Disadvantages of Dual Pneumatic Brakes of Mine Hoisting Machines
Abstract Generated abstract
This paper examines operational shortcomings in dual pneumatic two-stage brakes used on large mine hoisting machines, despite their generally favorable technical design. Using brake characteristic diagrams recorded during static and dynamic tests at a Kuzbass mine, the study evaluates idle-travel times, air-pressure regulation, braking moments, and stopping behavior under simulated service and safety-braking conditions. The results show excessive delays in safety-brake actuation, especially when the service brake is disabled, and identify restricted air-discharge passages and imperfect mechanical linkages in the pressure-regulator drive as principal causes. The paper recommends enlarging air-outlet cross sections, improving adjustable discharge control, and refining manufacture of regulator linkages to reduce response times and improve the safety performance of dual pneumatic brakes.
Full Text
SHORTCOMINGS OF DUAL PNEUMATIC BRAKES OF MINE HOISTING MACHINES
I. A. BALASHEV
In recent years in the Kuzbass, a number of mines equipped with powerful hoisting installations have been put into operation. One of the critical elements of a hoisting machine, on whose condition reliable and safe operation depends to a considerable degree, is the braking device. The modern type of brake for large machines is the pneumatic two-stage brake. In recent years our plants have substantially improved and perfected this type of braking device, producing it in the form of a dual brake. In this design arrangement, each drum of the hoisting machine is acted upon by its own independent brake drive with a common pressure regulator.
In addition, all brake control has been moved to the upper platform, to the operator, which considerably facilitates supervision and operation of the braking device.
If one adds to this that the actuating mechanism is made as a design with translational displacement of the brake shoes, which has a whole series of technical advantages over systems with swinging shoes, then it may be confidently stated that this braking device is, in technical and operational respects, the best brake for large hoisting machines.
Experience from numerous adjustments and tests of dual pneumatic brakes shows that, alongside their generally positive qualities, these designs contain a number of comparatively minor shortcomings that substantially affect their qualities as devices protecting the hoisting installation from accidents under all possible combinations of unfavorable circumstances during operation.
Such shortcomings include the following:
1) an impermissibly long idle-travel time when one safety brake acts while the service brake is disconnected. In this case the idle-travel time is from 0.7 to 1 sec. instead of the possible time of 0.2 sec.;
2) a long idle-travel time during safety braking, amounting to 0.3–0.4 sec. instead of 0.2 sec.;
3) a small range of possible regulation of the air pressure in the service-brake cylinder at the first stage during safety braking.
In order to illustrate the influence of the indicated shortcomings on the quality of braking, on the one hand, and, on the other hand, wishing to outline ways of eliminating these shortcomings, we shall present actual adjustment data from recent experience at Mine No. 1.
General Data
Hoisting machine of type \(2 \times 5 \times 2.3\), skips \(2 \times 8\ \text{t}\), maximum vessel speed \(5.6\ \text{m/sec}\), pneumatic two-stage brakes, duplicated. The kinematic diagram of the brake device is shown in Fig. 1.
The method of investigation and adjustment consisted in taking diagrams of the brake characteristics. For this purpose, a cord from a special indicator, designed by the author for adjusting the brakes of hoisting machines, was fastened to point \(a\) of the differential brake beam. During safety braking, point \(a\) has two motions: upward from the service brake and downward when the loads of the safety brake are lowered. Assuming that the amount of travel of the brake load produces proportional braking moments, it is possible to obtain the brake characteristic.
Fig. 1. Kinematic diagram of the brake of an \(8\ \text{t}\) skip hoisting machine of type \(2 \times 5 \times 2.3\), Mine No. 1, 5.V.51.
The braking impulse was given by breaking the protective circuit. For this purpose, the indicator has a special key, by means of which, simultaneously with the breaking of the protective circuit, a time marker is switched on; this makes it possible to record the origin of the coordinates of the characteristic, as well as the stopping time of the machine from the moment the braking impulse is given. A third record on the indicator records the change in pressure in the cylinder of the service brake.
The calculated values of the braking moments produced separately by the service and safety brakes, and their ratio to the maximum static moment, were determined in advance.
Calculated Data
Maximum static moment of the installation:
\[ M_{\text{st. max}} = (8 + 1.1)\,2.5 = 22{,}800\ \text{kgm}. \]
The braking moment produced by the service brake (TP), depending on the air pressure in the cylinder, is:
\[ M_{\text{tr}} = 2.6{,}6 \frac{\pi \cdot 45^2}{4 \cdot 1000} \cdot p \cdot 0.35 \cdot 2.4 \cdot 0.95 = 16.6\,p\ \text{t.m.} \]
On the basis of this dependence we obtain the value of \(M_{tr}\) of the service brake:
| Air pressure in the CBT, kg/cm\(^2\), \(P=\) | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Braking torque of one side \(M_1=\), in t·m | 16.6 | 33.2 | 49.8 | 66.4 | 83 |
| The same for two halves, in t·m, \(M_2=\) | 33.2 | 66.4 | 99.6 | 132.8 | 166 |
The margin of static reliability of the two service brakes at a minimum air pressure of 4 kg/cm\(^2\) will be
\[ m_{st}=\frac{132.8}{22.8}=5.85>3. \]
Consequently, the service brakes satisfy the requirement of a threefold margin of static brake reliability.
The safety brake, with eight loads suspended on each pull rod and with a total weight of 752 kg, develops a braking torque
\[ M_{tr}=2\cdot2\cdot29.5\cdot0.752\cdot0.35\cdot2.4\cdot0.95=72\ \text{t·m}. \]
The margin of static reliability will be
\[ m_{st}=\frac{72}{22.8}=3.16>3, \]
which also satisfies the specified requirements.
Experimental Check
Experiment 1. The machine is stationary, the brakes are released, and shims are placed under the armature of the service-brake electromagnet in order to create conditions for the operation of one safety load brake. In this experiment, artificial conditions were created for possible non-operation as a result—
Fig. 2. Static characteristic of the operation of one TP (without TR). Experiment 1.
—of sticking of the service brake, i.e., the first stage of braking was excluded from the safety-braking process. The results of the experiment are shown in Fig. 2. The inadmissibly long idle-travel time, which was brought down to 0.9 sec by prolonged adjustment of the three-way valve and of the throttle bolt, is noteworthy; the latter had been completely unscrewed, since at first this time was 1.42 sec. Thus, all adjustment possibilities were used, and nevertheless it was not possible to reduce the time below 0.9 sec.
A careful study of the air-outlet communications from the cylinders of the safety brakes showed that from both cylinders the air exits through tubes 3/4 inch in diameter, joins in a tee, and goes through a 1-inch tube to the three-way cock and through the same kind of tube to the pressure regulator, and then through a 1 1/2-inch tube to the muffler.
The counterflow movement of air from the TP cylinders through tubes of small diameter, though sufficiently long, at the critical velocity of air motion slows the process of brake actuation. A fundamental design change of this unit is required. The idle-travel time must be reduced to 0.2 sec instead of the inadmissible 0.9 sec, and this can be achieved by sharply increasing the flow sections of the pipelines, the three-way cock, and by installing a good adjustable bolt—needle, by means of which it would actually be possible to regulate the rate of air discharge from the cylinders of the safety brakes. In the version in which it exists at present, the needle bolt loses all meaning, and it has to be removed from the three-way cock as a superfluous part.
Experiment 2. With the machine motionless and released from braking, a braking impulse was applied in order to obtain the static characteristic of the brake. A typical characteristic of this experiment is shown in Fig. 3.
Fig. 3. Static characteristic of safety braking. Experiment 2.
The movement of the pressure regulator was adjusted to an air pressure at the first stage of 1.5 kg/cm². The results of the experiment are indicated in the same Fig. 3.
What attracts attention is the long idle-travel time, i.e., the time from the beginning of the application of the braking impulse to the beginning of the motion of the service-brake piston. In this case this time ranged within 0.28–0.32 sec, whereas for such quick-acting brakes it should be 0.15–0.2 sec.
This idle-travel time includes: the actual operating time of the KTP contactor, and of the braking electromagnet of the service brake, which moves the pull rod of the pressure regulator through a lever kinematic linkage. Despite increasing the load on the pull rod of the braking electromagnet, it was not possible to reduce this time. The lever kinematic linkage from the armature of the electromagnet to the pressure regulator undoubtedly requires more careful factory manufacture, with careful machining of the stop cam, roller, regulating pull rod, and other parts of this unit, so that the falling of the armature occurs without sticking.
Experiment 3. The dynamic characteristic of safety braking was taken while the machine was in motion. With the meeting of empty vessels in the mine shaft and at their full speed of motion—in this case...
more often 5.6 m/sec—an impulse was applied for braking, and an automatic record was made of the machine stopping time. The purpose of this test is to verify that the safety brake provides an average deceleration within the limits: when hoisting a normal load, not more than \(5\ \text{m/sec}^2\), and when lowering a load, not less than \(1.5\ \text{m/sec}^2\).
In its character the dynamic characteristic is analogous to the static one shown in test 2. For greater clarity, on the basis of the experimentally obtained characteristic in coordinates travel—time, a dynamic characteristic was constructed in coordinates moments—time; it is shown in Fig. 4.
Fig. 4. Dynamic characteristic of the safety brake of a skip 8 m machine. Mine No. 1, 5.8.1951.
Subsequent checks during hoisting and lowering of the load give nothing new and confirm the correctness of the calculated and adjusted experimental data.
At the same mine two machines of type \(1 \times 4 \times 2.5\) with single-cylinder drums have been installed, but with a brake system of the same type. Since the volumes of air in the TP cylinders of these machines are smaller, while the pipelines are the same as in the machine considered above, the idle-travel time of the safety brake (analogously to test 1) proved to be somewhat less—0.7 sec in one and 0.8 sec in the other. This also confirms the need to increase the flow cross sections in order to accelerate the escape of air in all dual brakes.
Similar results were obtained on several other new hoisting installations that have entered service during the last two years.
Conclusions
On the basis of the experimental data presented, as well as many others not cited here, it appears possible to draw conclusions aimed at improving the operating characteristics of two-stage dual brakes designed by domestic plants.
- It is necessary to revise the entire system of pipelines for the discharge of air from the cylinders of the safety brakes, including the three-way valve and the switching valve when a release device is present. The flow cross sections for the air outlet must be sharply increased, by two to three times. In addition, it is highly desirable to make the needle bolt adjustable, and, in the event that the electromagnet of the service brake fails to operate, the needle bolt should open and thereby accelerate the air discharge, so as to have an idle-travel time of no more than 0.2–0.3 sec in the event of jamming of the service-brake piston and operation of only one load (safety) brake, which is possible under machine operating conditions.
-
The most serious attention must be paid to the thoroughness of the manufacture (up to and including grinding) of the lever kinematic linkage from the rod of the service-brake electromagnet to the lever of the pressure regulator. It is desirable that the stop cam limiting the travel of the pressure regulator be adjustable or replaceable, in order to make it possible to easily regulate the air pressure at the first stage within the range from 1 to 2.5 atmospheres. The lowering of the electromagnet armature under the action of a load of 15–20 kg must occur freely.
-
Elimination of the shortcomings listed above will significantly improve the operating qualities of dual pneumatic brakes and will place them among the best modern brakes for hoisting installations in the mining industry.
Noted Misprints
| Page | Line | Printed | Should read | By whose fault the misprints were allowed |
|---|---|---|---|---|
| 7 | table 1 | $2 \times 2 \times 2{,}4$ | $2 \times 6 \times 2{,}4$ | Editors |
| 16 | fig. 4 | $X_0 = 79$ m | $x_0 = 7{,}9$ m | Author |
| 35 | 23 from bottom | for 10 | for 9 | Author |
| 92 | fig. 6 | 65% | 55% | Author |
| 93 | table 2, column 6 | $a_1$ | $a$ | Author |
| 131 | item 8 | installations | installation | Author |
| 131 | item 14 | installations | installation | Author |
| 132 | 7 from top | shunting | shunting | Author |
| 134 | 3 from bottom | identically | almost identically | Author |
| 163 | 16 from top | $r \rho / s^2$ | $r c / \rho^2$ | Editors |