Foothills Antique
Tractor and Engine Club

When it is not convenient to measure with the tape-line the length required, apply the following rule: Add the diameter of the two pulleys together, divide the result by 2, and multiply the quotient by 3 1/4; then add this product to twice the distance between the centers of the shafts, and you have the length required.

If possible to avoid it, connected shafts should never be placed one directly over the other, as in such case the belt must be kept very tight to do the work. It is desirable that the angle of the belt with the floor should not exceed 45 degrees.

It is also desirable to locate the shafting and machinery so that belt should run off from each shaft in opposite directions, as this arrangement will relieve the bearings from the friction that would result when the belts all pull one way on the shaft.

Multiply diameter of pulley in inches by 3.1416. This gives circumference of pulley, and this result multiplied by number of revolutions will give you belt speed in inches.

The horse power for a given speed will be directly proportioned to the width of the belt; that is, a 4-ply belt, 16 inches wide, running at a certain speed, will transmit eight times as much power as a 4-ply belt 2 inches wide, running at the same speed, and a belt 100 inches wide, ten times as much as a 10-inch belt of the same thickness, running at the same speed, etc.

Multiply the width of the belt in inches by its speed in feet and divide the result by 800. The final result will be the horse power for a 4-ply belt. For 1 6-ply belt, divide this result by 600; for 8-ply, divide this result by 400; for 10-ply, divide this result by 350.

To economically transmit a given horse power at a given belt speed, multiply the given horse power by 800 and the given width in inches by the given belt speed in feet and divide the first result by the second.

If the final result is one, or nearly one, a 4-ply belt is required, if one and one half, a 6-ply belt is required; if one and three-quarters to two, an 8-ply belt is required; if two to two and one-quarter, a 10-ply belt is required.

To find the width of a 4-ply belt required economically to transmit a given horse power at a given belt speed per minute: Multiply the given horse power by 800, and divide the results by the given belt speed.

To find the width of a 6-ply belt required: Multiply the horse power by 600- divide the result by belt speed.

To find the width of an 8-ply belt required: Multiply horse power by 400; divide result by belt speed.

To find the width of a 10-ply belt required: Multiply horse power by 350; divide result by belt speed.

The product of the diameter and speed of the driving pulley equals the product of the diameter and speed of the driven pulley; consequently, if the speed and the diameter of the driving pulley am given, multiply them together and divide by the diameter of the driven pulley to find the speed of the driven- or divide by the speed of the driven pulley to find its diameter.

Example -- The drive pulley on a tractor is 9 1/2 inches in diameter and runs at 1,000 RPM.; what size pulley must he used on a thresher cylinder shaft that must run at 1,100 RPM?

9 1/2 times l,000 equals 9,500; divided by 1,100, equals 8.64. Since pulleys are made only in certain standard diameters, use either the next size larger, 9" diameter, and raise the engine speed slightly, or use 8 1/2" pulley, considering that the slight slippage will reduce the effective speed to the correct number of revolutions per minute.

9 1/2 times 1,000 equals 9,500; divided by 6 equals 1,583 RPM.