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
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
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