Fig. 1,383.—Diagram illustrating rule for horse power transmitted by belts. A single belt travelling at a speed of 1,000 feet per minute will transmit one horse power; a double belt will transmit twice that amount, assuming that the thickness of a double belt is twice that of a single belt. This is conservative practice, and a belt so proportioned will do the work in practically all cases. The above rule corresponds to a pull of 33 lbs. per inch of width. Many designers proportion single belts for a pull of 45 lbs. For double belts of average thickness, some writers say that the transmitting efficiency is to that of single belts as 10 is to 7. This should not be applied to the above rule for single belts, as it will give an unnecessarily large belt.
Ques. Give a rule for obtaining the proper size of belt to deliver a given horse power.
Ans. A single belt travelling at a speed of one thousand feet per minute will transmit one horse power; a double belt will transmit twice that amount.
This corresponds to a working strain of 33 lbs. per inch of width for single belt, or 66 lbs. for double belt.
Many writers give as safe practice for single belts in good condition a working tension of 45 lbs. per inch of width.
Ques. What is the best speed for maximum belt economy?
Ans. From 4,000 to 4,500 feet per minute.
EXAMPLE.—What is the proper size of double belt for an alternator having a 16 inch pulley, and which requires 50 horse power to drive it at 1,000 revolutions per minute full load?