Paccar Braden PD12C, Инструкция по установке, обслуживанию и ремонту

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6
Motor
Control Valve
Pump
To Tank
Brake Valve
Static Brake
Medium
Pressure
Low
Pressure
High
Pressure
Figure 3
LOWERING 2
Th e static brake is released by the brake valve pilot pressure at
a pressure lower than that required to open the pilot operated
spool valve. Th is sequence assures that dynamic braking takes
place in the brake valve and that little, if any, heat is absorbed
by the friction brake.
Th e friction brake is a load holding brake only and has noth-
ing to do with dynamic braking or rate of descent of a load.
Th e overrunning brake clutch is splined to the primary sun
gear sha ft between the motor and the primary sun gear. It will
allow this sha ft to turn freely in the direction to raise a load
and lock up to force the brake discs to turn with the sha ft in
the direction to lower a load. Refer to fi gures 4 and 5.
Th e hydraulic cylinder, when pressurized, will release the
spring pressure on the brake discs, allowing the brake discs
to turn freely.
Dual Brake System – Operation
When hoisting a load, the brake clutch which connects the
motor sha ft to the primary sun gear, allows free rotation. Th e
sprag cams lay over and permit the inner race to turn free
of the outer race. Figure 4. Th e friction brake remains fully
engaged. Th e hoist, in raising a load, is not a ff ected by any
braking action. Figure 1.
When the li
ft ing operation is stopped, the load attempts to
turn the primary sun gear in the opposite direction. Th is re-
versed input causes the sprag cams to instantly roll upward
and fi rmly lock the sha ft to the fully engaged friction brake.
Figure 5.
When the hoist is powered in reverse, to lower the load, the
motor cannot rotate until su ffi cient pilot pressure is present to
open the brake valve. Figures 2 & 3. Th e friction brake with-
in the hoist will completely release at a pressure lower than
that required to open the brake valve. Th e extent to which
the brake valve opens will determine the amount of oil that
can fl ow through it and the speed at which the load will be
lowered. Increasing the fl ow of oil to the motor will cause
the pressure to rise and the opening in the brake valve to en-
large, speeding up the descent of the load. Decreasing this
fl ow causes the pressure to lower and the opening in the brake
valve to decrease thus slowing the descent of the load.
When the control valve is shi
ft ed to neutral, the pressure will
drop and the brake valve will close, stopping the load. Th e
friction brake will engage and hold the load a ft er the brake
valve has closed.
When lowering a load very slowly for precise positioning, no
oil fl ow actually occurs through the hoist motor. Th e pres-
sure will build up to a point where the brake will release suf-
fi ciently to allow the load to rotate the motor through its own
internal leakage. Th is feature results in a very slow speed and
extremely accurate positioning.
Th e friction brake receives very little wear in the lowering
operation. All of the heat generated by the lowering and stop-
ping of a load is absorbed by the hydraulic oil where it can be
readily dissipated.
Sprag Cams
Static Friction Brake Applied
Permits free sha ft rotation
while hoisting
Sprag Cams
Static Friction Brake Applied
Load attempts to rotate sha ft in opposite direction
Brake clutch locks sun gear sha ft to friction brake
Figure 4
Figure 5