BRADEN CH400B Installation Maintenance And Service Manual page 7

Figure 4
Static Friction Brake Applied
Permits free shaft rotation
while hoisting
Figure 5
Static Friction Brake Applied
Load attempts to rotate shaft in opposite direction
Brake clutch locks sun gear shaft to friction brake
Spring pressure on the steel end friction brake discs pre-
vent the brake discs from turning until the hydraulic cylin-
der and piston are pressurized, releasing the force on the
brake discs and creating clearance between the friction
and steel discs..
OPERATION
When hauling-in cable, or hoisting a load, the motor shaft
and hoist gear train turn freely as the sprag cams lay over
between the inner and outer races of the brake clutch.
See Figure 4.
The multiple disc friction brake remains fully engaged and
the hoist is not affected by any braking action when hoist-
ing. See Figure 1.
When hoisting is stopped, the load tries to turn the hoist
drum, gear train, and primary sun gear in the reverse di-
rection. This reversed input to the inner race of the brake
clutch causes the sprag cams to instantly roll upward and
lock the shaft to the fully engaged friction brake. See Fig-
ure 5.
When the hoist is powered in the pay-out or lowering di-
Sprag Cams rection, the motor cannot rotate until sufficient lowering
pressure is present to release the brake and open the
brake valve. See Figures 2 and 3. The friction brake will
completely release at a pressure lower than that required
to open the brake valve. The extent to which the brake
valve opens determines the amount of oil that can flow
through the motor, which is directly related to the drum
speed of the hoist. Increasing the flow of oil to the hoist
motor causes the pilot pressure to rise which increases
the opening in the brake valve, allowing more oil to flow
through the motor and increasing the drum speed. De-
creasing this oil flow causes the pilot pressure to drop,
reducing the opening in the brake valve which slows the
motor and hoist speed.
The friction brake receives little, if any, wear in the pay-out
or lowering operation. All of the heat generated by lower-
Sprag Cams
ing and stopping a load is absorbed by the hydraulic oil
where it can be readily dissipated.
When the control valve is shifted to neutral, pilot pressure
drops closing the brake valve spool, stopping the motor
and the load. The friction brake then engages and holds
the load after the brake valve has closed.
When lowering a load slowly for precise positioning, no
oil flow actually occurs through the pilot operated spool
in the brake valve. Pressure builds up to a point where
the friction 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.
HOIST OPERATION
The input section of the drive assembly is bolted to the
motor end support and cannot rotate. The drive housing
is the output member of the gear set and is bolted to the
hoist drum. The motor shaft is directly coupled to the pri-
mary sun gear through the inner race of the brake clutch.
The motor turns the primary sun gear which drives three
successive planetary gear sets, turning the drive housing
and the hoist drum.
In the haul-in direction, hydraulic oil flows through a large
check valve in the brake valve and turns the motor in the
free rotating direction of the brake clutch, driving the gear
train and hoist drum. The friction brake remains fully en-
gaged.
In the pay-out direction, oil flow through the motor is ini-
tially blocked by a spool in the brake valve. Oil pressure
supplied to the motor through the control valve is piloted
to the friction brake and the brake valve spool. The friction
brake is released at a lower pressure than that required to
shift the brake valve spool. When pressure is sufficient to
shift the brake valve spool, oil is allowed to flow through
the motor, rotating the hoist gear train and drum.
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