The pump (2) and motor (5) are joined in a closed hydraulic loop and most of oil circulates within the main oil
circuit. When the variable swash-plate is at right angle to the pump piston, the oil is not send to the motor (5). When
the variable swash-plate is tilted to forward or reverse, oil forced out of pump (2) at high pressure and send to the
And then the output shaft (6) rotates with the motor and oil is forced out of motor at low pressure and return to the
pump (2). On the other hand, oil is send to the main circuit through the filter (8) and excessive oil passes to the case
through the charge relief valve (4). The case relief valve (7) controls pressure in the HST case.
The check and high pressure relief valve (3) between the two lines in the main oil circuit monitors the oil pressure
in each line, it opens and close the oil into another line.