Pulleys with more tracks pull more load

The most common systems for transmitting power from a drive to a driven shaft are belt, gear, and chain drives. But V-belt drive systems, also called friction drives (because power is transmitted as a result of the belt's adherence to the pulley) are an economical option for industrial, automotive, commercial, agricultural, and home appliance applications. V-belt drives are also easy to install, require no lubrication, and dampen shock load.

Here's the catch: Standard friction drives can both slip and creep, resulting in inexact velocity ratios or degraded timing precision between input and output shafts. For this reason, it is important to select a belt appropriate for the application at hand.

Belt makeup

Belt drives are one of the earliest power transmission systems and were widely used during the Industrial Revolution. Then, flat belts conveyed power over large distances and were made from leather. Later, demands for more powerful machinery, and the growth of large markets such as the automobile industry spurred new belt designs. V-belts, with a trapezoidal or V shape, made of rubber, neoprene, and urethane synthetic materials, replaced flat belts. Now, the increased overall surface material of modern belts adheres to pulley grooves through friction force, to reduce the tension required to transmit torque. The top part of the belt, called the tension or insulation section, contains fiber cords for increased strength as it carries the load of traction force. It helps hold tension members in place and acts as a binder for greater adhesion between cords and other sections. In this manner, heat build-up is reduced, extending belt life. Prestretched tension-member cords (polyester, aramide, steel, fiberglass) also minimize stretch.

The bottom, or compression section, is designed to withstand compression. It is made from a tough rubber compound that exerts a wedging force against the pulley groove to increase adherence without deformation.

The protective cover (generally an elastic cover made of rubber-impregnated fabric that is slip-resistant and durable) is a heat-resistant layer that protects the belt's inner components.

The torque obtained depends on the belt's resistance to the applied tension and the degree of adherence to the inner walls of the pulley groove. For this reason, belt-drive systems should never be lubricated, as they depend on friction to transmit power — in contrast to chain or gear systems that function through pure contact pressure. Another tip: The inside face of the belt should never touch the bottom of the groove.

Horsepower

All belt sizes are classified by cross-section and length dimensions. The cross-section indicates the top width, depth, and V angle dimensions. Length is measured in terms of outside and inside (pitch) lengths.

In mechanical engineering, power is a measure of performance or capacity and is defined as the amount of work performed in a given time. Torque is related to power:

Adding a service factor to nominal horsepower (which is either motor power or horsepower to be transmitted) ensures drive efficiency.

Adjusted power rating

Basic power ratings are known for all belt sheave sizes and drive speeds. With the system presented here, a length correction and an arc correction factor adjust this basic power rating to the sheave speed ratio (which we’ll explain shortly) and the center distance. This is more accurate and realistic than formulas and tables that only show basic horsepower values.

For simplicity, a typical center distance is usually assumed for each sheave combination. For distances not listed, know that longer center distances mean slightly higher power ratings — and shorter center distances mean lower power ratings. For high ratio drives, small centerdistance changes may affect the power rating considerably.

Finally: Many standard sheaves are designed to operate up to 6,500 fpm as per industry standards. Drives exceeding this speed require a ductile iron sheave construction, and in such cases, extra manufacturer design help is highly recommended.