Forward Travel; Reverse Travel; Acceleration Valves; Push Valve - Kohler K91 Owner's Manual

(Refer to the Neutral, Forward
&
Reverse Schematic
Diagrams, Figs.
1
(Pg.
6 ) , 2
(Pg.
7), 3
(Pg.
8).
Power i s transmitted from the engine through a
flexible coupling or belt and pulley arrangement to
the pump shaft (shown at left of diagrams). Rotation
of the input shaft causes the main pump cylinder block
and the charge pump to turn, initiating the power
transmitting function. Oil from the reservoir is drawn
the pistons pass again to the low pressure side of the
valving plate and are refilled by the charge pump
with oil returning from the motor through the low
pressure l i n e The high pressure oil (shown in black)
closes the 'high pressure check valve and travels
through internal passages to the inlet side of the
piston motor.
In the motor high pressure oil acting on the back
of the piston forces the piston down the incline causing
the block to rotate and turn the output shaft.
by the charge pump through the inlet strainer and
forced, by way of the check valves, into the low Pres-
REVERSE TRAVEL
sure lines. Oil introduced by the charge pump fills the
When the control handle i s moved to the reverse
area in
back
of
the cylinder block
pistons,
holding
position, the pump variable swashplate i s tilted in the
them
against
their swash plates
in both the pump
and
opposite position which changes the direction of oil
the motor. flow. The high and low pressure circuits interchange
causing the motor to rotate in the opposite direction.
When the pump swash plate i s in neutral the pump
cylinder block pistons do not move in and out. Thus,
in neutral, no oil is being pumped from the variable
ACCELERATION VALVES
displacement pump.
Low pressure oil from the charge pump fills the
complete system, including the area around the ac-
celeration valves. These valves are
held
open by the
spring between them.
The
acceleration valves remain
open until high pressure oil is introduced behind one
of them.
Which
valve depends on the direction of
rotation as determined by the position of the pump
swash plate.
!
RELIEF VALVES
The
acceleration valves are placed in the circuit to
limit vehicle acceleration to a safe rate. With the con-
trol lever in' neutral, oil i s under low pressure through-
out the circuit When the control lever i s advanced,
the circuit pressure increases, causing the oil on the
high pressure side to bleed by the flats on the valve
lands and into the low pressure side. This high pres-
sure oil continues to bleed to the low pressure side,
by-passing the piston motor and holding the accelera-
tion rate to a safe level during the time
i t
takes for
the acceleration valve to close against its seat. The
delay in closing the valve i s due to the time it takes
the high pressure oil to
bleed
through the small orifice
Pressure from
70
to
150
is
controlled by
the
in the acceleration valve metering plug and into the
charge pressure relief valve.
This
exhausts the excess
charge oil not needed to make up leakage to the oil
area behind the valve.
filter and case reservoir. When an implement i s used,
the maximum charge pressure, from
550
to 700 PSI,
PUSH
VALVE
i s limited by
the
implement relief valve. When open the push or free wheeling valve allows
Some separated systems also incorporate a high
pressure relief valve which serves to prolong the life
of the hydrostatic unit. Located on the left side of the
tractor above the transmission, this relief valve has
been factory adjusted and will reset automatically
if
triggered.
FORWARD TRAVEL
In forward position the pump variable swash plate
tilts and, as the cylinder block continues to rotate, the
pistons cross over a valving plate under low pressure
and advance up the swash plate.
This
in turn pushes
them into the block, forcing the oil from the block
under high pressure. At the bottom of their stroke,
the oil to by-pass or flow from one side of the main
circuit to the other. When pushing, the motor becomes
a pump forcing the oil through the open valve and to
the other side of the motor.
The
direction of oil flow
depends on' the direction the unit
i s
pushed.
NOTE:
On
"B"
and "C" series tractors,
similar prior models, the hydrostatic pump and
motor are bolted together as a single unit. For
purposes' of description we will refer to such a
transmission system as a "standard system".
On
"D"
series tractors, and similar prior
models, the pump and motor are separated in-
corporating a manifold between them. In this
manual we refer to such a system as a "separ-
ated system".
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