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Power transmission device of a compressor

Power transmission device of a compressor description/claims

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-337229 filed on Dec. 14, 2006, the entire contents of which are incorporated herein by reference.

The present invention relates generally to a power transmission device having a torque limiting function, and more particularly to a power transmission device of a compressor for a vehicle air conditioner which is driven by an external drive source such as an engine through a belt.

A power transmission device for transmitting power to a compressor has a torque limiter or a power interrupting device for forestalling a trouble such as a breakage of a power transmission belt in the event of a seizure in the compressor. A conventional power transmission device which is disclosed, for example, in Japanese Patent Application Publication No. 2006-266483, has a pulley, a casing, a hub and a power shutoff member (as described in pages 4-6 and FIGS. 1-4 of the publication). To be more specific, the pulley is rotatably mounted to the casing, the hub is fitted in the pulley and the power shutoff member as a torque limiter is interposed between the hub and a rotary shaft. The front end portion of the rotary shaft has a thread portion, a large-diameter shaft portion and a shaft-side seating face. The thread portion has a thread on the outer circumference thereof. The diameter of the large-diameter shaft portion is larger than that of the thread portion. A shaft-side seating face is formed as a stepped portion between the thread portion and the large-diameter shaft portion. A washer is provided in contact with the shaft-side seating face. The casing and the rotary shaft are sealed by a shaft seal member (not shown) to prevent refrigerant gas and oil from leaking out of the compressor.

The power shutoff member functioning as a torque limiter is formed by a cylindrical body which includes a large-cylindrical portion and a small-cylindrical portion. The large-cylindrical portion is engaged with a part of the hub. The small-cylindrical portion has in the inner periphery thereof a thread portion which is engaged with the thread portion of the rotary shaft. The inner diameter of the large-cylindrical portion is slightly larger than that of the small-cylindrical portion and a cutout is formed at the transition between the inner peripheries of the large and small-cylindrical portions so as to form a breakable portion.

A torque produced in accordance with the rotation of the pulley is transmitted to the power shutoff member through the hub and then to the rotary shaft. If the seizure is occurred in the compressor, an excessively high axial force is applied to the engagement portion between the thread portion of the power shutoff member and the thread portion of the rotary shaft, so that the breakable portion of the power shutoff member is broken. Therefore, the torque transmission from the pulley is shut off, thereby preventing a belt connecting the pulley to the power source from being broken.

However, in the conventional power transmitting device disclosed in Japanese Patent Application Publication 2006-266483, heat is generated by sliding contact between the rotary shaft and the shaft seal member which is mounted on the rotary shaft of the compressor. The amount of the heat generated by the sliding contact is proportional to the coefficient of friction of the slide contact surfaces between the rotary shaft and the shaft seal member, load acting on the rotary shaft from the shaft seal member and the rotational speed of the rotary shaft. However, when the load is increased to enhance the seal performance of the shaft seal member or the load is increased due to the increase of the internal pressure of the compressor, the peripheral speed of the rotary shaft needs to be reduced so as to reduce the amount of the heat generated by the slide contact between the rotary shaft and the shaft seal member. The rotary shaft needs to be produced with a reduced diameter for reducing its peripheral speed. However, if the diameter of the rotary shaft is reduced, another problem may result. That is, reducing the diameter of the rotary shaft in the power transmission device having the power shutoff member will decrease the strength of the rotary shaft, so that breakage of the rotary shaft may occur before the breakable portion of the power shutoff member is broken. In other words, the power shutoff member may fail to function reliably.

The present invention is directed to a power transmission device of a compressor which permits the power shutoff member to function reliably.

In accordance with an aspect of the present invention, a power transmission device of a compressor includes a rotary shaft, a pulley, a hub, a power shutoff member, a spacer and a cylinder. The rotary shaft is rotatably supported by a casing of the compressor. The rotary shaft has an external thread portion. The pulley is rotatably mounted on the casing. The hub is connected to the pulley for rotation therewith. The power shutoff member is joined to the hub. The power shutoff member is interposed between the hub and the rotary shaft. The power shutoff member has an internal thread portion and shuts off excessive torque transmission between the pulley and the rotary shaft. The spacer disposed on the rotary shaft. The spacer has a seating surface. The cylinder is disposed between the power shutoff member and the rotary shaft. The cylinder has an external thread portion formed at the outer periphery thereof for engagement with the internal thread portion of the power shutoff member and an internal thread portion formed at the inner periphery thereof for engagement with the external thread portion of the rotary shaft. The cylinder has a contacting surface for contact with the seating surface of the spacer and a seating surface for contact with a contacting surface of the hub at the axial end portion of the hub.

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.

The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

FIG. 1 is a longitudinal cross-sectional view showing a power transmission device according to a first preferred embodiment of the present invention;

FIG. 2 is an enlarged longitudinal cross-sectional view showing the power transmission device of the compressor according to the first preferred embodiment of the present invention;

FIG. 3 is an illustrative view showing the transmission of torque and various radiuses of rotation of the power transmission device according to the first preferred embodiment of the present invention;

FIG. 4 is a graph showing the torque-axial force characteristics of the power transmission device according to the first preferred embodiment of the present invention;

FIG. 5 is a graph showing the relation between the radius of rotation and the coefficient of friction in the torque-axial force characteristics of the power transmission device according to the first preferred embodiment of the present invention;

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