The percentage of slip is the most important factor affecting the efficiency of belt transmission, but in addition to this we have journal friction, the resistance of the air, and with crossed belts the friction of the belt upon itself. These have been termed internal resistances, and their values for some of the most common arrangements of pulleys are given in [Table VII]. From this table it appears that the moment required to run a straight belt varies from 15 to 25 inch lbs. at 100 lbs. tension for all speeds. At 160 revolutions per minute and 1,000 lbs. tension, the required moment varied from 45 to 90 inch lbs., and at 18 revolutions per minute and at the same tension it varied from 80 to 150 inch lbs.
From the average of these quantities we find the moment of resistance to be expressed by the following formulæ for straight open belts between 2′′ journals:
At 160 r. p. m.:
| M = .053 S + 14.7, | (5.) |
At 18 r. p. m.:
| M = .11 S + 9, | (6.) |
in which
| M | = | moment of resistance in inch lbs. |
| S | = | sum of tensions. |
When a crossed belt does not rub upon itself, the resistance is the same as for an open belt.
The resistance offered by the introduction of carrying pulleys and tighteners is appreciable, and depends upon the pressure brought to bear against their journals. If the belt rubs against the flanges of the carrying pulleys, the resistance is very much increased, and this is often liable to occur in horizontal belts from a change of load. The friction on journals of carrying pulleys may be estimated by the formulæ already given if we substitute for S the pressure against their journals. In the experiments which were made upon internal resistances, the greatest resistance was offered by a quarter-twist belt 6 feet between journals on 20-inch pulleys.