Measurement of the Tension Loss in a Cable Traveling Over a Pulley, for Low-Speed Applications

Background

Wire ropes or cables are widely used solutions for force transmission in several industrial applications. Their hysteretic behavior may significantly influence control accuracy or the force transmission’s efficiency. Cables traveling through sheaves can suffer a relatively high tension loss, which this article addresses.

Objective

This paper aims to present a simple measurement method for the tension loss in cables traveling over sheaves on bearings.

Methods

The presented measurement method uses a cable-pulley system with a spring installed at one cable end. The pulley is moved in a zig-zag pattern. The force is measured on both cable ends; this way, the tension loss can be determined as a function of the cable tension. The force was measured with S-type load cells, which are highly sensitive to off-axis loads; this problem can be overcome by proving that the force measurement has a proportional error, which can be eliminated from the frictional coefficient. The measurements are compared to two models from the literature; one approximates the power loss of a cable drive by calculating the work of the cable’s inner friction, and the other is a cable bending model, which is used to determine the hysteretic energy of the cyclic bending.

Results

The result of the measurement evaluation is a coefficient of tension loss that contains the loss coming from the cable bending and the bearing friction. Four cable types and a steel strip with negligible bending hysteresis were measured, the latter for control measurement. It is demonstrated that a significant part of the tension loss originates from the inner friction of the cable and that it is equal to the hysteretic energy of the cyclic bending.

Conclusion

The presented method provides a robust measurement for the tension loss factor in cables traveling over pulleys. It is proven that the off-axis loads cause a proportional error in the force measured by S-type load cells, and this measurement error can be eliminated from the tension loss factor. The results demonstrated that the presented models can be used to predict the tension loss in cables traveling over sheaves.