The present invention relates to torque limiting mechanisms that transmit rotational power from an external power source to a driven apparatus.
A typical driven apparatus is directly coupled to a power source such as an electric motor or an engine and is driven by the power source. A torque limiting mechanism is sometimes located between the driven apparatus and the drive source. The torque limiting mechanism forcibly disengages the power source from the driven apparatus when a malfunction occurs in the driven apparatus, for example, when the apparatus 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 apparatus. Japanese Unexamined Utility Model Publication No. 63-19083 discloses an on-vehicle clutchless type compressor having such a torque limiting mechanism. The compressor has a rotary shaft and receives power of an engine through a pulley, which is coupled to the engine, and the rotary shaft. That is, a power receiver is press fitted to the rotary shaft. A pair of breakable pins project from the receiver. The pins are engaged with holes formed in the pulley. The pins are designed to simultaneously break when they receive an excessive load torque applied by the compressor. This disengages the pulley from the rotary shaft. Accordingly, the pulley goes into a freewheeling condition. The engine is thus not affected by the excessive load torque.
In the above prior art mechanism, it is difficult to improve both the fatigue strength of the pins and the predictability of the value of load torque at which the pins are broken. The load torque at which the pins are broken is determined by calculation. However, the pins are weakened by torque fluctuations that are smaller than the calculated breakage torque. Weakening of the pins eventually causes the pins to be broken by a load torque that is smaller than the calculated breakage torque. When the compressor is either turned on or off, the pulley rapidly applies torque to the pins. This accelerates the fatigue of the pins.
Further, since the pins are designed to be broken simultaneously, the breakage strength of each pin needs to be half the calculated breakage torque. This causes each pin to be more vulnerable to fatigue. As a result, the pins are more likely to be broken by a load torque that is smaller than the calculated breakage torque.
Although two or more breakable pins are usually used in the above mechanism, some mechanisms have a single breakable pin. In this case, the diameter of the pin is not necessarily small. However, as in the case of a mechanism having multiple pins, the single pin is fatigued by repeated torque fluctuations and is eventually broken by a load torque that is smaller than the calculated breakage torque.