Devices for Measuring Clutch Slip

S. I. Shubovich, A. V. Murin, V. T. Gorbenko

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

In testing hydraulic, electromagnetic, and other clutches that operate in the presence of a difference between the speeds of the driving and driven members, it becomes necessary to measure this difference in speeds—the so-called slip. At present there is no generally accepted method or standard instruments for measurements of this kind; therefore the description below of two devices developed and used by the authors of the present article in testing hydraulic clutches may be of some interest.

Mechanical Device

The operation of the mechanical device is based on the use of a differential mechanism, which, as is known, has the property of adding and subtracting angular velocities. A diagram of such a differential gear mechanism is shown in Fig. 1.

Fig. 1. Diagram of a differential gear mechanism.

The sun gear 1 and the carrier 9 of the mechanism are connected, respectively, with the shaft of the driving and driven members of the clutch being tested; sun gear 4, through transmission 7, 8, is connected by a flexible shaft with a mechanical revolution counter and a tachometer. The numbers of teeth of the differential gears—

the differential mechanism are selected so that, at a speed of the driving link \(n_1\) equal to the speed of the driven link \(n_2\), the sun gear 4 remains stationary; when there is a difference in speeds, gear 4 will rotate at a speed \(\Delta n = n_1 - n_2\), rpm, the magnitude of which is the absolute slip. To obtain the relative slip \(S\) (in percent), it is sufficient to divide the value \(\Delta n\) by the speed of the input link \(n_1\) and multiply by 100.

When working with the device described, the measurement of small slips \((S = 0—10\%)\), for which the accuracy of the tachometer readings is insufficient, is carried out from the difference in the counter readings over a limited time interval, measured with a stopwatch; at larger slips \((S > 10\%)\), it is carried out from the tachometer readings, which may be graduated either in units of absolute slip or in units of relative slip (%) provided that \(n_1 = \mathrm{const}\). To record the magnitude of slip when testing clutches under transient conditions, a tachogenerator is connected to the shaft of gear 7, the voltage of which is recorded by an oscillograph or another self-recording instrument.

The principal advantages of the mechanical device described are its high measurement accuracy over the entire range of slips (from 0 to 100%) and its reliability in operation. Along with this, one should note its substantial drawback, which consists in the fact that, in order to connect the differential mechanism to the shafts of the clutch being tested, bulky—and in some cases complex—mechanical transmissions with transmission shafts are required.

Electrical device

The operation of the electrical device for measuring slip is based on the use of two electric speed sensors of identical characteristics—tachogenerators—connected for joint operation. One tachogenerator is driven by the shaft of the driving link of the clutch, and the second by the shaft of the driven link.

When the tachogenerators operate in an opposing-connection circuit (Fig. 2), the voltage in the circuit will be proportional to the slip. According to the circuit (Fig. 2), each of the tachogenerators may, at the experimenter’s discretion, be connected to the millivoltammeter, which makes it possible periodically to monitor the number of revolutions of the shafts of the clutch being tested.

Fig. 2. Electrical connection diagram of the tachogenerators.

In the laboratory of the Department of Applied Mechanics of TPI, standard ET-7 tachogenerators and an MVA-47 magnetoelectric millivoltmeter/ammeter system (accuracy class 0.5) were used to measure slip during tests of the hydraulic clutch; these were connected according to the circuit in Fig. 2. The resistance \(R_1\) was selected so that, when measuring the specified slip values, the entire scale of the instrument was used. The resistance \(R_2\) (equal to the armature resistance of the tachogenerator) was connected into the circuit in order to preserve the scale factor of the instrument when measuring the rotational speeds \(n_1\) and \(n_2\).

The device described was tested in order to establish the accuracy of the measurements. These tests showed that the deviations of the slip values measured with the tachogenerators (in the range \(S = 10\text{—}15\%\)) amount to approximately 5% of the values measured by the mechanical device described above, whose readings were considered reliable. At small slips the errors are considerably larger because of the nonlinearity of the tachogenerator characteristics. Therefore, when such an electrical device is used to measure small slips, experimental calibration of the instrument readings is necessary for this range of slip values.

Thus, the electrical method described for measuring slip provides acceptable measurement accuracy and has a number of advantages in comparison with a mechanical device, namely: 1) the tachogenerators can be connected directly (without transmissions) to the shafts of the clutch under test; 2) the recording instrument can be brought out to the test-stand control panel; 3) during testing it becomes possible to monitor the rotational speeds of the driving and driven shafts and, along with visual observations, to record slip or speeds on an oscillograph.