The total power expended per revolution is about 2,000 ft. lbs., therefore .03 is lost.
Under light loads, the internal resistance, which is nearly constant in amount, may be a large percentage of the power transmitted, while under heavy loads the percentage of slip may become the principal loss.
It would be difficult to work out, or even to use, a general expression for the efficiency of belt transmission, but, from the foregoing, it would seem safe to assume that 97 per cent. can be obtained under good working conditions.
When a belt is too tight, there is a constant waste in journal friction, and when too loose, there may be a much greater loss in efficiency from slip. The allowance recommended of 2 per cent. for slip is rather more than experiment would indicate for any possible crawl or creep due to the elasticity of the belt, but in connection with this, there is probably always more or less actual slip, and we are inclined to think that in most cases this allowance may be divided into equal parts representing creep and slip proper. Under good working conditions, a belt is probably stretched about 1 per cent. on the tight side, which naturally gives 1 per cent. of creep, and to this we have added another per cent. for actual slip in fixing the limit proposed.
The indications and conclusions to be drawn from these experiments are:
1. That the coefficient of friction may vary under practical working conditions from 25 per cent. to 100 per cent.
2. That its value depends upon the nature and condition of the leather, the velocity of sliding, temperature, and pressure.
3. That an excessive amount of slip has a tendency to become greater and greater, until the belt finally leaves the pulley.
4. That a belt will seldom remain upon a pulley when the slip exceeds 20 per cent.
5. That excessive slipping dries out the leather and leads toward the condition of minimum adhesion.