XII
BELT CREEP
The question of the minimum amount of slip of a belt in transmitting power from one pulley to another reduces itself to a question of creep, for it is possible to have belts large enough so that with proper tensions there will be no regular slip. With a difference in tension on the two sides and of elasticity in the belt, creep, however, is bound to take place. What does it amount to and what allowance should be made for it? asks Prof. Wm. W. Bird of the Worcester Polytechnic Institute in his paper under the above title.
Fig. 84.
In Fig. 84 let A be the driver and B the driven, T1 the tension in the tight side of the belt and T2 in the slack side, the pulleys and belt running in the direction indicated. One inch of slack belt goes on to the pulley B at o; at or before the point p it feels the effect of increased tension and stretches to 1 + s inches. It now travels from p to m and goes on to pulley A while stretched. At or before reaching the point n, as the tension decreases, it contracts to one inch and so completes the cycle.
With a light load the belt creeps ahead of the pulley B at or near the point p. If the load is heavy, the creep works towards the point o and the belt may slip; this also takes place when the belt tensions are too light even with small loads.
The point may be easily appreciated by imagining the belt to be of elastic rubber. Professor Bird gives formulas for calculating the creep, and tests made at the Polytechnic to determine the modulus of elasticity. He concludes that the answer to his opening question is that for the common leather belt running under ordinary conditions the creep should not exceed one per cent. While this is sometimes called legitimate slip, it is an actual loss of power and cannot be avoided by belt tighteners or patent pulley coverings.
The smooth or finished side should go next to the pulley because the actual area of contact is greater than when the rough side is in contact; consequently, the adhesion due to friction is greater. Moreover, the smooth side has less tensile strength than the rough side, so that any wear on that side will weaken the belt less than wear on the other side would.