The present invention relates to a driving device for driving a driven shaft with small variations in the speed of rotation thereof even when the speed of rotation of a driver shaft is varied, the driving device being employed, for example, to drive accessories of an automotive engine from its crankshaft where large variations in the speed of rotation of the accessories are undesirable.
AC generators serving as power supplies for electric systems associated with international combustion engines normally are driven by the crankshafts thereof at a speed directly proportional to the speed of the crankshaft. The AC generator is designed to generate enough electric power even when the engine operates in a low-speed range. Therefore, when the engine operates at a high-speed, the AC generator produces excessive electric power and puts an undue load on the engine. Various proposals have heretofore been made to eliminate these shortcomings. According to one attempt, the driven pulley mounted on the generator comprises a variable-diameter pulley with its effective diameter varied according to the electromotive force of the generator for driving the generator at a constant speed (see Japanese Patent Publication No. 60-38933). Another proposal includes pulleys with clutches, the pulleys being mounted on a crankshaft and a camshaft, respectively, these pulleys are operatively coupled to the pulley of a generator by a single belt. During low-speed operation, the clutch of the pulley on the crankshaft is engaged to drive the generator with the crankshaft, and during high-speed operation, the clutch of the pulley on the camshaft is engaged to drive the generator with the camshaft at 1/2 of the speed of rotation of the crankshaft (see Japanese Laid-Open Patent Publication No. 50-121603 and U.S. Pat. No. 4,296,717). According to still another known arrangement, the outer and inner gears of an internal gear pump are used as shaft couplings, and the rotational speed of the generator which is driven is varied by controlling the flow rate of the gear pump (see Japanese Laid-Open Patent Publication No. 56-6020). A1so see another arrangement disclosure in U.S. Pat. No. 4,412,460 for driving a generator at two different speeds.
In the proposal first described above, a slide shaft is disposed in a rotor shaft of the generator and is magnetized by a current flowing through the rotor. The slide shaft is axially moved dependent on the current to move the sheave of the pulley. Therefore, the rotor shaft of the generator must be of a double-shaft construction which is complex and expensive. The second described system requires the clutches to be coupled respectively to the crankshaft and the camshaft and hence is expensive. Moreover, the clutch on the crankshaft results in a substantial increase in the length of the crankshaft. The axial length of the engine is therefore increased, and the engine compartment must be enlarged to accommodate the engine. The third described arrangement is disadvantageous in that the temperature of lubricating oil rises because the oil is forced to flow by the gear pump is restricted by a valve.