INDUCTIVE TORQUE METER WITH REPLACEABLE ELASTIC ELEMENTS

A. V. Murin, Yu. I. Kapustin

(Presented by the scientific seminar of the Department of Applied Mechanics)

In studying the operation of machines or of their individual assemblies, it is very often necessary to measure torques. For measuring moments that vary with time, the deformations of elastic elements transmitting the torque are usually used. The measurement of elastic deformations, proportional to the transmitted torque, is carried out by various methods [1—6], but of particular interest is the use, for this purpose, of inductive sensors, which make it possible to obtain a powerful electrical signal that can be recorded by means of a loop oscillograph without the use of amplifying devices. This circumstance is of great importance when investigations are carried out directly under production conditions.

Known designs of torque meters with inductive sensors are fairly complex [1—3; 7]. Some of them are intended for measuring moments only within a narrow range; for others, expansion of the measurement range is associated with complete disassembly of the structure and replacement of the principal, rather complicated parts of such a device. In carrying out investigations, however, it is often necessary to use one and the same device for measuring moments over a wide range. A necessary condition in this case, naturally, is to ensure maximum sensitivity of the device in any range of measured moments, with minimum expenditure of time on assembly work connected with replacing the elastic elements of the device and on manufacturing such replaceable elements. In this connection, we have developed, manufactured, and tested an inductive torque meter with replaceable elastic elements, simple in design, which makes it possible, when necessary, easily and quickly to replace the elastic elements without removing the torque meter from the installation and without fully disassembling it. A distinctive feature of the proposed device is also the simplicity of the design of the replaceable elastic elements used in it.

The arrangement of the developed torque meter can be seen from Fig. 1. Disk 2 is fastened by welding to hollow shaft-bushing 1. Disk 3 is mounted on shaft-bushing 1 on two roller radial bearings 4. Disks 2 and 3 are connected by rods 5, ending, as can be seen from Fig. 1, in flanges. Rods 5 serve as the elastic elements of the measuring device. Thus, transmission of the measured moment from shaft-bushing 1, which is rigidly connected

Fig. 1. Design of the torque meter.

with the shaft of the machine or of its unit, disk 3 is effected through two elastic elements 5. In turn, disk 3 is connected with the adjacent link to which the torque is transmitted, in this version by means of three pins 6, which are parts of a pin coupling with an intermediate element. Of course, other methods of coupling disk 3 with the machine links that set it in motion are also possible. Under the action of the transmitted torque, elastic elements 5 are deformed and disk 3 turns through a certain angle relative to disk 2. These angular displacements of disk 3 are detected by means of two inductive differential transducers connected into a differential measuring electrical circuit (Fig. 2), which has a number of advantages over a bridge circuit [3]. The cores 7 of the inductive transducers, assembled from sheet transformer steel, together with their windings 8, are fastened in pairs on two plates made of aluminum alloy,

Fig. 2. Electrical circuit for connecting the inductive transducers: SN—voltage stabilizer, K1–K4—current-collecting rings, D1–D4—working transducers, D—trimming choke.

which are mounted diametrically on disk 2 (Fig. 1). Two armatures 9, also made of sheet transformer steel and fastened on brackets 10 of aluminum alloy, are mounted on disk 3 so that between them and cores 7 there is an initial gap of about 0.1 mm.

Angular displacements of disk 3 cause a change in the air gap between armatures 9 and cores 7 of coils 8, which leads to a change in their magnetic flux and, as a result, to a change in the current flowing in the measuring diagonal of the electrical circuit. This change in current is recorded by a milliammeter or fed to the vibrator of an oscilloscope (Fig. 2). The transducer windings are made of PEV-2 wire 0.14 mm in diameter and have 3150 turns each. The active resistance of each coil is 180–190 ohms.

Figure 3 presents the static calibration curve of the torque meter, obtained with the supply voltage of the primary winding of the working transformer Tr2 equal to 100 volts, at a frequency of 50 hertz (lines 1 and 2). The voltage at the output of this transformer (between its end terminals) with this primary-winding supply voltage is 20 volts (Fig. 2). Line 1 on the calibration curve connects the points obtained during loading, and line 2 those obtained during unloading. This calibration curve was obtained with the diameter of the elastic elements equal to 8 mm. With this diameter of the rods of the elastic elements, the magni-

the measured torque reaches 11 kgm. The smallest gaps between the coil cores and the armatures corresponding to such a load are about 0.02 mm. As can be seen from Fig. 3 (lines 1 and 2), when the transducers are supplied with current of industrial frequency, the calibration curve has a certain nonlinearity. The use of an electrical circuit with a current frequency of 500 Hz for the supply made it possible to eliminate this drawback (line 3, Fig. 3). Naturally, at such a frequency, in order to maintain a sufficiently high level of the output signal, it was necessary to increase the supply voltage. The voltage at the output of the working transformer Tr₂, between its end terminals, was then 230 volts (the parameters of the transducers and elastic elements were the same as when obtaining graph 1—2, Fig. 3).

Fig. 3. Calibration curve of the torque meter: 1 — loading, 2 — unloading.

Measurements with the proposed device of torques in the range 4—11 kgm (when using, for the supply, a current of frequency 500 Hz and an elastic-element rod diameter equal to 8 mm) are performed with an accuracy of 1—3%; measurements of smaller torques are less accurate.

Figure 4 presents the dynamic characteristic of the torque meter—an oscillogram of natural oscillations with the diameter of the rods of the elastic elements 8 mm and the other basic dimensions indicated in Fig. 1. The time marks on this oscillogram are made every 0.01 sec. The natural-oscillation frequency of the torque meter with this diameter of the elastic elements is 285—290 Hz. The frequency of forced oscillations—oscillations that can be recorded without distortion in this case, in accordance with [1]—must not exceed 29 Hz. Obviously, by increasing the diameter of the rods of the elastic replaceable elements, it is possible, for this torque meter, both to expand the range of measured torques and to improve its dynamic characteristic.

The simplicity of the design, the possibility of rapidly replacing the elastic elements without completely disassembling the entire torque meter, and also the possibility of recording the output electrical signal without the use of amplifying devices, by means of type VIII and even type V vibrators, allow us to recommend the developed torque meter for measuring torques at low frequencies of their variation.

Fig. 4. Oscillogram of the natural oscillations of the torque meter
(time marks every 0.01 sec.).

In conclusion it should be noted that the use of the described torque meter can be extended, bearing in mind that a similar design for measuring torque can be built into a pulley, sprocket, half-coupling, etc.

REFERENCES

1. N. P. Raevskii. Methods of Experimental Investigation of Mechanical Parameters of Machines. Publishing House of the Academy of Sciences of the USSR, 1952.

2. N. P. Raevskii. Transducers of Mechanical Parameters of Machines. Publishing House of the Academy of Sciences of the USSR, 1959.

3. A. M. Turichin. Electrical Measurements of Nonelectrical Quantities. Gosenergoizdat, 1959.

4. A. G. Sosnin. Experimental investigation of a two-drive scraper conveyor. Problems of Mine Transport. Collection of articles, Issue 2. Ugletekhizdat, 1957.

5. Yu. N. Ledenev et al. A dynamometer for measuring torques on large-diameter shafts. Vestnik Mashinostroeniya, No. 11, 1964.

6. I. I. Margolin, I. P. Ivanov. A device for measuring torques. Proceedings of Higher Educational Institutions, Mining Journal, No. 8, 1962.

7. N. Ya. Bilichenko, E. Kh. Zavgorodnii. Measurement of rotating moments of drive shafts. Izmeritel’naya Tekhnika, No. 1, 1960.