Abstract:
Cold temperature performance of a pump for a controlled braking systems is enhanced, and internal flow restrictions of the pump are minimized, by placing the poppet return spring in a separate cavity out of the flow path through the pump. The separate spring cavity also allows travel of the poppet to be limited, thereby also limiting the loss of volumetric efficiency due to flow forces generated by cold, viscous fluid pushing the poppet an increasing distance from the seating area.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a pump with improved cold temperature performance for use in controlled vehicle braking systems, such as adaptive braking systems, traction control systems, and vehicle stability enhancement systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Controlled braking systems, such as adaptive braking systems, traction control systems, and vehicle stability enhancement systems, use a pump to force brake fluid to the vehicle brakes during such controlled operation. The pump must be self priming and must be able to force brake fluid, even cold viscous brake fluid, to one or more of the vehicle brakes in a relatively short time period. For example, a driver performing a quick steering maneuver which causes the vehicle to oversteer will require a counter-braking moment on the opposite front wheel to occur almost instantaneously in order to correct the skid condition. Accordingly, the pump must quickly extract brake fluid from the reservoir and force it to the appropriate brake under increasing pressure loads. The problem of quick response is particularly acute during cold weather operation, where the viscosity of the brake fluid places severe limitations on pump performance. Accordingly, to enhance cold weather performance, it is desirable to minimize internal restrictions within the pump, provide a pump having a relatively high compression ratio, and limit the travel of the pump inlet poppet.  
         SUMMARY OF THE INVENTION  
         [0003]    According to the present invention, internal flow restrictions of the pump are minimized because the poppet return spring is placed in a separate cavity completely out of the flow path through the pump. The spring is a simple compression spring instead of the complex barrel spring used in prior art designs. Any viscous drag created by the spring and separate spring cavity is minimized by incorporating a slotted head on the poppet stem, in addition to grooves on the stem that permit fluid displacement between the spring cavity and the flow path through the pump. The spring cavity is closed by a wear button which extends from the piston and is engaged by an eccentric bearing to drive the pump piston, thereby enabling the rest of the assembly to be made of a softer steel. Poppet travel is limited by a step on the poppet stem or separate poppet retainer which contacts a corresponding face on the housing. Accordingly, the problem of the poppet continuing to open more and more at cold temperatures in response to increased brake fluid viscosity and resulting flow forces on the poppet is eliminated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a fragmentary cross sectional view taken through a pump assembly make according to the teachings of the present invention;  
         [0005]    [0005]FIG. 2 is an enlargement of a portion of FIG. 1, to better illustrate some of the components of the pump; and  
         [0006]    [0006]FIG. 3 is a view similar to FIG. 2, but illustrating another embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0007]    Referring now to the drawings, a pump assembly generally indicated by the numeral  10  made according to the present invention is mounted in a bore  12  of a typical housing  14  of a brake pressure modulator. The pump  10  includes a housing sleeve  16  defining a bore  18  therewithin. The housing sleeve  16  is mounted in bore  12 , which is closed by a plug  20 . The housing  14  includes an inlet passage  22 , which is communicated with a fluid reservoir, and an outlet passage  24 , through which the pumped fluid is communicated. The bore  18  extends through open end  26  of the housing sleeve  16 , and the opposite end  28  of the sleeve  16  is provided with an outlet opening  30 , which communicates bore  18  with the outlet passage  24 . A spring loaded, one way check valve  32  permits fluid to flow from bore  18  to the outlet passage  24 , but prevents flow in the reverse direction.  
         [0008]    A pump piston  34  is slidably mounted in the bore  18  and is movable toward and away from the end  28  of the sleeve  12  to effect pumping of fluid. Pump piston  34  defines a chamber  36 , which terminates in an open end  38  circumscribed by a valve seating area  40 . Accordingly, the chamber  36 , the bore  18  and the outlet opening  30  define a flow path communicating the inlet passage  22  with the outlet passage  24 . The valve seating area  40  divides the flow path into an inlet section  42  communicated with inlet passage  22  and a pumping section  44  communicated with outlet opening  30 . The end of the chamber  36  opposite the valve seating area  40  terminates in a wall  46  through which an aperture  48  extends. The aperture  48  communicates the inlet section  42  with a spring cavity  50  defined within pump piston  34 . The end  52  of pump piston  34  opposite end  38  terminates in an opening. End  52  is closed by a wear button  54 , which defines the end of spring cavity  50 . The wear button  54  is made of a wear resistant material as compared to the material from which the remaining components of the pump  10  are made.  
         [0009]    Communication through the aforementioned flow path is controlled by a poppet generally indicated by the numeral  56 , which cooperates with the valve seating area  40 . The poppet  56  includes a stem  58  which extends through the inlet section  42  and the aperture  48 , and into the spring cavity  50 . The poppet  56  further includes a circumferentially extending, radially outwardly projecting head  60  which projects from the stem  58  and engages and moves away from the valve seating area  40  during operation of the pump  10 . It will be noted that the volume of the stem  58  extending through the chamber  36  is relatively small, thereby maximizing the volume of the chamber  36  through which fluid may communicate during normal operation of the pump  10 .  
         [0010]    The end  62  (FIG. 2) of the poppet opposite the end which caries the head  60  terminates within the spring cavity  50 . End  62  is provided with a circumferentially extending groove which receives radially inwardly extending portion  64  of a sleeve  66  which is split longitudinally (not shown) so that it may be snapped over the end  62  of the stem  58  with the inwardly extending portion  64  received in the groove on the stem  58 . Alternately, the retainer  66  may be slipped over the end  72  of poppet  58  with the radially outward projection  73  being formed after assembly by a suitable peening, staking, or heat staking process. The sleeve  66  carries a radially outwardly projecting, circumferentially extending shoulder  68  that faces a corresponding radially extending surface  70  on the piston  34  defining the end of the spring cavity  50 . A spring  72  acts between the shoulder  68  and the surface  70 , thereby urging the poppet to the left viewing the Figures, so that the poppet head  60  will be urged into engagement with the valve seating area  40 . However, the travel of the poppet  56  away from the valve seating area will be limited to that attained when front face  71  of retainer  66  engages surface  70 . This method of restricting poppet travel is low cost and minimizes tolerance stack-ups so that tight manufacturing tolerances may be maintained for the travel limit. It will also be noted that the spring  72  is a simple compression spring, and not the more complex springs required in prior art designs. Longitudinally extending, circumferentially spaced grooves  74  are provided on the outer circumferential surface of the portion of the stem  58  that extends through the aperture  48  to facilitate flow of fluid from the inlet section  42  of the chamber  36  into the spring cavity  50 . Circumferentially spaced slots  76  are provided around the radially outwardly extending portion of the sleeve  66  to minimize viscous drag on the poppet  56 .  
         [0011]    Reciprocation of the pump piston  34  is effected by rotation of a shaft  78  which is rotatably mounted in the housing  14  by bearings  80 , 82 . The shaft  78  may be rotated by any appropriate device, such as an electric motor (not shown). Shaft  78  includes an eccentric portion  84 , the motion of which is transferred to wear button  54  by an eccentric bearing  86  to thereby reciprocate the piston  34 . A suitable circumferential retaining clip  88  is preloaded into piston grooves  90  to keep the faces of the wear buttons  54  tight against the bearing  86  riding on shaft eccentric  84  during the pump suction stroke.  
         [0012]    In the embodiment of FIGS. 1 and 2, the stem  58  and head  60  are integral and may be made from, for example, molded plastic. The poppet  56  is assembled with the piston  34  by inserting the stem  58  through the aperture  48  installing the spring  72  on the stem  58 , and then snapping the sleeve  66  on the stem or alternately forming the extended radial end  73  of the poppet stem  58  by a suitable peening or staking operation as the final assembly step. In the embodiment of FIG. 3, the head  60  and stem  58  are separate pieces and the separate sleeve  66  is not necessary, since the stem  58  may be formed with a counterbored head  88  and the head  60  may be installed on the stem  58  after the stem has been installed in the aperture  48  and then subsequently swaged to form an enlargement of counterbore  88  as shown. Alternately, the stem  58  and poppet head  60  may be attached by a simple press fit.  
         [0013]    In operation, when fluid must be pumped by the pump  10 , the aforementioned electric motor (not shown) is started to turn the shaft  84  to thereby reciprocate the piston  34 . As the piston  34  moves in its compression stroke (that is, the piston  34  moves to the right viewing the Figures, into the pumping section  44 ), fluid is forced past the check valve  32  and into the outlet passage  24 . When the piston  34  passes through the top dead center position, the volume of the pumping section  44  is minimized and thereafter begins to increase. Accordingly, because of the reduced pressure in pumping section  44  and the relatively low force of return spring  72 , the piston  34  withdraws from the head  60  of the poppet  56 , permitting fluid to flow past the valve seating area  40  and into the pumping section  44 . Thereafter, the piston  34  passes through the bottom dead center position and begins a compression stroke, thereby forcing the head  60  back into engagement with the valve seating area  40  due to the action of the spring  72  and the increase in pressure in the pumping section  44 .  
         [0014]    It will be noted that the compression spring  72  is displaced from the flow path through the chamber  36 , and the volume of the stem  58  is relatively small. Accordingly, the flow path is relatively unobstructed, thereby minimizing the internal flow restrictions of the pump, thereby decreasing response time and increasing the efficiency of the pump, particularly under cold temperature conditions. In addition, the fluid volume close to the pumping chamber is maximized, further improving cold temperature pumping due to the vacuum induced volumetric expansion effects occurring within the fluid. Internal restrictions within the pump when cold, viscous fluid must be pumped are critical performance factors. Furthermore, the distance that the poppet head  60  is allowed to move away from the valve seating area  40  is limited since when face of retainer  66  hits the radial wall  70 , or the equivalent step  79  in alternate design stem  58  hits the radial wall  70 , the head  60  cannot move further away from seating area  40 . In prior art designs, the colder and more viscous the fluid the greater the resulting flow forces, which tend to push the poppet head further away from the valve seating area  40 . Accordingly, volumetric efficiencies were lost as more of the stroke of the piston  34  will be required merely to cause the poppet head  60  to close against the valve seating area  40  after the piston moves past bottom dead center. Accordingly, this loss of volumetric efficiency is limited.