This section is from the book "Machine Shop Work", by Frederick W. Turner, Oscar E. Perrigo, Howard P. Fairfield. Also available from Amazon: Machine shop work.
The belt manufacturer has helped to solve this problem by producing belting suited to the machine constructor's needs. Most conditions of temperature, humidity, and pliability have been met by the belt-maker. Many experiments have been made and published by engineers to show what a given belt may be expected to do under varying conditions of heat, cold, and dampness.
Machines designed for heavy roughing cuts are often provided with a complete geared driving mechanism in which all the speed changes are made through trains of gearing and engaging clutches.
Instead of driving by means of trains of belting there is an increasing tendency toward the use of direct driving. This is usually done by direct connection of an electric motor to the driving mechanisms. In shops where the machines are fitted with individual motors, it may be so complete that no belting is to be observed.