An interesting feature of these and subsequent experiments is the progressive increase in the sum of the belt tensions during an increase in load. This is contrary to the generally accepted theory that the sum of the tensions is constant, but it may be accounted for to a large extent by the horizontal position of the belt, which permitted the tension on the slack side to be kept up by the sag. That this is only a partial explanation of the phenomenon, and that the sum of the tensions actually increases as their difference increases for even a vertical position of the belt, will be shown by a special set of experiments. If a belt be suspended vertically, and stretched by uniformly increasing weights, it will also be found that the extension is not uniform, but diminishes as the load is increased, or, as already stated, the stress increases faster than the extension. A little reflection will show that when this is the case the tensions must necessarily increase with the load transmitted.

TABLE II.

DOUBLE BELT 21⁄4′′ WIDE BY 5⁄16′′ THICK, AND 32 FT. LONG, WEIGHING 91⁄2 LBS., ON 20′′ CAST-IRON PULLEYS. THIS BELT HAD BEEN USED ON A PLANING MACHINE, WAS QUITE PLIABLE, DRY, AND CLEAN. 160 R. P. M.
No.
of
Exper-
i’nt.		Sum of Tensions T + tT - t
Work-
ing.		TtT ⁄ tPer-
cent-
age
of
Slip.		Velocity
of Slip
in ft.
per
min.		Arc
of
con-
tact.		Coef-
ficient
of
Friction.		Remarks.
Initial.Work-
ing.		Final.
105100104 4072 32 2.25 .31.2177° .263
106 110 6085 25 3.40 .83.2177 .395
107 122 80101 21 4.811.76.8176 .511
108 138 100119 19 6.264.317.2175 .600
109200208 80144 64 2.25 .41.6179 .260
110 212 100156 56 2.81 .72.8179 .331
111 216 120168 48 3.501 4 179 .401
112 220 140180 40 4.501.87.2178 .484
113 230 160195 35 5.574.417.6178 .553
114300308 120214 94 2.28 .41.6180 .262
116 316 160238 78 3.05 .83.2180 .355
118 322 200261 61 4.281.66.4179 .465
119 330285220275 55 5 2.610.4179 .516
121400404 160282 122 2.31 .72.8180 .267
124 410 220315 95 3.371.56 180 .387
125 412 240326 86 3.792.39.2180 .424
126 414 260338 78 4.333.714.8179 .469
127 416370280348 68 5.1210.140.4179 .523Belt almost slipped off.
128500516 200358 158 2.27 .52 180 .261
131 520 260390 130 3 1.14.4180 .350
133 525 300412.5112.53.671.87.2180 .414
134 525 320422.5102.54.112.710.8180 .450
135 525460340432.592.54.675.120.4180 .490
136100105 4072.532.52.02 .2 .8177 .228Here the belt was coated with “Sankey’s Lifeof Leather,” and run until in good working condition before noting experiments.
137 110 6085 25 3.40 .41.6177 .396
138 125 80102.522.54.56 .62.4176 .494
140 150 120135 15 9 1.87.2174 .723
141 164 140152 12 12.72.810.8172 .779
142 180 160170 10 17 5 20 170 .954
144 215 200207.57.527.77.329.2166 1.15
146 250 240245 5 49 10.642.4158 1.41
147 27090260265 5 53 17.770.8158 1.44
149100105 4072.532.52.02 .2 .8177 .228Three days later without any additional dressing.
150 110 6085 25 3.40 .31.2177 .396
151 120 80100 20 5 .41.6176 .524
153 150 120135 15 9 .72.8174 .723
155 182 160171 11 15.51.24.8172 .913
156 202 180191 11 17.33 12 172 .950
157 216 200208 8 26 5.823.2167 1.12
158 232 220226 6 37.37 28 161 1.29
159 252 240246 6 41 9.839.2161 1.32
161 292 280286 6 47.713.754.8161 1.37

A piece of belting 1 sq. in. in section and 92 ins. long was found by experiment to elongate 1⁄4 in. when the load was increased from 100 to 150 lbs., and only 1⁄8 in. when the load was increased from 450 to 500 lbs. The total elongation from 50 to 500 lbs. was 111⁄16′′, but this would vary with the time of suspension, and the measurements here given were taken as soon as possible after applying the loads. In a running belt the load is applied and removed alternately for short intervals of time, depending upon the length and speed of the belt, and the time for stretching would seldom be as great as that consumed in making the experiments just mentioned.

The differences between the initial and final tensions unloaded, as given in the tables, show the effect of extension or contraction during the course of the experiments made at a fixed position of the pulleys. The percentage of elongation which a belt undergoes in passing from its loose to its tight side, is the measure of the slip which must necessarily take place in the transmission of power. This is a direct loss, and within the assumed working strength of 500 lbs. per sq. in. for cemented belts without lacings, experiment indicates that it should not exceed 11⁄2 or 2 per cent. When, therefore, an experiment shows less than 2 per cent. of slip, the amount may be considered as allowable and proper, and the belt may be relied upon to work continuously at the figures given.

[Table III.] gives the results of experiments upon a soft and pliable rawhide belt made by the Springfield Glue and Emery Co. This belt had been used by the Midvale Steel Co. for a period of seven months, at its full capacity, and was sent in its usual working condition to be tested. It had been cleaned and dressed with castor oil at intervals of three months, and was received three weeks after the last dressing. Commencing with the light initial tension of 50 lbs. on a side, it was found impossible with the power at command to reach a limit to the pulling power of the belt, and in order to do so the experiment was made of supporting the slack side of the belt upon a board to prevent sagging.

TABLE III.

RAWHIDE BELT 4′′ WIDE BY 9⁄32′′ THICK AND 31 FT. LONG, WEIGHING 15 LBS. 160 R. P. M. ON 20′′ CAST-IRON PULLEYS.
No.
of
Exper-
i’nt.		Sum of Tensions T + tT - t
Work-
ing.		TtT ⁄ tPer-
cent-
age
of
Slip.		Velocity
of Slip
in ft.
per
min.		Arc
of
con-
tact.		Coef-
ficient
of
Friction.		Dura-
tion
of run
at time
of
experi-
ment.		Remarks.
Initial.Work-
ing.		Final.
171100118 4079 39 2.03 .2 .8177° .229
173 140 80110 30 3.67 .41.6176 .423
175 168 120144 24 6 .62.4174 .590
177 202 160181 21 8.62 .83.2172 .661
179 232 200216 16 13.51 170 .897
181 268 240254 14 18.11.24.8167 .993
183 302 280291 11 26.51.45.6163 1.15
184 318110300309 9 34.31.66.4160 1.27
185100150115140145 5 29 1.66.4180 1.02 Slack side of belt running on a board to prevent sagging.
186200258 240249 9 27.41.24.8180 1.05
188 290 280285 5 57 2.28.8180 1.29
189300412 400406 6 67.71.76.8180 1.34
190 428 420424 4 106 1.87.2180 1.48
191 446275440443 3 148 3.313.2180 1.59
192400570360560565 5 113 2 180 1.47
329100110 4075 35 2.14 .3 .6177 .246 10′′ cast-iron pulleys.
330 135 80107.527.53.90 .61.2175 .446
331 198 160179 19 9.421 171 .751
332 275 240257.517.514.71.5 169 .911
334 345 320232.512.518.62 165 1.01
336 420110400410 10 41 3.26.4162 1.31
339200230 160195 35 5.86 .81.6176 .576
340 360 320340 20 17 1.63.2171 .949
341 435 400417.517.523.82 169 1.07
342 505 480492.512.539.42.75.4165 1.28
343 590200560575 15 38.35 .0168 1.24
344300400 320360 40 9 1.42.8175 .719
345 450 400425 25 17 1.73.4173 .938
346 520 480500 20 25 2.14.2171 1.08
347 600 560570 10 57 3 162 1.431min.
348 600280560570 10 57 3.46.8162 1.435min.
350400500 400450 50 9 1.63.2176 .715
352 605 560577.517.521.32.34.6169 1.04
353 680 640660 20 33 3.26.4171 1.171min.
354 680 640660 20 33 3.77.4171 1.175min.
355 680 640660 20 33 4.18.2171 1.1710min.[4 m. later.
356 680 640660 20 33 6.112.2171 1.1715min.Belt slipped off
357 600 560580 20 29 .0 .0171 1.1320min.Continuing.
358 600 560580 20 29 17.234.4171 1.1325min.
359 530 480505 25 20.25.210.4173 .95530min.
360 530350480505 25 20.22.85.6173 .95535min.
361500570 400485 85 5.711.32.6178 .561
364 700 640670 30 22.32.34.6174 1.02
365 755 720637.517.536.43.26.4169 1.22
366 820 800810 10 81 6.613.2162 1.55 Belt slipped off 2 m. later.
367 750 720735 15 49 5.110.2168 1.321min.
368 750 720735 15 49 .1 .2168 1.325min.
369 690 640665 25 26.6 .2 .4173 1.09 Belt slipped off 3 m. later.
370 610 560585 25 23.414.428.8173 1.051min.
371 610 560585 25 23.4 .0 .0173 1.054min.
372 550 480515 35 14.77.414.8175 .8801min.
373 550410480515 35 14.72.34.6175 .8805min.
374600680 480580 100 5.81.5 178 .566
376 755 640697.557.512.12.14.2177 .807
378 850 800825 25 33 2.85.6173 1.161min.
379 850 800825 25 33 3.5 173 1.165min.Belt slipped off 5 m. later.
380 780 720750 30 25 8.817.6174 1.061min.
381 680 560620 60 10.311.222.4177 .7555min.
382 680 560620 60 10.32 177 .755 After running 5 minutes at T - t = 560.
383 730 640685 45 15.22.5 176 .8861min.
384 730 640685 45 15.22.44.8176 .8865min.
385 780 720750 30 25 4.69.2174 1.061min.
388 780550720750 30 25 8.817.6174 1.065min.
389 780 720750 30 25 4 174 1.061min.Belt scraped.
390 780 720750 30 25 6.412.8174 1.065min.
391 730 640685 45 15.23.77.4176 .8861min.Belt slipped off 2 m. later.
392 730550640685 45 15.23.97.8176 .8865min.
396600680 400540 140 3.862 .45170 .432 18 r. p. m. 10′′ cast-iron pulleys.
397 820 720770 50 15.417.23.87176 .890
398 750 640695 55 12.6 .53.37177 .874
399 700 560630 70 9 9.42.17177 .711
400 670 480575 95 6.054.51.12178 .579
401 630550400515 115 4.483.5 .75178 .483
402 830 720775 55 14.1 .65.85177 .856
403 630 320475 155 3.061.5 .30179 .358
404 610 60335 275 1.22 .7 .16180 .063
408600610 120365 245 1.49 .2 .09180 .127 20′′ cast-iron pulleys. 18 r. p. m.
413 660 400530 130 4.081 .45179 .450
415 710 560635 75 8.461.9 .86177 .691
416 750 640695 55 12.63.21.44177 .820
417 800 720760 40 19 3.81.71175 .964
418 340 200274 70 3.91 .6 .27177 .441
419300380 280330 50 6.61.2 .54176 .614
421 450 400425 25 17 3.21.44173 .938
423 515 480497.517.528.44 1.8169 1.13
425 580 560570 10 57 5 2.25162 1.43
427 695 680687.57.591.77 3.15155 1.67