8.12 : Frictional Forces on Flat Belts

Flat belts are commonly used in various industrial applications for transmitting power from one pulley to another. When a flat belt is wrapped around a set of pulleys, it experiences different tensions at the driving pulley ends due to the friction between the belt and pulley surface. When the pulley moves in a counterclockwise direction, the tension T₂ on the opposite side of the pulley where the belt is moving away from is higher than the tension T₁ on the side where the belt is moving towards.

To estimate the tensions in a flat belt, the total angle of belt-to-surface contact and the coefficient of friction between the surfaces must be known. A free-body diagram of a differential element AB of the belt can be drawn assuming impending motion.

The frictional force opposes the sliding motion of the belt, causing the magnitude of the belt tension acting at point B to increase by dT. This differential tension can be used to relate the tensions at different points along the belt.

To determine the relationship between the belt tensions, the horizontal and vertical force equilibrium equations, along with cosine and sine approximations, are used.

These equations consider the forces acting on the differential element AB in the horizontal and vertical directions. The product of two differentials compared to the first-order differentials is neglected in the vertical equilibrium equations, while the horizontal equilibrium equation is simplified further. The equilibrium equations are combined and integrated between corresponding limits.

The result gives an expression correlating the belt tensions.

This equation applies to flat belts passing over any curved contacting surface.

The frictional forces on flat belts are crucial in determining the performance and efficiency of the belt-driven system. Over-tensioning the belt can cause premature wear and reduce efficiency, while under-tensioning can cause slippage and reduce power transmission. Proper tensioning is achieved by adjusting the distance between the pulleys or using a tensioning device.