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     This is a Uchida open circuit pump, from a tiny Yanmar excavator, model reference AP2D14LV1RS7-955-0. The pump itself isn't that  much complicated, but it is a good example of a direct torque limiter control, as well as an extremely compact unit design.

    The main pump is actually a double axial piston pump, that uses a single ten-bore barrel (five pistons for each pump), with the kidney pressure ports arranged in two rings.  The valve plate also has two separate pressure kidneys and a single suction port. The swashplate is common for both pumps. A double gear pump is mounted on the back of the piston pump.

   The most interesting thing about this four-pump assembly is its torque limiting control, which provides the sum of torque limiting function for both piston pumps and the larger gear pump. The swashplate of the piston pump is biased by a very strong double coil spring, and if you look at the end cover, you'll see that it is destroked not by one, but by three servo-pistons, each connected to a separate pump. In this arrangement the swashplate position is affected by the sum of force each servo-piston is creating, along with the rotary group piston induced force that acts directly on the swashplate. The size of the servo-pistons, the double spring rate and the valve plate pressure carry-over angle  are selected in a way that the piston pump gradually destrokes as the outlet pressures rise, allowing the three pump combination to follow a desired constant power curve.

   The main advantage of this torque limiting system is its simplicity - the torque limitation is done by the servo-pistons directly, without intermediate spools or the necessity for a swashplate position feedback. However the fact that the bias spring has to have a large rate makes this system suitable only for relatively small displacements.

   The whole unit is extremely compact. Check out the double gear pump - the body of the first pump has the suction and pressure lines of the small tail pump machined into it! The tiny pump sends the oil to the relief vlave, mounted inside the piston pump end cover, and feeds the two solenoid operated valves  mounted on the end cover as well (picture). The DC solenoid valves connect the pilot pressure to their respective outlets when energized. Without the current supply the outlets are vented to the pump casing.

    As you can see, the efficient Japanese engineers have once again come up with an extremely compact hydraulic pump design, which is simple enough and efficient enough for the presupposed purpose.

Uchida hydraulic pump
Uchida hydraulic pump
Uchida hydraulic pump
Uchida double pump barrel
Uchida double pump barrel
Uchida double pump valve plate
Swashplate and the bias spring
End cover
Servo pistons (torque control)
pilot solenoid connections
End cover
Relief valve
Relief valve spool detail
Gear pump body detail