Thirty-six strands of rubber, weight about 56 grammes, at 150 turns give a torque of 4 oz. on a 5-in. arm, but an end thrust, or end pull, of about 3½ lb. (Ball bearings, or some such device, can be used to obviate this end thrust when desirable.) A series of experiments undertaken by the writer on the torque produced by twisted rubber strands, varying in number, length, etc., and afterwards carefully plotted out in graph form, have led to some very interesting and instructive results. Ball bearings were used, and the torque, measured in eighths of an ounce, was taken (in each case) from an arm 5 in. in length.
The following are the principal results arrived at. For graphs, see Fig. 16.
§ 6. A. Increasing the number of (rubber) strands by one-half (length and thickness of rubber remaining constant) increases the torque (unwinding tendency) twofold, i.e., doubles the motive power.
B. Doubling the number of strands increases the torque more than three times—about 3-1/3 times, 3 times up to 100 turns, 3½ times from 100 to 250 turns.
C. Trebling the number of strands increases the torque at least seven times.
The increased size of the coils, and thereby increased extension, explains this result. As we increase the number of strands, the number of twists or turns that can be given it becomes less.
D. Doubling the number of strands (length, etc., remaining constant) diminishes the number of turns by one-third to one-half. (In few strands one-third, in 30 and over one-half.)
Fig. 16.—Torque Graphs of Rubber Motors.
| Abscissæ = Turns. | Ordinates = Torque measured in 1/16 of an oz. Length of arm, 5 in. |
| A. | 38 strands of new rubber, 2 ft. 6 in. long; 58 grammes weight. |
| B. | 36 strands, 2 ft. 6 in. long; end thrust at 150 turns, 3½ lb. |
| C. | 32 strands, 2 ft. 6 in. long. |
| D. | 24 "" " |
| E. | 18 " " " weight 28 grammes. |
| F. | 12 " 1 ft. 3 in. long |
| G. | 12 " 2 ft. 6 in. long. |