Axial Piston Pumps

This type of pump consists of several units of small reciprocating pumps located in its housing.

Based on the delivery capacity, this pump is classified into two types, namely:

  • Fixed displacement axial piston pump
  • Variable displacement axial piston pump.

The Construction of a fixed displacement axial piston (straight shaft) pump

This pump has of the following components:

  • Housing
  • Front bearing that supports the drive shaft
  • Rotating group
  • Splined-end drive shaft
  • Valve plate consisting of inlet and outlet ports. This plate also functions as the back cover on which the direction of rotation and part number are indicated.

The rotating group includes:

  • Splined block cylinder
  • Splined spherical washer
  • Spring block cylinder
  • Spring that presses the transmitting pin
  • Nine (9) pistons with their shoes
  • Swash Plate/Cam Plate
  • Shoe retainer Plate.


When the piston moves to the right, the atmospheric pressure in the reservoir forces the fluid to flow through the inlet check valve to the pump cylinder.

As the shaft swash plate forms an angle against the cylinder block, the piston will move reciprocally in the bore during rotating.  The fluid inlet on the valve plate is located in such a way that the fluid flow will enter the bore when the piston moves backward.  When the cylinder block continues rotating, the piston will be forced to move into the bore and to force the fluid to exit through the outlet valve.


Although the specification stated in this STMG is applicable at the time it was published, the contents can be revised in case of any changes.  See the document provided to find the new specification.


   Output flow


Case drain flow




Relief Valve setting


132 GPM (499,7 ltr/min) at 2060 RPM against 1000 PSI

(70,3 kg/cm²) with SAE 10W oil at 150ºF(65ºC)

4 + 1 GPM ( 15,0 +  3,6 ltr/min ) at  2060 RPM and

2500 PSI (175,8 kg/ cm² ) and oil at 180 ºF ( 80 ºC )

CCW as seen in the cabin

205 horsepower

2500 PSI ( 175,8 kg/cm² )

232  lbs. ( 105 kg )


The torque required for rotating the the pump

Maximum Breakaway

Maximum Continuous

Spanner Nut

Retraction Bearing Cap Screws

Anti-tip Bearing Cap Screws



40 lb. ft. (5,53 mkg )

30 lb. ft. ( 4,84 mkg )

60 + 5 lb. ft. ( 8,29 + 0,69 mkg )

60 + 2 lb. ft. ( 1,66 + 0,27 mkg )

60 + 2 lb. in. ( 20,74 + 2,76 mkg )



Slipper to Cam

Anti-tip Bearing to Barrel

Impeller to Head



.003  + .001 inch  ( 0,076 + 0,025 mm )

.006  + .003 inch  ( 0,152 + 0,076 mm )

.012  + .003 inch  ( 0,293 + 0,076 mm )

Tool Group 2P2325
988 Service Manual Hydraulic System

Testing Implement Hydraulic System & Components (Service Manual)

STMG 99, “ Flow Meter Tee-Test Procedure “




The components of a piston pump

  1. Inlet housing
  2. Inlet port
  3. Spanner nut dan lock
  4. Impeller
  5. Arrow indicating direction of rotation
  6. Drive shaft
  7. Head
  8. Impeller shims for Dimension “ A “
  9. Teflon seal
  10. Outlet port
  11. Small tapered roller bearing
  12. Port plate
  13. Pum Housing
  14. Spiral snap ring
  15. Anti-tip bearing spacer
  16. Snap ring
  17. Spring
  18. Barrel
  19. Anti-tip Bearing
  20. Shims for dimension “B”
  21. Slipper
  22. Piston
  23. Housing drain port
  24. Retraction bearing
  25. Slipper retraction plate
  26. Cam
  27. Retraction bearing shims for dimension “ C “
  28. Lip seal
  29. Large tapered roller bearing

A. Impeller to inlet housing clearance

B. Anti-tip bearing to barrel clearance

C. Slipper to cam face clearance


The oil is forced from the hydraulic reservoir to the inlet port by a centrifugal pump mounted on the drive shaft.  The oil enters the centrifugal pump in the center of the impeller and is held between the impeller blades.  The centrifugal force throws the oil out at a high speed, and the head directs the oil to the kidney-shaped inlet port of the impeller port plate, producing a supercharging effect of 20 psi.

Note: The pressure will range from 6 psi (low idle) up to 20 psi (high idle).  The barrel that is integral with the drive shaft has 9 pistons at various positions because of the cam angle shape at which the slipper travels.

When the piston starts traveling downward (leaving the port plate), the barrel will revolve considerably to open the inlet port of the port plate, allowing the oil to flow to the cylinder.  The oil may flow to the cylinder during the whole downward travel of the piston.  Before the piston turns back, the barrel rotates through the opening of the inlet port of the port plate, and blocks the oil flows to and from the cylinder.

When the barrel rotates further, the piston will start traveling upward (toward the port plate), and the cylinder will open, facing the outlet port of the port plate.  The piston will continue to travel upward, forcing the oil to leave the cylinder through the outlet port of the port plate.   Then, the oil is redirected by the head into the hydraulic system’s auxiliary devices.

The relief valve is maintained at 2500 PSI setting.

The port plate is doweled on the head and is maintained stationary in the pump.  The two kidney-shaped bores are the inlet port (26) and the outlet port (24).  The strutting part on the outlet is used to support the parts of the plate that are exposed to high pressure.

The slotted-surface of each port end is used to prevent any shock when opening the cylinder during the process of opening the port.  The contact surfaces of the barrel and the port plate is lubricated by the leakage through the sills (29) flowing into the oil slot (27) and then leaves the leveled radial slot (25) to lubricate the outer trust pad (28).

The sill is designed in such a way that the high oil pressure provides a hydrostatic barrier that counterbalance the load generated by the oil pressure in the cylinder.  The bores that penetrate the port plate on the oil groove are used to release any high oil pressure produced between the port plate and the head.

The anti tip bearing (10) is held on the bearing by a Spacer (14). The purpose of anti tip bearing is to protect the pump from overspeeding.  In overspeeding condition, there may be a possibility for the barrel to lift and scratch the port plate.  The anti tip bearing will prevent oversloping by traveling on the external shoulder of the barrel.


  1. Shaft (Splined end)
  2. Pump Housing
  3. Plate indicating the direction of rotation and part number, and constitutes the head
  4. Inlet housing
  5. Arrow indicating the direction of rotation
  6. Tapered Roller Bearing
  7. Cam
  8. Slipper Retraction plate
  9. Piston & Slipper Assembly
  10. Anti Tip Bearing
  11. Tapered Roller Bearing
  12. Retraction Bearing
  13. Barrel
  14. Anti Tip bearing Spacer
  15. Port Plate
  16. Impeller
  17. Lock & Spanner Nut

The Slipper Retraction plate (8) is held in place by the Retraction bearing (12) that is bolted to the Cam (7). When a cylinder is in intake position, the Retraction plate will force the Slipper to follow the surface of the Cam Face and pull the piston together.

The Slipper (18) encloses the ball piston (19) and constitutes an integral assembly. The piston is hollow and has a drill bore (20) penetrating through the ball toward the slipper. Any hollow of the piston allows the pressurized oil to flow to the lower part of the slipper to lubricate the slipper and the surface of the Cam (21).

The pressure oil also works on the lower surface of the slipper to counterbalance the pressure working on the piston. This practically limits the metal contact on the Cam.

The Spring (23) mounted in the center of the barrel assures an accurate position between the surface of the barrel and the surface of the port plate. The kidney-shaped slot at the barrel end allows the oil to enter and exit the cylinder during the intake and discharge courses.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.