Principle Of Operation - Bolens QS Series Service Manual

OS, QT
&
1900 SERIES
Page 3-2
REV.
12/78
DRIVE TRAIN (Continued)
PRINCIPLE OF HYDROSTATIC OPERATION
EATON
The Eaton Model 11 hydrostatic
transmission is composed of
four(4) major components; a
charge pump; a variable
displacement, radial ball-piston
pump; a fixed displacement,
radial ball-piston motor; and a
system of valves located between
- the pump and motor.
The transmission converts me-
chanical energy at its input shaft
into fluid flow under pressure
from its pump. transfers this fluid
SUNDSTRAND
Engine power is transmitted to the
pump input shaft of the transmission
by the tractor drive shaft: Rotation
of the input shaft causes the main
pump block and the charge pump to
turn, thus initiating the power trans-
mitting function. Oil from the reser-
voir is drawn by the charge pump,
through the inlet filter, and forced
into the low pressure line by way of
the low presure check
valves.
Oil
introduced into the low pressure line
by the charge pump fills the area
in back of the pistons in the pump
section. When the variable swash-
P.Iate is in the vertical or neutral
position the pistons do not
move
in
or out resulting in no oil flow to the
pistons in the motor section. Thus
the tractor will not move. By tilting
the variable swashplate the pistons
are forced in and out of the cylinder
block, as they rotate across the
angled swashplate forcing the oil
from the c;ylinder block under high
pressure. The further the swashplate
is tilted the longer the stroke of the
pistons, causing a greater volume
of oil being pumped to the motor
section, thus increasing the speed
of the tractor. At the bottom of their
stroke, the pistons pass again to the
low pressure side of the valving plate
energy to its motor which in turn
converts it back to mechanical
energy at its output shaft. The
purpose of this transformation is
to prqvide a means of having an
infinitely variable output speed,
with easy reversing, all with a
constant input speed. In
operation, the pressure of the
fluid in the hydrostatic .
transmission closed loop varies
as the "load" increases or
decreases.
and are refilled by the charge pump
and oir returning from the motor
through the low pressure line. The
high pressure oil
travels
through
internal passages to the inlet side of
the piston motor. In the motor
section, high pressure oil acting on
the back of the piston forces the
piston "down the stationary tilted
swashplate causing the cylinder
block to rotate and turn the output
shaft." When the variable swashplate
is tilted in the opposite position, the
pressures are reversed and the motor
cylinder block will rotate in the
opposite direction, thus reversing the
direction of the tractor.
The acceleration valves are placed in
the circuit to limit vehicle acceler-
ation to a safe rate. With the control
lever
in neutral, oil is under low
pressure throughout the circuit.
When the control lever is advanced
rapidly from the neutral to the for-
ward position, the circuit pressure
increases and the forward accelera-
tion
valve
bleeds some of the high
pressure oil to the low pressure side
of the circuit thus limiting vehicle
acceleration. As the vehicle continues
to accelerate at the safe rate, high
pressure oil leaking .Past the
If the load increases, so does the
pressure of the fluid in the trans-
mission. This change in load and
pressure automatically changes
the torque output of the trans-
mission. The greater the load
(pressure), the more torque the
transmission produces to match
the load requirements. If the load
is lessened, pressure decreases
and torque output drops.
acceleration valve body forces
the
valve
down against its seat,
stopping the bleeding of oil to the
low pressure side. The acceleration
valve will remain in this position as
long as the control lever position is
unchanged. When the control lever is
moved
into
reverse
position, the
reverse
acceleration
valve
performs
in the same way.
The charge pump relief valve is a
simple spring loaded valve. The func-
tion of this
valve
is to regulate the
supply of low pressure oil
to the pistons in the pump section.
The excess charge oil is bypassed
by this
valve
to the implement
circuit.
When implement hydraulic cylinders
are actuated, the pressure supplied
by the charge pump is limited by the
implement relief
valve.
The reason
for this is to permit the charge pump
to supply high pressures for inter-
mittent periods to permit sufficient
pressures for implement cylinder
actuation. When the implement relief
valve opens, the oil is by-passed to
the case and pumped into the trans-
axle.
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