Abstract:
A balanced rotary vane pump for a motor vehicle power steering system includes a rotor having fifteen vanes disposed in a three lobe cam ring. Three inlet ports and three outlet ports disposed in equally spaced pairs provide fluid communication to the three lobes of the cam ring. Because the subject vane pump provides a significantly larger number of pump pulses per revolution which are more closely spaced in time and may define smaller volumes, the pulsatile nature of the output and thus vibration as well as the excitation of sympathetic vibration is greatly reduced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to vane pumps and more specifically to a balanced vane pump for automotive power steering systems having a three lobe cam ring and three pairs of inlet and outlet ports. 
     2. Description of Related Art 
     Vane pumps typically used in vehicular power steering systems in automobiles, sport utility vehicles, pickup trucks and the like represent a significant source of noise because they provide a pulsatile output. Such a rippling or pulsing output interacts with the hydraulic circuit to create harmonic pressure pulsations which may be characterized as fluid borne noise. Currently utilized balanced vane pumps have two inlet ports and two outlet ports disposed in diametrically opposed pairs. So configured, a completely hydraulically balanced device is achieved. That is, for every force and compressive action occurring at one specific circumferential location or vane on the rotor, the same force or action is occurring at a diametrically opposed location. Generally speaking therefore, the forces in the pump and particularly those against the rotor and vanes relative to the axis of rotation of the rotor cancel each other. 
     A conventional power steering pump has ten vanes and a rotor which rotates within a cam ring having two oppositely disposed lobes. A pumping region or volume is formed between any two adjacent vanes twice in each revolution, which allows each pumping volume to pump twice per revolution. There are, thus, twenty flow pulses generated per revolution, which represent potential sources of noise. 
     With increased emphasis on reducing noise, vibration and harshness (NVH) in motor vehicles, all operating systems and components have come under scrutiny. Components of power steering systems which are typically hydraulic and energized by a rotary vane pump have been included in such examination. The present invention relates to such a device. 
     SUMMARY OF THE INVENTION 
     A balanced rotary vane pump for a motor vehicle power steering system includes a rotor having fifteen vanes disposed in a three lobe cam ring. Three inlet ports and three outlet ports disposed in equally spaced pairs provide fluid communication to the three lobes of the cam ring. Because the subject vane pump provides a significantly larger number of pump pulses per revolution which are more closely spaced in time and may define smaller volumes, the pulsatile nature of the output and thus vibration as well as the excitation of sympathetic vibration is greatly reduced. 
     It is thus an object of the present invention to provide a rotary vane pump having three equally spaced pumping lobes in a cam ring and three pairs of inlet and outlet ports. 
     It is a further object of the present invention to provide a rotary vane pump for vehicular power steering systems having reduced noise output. 
     It is a still further object of the present invention to provide a balanced rotary vane pump having fifteen vanes disposed in a three lobe cam ring. 
     It is a still further object of the present invention to provide a rotary vane pump for motor vehicle power steering systems which provides a higher number of pumping pulses per revolution of the rotor than currently available units, thus reducing noise generation. 
     Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers refer to the same component, element or feature. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a three lobe balanced rotary vane pump according to the present invention; 
     FIG. 2 is a cross-sectional view of a cam ring and rotor of a three lobe balanced rotary vane pump according to the present invention; 
     FIG. 3 is an end view of a lower pressure plate of a three lobe rotary vane pump according to the present invention showing the relative positions of the inlet and outlet ports; 
     FIG. 4 is an end view of the upper pressure plate showing the relative angular positions of the inlet and outlet ports; and 
     FIG. 5 is a schematic, cross-sectional view of a cam ring and rotor of a three lobe balanced rotary vane pump according to the present invention illustrating a preferred profile of the cam ring surface. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 1 and 2, a three lobe balanced rotary vane hydraulic power steering pump according to the present invention is illustrated and generally designated by the reference number  10 . The pump  10  includes a housing  12  defining a cylindrical region  14  containing the mechanical components of the pump  10 . The housing  12  also includes at least three bosses  16  defining through apertures  18  each adapted to receive a mechanical attachment device such as a bolt (not illustrated) which can be threaded into an engine block (also not illustrated) to secure the housing  12  thereto. So configured, the conventional bracket typically used to support a power steering pump can be eliminated. 
     Centrally received within the circular region  14  and supported for rotation therein is a shaft  20  which extends out the front of the pump  10  and receives a pulley (not illustrated) which is driven by an engine belt (also not illustrated). The shaft  20  include male splines  22  which engage female splines  24  within a rotor  26 . The rotor  26  is fixed in position on the shaft  20  by a snap ring  28 . The rotor  26  defines fifteen equally spaced radial slots  30  which each receive a blade or vane  32 . The rotor  26  and vanes  32  are received within a cam ring  34  having an undulating inner surface  36  which defines three lobes  38 . 
     Referring now to FIGS. 1,  3  and  4 , disposed at each end of the cam ring  34  and also received within the cylindrical region  14  of the housing  12  is a first or upper pressure plate  40  and a second or lower pressure plate  60 . The upper pressure plate  40  includes three arcuate outlet ports  42  which communicate with passages in the housing  12  as well as arcuate passageways  46  which assist in cold start priming of the pump  10 . Additional groups of passages  48  are coupled to the outlet port through passages  52 . The upper pressure plate  40  also includes a pair of diametrically opposed through openings  54  which receive a pair of axially disposed alignment pins  56 . 
     Disposed adjacent the opposite end of the cam ring  34 , the lower pressure plate  60  includes three equally spaced outlet ports  62 , three equally spaced inlet ports  64 , and three arcuate, diametrically opposed channels  66  located radially and angularly at positions which facilitate communication with terminal holes at the radial base of the slots  30  in the rotor  26 . The lower pressure plate  60  also includes a pair of diametrically opposed through passages  68  which receive the alignment pins  56 . One or more registration lugs (not illustrated) project from the back face of the lower pressure plate  60  and engage complementarily configured recesses in the interior of the housing  12 . Cooperation between the lugs and recesses inhibits rotation of the lower pressure plate  60  and the alignment pins  56  maintain the cam ring  34  and the upper pressure plate  40  in proper alignment and inhibit rotation thereof. 
     Adjacent the front or upper pressure plate  40  is an upper or outer bushing  70  which supports the shaft  20  for rotation on its axis. The bushing  70  is supported by a pump cover  72 . An O-ring seal  74  is disposed between the cover  72  and the housing  12  and a wire snap ring  76  retains the cover  72  in secure, fluid-tight disposition within the housing  12 . 
     Adjacent the lower pressure plate  60  is an inner seal  80  and an outer seal  82 . A Belleville spring  84  develops an axial force between the inner surface of the housing  12  and the various components within the pump  10  and forces them into proximity to minimize fluid leakage therebetween. A bushing  86  is supported within the housing  12  and rotatably supports the shaft  20  and a shaft seal  88  prevents loss of hydraulic fluid from the interior of the pump  10 . 
     Referring now to FIG. 5, a schematic illustration of a preferred profile of the interior surface  36  of a cam ring  34  according to the present invention which defines three pumping lobes  38  is illustrated. As shown in FIG. 5, beginning 8° clockwise from a 0° reference point, the surface  36  begins 36° of rise to a point 44° clockwise of the 0° reference point. The major diameter of the lobe  38  then dwells for 27.5°. At 71.5° from the 0° reference point, the surface  36  falls for 35.5° to an angular position 107° from the 0° reference point. The cam surface  36  then dwells at a minor diameter for 21° extending to 128° from the 0° reference. Note that the rise, fall and dwells extend over exactly 120°. The cam ring  34  defines two additional lobes  38  about the remaining 240°. 
     In operation, a three lobe, balanced vane pump according to the present invention provides greatly reduced flow pulsations and thus reduces sympathetic vibration of the components such as hoses and mechanical components of the power steering system resulting in reduced noise, vibration and harshness of the overall power steering system. 
     Pumping volume, that is, pump displacement of vane pumps is given by the following formula:        Q   =           B   pf     ×     N   v     ×     V   v     ×     N   p       1000     .                            
     In this equation Q is the pump output in liters per minute, B pf  equals the balanced pump factor, N v  equals the number of vanes, V v  equals the volume between adjacent vanes in milliliters and N p  equals pump rpm. In conventional ten vane pumps, the balanced pump factor (B pf ) equals two and the number of vanes (N v ) is ten. 
     The same equation applies to the three lobe, balanced vane pump  10  of the present invention. However, the number of vanes (N v ) increases to fifteen and the balanced pump factor (B pf ) is three. Accordingly, it will be readily appreciated that rather than twenty pulses per revolution generated in a conventional ten vane, two lobe pump the three lobe, balanced vane pump of the present invention produces forty-five flow pulses. The pulses are thus more closely spaced in time and depending upon the geometry of the cam ring  34 , may be of slightly smaller magnitude. Both of these factors reduce pulsations and thus sympathetic vibration and provide improved NVH performance of the pump  10 , specifically, and the entire power steering system, generally. 
     The foregoing disclosure is the best mode devised by the inventors for practicing this invention. It is apparent, however, that apparatus incorporating modifications and variations will be obvious to one skilled in the art of rotary vane pumps. Inasmuch as the foregoing disclosure presents the best mode contemplated by the inventors for carrying out the invention and is intended to enable any person skilled in the pertinent art to practice this invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.