VARIABLE GEARS This article is concerned with variable transmission of power, and not merely variable speed transmission, otherwise it would have to include the consideration of a multitude of appliances such as speed indicators, integrators and many kinds of calculating machines.
Power transmission gears may be divided into two classes (I) that in which there is a step by step change and (2) that in which the change is gradually effected and an infinite range of velocity ratio is possible.
Step by step variable gears are familiar to all who have passed through an engineer's workshop, in which lathes, drilling machines and other tools are operated by stepped belt pulleys or trains of wheels of different size, these devices being used to effect variable transmission. A familiar example on the roads is the motor-car, in which with very rare exceptions the change of speed is effected by means of a gear box containing a number of different sized pairs of tooth wheels, arranged so that a change of speed can be effected at will by operating a gear lever. On the other hand infinitely variable transmission is less familiar and until com paratively recent times had no particular application except for very light powers. The increasing employment of infinitely vari able transmission is due to the fact that the electric motor, steam turbine, and most of all the internal-combustion engine (q.v.) have for many purposes superseded the steam engine. One feature of the greatest value in the steam engine is its flexibility ; the inferiority in this direction of other kinds of motive power being very marked. For this reason there have been attempts in recent years to find either by frictional, electric, hydraulic or other mechanical means some practicable and effective form of infinitely variable transmission.
Before proceeding to describe various types of variable trans mission it must be understood that it is quite possible to vary the speed of transmission by a slipping clutch and claims are often put forward for such a clutch as effecting infinitely variable trans mission. This, however, is not correct, being only a case of varia tion in speed by a direct loss of energy and not true variable trans mission. The true test of variable transmission is as to whether
it fulfils, at any rate approximately, what is called the Law of Work that "what is lost in force is gained in speed" and vice versa. Thus, for instance, apart from the unavoidable loss of efficiency due to friction, either a train of wheels or a combination of belt pulleys obey the law of work and overcome a greater resistance by a corresponding reduction of speed. On the other hand, if a lesser resistance is to be over come the rate of operation can be proportionately increased by suitable alteration of the train of wheels employed.
Classification of Methods.— In order to understand the dif ferent types of variable trans mission they may be classified under the following heads; Fric tional, Mechanical, Pneumatic, Electric, and Hydraulic.
Frictional.—Fig. i shows the step pulley system, the lower series operating in the headstock of the lathe, and the upper deriv ing motion from the driving shaft. As the belt is shifted from right to left off and on the different pulleys the speed of the lathe can be increased; or when from left to right the opposite effect is obtained. Corresponding to step by step movement, fig. 2 shows an infinitely variable transmission by pulleys which forms part of the cotton spinning device of Houldsworth invented just a century ago. Here the variation of speed between the driving and driven pulleys is obtained by shifting the belt to and fro along drums of nearly conical form. A modern form of variable trans mission was in 1910 and later applied successfully to motor bi cycles. Each pulley has a groove of "V" shape, the driving and driven pulleys being both capable of expansion and contrac tion. This expansion or contraction is effected by shifting in one direction or another the conical movable side of the pulley. The closer the sides come together the greater will be the effective diameter of the pulley. Powerful springs ensure that the driving belt shall be gripped on both pulleys, an idle wheel running be tween two pulleys accurately to secure the correct location of the conical surface.