The equation for this belt may be written:
| M = .35 S + 58, | (7.) |
but the introduction of a carrying pulley reduced the resistance to no more than what might be expected from the same number of journals with a straight belt.
With quarter-twist belts the resistance lies chiefly in slip, which occurs as the belt leaves the pulleys, and this naturally depends upon the distance between journals in terms of the diameters of the pulleys.
The effect of time upon the tension of the belt used in [Table VIII.] is plainly shown by experiments 588 to 613 inclusive, between which the pulleys remained at a fixed distance apart, and the belt slowly stretched from a tension of 380 to 280 lbs.
To estimate the efficiency of belt transmission for an average case, we may assume 40 in. lbs. as the moment of internal resistance for a belt whose tension is 500 lbs. and 40 in. lbs. statical moment = about 20 ft. lbs. per revolution. If the belt is transmitting 400 lbs. with two per cent. of slip on 20 in. pulleys, then .02 × 400 × 5 = 40 ft lbs. are lost per revolution in slip, making a total loss of 60 ft. lbs. per revolution.
TABLE VIII.
SHOWING THE INCREASE IN THE SUM OF THE TENSIONS ON A VERTICAL BELT 4′′ WIDE BY 1⁄4′′ THICK, AND 24 FT. LONG, ON 20′′ CAST-IRON PULLEYS, AT 120 R. P. M.
| No. of experi- m’nt. | Scales A. [44] | Scales B. [44] | Ten- sion T + t | Ten- sion T - t | T | t | Incre- m’nt of T + t | Per- cen’e of Incre- ment. | Date. | ||
| 578 | 93 | 101 | 194 | 16 | 105 | 89 | 0 | 5-15-1885. | |||
| 579 | 70 | 142 | 212 | 144 | 178 | 34 | 18 | ||||
| 580 | 67 | 170 | 237 | 206 | 221 | .5 | 15 | .5 | 43 | ||
| 581 | 66 | 180 | 246 | 228 | 237 | 9 | 52 | ||||
| 582 | 66 | 188 | 254 | 244 | 249 | 5 | 60 | .323 | |||
| 583 | 91 | 101 | 192 | 20 | 106 | 86 | -2 | ||||
| 584 | 202 | 210 | 412 | 16 | 214 | 214 | 0 | 5-15-1885. | |||
| 585 | 167 | 250 | 417 | 166 | 292 | .5 | 292 | .5 | 5 | ||
| 586 | 145 | 300 | 445 | 310 | 376 | .5 | 376 | .5 | 33 | .171 | |
| 587 | 185 | 195 | 380 | 20 | 200 | 200 | -32 | ||||
| 588 | 190 | 199 | 380 | 0 | 190 | 190 | 0 | 5-18-1885. | |||
| 589 | 133 | 250 | 393 | 214 | 303 | .5 | 89 | .5 | 13 | .033 | |
| 590 | 177 | 177 | 354 | 0 | 177 | 177 | 0 | 5-19-1885. | |||
| 591 | 156 | 203 | 359 | 94 | 226 | .5 | 132 | .5 | 5 | ||
| 592 | 138 | 235 | 373 | 194 | 283 | .5 | 89 | .5 | 19 | ||
| 593 | 135 | 250 | 385 | 230 | 307 | .5 | 77 | .5 | 31 | ||
| 594 | 128 | 275 | 403 | 294 | 348 | .5 | 34 | .5 | 49 | ||
| 595 | 125 | 300 | 425 | 350 | 387 | .5 | 37 | .5 | 71 | ||
| 596 | 123 | 325 | 448 | 404 | 426 | 22 | 94 | .333 | |||
| 597 | 168 | 168 | 336 | 0 | 168 | 168 | -18 | ||||
| 598 | 143 | 143 | 286 | 0 | 143 | 143 | 0 | 5-25-1885. | |||
| 599 | 140 | 148 | 288 | 16 | 152 | 136 | 2 | ||||
| 600 | 130 | 160 | 290 | 60 | 175 | 115 | 4 | ||||
| 601 | 122 | 170 | 292 | 196 | 194 | 98 | 6 | ||||
| 602 | 116 | 180 | 296 | 28 | 212 | 84 | 10 | ||||
| 603 | 112 | 190 | 302 | 156 | 229 | 73 | 16 | ||||
| 604 | 108 | 200 | 308 | 184 | 246 | 62 | 22 | ||||
| 605 | 105 | 210 | 315 | 210 | 262 | .5 | 52 | .5 | 29 | ||
| 606 | 102 | 220 | 322 | 236 | 279 | 43 | 36 | ||||
| 607 | 100 | 230 | 330 | 260 | 295 | 35 | 44 | ||||
| 608 | 99 | 240 | 339 | 282 | 310 | .5 | 28 | .5 | 53 | ||
| 609 | 98 | 250 | 348 | 304 | 326 | 22 | 62 | ||||
| 610 | 98 | 260 | 358 | 316 | 337 | 21 | 72 | ||||
| 611 | 99 | 270 | 369 | 342 | 355 | .5 | 13 | .5 | 83 | ||
| 612 | 100 | 280 | 380 | 360 | 370 | 10 | 94 | .357 | |||
| 613 | 140 | 140 | 280 | 0 | 140 | 140 | -6 | ||||
[44] Scales a recorded the reduction of the load on the testing device for vertical belts by the tension of the loose part of the belt (t.). Scales b, by that of the tight side of the belt (T).