JLG 680S, Service And Maintenance Manual

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SECTION 4 - BOOM & PLATFORM
4-50 3121234
4.12 ROTARY ACTUATOR
Theory of Operation
The rotary actuator is a simple mechanism that uses the sliding
spline operating concept to convert axial piston motion into
powerful shaft rotation. Each actuator is composed of a housing
with integrated gear ring (1) and only two moving parts: the cen-
tral shaft with integrated bearing tube and mounting flange (2),
and the annular piston sleeve (3). Helical spline teeth machined
on the shaft engage matching splines on the inside diameter of
the piston. The outside diameter of the piston carries a second
set of splines, of opposite hand, which engage with matching
splines in the housing. As hydraulic pressure is applied, the pis-
ton is displaced axially within the housing similar to the opera-
tion of a hydraulic cylinder while the splines cause the shaft to
rotate. When the control valve is closed, oil is trapped inside the
housing, preventing piston movement and locking the shaft in
position.
The shaft is supported radially by the large upper radial bearing
and the lower radial bearing. Axially, the shaft is separated from
the housing by the upper and lower thrust washers. The end cap
is adjusted for axial clearance and locked in position by set
screws or pins.
The actuators are equipped with factory installed counterbal-
ance valves, which performs four major functions.
• Protects the actuator in the event of overload
• Enables the actuator to hold position without drifting
when external loads are applied
• Reduces hydraulic backlash by pressuring the hydraulic
fluid
• Provides a constant controlled rate of rotation in over-cen-
ter load conditions
3
2
1
Bars indicate starting positions of piston and shaft.
Arrows indicate direction they will rotate. The hous-
ing with integral ring gear remains stationary.
Applying fluid pressure will displace the piston axially
while the helical gearing causes the piston and shaft to
rotate simultaneously.
The double helix design compounds rotation: shaft rota-
tion is about twice that of the piston. Applying pressure
to the opposite port will return the piston and shaft to
their original starting positions.