Abstract:
The invention provides an axial piston pump/motor in which clearance at the port face between a stationary fluid distribution block and a rotating cylinder barrel is controlled in accordance with impedanced working fluid pressure applied to a hydrostatic bearing.

Description:
This is a continuation of application Ser. No. 370,375, filed June 15, 1973 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to improvements in axial piston pumps or motors and comprises means for controlling the clearance between the port-face and the rotating barrel of such pumps or motors (hereinafter referred to as pumps). 
     In axial piston hydraulic pumps, where distribution of the working fluid to and from the working cylinders carried by a rotating barrel is effected by co-operating port-faces on the barrel and on the casing, it is usual to employ minimal porting at minimal operating radius to minimize face leakage and, more importantly, port-face fluid-shear drag and heating which can become critical at low through-put and low pressure-leakage. Also, of course, the effective port-face areas have to balance the combined piston areas, as under-balance would allow rubbing contact and wear, whilst over-balance increases leakage and therefore volumetric efficiency loss. Further, there is a substantial imbalance variation as the cylinder ports successively come into and out of communication with the delivery port of the casing port-face. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method of controlling the port-face running clearance to an optimum value at high pressures and deliveries, despite substantial piston/port-face imbalance, yet to allow increase of clearance at low or zero output pressures so as to reduce considerably idling, or downtime, heating and drag-loss at the port-face. 
     To this end what we propose is a hydraulic thrust bearing acting between the casing head and directly or indirectly upon the cylinder barrel so as to be in opposition to the port-face thrust. This hydrostatic counterthrust is characterised by the provision of an axially floating thrust-plate having an annular fluid chamber between the floating plate and the casing, the annular area of the said fluid chamber being less than that of inner and outer sealing lands by which it is bounded. 
     In one preferred construction, a similar, but preferably smaller, hydrostatic thrust unit opposes the counterthrust to control the system stability, but in this case the annular area of the fluid chamber is greater than that of inner and outer lands by which it is bounded. 
     Whereas the hydrostatic counterthrust is pressured from any of the casing ports via a check-valve controlled gallery and an impedance, the opposing thrust unit is pressured from a constant pressure source via an appropriate impedance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary, partially diagrammatic, cross-sectional view taken transversely on the vertically disposed axis of pump/motor including a clearance control system in accordance with the invention, and 
     FIG. 2 is a view similar to that of FIG. 1, but showing a modification. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The pump barrel 1 is cast integrally with a sleeve 2 by which it is supported from an internal annular shoulder 3 formed in the casing 4 upon an annular hydrostatic bearing 5. 
     The pump barrel is rotated in the normal way by its interconnection with a swash plate or carrier therefor by pistons and connecting rods. 
     The port-face 7 between the barrel 1 and the casing 4 is that which is controlled in accordance with this invention. It is essential that the clearance at this face be minimal when pumped fluid pressures are high, and a good deal greater under idling or low pressure conditions. 
     To this end the hydrostatic annular bearing pad or plate 5 (of generally Z shaped cross-section) includes a fluid chamber 8 (between lands 8a, whose combined annular area is greater than that of the chamber) pressurized by pump outlet pressure through a check valve-controlled impedance 9. This pad 5 applies a lifting force to the upper flange 10 of the sleeve 2 and thus tends to minimise the clearance at the face 7. Springs 11 ensure that such change of port face clearance is progressive. 
     An auxiliary thrust hydrostatic annular bearing pad 12 abutting an upwardly facing shoulder 13 of the sleeve 2 is pressurised through an impedance 14 from a constant pressure source. This pad, whose fluid chamber area is greater than that of the lands by which it is bounded, controls the stability of the clearance control system. 
     In operation, with the pump developing a pressure greater than a predetermined fraction of the constant pressure, the counterthrust annulus 5 overpowers the auxiliary thrust annulus 12 plus any differential load at the port-face thus pulling the port-face to the close clearance determined by the limit of the auxiliary annulus collapse; and at any higher pressure the system stability maintains the port-face clearance at, say, seven tenths of a thousandth of an inch and precisely to within a range of approximately one-ten-thousandth of an inch. 
     When the pump pressure falls below the preset figure, the auxiliary-thrust annulus 12 overpowers the counterthrust annulus 5 to the limit of its collapse, corresponding to the desired open clearance of, say, five to six thousandths of an inch of the port-face for minimum heating and drag. 
     In a modified form of port-face control illustrated in FIG. 2, the bearing pad 12 is omitted and thrust against the upwards force applied to the flange 10 by the bearing pad 5 is supplied by more powerful springs 11 carried on adjustable threaded pins 14 through the intermediary of a crown plate 15. 
     The combined rate of the springs 11 is such that at zero or very low pumping pressures the port-face clearance is relatively very large, typically 5 times as great as the clearance at highest pressure when the active supporting load in the annular space 3 is balanced by the separating force at the active port-face together with the maximised load of the compressed springs 11. 
     For certain pump applications it is advantageous for the spring 11 to have variable rate so that the increase of port-face clearance at low pressures is non-linear. 
     Advantage can be taken by increasing the counterthrust area, to increase the cylinder-port and port-face areas well in excess of piston area balance with a view to increasing throughput without incidence of cavitation in the cylinder ports and cylinders.