1. Field of the Invention
The present invention relates to an electromagnetic clutch that cuts and supplies power and that is particularly suitable for driving a compressor in a refrigeration cycle for an automobile air conditioning apparatus.
2. Description of Related Art
One type of known electromagnetic clutch has been generally utilized as a coil-fixed type clutch which has an electromagnetic coil on a fixed member for generating an electromagnetic attractive force. This coil-fixed type electromagnetic clutch forms a passage for supplying current to the electromagnetic coil without an additional current supply member, because the electromagnetic coil is installed in the fixed member.
However, a magnetic circuit, through which flux passes, is formed through the fixed member, the rotational member and an armature magnetically attracted by a drive-side rotational member because the electromagnetic coil is installed in the fixed member. Thus, magnetic gaps are formed among these three circuit components. Therefore, the size of the electromagnetic coil may be increased, or it may consume more electricity, due to greater magnetic loss and lower magnetic efficiency.
To solve this problem, a coil-rotation type which includes the electromagnetic coil on the rotational member is proposed in, for example, JP-U-1-131028. According to the coil-rotation type, the magnetic circuit comprises only the rotational member and the armature magnetically attracted by the rotational member. Therefore, such coil-rotation type has an advantage because the magnetic loss is much smaller and its magnetic efficiency is much higher than the coil-fixed type.
However, according to the electromagnetic clutch disclosed in the above official gazette, a friction plate made out of magnetic substance is joined to the drive-side rotational member made out of magnetic substance, and an armature made out of magnetic substance is facing the friction plate keeping a small gap between the armature and the friction plate. Therefore, magnetic flux of the magnetic circuit, generated by supplying current to the electromagnetic coil, passes through a joint portion between the rotational member and the friction plate.
The rotational member and the friction plate are usually joined by the spot welding or the like, and small gaps exist at the joint portion other than welded points in the circumferential direction. Such the small gaps may cause degradation of the magnetic efficiency.
Furthermore, assembling productivity is low because a positioning of the friction plate against the rotational member in the radial direction can not be made.