The main pressure pump provides all the oil volume required; however,
it does not furnish all the pressure required.
Located in the line between the main pressure regulator and the
hydrostatic pump are two supercharge check valves.
One of the two
check valves remains open at all times to provide additional oil to
the hydrostatic pump circuit, as required, to replenish oil lost
through leakage, during overload, or in control functions. The other
valve remains closed.
The direction of rotation of the hydrostatic
pump determines when each of the valves opens or closes.
control pumps further pressurize the oil. The output of the control
c. The Control Circuit (fig. A-14 and A-15). The servo sleeve is
a rotating sleeve valve with ports to direct control pressure to the
The servo sleeve centers on the same axis as the
hanging ring (mechanism that controls the pump piston stroke) and
connects to the hanging ring by a pin that transmits the partial
rotation of the hanging ring to the servo sleeve.
Centered with the servo sleeve is the manually controlled servo valve
that, when rotated, directs control pressure to one or the other of
the two control pistons. Clockwise rotation of the servo valve from
its centered position gives steer in one direction; therefore,
rotation counterclockwise gives steer in the other direction.
control pistons have no pressure acting on them when no steer is
applied, and their oil lines open to exhaust through the servo valve.
When applying steer, the servo valve rotates to the extent required
to produce the desired steer.
This directs pressure to the control
piston which controls the pump piston stroke.
For steer in one
direction, it is one of the two control pistons; consequently, for
steering in the other direction, it is the other control piston. The
piston opposite the pressurized piston remains exhausted to the
(See figures A-16 through A-19 for oil circuits during
left steer and right steer.)
The pressurized control piston rotates the hanging ring to a position
that causes the hydrostatic pump pistons to reciprocate. The length
of their stroke corresponds to the degree of steer applied, and the
hydraulic output of the hydrostatic pump is directly proportional to