Measurement of small friction moments

Abstract

The most widespread methods of those examined by us for measuring small friction moments consist at present of the pendulum and calibrated spring methods for laboratory research, and of the self-braking method for industrial use. This difference is due to the fact that the self-braking method is speedier, but it can only determine the mean value of the friction moment over a given time interval, which is often insufficient for research work. The pendulum method possesses a theoretically high sensitivity; however, the measurement of small moments entails several technical difficulties, the most important of which is the impossibility of evaluating the moment due to the system's residual unbalance and to the error in determining arml. For measuring friction moments of 10−3−10−4g·cm preference should be given to the calibrated spring method with the rotation axis of the system beingplaced in the vertical position. An essential defect of mechanical methods for determining the instantaneous values of the friction moment consists in the limited range of the measured moments. By means of electromechanical methods it is possible to measure arbitrarily small friction moments. However, they have to be calibrated by means of mechanical instruments. Induction instruments are considerably simpler in design than those of the moving-coil type, but they are less sensitive and have a large relative error. The use of electromechanical instruments is most suitable for comparison measurements and for production testing, due to their high productive capacity.