APPLICATION OF AN ELECTROMAGNETIC TRANSDUCER FOR MEASURING THE STARTING TORQUE OF INSTRUMENT BALL BEARINGS

I. G. Leshchenko, E. F. Kolotii, Yu. A. Bannov

(Presented by the scientific seminar of the Faculty of Automation and Computer Engineering)

At present, quality control of ball bearings by friction moments is carried out under factory conditions selectively and, for the most part, only by mechanical methods, which do not make it possible to obtain high productivity in bearing testing at a specified measurement accuracy [1]. At the same time, instrument ball bearings of small dimensions present a particular difficulty for inspection, when the permissible friction moments range from 0.1 to 2.5 g·cm. The transition from selective inspection to continuous inspection requires, in principle, new methods with increased productivity, on the basis of which it would be possible to create the corresponding automatic machines that allow the entire cycle of the required bearing tests to be performed.

Below, one of the possible methods is described for electromagnetic measurement of the static friction moment (starting torque) of ball bearings with a vertical axis of rotation and with a load corresponding to the technical specifications.

The basic design of the device and the measuring circuit for determining the friction moments are shown in Fig. 1. The bearing under test 1 is mounted on shaft 2, fixed rigidly to the base of the instrument. The required axial load on the outer ring of the bearing is provided by a removable metal cup 3, whose center of gravity is below the plane of the ball bearing for stable motion and uniform distribution of the load. Attached to the cup on two light brackets 4 are identical cores 5 made of ferromagnetic material of the Armco type, which are attracted by coils \(K_1\) and \(K_2\), creating a torque of the outer ring when current passes through the coils. A shutter with a narrow slit 6, forming part of a photoelectric transducer, is also fastened to this same movable part. In addition, the device provides for an electric motor ED with an electromagnetic clutch EM, intended for automatic rotation of the movable part 7 to specified positions. The second projection of the figure shows the basic measuring circuit.

In the initial state, the movable part of the apparatus, connected with the outer ring of the bearing by friction forces, is set in a position in which the photoresistor FS, connected into the circuit of the photorelay, is illuminated through the slit of the shutter by lamp \(L_1\). In this case the contacts of photorelay FR 1—1 and 3—3 are closed. The potential on the grid of the triode ra-

is brought to zero, the tube is “closed,” and the current in the anode circuit, which creates the magnetomotive force in the identical coils \(K_1\) and \(K_2\) connected in series, is absent.

Fig. 1.

When the installation is started for measurement by pressing the “start” button, the capacitor \(C_1\) begins to charge through the resistor \(R_3\) from the voltage divider \(R_1 \div R_2\); the potential on the grid of the tube rises over the required interval of time, determined by the charging time constant

\[ \tau_{\text{ch}} = C_1 \left( R_3 + \frac{R_1 \cdot R_2}{R_1 + R_2} \right), \]

while the current in the anode circuit increases, practically according to a linear law. As the current in the coils \(K_1\) and \(K_2\) increases, the attractive force increases, and consequently so does the torque; as soon as this torque exceeds by a small amount the static friction torque of the bearing, the moving part breaks away from its position and blocks the photoresistance \(FS\) with a shutter. The photorelay \(FR\) operates, opens contacts 1—1 and 3—3, and closes contacts 2—2 and 4—4, as a result of which the capacitor \(C_1\) discharges through the resistor \(R_4\), and the drive rotates the moving part of the installation, together with the magnetizing coils and the outer ring of the bearing with the load, through one tenth of a circumference, as specified by the technical requirements for instrument ball bearings.

At the moment the photorelay operates, the inspector notes the breaking-away torque on a milliammeter calibrated in units of torque.

For convenience and for a more accurate reading of the maximum current, a capacitor \(C_2\) is connected in parallel with the milliammeter; after the anode circuit is switched off, this capacitor will discharge through the instrument and thus eliminate the possibility of a rapid decrease in the current. The instrument is designed for the maximum permissible starting torque, which is additionally monitored by the signal lamp \(L_2\) (“Reject”), connected into the contact circuit of the maximum-current relay. After the moving part is turned to a new initial position for measuring the starting torque, the measuring circuits are automatically switched on and the measurement process is repeated. The installation is switched off by the “Stop” button, or automatically when a starting torque greater than the permissible value appears.

Since, in flow testing of bearings, it is usually necessary to sort them only into serviceable and rejected ones, this device and the principle of monitoring static friction moments can be used in the creation of integrated automatic installations for testing ball bearings and other types of bearings under production conditions, especially since the accuracy of measuring torques other than the nominally permissible one does not play an essential role. The sensitivity of the model of the installation developed at the Department of Electrical Measuring Engineering of the Tomsk Polytechnic Institute is, at a nominal starting torque of \(2.5\) g·cm, \(4\ \mu\text{A}/\text{mg}\cdot\text{cm}\). The calculated productivity for sorting serviceable bearings is on average 60–100 bearings per hour, which considerably exceeds the currently existing norms.

REFERENCES

1. R. M. Matveevskii. Investigation of Friction in Instrument Ball Bearings. Mashgiz, Moscow, 1957.