Abstract:
A power steering pump for an automotive vehicle includes a housing having an inner wall with an axial groove. A cam plate that defines a cam chamber is received in the housing and includes a rib received in the groove. The groove and rib are disposed near the outlet port of the cam chamber and are dimensioned such that, during operation, distortion of the cam plate directs the rib into locking engagement with the groove while maintaining clearance between the housing and remainder of the cam plate to reduce propagation of vibration and noise.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to a power steering pump for an automotive vehicle and, more particularly, to a power steering pump that includes a cam plate accurately located within a housing. 
     BACKGROUND OF THE INVENTION 
     A power steering system of an automotive vehicle comprises a pump for supplying hydraulic fluid under pressure to a power steering gear assembly. A typical power steering pump comprises a rotor having retractable vanes and rotating within a cam chamber. The circumference of the cam chamber is defined by a cam plate that is interposed between upper and lower pressure plates and assembled within a housing. The housing is connected to the steering gear assembly for receiving spent fluid. Suction passages within the housing convey the fluid to inlets to the cam chamber formed in the pressure plates. Pressurized fluid is discharged through outlets in the pressure plate to discharge passages that are connected to the gear assembly. Typically, a flow control assembly regulates the output from the pump and recycles excess fluid through a bypass port to the suction passages. During operation, the rotor is driven by the engine through a belt and pulley mechanism. The rotor draws fluid from the suction passage through the inlets, compresses the fluid, and discharges pressurized fluid through the outlets to the discharge passages that lead to the steering gear assembly. 
     The cam plate must be accurately aligned in the housing to register the cam chamber with the inlets and outlets. This is typically accomplished by alignment pins extending through bores in the cam plate, or between the perimeter of the cam plate and the housing. Furthermore, the cam chamber typically includes two inlet regions for receiving low pressure fluid and two outlet regions for discharging high pressure fluid. The outlet regions are diametrically arranged to balance the forces created by the high pressure fluid, and similarly the inlet regions are diametrically arranged. In general, it is desired to minimize the radial width of the cam plate to reduce the weight and cost of the pump. However, because of the high pressure of the fluid at the outlet regions, which may exceed 2,500 psi, and the differences in pressures with the inlet regions, forces develop within the cam plate that tend to distort the cam chamber, or even create stresses that may cause catastrophic fracture of the plate. Also, vibration of the cam plate during operation results in noise. Moreover, pressure distortion of the cam plate, and vibration of the alignment pins, increases the noise emanating from the pump. 
     Therefore, a need exists for a power steering pump that includes a cam plate that is accurately aligned within the pump housing to register the cam chamber with the inlets and outlets, and further that is able to reduce vibration and noise, including noise due to vibration of the cam plate and distortion of the cam chamber. There is also a need to reduce the size and number of components of the pump, while minimizing the vibration and noise, to thereby reduce the size and cost of the pump. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention provides a power steering pump for an automotive vehicle that includes a housing having an inner housing wall about an axis, and a cam plate received in the housing. The cam plate defines a cam chamber and includes an perimeter spaced apart from the inner housing wall. The pump also includes a rotor coaxially received in the cam chamber and comprising retractable vanes that cooperate with the cam chamber to pressurize fluid. One or more outlets communicate with the cam chamber for discharging pressurized fluid. In accordance with this invention, the inner housing wall includes an axial groove and the cam plate includes a locator rib received in the groove. The groove and rib are disposed adjacent the outlet port. In the absence of pressurized fluid within the cam chamber, the outer surface of the cam plate about the rib is radially spaced apart from the housing inner surface by a first dimension, and the rib is radially spaced apart from the groove by a second dimension that is less than the first dimension. During operation, the retractable vanes of the rotor cooperate with the cam chamber to pressure fluid adjacent the outlet port. Distortion of the cam plate due to operating fluid pressures directs the rib into a locking engagement within the groove, while maintaining clearance between the housing and the remainder of the cam plate outer surface. This not only accurately positions the cam plate within the housing, but also, secures the cam plate to reduce vibration. Also, the remaining clearance inhibits vibration propagation from the cam plate to the housing. Thus, this invention provides a power steering pump that exhibits reduced noise during operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be further described with reference to the drawings wherein: 
     FIG. 1 is a cross-sectional view a power steering pump in accordance with this invention; 
     FIG. 2 is a cross-section of the pump in FIG. 1 taken along lines  2 — 2  in the direction of the arrows; and 
     FIG. 3 is a cross sectional view of the pump in FIG. 1 taken along lines  3 — 3  in the direction of the arrows. 
     FIG. 4 is an enlarged view showing the region within circle  4  of FIG. 2 prior to operation; and 
     FIG. 5 is an enlarged view comparable to FIG. 4 showing the region during high pressure operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with a preferred embodiment of this invention, referring to the Figs., a power steering pump  10  is adapted for use in a power steering system of an automotive vehicle for providing pressurized fluid to a power steering gear assembly. Pump  10  comprises a canister housing  12  that defines a compartment surrounded by an inner wall  14  generally cylindrical about an axis  16 . Received within the compartment is a cam plate  18 , a lower pressure plate  20  and an upper pressure plate  22 . Plates  18 ,  20  and  22  are enclosed by a cover  24  secured by a retaining ring  26 . 
     Cam plate  18  defines a cam chamber  28 . A rotor  30  is disposed within cam chamber  28  and mounted on a shaft  34  that extends through housing  12 . Shaft  34  is adapted to be driven by the vehicle motor through a belt and pulley arrangement and rotates rotor  30  about axis  16 . Rotor  30  includes radially slideable vanes  32  that engage the surface of cam chamber  28  to pump fluid as rotor  30  is rotated. 
     The several elements of pump  10  cooperate to define suction passages  36  for conveying relatively low pressure fluid to cam chamber  28  and discharge passages  38  for conveying pressurized fluid from cam chamber  28 . Fluid enters the cam chamber from the suction passages through inlets  40 . Fluid exits to discharge passages  38  through outlets  42 . It would be appreciated that only a portion of the suction passages  36  and the discharge passages  38  are depicted in the figures, and that additional passages are provided for conveying the fluid from the pump inlet and to the pump outlet. Also, pump  10  may include bypass porting to regulate the output from the pump and return a portion of the pressurized fluid to the suction passages. It is a feature of this embodiment that outlets  42  are diametrically opposed about axis  16  for balancing the pressures therebetween during operation. Similarly, inlets  40  are diametrically opposed. During operation, the fluid pressures within cam chamber  28  in the outlet regions adjacent outlets  42  are substantially higher than in the inlet regions adjacent inlet  40 . Moreover, the relatively high discharge pressures and the non-uniform pressure distribution tend to distort the size and shape of the cam chamber. 
     In accordance with this invention, cam plate  18  comprises perimeter  44  spaced apart from inner wall  14  of housing  12 . Also, housing  12  comprises axial grooves  46 , and cam plate  18  comprises axial locator ribs  48  received in grooves  46 . Preferably, the rib has a semicircular cross section perpendicular to the axis, and the groove has a similar semicircular cross section sized and shaped to conform to the rib. Referring particularly to FIG. 4, perimeter  44  of cam plate  28  is spaced apart from inner wall  14  to provide clearance for assembling the cam plate into the housing. Except at rib  48 , the cam plate is separated from the housing by a first radial dimension a. In contrast, ribs  48  are spaced apart from grooves  46  by a second radial dimension b that is less than a. By way of example, clearance a may typically be within the range of 0.3 mm to 0.6 mm; whereas clearance b is preferably less than 0.05 mm, and by way of example, a clearance b of 0.015 mm has been achieved. In addition, lower pressure plate  20  includes ribs, and upper pressure plate  22  includes ribs  50  in FIG. 3, that are also received in grooves  46  and register with ribs  48 , thereby providing accurate circumferential arrangement of the pressure plates and cam chamber  28 . It is a feature of this invention that grooves  46  and ribs  48  are disposed adjacent outlets  42  and the associated regions of cam chamber  28  that contain pressurized fluid during operation. In this manner, ribs  48  provide reinforcement of cam plate  18  in regions where distortion tends to occur. 
     During operation, shaft  34  is driven about axis  16  to rotate rotor  30  within cam chamber  28 . Relatively low pressure fluid is drawn into cam chamber  28  through inlets  40 . Within the cam chamber, the fluid is compressed by vanes  32 , and the pressurized fluid is discharged through outlets  42 . The pressurized fluid adjacent outlets  42  tend to distort cam plate  18  radially outward. Referring to FIG. 5, as the cam plate becomes distorted, rib  48  extends into contact with groove  46 . Thus, rib  48  registers with groove  46  to prevent circumferential deflection of the cam plate and maintain alignment of the cam plate with the upper and lower pressure plates  20  and  22  in the desired arrangement with the inlets  40  and outlets  42 . In addition, as rib  48  engages groove  46 , housing  12  provides additional reinforcement of cam plate  18  in the regions of the pressurized fluid to prevent further distortion of the cam plate. Still further, because greater clearance is provided between outer surface  44  and inner wall  14  of housing  12  about rib  48 , upon contact between rib  48  and groove  46 , radial clearance c remains between the cam plate and the housing. This remaining clearance isolates the cam plate from the housing to inhibit propagation of vibrations and reduce noise associated therewith. 
     Therefore, this invention provides an improved power steering pump for an automotive vehicle that includes a rib protruding from the perimeter of the cam plate and received in an axial groove in the housing. The rib together with similar locating ribs on the upper and lower pressure plates, serves to accurately locate the elements within the housing, including the inlets and outlets to and from the cam chamber. In addition, clearance is reduced between the rib and groove radially relative to the surrounding surfaces of the cam plate in the housing. Thus, during operation, distortion of the cam plate attributed to high pressures developed within the cam chamber cause the rib to engage the groove, while still providing clearance about the majority of the cam plate. This assists in isolating vibrations and noise generated within the cam chamber from propagating to the housing. This reduces vibration and noise associated with the operation of the pump. Still further, the locating ribs are integral with the cam plate and are disposed adjacent high pressure regions within the cam chamber. This increases the radial width of the cam plate, and thus the strength, in those regions that experience increased stress due to high pressures within the cam chamber. In addition, the housing reinforces the cam through the ribs in the high pressure regions where strengthening is critical. However, the clearance between the cam plate and the housing in other regions, including low pressure regions, provides a barrier to vibrations that would otherwise propagate and produce noise. This is accomplished while eliminating locator pins or other components required for location of the cam plate, thereby reducing the number of components and cost for the power steering pump. 
     While this invention has been described in terms of certain embodiments thereof, it is not intended to be so limited, but rather only to this extend set forth in the claims that follow.