Paccar Braden PD12C - Dual Brake System - Operation

6
THEORY OF OPERATION
The static brake is released by the brake valve pilot
pressure at a pressure lower than required to open
the pilot operated spool valve. This sequence ensures
dynamic braking takes place in the brake valve and little,
if any, heat is absorbed by the friction brake.
The friction brake is a load holding brake only and has
nothing to do with dynamic braking or rate of descent of
a load.
The overrunning brake clutch is splined to the primary sun
gear shaft between the motor and the primary sun gear.
It will allow this shaft to turn freely in the direction to raise
a load and lock up to force the brake discs to turn with
the shaft in the direction to lower a load. Refer to gures
4 and 5.
The 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 shaft to the primary sun gear, allows free rotation.
The sprag cams lay over and permit the inner race to turn
free of the outer race. Figure 4. The friction brake remains
fully engaged. The hoist, in raising a load, is not a󰀨ected
by any braking action. Figure 1.
When the lifting operation is stopped, the load attempts
to turn the primary sun gear in the opposite direction.
This reversed input causes the sprag cams to instantly
roll upward and rmly lock the shaft 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󰀩cient pilot pressure is
present to open the brake valve. Figures 2 & 3. The friction
brake within the hoist will completely release at a pressure
lower than required to open the brake valve. The extent
to which the brake valve opens will determine the amount
of oil owing through it and the speed at which the load
will be lowered. Increasing the ow of oil to the motor will
cause the pressure to rise and the opening in the brake
valve to enlarge, speeding up the descent of the load.
Decreasing this 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 shifted to neutral, the pressure
will drop and the brake valve will close, stopping the load.
The friction brake will engage and hold the load after the
brake valve has closed.
When lowering a load slowly for precise positioning, no oil
ow actually occurs through the hoist motor. The pressure
will build up to a point where the brake will release
su󰀩ciently to allow the load to rotate the motor through its
own internal leakage. This feature results in a slow speed
and accurate positioning.
The friction brake receives little wear in the lowering
operation. All of the heat generated by the lowering and
stopping of a load is absorbed by the hydraulic oil where it
can be readily dissipated.