Power Transmission by Belt: Part II

Editor's note: This is the second of a two-part series on belt power transmission.


| April 2005



Chart1

Continuing our discussion of belt power transmission, we now apply the 18 considerations for a successful flat belt drive that were developed in “Power Transmission by Belt: Part I,” Gas Engine Magazine, April 2005.

Lets assume we want to run a 10 kilowatt, 1,800 RPM DC generator with our antique 30 HP tractor. We are told we'll need about 17 HP to power the generator at the full 10-kilowatt output. The tractor's belt pulley capability will undoubtedly be sufficient since this rating is always less than the engine's rating by just a few horsepower. This is because of the slight amount of loss attributable to gears and bearings. The rated speed of the belt pulley is 1,000 RPM. The pulley is 10 inches in diameter and 7 inches wide.

Some time ago, we mounted a generator in a shed with a hinged door on the side for a belt to enter. We placed the generator in such a position that the following criteria were met: 1) the pulley-end of the generator will be in a people-safe area; 2) the tension side of the belt will be on the bottom when the generator is being turned in the proper direction; 3) the tractor can easily be maneuvered and placed so that the belt length will fall into an acceptable range. Ten feet is about all the pulley-to-pulley separation we can get in this case. Certainly not very far, but it will be okay.

First we'll check belt speed. The tractor pulley may have to be changed if the belt speed is not in our desired range at the rated speed of 1,000 RPM. Factory installed pulleys are normally sized to give belt speeds in the correct range. Belt speed is calculated by multiplying pi (3.14) by the diameter of the pulley by the RPM. In our case the belt speed is 31,400 IPM or about 2,600 FPM. This is good. Since the RPM of the generator shaft must be 1.8 times the tractor shaft, the load pulley must be smaller than the PTO pulley by a factor of 1/1.8.

Cross-multiplying and dividing both sides by 1,800 gives approximately 5-1/2 inches for the generator pulley diameter.

A 5-1/2-inch pulley would be nice, but most likely a 5- or 6-inch pulley will be located. Either one could be used, but the following concerns must be evaluated: The larger pulley is better as far as power transmission is concerned, but the tractor pulley will now need to run at 1,080 RPM. If 5 inches is used the tractor pulley can run at 900 RPM. The question in this latter case is – can the tractor supply 17 HP at this reduced speed?