This requirement frequently requires calculations to be made by the erectors to determine the proper size pulleys to employ to obtain the desired speed.
Fig. 2,803.—Home made belt clamp. It is made with four pieces of oak of ample size to firmly grip the belt ends where the bolts are tightened. The figure shows the clamp complete and in position on the belt and clearly illustrates the details of construction. In making the long bolts the thread should be cut about three-quarter length of bolt and deep enough so that the nuts will easily screw on.
Example.—What diameter of engine pulley is required to run a dynamo at a speed of 1,450 revolutions per minute the dynamo pulley being 10 inches in diameter and the speed of engine, 275 revolutions per minute?
The diameter of pulley required on engine is 10 × (1,450 ÷ 275) = 53 inches, nearly.
Rule.—To find the diameter of the driving pulley, multiply the speed of the driven pulley by its diameter, divide the product by the speed of the driver and the answer will be the size of the driver required.
Example.—If the speed of an engine be 325 revolutions per minute, diameter of engine pulley 42 inches, and the speed of the dynamo 1,400 revolutions per minute, how large a pulley is required on dynamo?
The size of the dynamo pulley is 42 × (325 ÷ 1,400) = 9¾ inches.
Rule.—To find the size of dynamo pulley, multiply the speed of engine by the diameter of engine wheel and divide the product by the speed of the dynamo.