Continuously Variable Transmission
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Continuously Variable Transmission
a mechanism for smoothly changing a gear ratio, that is, the ratio between the frequency of rotation of the driving element and the rate of the driven element. It is used in transportation machines, lathes, instruments, and so forth. Compared to the adjustment of speed in steps, the continuously variable transmission increases machine productivity, facilitates automation, and permits control during operation. It forms a part of a speed regulator, which consists of one or more such transmissions and an apparatus to ensure that they function. There are electrical and mechanical types.
Depending on the type of their transmission elements, the mechanical continuously variable transmissions may have a fluid operating link (hydraulic), a flexible link (belt and chain), or a rigid link. Those with flexible and rigid elements are divided according to their mode of operation into fric-tional and geared-type with either continuous or pulse operation. The term “continuously variable transmission” is usually applied to mechanical transmissions having flexible and rigid elements.
Electric continuously variable transmissions in the form of generator-motor systems are used in transportation machines and for other purposes where considerable power is transmitted.
Hydraulic continuously variable transmissions may be hydrostatic (or positive-displacement action) or hydro-dynamic. In order to reduce the rate of rotation with constant torque and a relatively low efficiency, slip clutches such as hydrodynamic clutches are employed.
Friction-type continuously variable transmissions with a flexible link and separable conical pulleys provide for small changes in the gear ratio as the load changes and have excellent reliability, but they are large in size. In this type (with two pairs of cones connected by a V-belt or a special roller chain) the gear ratio is varied by forcing one pair of cones closer together as the other is simultaneously separated by a control mechanism, by forcing the axial displacement of one pair of cones while the other is acted on by springs, and by varying the interaxial separation between one spring-opposed pulley and another fixed pulley.
Continuously variable transmissions with gear teeth and a flexible link give excellent performance but are complicated to manufacture. The basic elements of this transmission are separable toothed cones and a lamellar chain. The links of the chain have transverse openings which carry packets of thin laminae. The depressions on one cone are arranged to be opposite the projections on the other so that as the laminae are shifted axially they take on the shape of the teeth, thus meshing the gears.
Friction-type continuously variable transmissions with a rigid link are compact and have a typically stiff mechanical performance but need substantial forces to squeeze the operating portions together and thus create the required friction between them; their performance is less reliable because of possible skidding and damage to the working surfaces. The efficiency and long life of these transmissions depends to a large extent on the geometrical slippage that occurs as a result of the unequal velocities of the driving and driven elements along the line of contact. The greater the relative slip velocity Vst along the line of contact, the lower is the efficiency and the greater the wear of the rubbing surfaces.
In Figure 1 diagrams are shown of several continuously variable transmissions arranged in the order of decreasing geometrical slippage. The multidisc transmission (Figure 1, a), despite a disadvantageous geometrical slippage arrangement, is widely used for medium and high power (up to hundreds of kilowatts) because of favorable conditions for forming an oil wedge at the contact areas and the existence of a large number of tight contacting surfaces. In the faceplate-type of continuously variable transmission (Figure 1, b) with a conical roller, there is no geometrical slippage when the peak of the cone A is at the point A1, and in other positions it is substantially less than for a transmission with a cylindrical roller (Figure 1, c). In the toroidal design of a continuously variable transmission (Figure 1, d) there is very little geometrical slippage at any of the roller positions and practically none when the peak A of a conical surface that arbitrarily replaces the spherical surface of the roller is located at points A1 and A2 on the geometric axis of the cups. Transmissions of this type are built with two or three rollers, have high efficiency, and are compact. Their drawbacks are the complexity of their manufacture and repair and poorer reliability. The point contact type of continuously variable transmission uses steel spheres as an intermediary; and the position of their physical or geometric axes is varied by a control mechanism.
In the pulse type of continuously variable transmissions the rotary motion of a drive shaft is converted into a rocking (vibratory) or irregular rotary motion of intermediate elements from which motion is transferred to the driven shaft through a free-wheeling mechanism. The gear ratio is established by a control mechanism that varies the vibration amplitude or the velocity of the intermediate links. The velocity irregularity of the driven element is partly smoothed out by its inertia.
REFERENCESDetail mashin: Spravochnik, 3rd ed., vol. 3. Moscow, 1969.
Kratkii spravochnik mashinostroitelia. Moscow, 1966.
N. IA. NIBERG