A variety of refrigerant compressors for use in vehicle air conditioning systems are currently available. One popular vehicle compressor design is the axial type. In this type of compressor, a number of cylinders are equally angularly spaced about and equally radially spaced from the axis of a central drive shaft. A piston is mounted for reciprocal sliding motion in each of the cylinders. The pistons are connected to a swash plate or wobble plate, which is in turn operatively connected to the drive shaft.
During operation of the compressor, rotation of the drive shaft imparts a wave-like reciprocating motion to the wobble plate. This nutating driving action of the wobble plate serves to impart a linear reciprocating motion to the pistons. The pistons serve to compress the refrigerant gas and circulate it through the air conditioning system to cool the air being directed into the vehicle passenger compartment.
The development of a new refrigerant compressor for a vehicle air conditioning system presents unique challenges to an engineer. In recent years, a key objective of automotive air conditioning engineers has been to reduce the size of the compressor as much as possible in order to reduce the overall weight of the vehicle, as well as to lower the cost and to conserve valuable space. It is, of course, also desirable to improve the operating efficiency and quietness of the compressor.
It can be appreciated that substantial momentum forces are capable of being produced in a compressor due to the extreme accelerations imparted to the compressor pistons. Accordingly, if there can be a reduction in the mass of the piston, and a concomitant reduction in the forces generated by the piston action, not only is substantial energy saved (and efficiency increased) due to a reduction in the force required to drive the pistons, but benefits in the other key areas are also gained.
Historically, pistons for axial type compressors have been of unitary construction, machined from a durable metal body, such as a steel alloy. This prior art construction provides the desired strength, but unfortunately has an undesirably high mass. While improvements in hardening and machining techniques have allowed lighter weight metals, such as aluminum alloy, to be substituted, the unitary construction still dictates that a relatively high mass structure be provided.
In recent efforts to address this problem, some progress has been made in producing an improved swash plate compressor piston of composite construction. Others have recognized the main advantage of a composite piston assembly design; that is, a piston capable of operating at extreme accelerations, while minimizing the mass of the assembly and thus reducing the power input required to drive it. This concept is applied to a piston of the rocking or angulating type; that is, the piston head and connecting rod are combined into a single, integral unit, so that during operation the piston assumes a constantly changing, inclined posture relative to the cylinder.
The composite piston of this prior art includes a reinforced plastic piston head that is molded to the distal end of an aluminum shank or piston rod. Various structure is provided to assure a secure connection. Additionally, the proximal end of the rod includes one part of a ball and socket joint providing the connection to the wobble plate of the compressor.
One successful prior art composite piston assembly is illustrated and claimed in co-pending patent application of Shontz, Gavlak and Ebbing, U.S. Pat. No. 5,022,313 issued Jun. 11, 1991, and assigned to the assignee of the present invention.
Thus, while the composite piston assembly represents a significant advance in the art, further improvements are still possible. As one might suspect, the fact that the angulating piston assembly assumes an inclined posture relative to the cylinder bore during piston reciprocation, redesign of the piston continues to be an area where the greatest challenges are present. It is this area where most operational difficulties, mainly sealing problems, can arise. This is especially true under severe service.
The angle of inclination of the piston usually is at a maximum at bottom dead center and becomes approximately zero at top dead center. As mentioned above, as the piston reciprocates between these extreme positions, the angle of inclination is constantly changing. As a matter of fact, this environment and extreme action makes designing a highly efficient piston, and piston ring for effective sealing with the cylinder bore, very elusive.
In the past, especially with respect to angulating pistons of the type described, it is usually necessary to fully machine the piston ring. This is an expensive process as tight tolerances must be maintained in order to provide the required relationship between the piston head and piston ring for proper sealing between the piston head and the cylinder bore.
More particularly, a tight tolerance must be maintained on the outside diameter of the piston ring not only to properly mate with the cylinder, but also so that the ring may be inserted and held firmly in the proper position within the mold cavity for casting of the piston head. The piston ring must also be relatively thick in cross-section so that the ring maintains its annular shape and does not collapse. The additional material that must be utilized to construct this type of prior art ring adds significantly to the overall cost.
Another difficulty in utilizing this type of fully machined piston ring is that it has a tendency to shrink with the molten piston head, as the resin/glass fiber reinforced material forming the head begins to cool in the mold. This disadvantageously results in some variation in the final outside diameter of the piston ring, often resulting in a poor fit within the cylinder bore and a loss in efficiency due to reduced compression of the refrigerant gas.
A need is therefore identified for an improved angulating piston assembly and a new approach to designing the piston ring subassembly for such an angulating piston. Further, a need is also identified for an improved method of forming and assembling an angulating piston assembly, including the novel piston ring subassembly.