The present invention relates to a torque limiting mechanism that is located between a first rotor and a second rotor, and more particularly, to a torque limiting mechanism that discontinues power transmission from one of the rotors to the other when an excessive torque is generated in the driven one of the rotors.
A typical torque limiting mechanism is located between a driven device such as a compressor and a drive source such as an engine and a motor. The torque limiting mechanism forcibly disengages the driven device from the power source when a malfunction occurs in the driven device, for example, when the driven device is locked. That is, the mechanism prevents the power source from being affected by an excessive load torque due to the malfunction in the driven device.
Japanese Unexamined Utility Model Publication No. 63-19083 discloses an on-vehicle clutchless type compressor that has such a torque limiting mechanism. The torque limiting mechanism is located between a pulley, which is coupled to an engine, and a drive shaft of a compressor. A power receiver is secured to the drive shaft. A pair of shear pins project from the receiver. The pins are engaged with holes formed in the pulley. When stress applied to the pins exceeds a predetermined limit level due to, for example, a malfunction of the compressor, the pins simultaneously break to disconnect the engine from the compressor.
A torque limiting mechanism that has no breakable couplers like pins also has been proposed. This mechanism has a pair of coupler members that are engaged with each other to couple a drive source with a driven device. When there is an excessive load torque, an elastic member is deformed to disengage the coupler members from each other, which discontinues power transmission (for example, Japanese Unexamined Patent Publications Nos. 10-267047 and 10-252772).
Shear pins in a torque limiting mechanism are designed to break at a predetermined level of torque, or a breaking torque. However, the pins may break at a load that is smaller than the breaking torque. This is because the shear pins get gradually fatigued by stresses due to repetitive fluctuation of normal load torque, which is smaller than the breaking torque, and, as a result, the level of limit stress, at which the shear pine are broken, is gradually lowered. To prevent such undesired breaking due to fatigue, the diameter of each shear pin may be increased. However, if the diameter is increased, it is difficult to cause a new shear pin, which is not fatigued, to break at a desired breaking torque.
A torque limiting mechanism that has breakable members requires no means to maintain a discontinuation of power transmission. However, a torque limiting mechanism that has two coupler members requires means for maintaining a discontinuation of power transmission, which complicates the structure.
Accordingly, it is an objective of the present invention to provide a torque limiting mechanism that reduces variations of a level of load torque at which power transmission is discontinued and maintains discontinuation of power transmission with a simple structure.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a torque limiting mechanism is provided. The mechanism includes a first rotor that has an elastic member, a second rotor that has an engaging portion, a coupler member that is located between the rotors and an urging member that urges the coupler member such that the coupler member is disengaged from the engaging portion. The coupler member is engaged with the elastic member and with the engaging portion such that power is transmitted between the rotors. When power is transmitted between the rotors, the elastic member maintains the coupler member engaged with the elastic member and with the engaging portion. When load generated either in the first rotor or in the second rotor due to power transmission exceeds a predetermined level, the elastic member is elastically deformed such that the coupler member is disengaged from the elastic member. When disengaged from the elastic member, the coupler member is disengaged from the engaging portion by the urging member.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.