7
The static brake is released by the brake valve pilot pres-
sure at a pressure lower than that required to open the
pilot operated spool valve. This sequence assures that dy-
namic 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
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.
Dual Brake System – Operation
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 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 up-
ward 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 and 4). 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 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 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 pres
-
sure will build up to a point where the brake will release
sufficiently to allow the load to rotate the motor through its
own internal leakage. This feature results in a slow speed
and extremely accurate positioning.
The friction brake receives little wear in the lowering op-
eration. 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.
Static Friction Brake Applied
Figure 5
Hoisting
Permits free
shaft rotation
while hoisting
Sprag
Cams
Static Friction Brake Applied
Figure 6
Stopped,
Holding
Load
Sprag
Cams
Load attempts to rotate shaft in opposite direction.
Brake clutch locks sun gear shaft to friction brake.
