An analytical solution of the rope-sheave contact in static conditions based on a bristle model

Abstract

This paper presents an analytical solution for the rope-sheave contact problem in static conditions, also applicable to belt-pulley problems and similar mechanisms. The rope is assumed under constant and unequal loads at the two ends, as in an elevator when the brake is acting on the drive sheave and the weights of the cabin and counterweights are different. The rope is assumed to behave as a rod without bending stiffness. Assuming a bristle model for the rope-sheave contact and Coulomb friction, the balance of forces in a differential slice of the rope is used to obtain a closed-form solution for the axial force field and the normal and tangential contact force fields. This analytical solution is found first in the case of a bristle model with tangential flexibility but axially rigid. Then, the model is extended to axially flexible bristles, that allows the rope to sheave relative penetration. The solution is valid until one cross-section of the rope achieves the saturated tangential friction force. The value of the axial load when this condition is met is identified. If the high-load further increases, the rope in contact with the sheave is separated in two areas, the area next to the low-load remains fully stuck to the sheave while the area next to the high-load shows exactly the saturated friction force. This paper calculates the angle of separation between the two areas and the axial force field and normal and tangential contact force fields, being all these functions piece-wisely defined. Finally, the paper presents numerical results for a particular example of all the analytical solutions.