The practical test showed no ill effects except from No. 3, the sticky dressing, which ripped and tore the surface of the belt. The high initial tensions caused overheating of the journals, even though we kept them flooded with oil. On the low initial tensions there was no tendency to heat, even when the maximum horse-power was being transmitted by dressing No. 1. In the latter case we oiled the bearings once in every two or three runs (a "run" comprised all the readings for one initial tension), while in the former we oiled the bearings after each reading and sometimes between them; even then we were afraid that the babbitt would get hot enough to run. The readings for each run varied in number from two to a dozen, but only the one giving the maximum horse-power was used in drawing the curves. The belt speeds during the tests varied between 2000 and 2500 feet per minute, most of the tests being made at about 2200 feet per minute.
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.
In Fig. 84 let _A_ be the driver and _B_ the driven, _T__{ 1} the tension in the tight side of the belt and _T__{ 2} 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.