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 static friction brake is a load holding brake only and
has nothing to do with dynamic braking or rate of
descent of a load.
The 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. Figures 5 and 6.
The hydraulic cylinder, when pressurized, will release
the spring pressure on the brake discs, allowing the
brake discs to turn freely.
D
UAL
B
RAKE
S
YSTEM
– O
PERATION
When hoisting a load, the brake clutch which connects
the motor shaft to the primary sun gear, allows free rota-
tion. The sprag cams lay over and permit the inner race
to turn free of the outer race. Figure 5. The static friction
brake remains fully engaged. The hoist, in raising a
load, is not affected by any braking action. Figure 2.
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 firmly lock the shaft to the fully engaged
friction brake. Figure 6.
When the hoist is powered in reverse, to lower the load,
the motor cannot rotate until sufficient pilot pressure is
present to open the brake valve. Figures 3 & 4. The fric-
tion 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 hoist 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 pres-
sure 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 very slowly for precise position-
ing, very little oil flow actually occurs through the hoist
motor. The pressure will build up to a point where the
static brake will release sufficiently to allow the load to
rotate the motor through its own internal leakage. This
feature results in a very slow speed and extremely
accurate positioning.
The friction brake receives very little wear in the lower-
ing 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.
7
Figure 5
Figure 6
Static Friction Brake Applied
Static Friction Brake Applied
Sprag
Cams
Permits free
shaft rotation
while hoisting
Load attempts to rotate shaft in opposite direction.
Brake clutch locks sun gear shaft to friction brake.
Sprag
Cams
Hoisting
Stopped,
Holding
Load
