or another, the tachometer being checked is connected. The exact determination of the number of revolutions of this shaft is sometimes carried out with the aid of a special recording device.

In view of the high cost of these devices, which have to be used comparatively infrequently, they cannot be regarded as a necessary accessory even of large stations or turbine-building works; therefore we shall not dwell on them in greater detail, referring those interested to the relevant literature³.

5. Hand Tachometers.—For checking the readings of the tachometer installed at the turbine, and also when testing small turbines that do not have their own tachometer, hand tachometers may be used.

Tachometers with pendulums are the most widespread and indeed the most perfect of the hand instruments. One of the important requirements imposed on hand instruments is small size and weight, in view of which, in most cases, 2, 3, and even up to 4 measuring ranges are combined in them.

Fig. 33.

Fig. 33 shows a tachometer for speeds from 200–8000 per min., divided into 3 groups. The tip pressed against the

to the end of the machine shaft, and the corresponding projecting end of the axle is fitted onto it. The middle axle is connected directly with the axle with the pendulums; the right-hand axle is connected with it by a gear train of \(1:3\), and the left-hand one by \(10:3\).

The tips are fitted on, or else are three-sided, steel or rubber ones—the latter are better. To avoid the tip slipping on the axle of the instrument, the latter is provided at its end with a slot, Fig. 33, into which the corresponding transverse key of the tip fits.

When using the instrument, care must be taken that the tip does not slip on the core of the shaft. The direction of rotation is immaterial.

Fig. 34 shows an instrument of improved design: to engage the corresponding transmission between the pendulum axle and the tip axle, one must move the button located on top, whereupon

Fig. 34.

the mark on the plate connected with it indicates the limits for which the instrument is set in the given case. The setting is made before pressing the tip against the turbine shaft; in doing so, care must be taken that the measured number of revolutions is not greater than the upper limit set in the given case. If the number of revolutions of the turbine is unknown, one begins with the upper limit. If the pointer does not deflect at it, the instrument is switched to the next one, until the desired deflection is obtained.

Fig. 35.

In Fig. 35 is shown the mechanism of this instrument: \(a,a\)—four massive little cylinders—the pendulums, which move apart during rotation and

§ 5. Hand Tachometers.

…moving a toothed rack concealed in the hollow axis of rotation; the rack is meshed with a pinion, on whose axis the pointer b is mounted; c, c are two springs returning the pendulums and the pointer to their initial position; d is a button by which the toothed gear trains are shifted in order to obtain the corresponding transmission ratio between the axes; to reduce friction and wear, the thrust bearing e, which takes up the pressure on the shaft, is made with balls.

At the present time there exist still more convenient designs with automatic setting of the required measuring ranges. In some instruments this is achieved by the action of the centrifugal force of the pendulums, which presses back the corresponding latch, whereupon the released spring switches the corresponding transmission mechanism for the higher ranges. Against this design one may cite excessive complexity and fragility. In other instruments, as in that shown in Fig. 36, the idea of automatic setting consists in the fact that, for a reliable frictional connection of the tip axis with the axis of the pendulums, at a larger number of revolutions of the tip a sufficiently smaller pressing force is required than at a smaller number of revolutions.

Fig. 36.

In the instrument according to Fig. 36, a spring presses outward the slides with the transmission wheels. If the tip of the instrument is pressed lightly against the end face of the rotating shaft, the axis is displaced hardly at all, but the mechanism is engaged for the higher measuring range. If, in this case, the pointer does not move, this indicates that the measured number of revolutions is lower. Then the pressure is increased; the slides move somewhat and connect the next, faster transmission and the corresponding lower measuring range. Still stronger pressure engages the next still lower range, and so on. If the pressure is reduced, the spring pushes the slides forward and sets the instrument to a higher number of revolutions.

Together with the displacement of the axis, a small plate with the corresponding measuring ranges appears in the little window. Thus, the instrument in Fig. 36 has three ranges: 1000–4000, then 300–1200, and finally 100–400. The pawl shown below serves so that, if desired, one may lock—

Chapter I. Measurement of Speed. § 5.

…to cover a certain range of measurement, for which the pawl is pressed somewhat to the side.

Hand tachometers with pendulums are normally made for up to 12,000 rev./min., but there are instruments also for up to 16,000 and even up to 24,000, which is achieved by introducing the corresponding gearing.

Special attachment. When pressing the tachometer against the shaft by hand, besides tachometers with automatic setting of the measuring range of the kind shown in Fig. 36, it may be pressed with the tip either too strongly, which causes excessively rapid wear and also affects its sensitivity and accuracy, or too weakly, in which case the readings will be lower than the actual values.

To avoid these shortcomings, especially for attaching more precise control tachometers, the following electromagnetic coupling may be used, Fig. 37⁴: the axis of the tachometer

Fig. 37.

is clamped in the sleeve $a$, which is connected by means of the flexible shaft $g$ to the disk $b$, having two pins. The instrument is applied to the end of the turbine shaft by means of the core $c$; then direct current is switched on, which, entering through one of the sliding ring contacts $d$, passes into the electromagnet $e$ and returns through the other contact $d$; the electromagnet $e$ is attracted to the end of the shaft, the core $c$ is pressed in, compressing the helical spring, and the pins $b$ enter the corresponding holes in the end face of the shaft. The electromagnet provides only the attachment, while the torque is transmitted by these pins without any axial pressure on the tachometer.

Other hand tachometers.—In addition to tachometers with weights, some works manufacture instruments with eddy currents. The principle of these instruments is exactly the same as that of the constant tachometers described above. There is only a certain difference in construction, chiefly in the absence of a pointer and in the use of a movable scale.

Figures 38 and 39 show the external view and section of such a hand tachometer⁵: the bell-shaped steel magnet $m$ and the iron core $k$ are fastened to the axis $a$ by means of magnetic insulation.

§ 5. Hand Tachometers.

Into the annular space between the magnet and the core enters a light aluminum bell \(g\), the axis of which rests at one end in a recess at the end of the axle \(a\), and at the other—in a fixed frame. To the bell \(g\) is connected a ring \(r\), on which division scales are marked; they are observed through the little window \(f\) with a thread across the middle. Two spiral springs \(s\), wound in opposite directions, are fastened

Drawing 38 and 39.

at one end to the axis of the bell, and at the other—to the fixed frame. The scales for the left and right directions of rotation are common, but for one the numbers run from the common zero to the left, and for the other—to the right, which is sometimes confusing in readings, especially at numbers of revolutions close to the middle of the scale; for example, for the instrument shown in Drawings 38 and 39, indicating from 0 to 500, near 250 revolutions; when taking a reading it is necessary to remember in which direction the scale moved from 0.

In general the instrument is fairly accurate. For very large numbers of revolutions, not to mention several measuring ranges, it cannot be built, which substantially hinders its spread.

Although there exist—but they have not become widespread because of their bulkiness and heaviness, and also because of the narrowness of the limits of measurement—hand bifilar tachometers, we therefore do not cite them.

6. Tachographs.—A tachograph is an instrument measuring the speed of rotation of a machine, with a self-recording device. The purpose of a tachograph is to record fluctuations in the speed of rotation.

The rotation of steam turbines, even slow-running ones, is so uniform that the speed fluctuates noticeably only when the load changes.

Tachographs may be divided into two principal groups: the tachometers described above, equipped with a self-recording device, and tachographs proper, specially designed, particularly sensitive instruments. We shall begin with the former.

Recording tachometers. These are almost exclusively instruments with centrifugal pendulums. They are used chiefly as monitoring instruments at large stations. For the actual testing of the degree of regulation of a machine when the load changes, they give a somewhat insufficiently fine diagram; nevertheless, special types among them are also applicable for this purpose.

Fig. 40.

Figure 40 shows a sample of such an instrument, manufactured by the Horn works; for steam turbines it is made only with a different measuring range and with narrower limits, about \(\pm 10\%\) of the mean value corresponding to the normal number of revolutions of the given turbine.

The slide, to the end of which a special pen is attached, is displaced by the same deflection of the centrifugal pendulums as the pointer. The pen consists of a small cylindrical vessel with a fine capillary orifice and a tube at the bottom. Recording is made on a paper strip, 60 mm wide, with a total length of 50 m, rewound by means of a clockwork mechanism from one spool to another. The speed of motion of the paper can be set as desired, by moving a special lever, from 0 to 10 mm/min. The scale of ordinates is 1 mm \(=0.5\%\) of the mean number of revolutions. It is most convenient to drive the tachograph by means of a belt transmission.

In acceptance tests, the determination of the regulation of a turbine can also be carried out with a hand tachograph, Fig. 41. This is the same hand tachometer, only of larger dimensions, connected with a writing attachment and a clockwork mechanism. As the speed increases, the pen moves away from the center. The mean speed on the tachogram is 120 mm/min with a paper disk diameter of 90 mm. It is convenient to order the speed limits from the factory from 1 to 1½ times; for example, at 3

§ 6. Tachographs.

in the ranges: 1000–1500, 1400–2100, and 2000–3000 rev./min. The scale of speeds is obtained in this case as 0.75 mm = 1%. The weight of the instrument is about 1 pood.

Drawing 41.

Horn’s tachograph. For obtaining more accurate diagrams, the most convenient is the well-known Horn tachograph, drawings 42–45.

Drawing 42 gives a side view with the casings cut away over the centrifugal pendulums and the writing mechanism; drawing 43 is an end view of the writing mechanism with a section through its casing, which is removed while the instrument is in operation; drawing 44 is likewise a top view with the casings cut away, and drawing 45 is a perspective view, both casings being represented conventionally as transparent, as though made of glass.

In essence this is the same tachometer with centrifugal pendulums $k,k$, stretching, as they diverge, the springs $f,f$, only it is very powerful and sensitive; the lever mechanism $b,c$ transmits the displacement of the pendulums through a bored shaft and a ball-and-socket joint of the slide $t$ with a specially shaped pen $S$; the pawl $h$ serves to lower the pen until it touches the paper and to raise it back again; the paper tape $i,i,i$ is rewound from drum $P$ to $Q$, passing around drum $W$, along whose generatrix the pen travels, and a guide roller; the width of the paper tape is 60 mm, and of its useful part, i.e. the greatest height of the diagram, 48 mm.

In order to impart motion to the tape, the small lever $e$ must be lowered into the horizontal position, whereby the friction wheel $F$ is pressed against the wheel $E$, which receives continuous rotation from the main shaft of the instrument by means of a cord transmission made of a thin helical spring. The axle of roller $F$ is connected by a worm gear with roller $y$, and the latter rotates, by means of the same kind of helical spring $s,s$, the roller $Q$; the lever $V$ serves for shifting wheel $F$ relative to the axis of wheel $E$: the closer to the center, the lower the speed of motion of the paper. The mean speed of the main axis of the instrument with the pendulums must be set at 500 rev./min.; so considerable a speed makes the instrument sufficiently powerful even with not especially heavy pendulums, which, in turn, ensures sufficient sensitivity.

The speed of the paper can be set from 1 to 20 mm/sec, as indicated by pointer \(Z\) on scale \(g\). Wheel \(F\) is placed to the right or to the left of the axis of wheel \(E\), depending on the direction of rotation.

Figs. 42–44.

In order to mark on the tachogram certain moments, for example changes in load, there is a striker \(L\), which, when pressed quickly, makes a dot on the paper; it is actuated either directly by hand or by means of electromagnets, through which current is passed at the required moments.

§ 6.

In order to obtain the required average speed of 500 revolutions per minute, the instrument is usually provided with a set of light aluminum stepped pulleys, three diameters on a common hub, from 209 to 500 mm in steps of 5%. For the sake of greater sensitivity of the instrument one must try to make the circumferential force as small as possible, i.e., to take as large a pulley diameter as possible.

The tachograph is connected to the turbine by means of a belt or woven canvas band, permitting a small amount of stretch. The joint should be as smooth and imperceptible as possible, and on a belt the ends should be glued together.

The driving pulley on the turbine shaft may be made of the lightest possible kind, of small width, 30–50 mm. It is important only that the pulley be perfectly round and that there be no wobble. It is convenient to use a wooden pulley, gluing it simply from two mutually perpendicular rows of one-inch boards and boring its bore for fastening

Drawing 45.

to the shaft—turning it in the direction of rotation of the machine. It is often most convenient to attach it directly to the end face of the free end of the turbine shaft. Often, instead of a pulley, one may use a coupling or even simply a smooth portion of the shaft, if its diameter, at the given number of revolutions, gives the tachograph the required speed with one of its standard pulleys.

The place on the shaft from which rotation is taken, in steam turbines, in view of the uniformity of their rotation, is of no essential importance.

Chapter I. Measurement of Speed. § 6.

values. With turbodynamos it is only necessary to avoid taking the rotation from the dynamo shaft if the latter is connected to the turbine by an elastic coupling.

A good tachograph usually has a set of springs of different stiffness for maximum non-uniformities of $\pm 3$, $\pm 6$, and even $12\%$. The latter have to be used in the tests described below with retarded rotation, for determining the magnitude of the mechanical resistances.

Horn tachographs are also made with a completely uniform feed of the paper by means of a clockwork mechanism; but for turbines they have to be used only in the aforementioned tests with retarded rotation.

Checking the Horn tachograph. Although the apparatus described is distinguished by a very well-considered and expedient design and careful workmanship, nevertheless, for more accurate investigations it is still useful to check it—or, more precisely, to establish the scale of the ordinates of the tachograms, i.e. the deviations of the graph paper strip.

The author performs this check in the following way: from a machine rotating as uniformly as possible—best of all from a steam turbine or a high-speed electric motor—and making an exactly known number of revolutions per minute, the tachograph is set in motion, selecting the gear ratio of the pulleys so that the instrument makes exactly 500 r.p.m.

If the pen point does not then settle on the middle zero line of the tachogram strip, the pen is shifted on the half-coupling until the indicated coincidence is obtained; or, if this proves impossible, the number of revolutions of the tachograph shaft is changed until the pen stands opposite the zero line. In the latter case, when calculating the transmission ratios for driving the tachograph, for the given apparatus and set of springs, the number obtained is taken instead of 500 revolutions.

After this the belt is shifted to the adjacent step of the pulley, which corresponds to a change in the number of revolutions of the instrument by $5\%$, and one observes whether the pen point has stood on the corresponding line on the paper; if not, a correction factor is calculated—or, what is the same thing, the true ordinate scale is found from the observed displacement of the pen. If springs giving deviations of $\pm 6\%$ are used, it is useful to find the scale for $+5\%$, and then also for $-5\%$. With springs for $\pm 3\%$, the initial position of the pen must, of course, be taken not at 0, but near one of the edges of the strip; otherwise the $5\%$ deviation, caused by the difference in the diameters of the stepped pulley, will not fit on the tachogram. It is also useful to check the diameters of the pulleys, and if the difference accidentally differs from $\pm 5\%$, to take this into account.

Directions for use. The pen, consisting of a hairlike metal tube, becomes clogged very easily. It is therefore recommended to use special liquid ink that does not oxidize metals, and each time after

§ 6. Handling the Horn tachograph.

after use, pour out of the small reservoir near the pen the remaining ink and thoroughly rinse the entire pen with water, blow through the tube and, if necessary, clean it with the fine needle supplied with the instrument.

The pen becomes clogged so easily that the instrument is supplied with a spare one for quick replacement, so as not to interrupt the test.

Next, one must see to the proper lubrication of all parts, admitting oil into the corresponding openings. Into the two main bearings with ring lubrication it is necessary to pour a sufficient quantity of oil, and after the test to drain it through special little cocks. The oil should be of the same kind—not thickening—as indicated above for tachometers.

In order that the bearings not wear in, the belt from the turbine should not be tightened too much; and to eliminate slipping it is useful to sprinkle the pulleys lightly, especially the small driving pulley, with finely powdered rosin. A canvas belt is good precisely because, being sufficiently flexible and sufficiently strong, it cannot give excessive tension.

Accuracy of the instrument. Although the masses of the moving parts of the instrument have been reduced, through the use of aluminum and steel, to the smallest possible value in comparison with the live force of the weights, nevertheless the pen of the instrument lags somewhat behind the actual changes in the speed of the machine. The lag occurs for two reasons: on the one hand, from the transmission—the elasticity of the belt or band and the live force of the pulley on the instrument; on the other hand, from the inertia of the weights and of the other moving parts of the instrument. These circumstances affect the tachogram in the sense of smoothing it; the speed fluctuations found from the tachogram may be regarded as somewhat diminished. As for the displacement of the abscissae, the lag of the record, since it has approximately the same magnitude for the beginning and the end of the phenomenon, practically does not affect the accuracy of the tachograms.

Examples of tachograms. In Fig. 46 is given a photograph of a tachogram taken from a Laval turbo-dynamo, normally developing 100 kW at 1050 rpm, with the dynamo exhausting steam into the atmosphere*). As

Fig. 46.

we see, with an instantaneous change of load from 0 to \(1/2\) and back, the change in the number of revolutions is somewhat more than \(2\%\); with a change of load from \(1/2\) to \(1/4\) and back to \(1/2\), the change is only about \(1\%\).

*) K. Mewes, Dampfturbin, Berlin 1904. S. 243.

In operating Laval turbines with a condenser, the tachograms show still smaller oscillations.

Fig. 47.

Fig. 47 shows a photographic print, reduced threefold, of a tachogram of a Parsons turbodynamo of the Brown-Boveri & Co. works, rated at 300 kW and making 2700 r.p.m. \(^{8}\). The tachogram shows that the number of revolutions at full load and at no load differs by only 2%; with an instantaneous change of load from full to 0, the speed jump reaches 3%, but after 3.5 sec. it is already smoothed out, and after 6 sec. the speed is completely established.

Figs. 48 and 49 show tachograms taken in 1900 from an English Parsons turbodynamo of 1000 kW, with the turbine operating with a centrifugal governor \(^{9}\); the mean normal number of revolutions is 1500. The load was changed by 25% from normal. The jump, the ordinate \(a\), amounted to 1.8% of the normal speed; the quantity \(b\)—the over-regulation—was about 0.8%; the difference between the preceding and the subsequent speed, the quantity \(a-b\), was not more than 1%. The time required for the establishment of the new speed was 10 to 15 sec. With a reduction of the load by the same 25%, Fig. 49, the character of the phenomenon is exactly the same, and the numerical values are also close. The difference in the numbers of revolutions at full load and at no load was about 4%.

Figs. 48 and 49.

unloading	loading
\(+10\%\)	\(+10\%\)
\(+5\%\)	\(+5\%\)
0	0
\(-5\%\)	\(-5\%\)
\(-10\%\)	\(-10\%\)

When the same turbodynamo was operated with an electric governor, interesting tachograms were obtained, Figs. 50 and 51. As can be seen, their character is just the reverse of normal regulation: with an in-

\(^{8}\) Z.V.d.I. 1902, S. 1169.
\(^{9}\) Z.V.d.I. 1900, S. 883.

with an increase in load the number of revolutions increases, and conversely. With a successive change of load by 25% from normal, the greatest throw, the quantity \(d\), was about 1.3%; the throw before the action of the governor, the quantity \(a\), was from 0.2 to 0.3%. The overspeed,

Fig. 50 and 51.

the quantity \(b\), was also from 0.2—0.3%. The difference between the established numbers of revolutions before and after the change of load, the quantity \(c\), was only about 0.8%.

instantaneous loading from 0 to 500 kW.

instantaneous unloading from 500 kW to 0.

Fig. 52 and 53.

In Figs. 52 and 53 are shown tachograms of a 500-kW turbine dynamo of V. K. E.—Kertis (General Electric Company—A. E. G.), taken in 1904.⁶ The turbine was regulated by changing the number of work—

Chapter I. Measurement of Speed. § 6.

…of the nozzles. With a rapid change from 0 to full load, or back again, the fluctuation in the number of revolutions is only 3%. No overspeeding is observed at all. The time required for establishing the new number of revolutions is only 2–2½ sec.

Fig. 54.

Fig. 54 shows a photographic image of the tachogram of the latest “Zelly” turbodynamo of the Augsburg–Nuremberg works, rated at 1050 kW.¹¹ The turbine regulation is mixed, by throttling and by changing the number of active nozzles.

As may be seen, at full load the fluctuation in the number of revolutions is apparently about 1%, i.e., the degree of nonuniformity of the turbine is apparently \(1/100\). However, if one counts the number of teeth, it turns out that there are about 500 per minute, i.e., a number equal to the revolutions of the tachograph, not of the turbine, which made 1500 rev./min. In view of this, these fluctuations should rather be attributed to a certain defect of the tachograph—or, more precisely, of bringing it into action.

Figs. 55 and 56.

¹¹ Catalogue of the M.A.N. works, Mitteilung Nr. 20, S. 14.

1. Forsch. H. 12, S. 10. ↩

2. Z.V.d.I. 1911, p. 1795. ↩

3. Z.V.d.I. 1909, S. 483; Forsch. H. 100; Fr. Pflug, Geschwindigkeitsmesser, Berlin, 1908, S. 150. ↩

4. Z.V.d.I. 1910, p. 2174. ↩

5. Forsch. H. 100, p. 75. ↩

6. Z.V.d.I. 1904, S. 1212. ↩