Oshkosh JLG 1500SJ, Service And Maintenance Manual

Page: 337 / 706

SECTION 4 - BOOM & PLATFORM
3121262 – JLG Lift – 4-89
4.19 ROTARY ACTUATOR
Each actuator is individually serial numbered. The serial num-
ber is a five or six digit number and must be provided before
parts and/ or service issues can be addressed.
The serial number can be found on the Identification (ID) Tag
that is affixed to all actuators. The tag is a thin, silver colored,
plastic material with a self-adhesive backing. Information is
imprinted in black. The tag is located either on the side plate
or on the housing tube of the actuator.
Additionally, the serial number of the actuator is stamped
onto the side plate or the housing tube. It may be necessary to
remove paint to expose the serial number.
Theory of Operation
The rotary actuator is a simple mechanism that uses Helac’s
sliding spline technology which converts axial piston motion
into powerful shaft rotation. As seen in the illustration below
left, each actuator is composed of a housing with an integral
ring gear (1) and only two moving parts: the central shaft (2),
and the annular piston sleeve (3). Note the actuator shaft fea-
tures an integral mounting flange and bearing which are not
shown in the illustration.
Helical spline teeth machined on the shaft engage matching
splines on the inside diameter of the piston. The outside diam-
eter of the piston carries a second set of splines, of opposite
hand, which engage the matching splines of the housing’s ring
gear.
As hydraulic pressure is applied, the piston is displaced axially
within the housing - similar to the operation of a hydraulic cyl-
inder - while, simultaneously, 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 firmly in position.
The shaft is supported radially by the large upper radial bear-
ing and the lower radial bearing (see drawings on pages 8 and
9). 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.
Configurations of parts may be slightly different depending on
model.
Many actuators are equipped with counterbalance 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
Applying fluid pressure will displace the piston axially while
the helical gearing causes the piston and shaft to rotate simul-
taneously. The double helix design compounds rotation: shaft
rotation is about twice that of the piston. Applying pressure to
the opposite port will return the piston and shaft to their orig-
inal starting positions.
Bars indicate starting positions of piston and shaft. Arrows indicate direction
they will rotate. The housing with integral ring gear remains stationary. For clar-
ity, the shaft flange, bearings, and end cap are not shown. 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 rotation is about twice that of the piston. Applying
pressure to the opposite port will return the piston and shaft to their original
starting positions.
Figure 4-26. Actuator Theory of Operation