Primarily, the clutch is used to allow the use of change-speed gearing; or, stated in the reverse way, the form of the transmission determines whether a clutch must be used or not, there being cases in which it is not used. Thus, where the frictional form of transmission is used, no clutch is necessary, the frictional disks acting as a clutch and rendering another one superfluous. So, too, with the form of transmission known as the planetary gear,no master clutch is needed.
On the other hand, the reverse of this does not always hold. Any form of clutch may be used with the various other forms of transmission, as the sliding gear; in fact, in actual practice every known kind of a clutch will be found coupled with the sliding-gear transmission.
Broadly considered, there are five classes of transmissions used. In cases where the use of any one of these forms eliminates the final drive, this from its very nature does not alter the facts, but simply calls for a different and more detailed treatment. The five classes are:
Planetary or epicyclic
Belt and cables
Combinations of two or more
The features of the 1916 transmissions which stand out from previous years are: reduced sizes; simpler, lighter construction; greater compactness and greater accessibility. The smaller sizes have brought about the simplification and lighter weight, and in turn have been produced in answer to the popular demand for lighter weight cars. In part, simplification has been produced by unit power plants, now so popular.