Power Transmission by Belt: Part I

Editor’s note: This is the first of a two-part series on belt power transmission.


| March 2005



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At one time, perhaps up through the 1940s, it was very common for an industrial complex to have one large power unit to power the whole operation by means of one or more long lineshafts. A flat belt drive was then used for each individual machine. Steam engines were commonly used as the main power unit, but gas engines were also well represented. Diesel engines started to come on strong in the 1930s and 1940s, but the whole concept started to lose favor as individual electric motors became smaller and cheaper and electricity became more readily available.

By the 1950s, it was common for each lathe, milling machine, sewing machine, etc., to have its own electric motor. Ironically, each machine still had its own internal belt drive from its motor.

Farm uses for belt power continue to be strong, and today every engine show and festival features some kind of apparatus that uses a long, flat belt.

The subject is not particularly technical, but is complicated by many factors, some of which are the numerous rules, formulas and superstitions involved in the belt-power world. Our goal is to boil it all down to a set of simplified rules that always work.

Getting Started

First, the basics: The best way, in my opinion, to understand the mechanics of power transmission by belt is to study Figure 1.

A weight hangs on the axle of a pulley that is suspended from the ceiling by a rope or belt, as shown. Each side of the rope holds one half of the weight. In this case, the initial tension on each side of the rope is 100 pounds. Now, grasp the crank handles on each side of the pulley and apply clockwise torque to the pulley as shown by the arrows. Don’t pull down or lift … just turn! The effect is to increase the tension T1 and decrease T2. The amount added to T1 is subtracted from T2. When T2 gets small enough, the belt slips and the pulley spins. This is a most important concept.