6
Motor
Control Valve
Pump
To Tank
Brake Valve
Static Brake
Medium
Pressure
Low
Pressure
High
Pressure
Figure 3. Lowering (2)
The static brake is released by the brake valve pilot pressure at
a pressure lower than that required to open the pilot operated
spool valve. This sequence assures that dynamic braking takes
place in the brake valve and that little, if any, heat is absorbed
by the friction brake.
The friction brake is a load-holding brake only and has noth-
ing 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 Figures 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 affected 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 re-
versed input causes the sprag cams to instantly roll upward
and firmly 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 sufficient pilot pressure is pres-
ent to open the brake valve (Figures 2 and 3). The friction
brake within the hoist will completely release at a pressure
lower than that required to open the brake valve. The extent
to which the brake valve opens will determine the amount of
oil that can flow through it and the speed at which the load
will be lowered. Increasing the flow 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 flow 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
flow actually occurs through the hoist motor. The pressure
will build up to a point where the brake will release sufficient-
ly to allow the load to rotate the motor through its own inter-
nal leakage. This feature results in a slow speed and accurate
positioning.
The friction brake receives little wear in the lowering opera-
tion. 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.
Sprag Cams
Static friction brake applied
Permits free shaft rotation
while hoisting
Sprag Cams
Static friction brake applied
Load attempts to rotate shaft in opposite direction.
Brake clutch locks sun gear shaft to friction brake.
Figure 4
Figure 5
