JLG 1500SJ Service Manual - Manual (Page 335)

SECTION 4 - BOOM & PLATFORM
3121262 – JLG Lift – 4-89
4.19 ROTARY ACTUATOR
Each actuator is individually serial numbered. The serial
number 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 neces-
sary 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 pis-
ton motion into powerful shaft rotation. As seen in the illus-
tration 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 features an integral mounting
flange and bearing which are not shown in the illustration.
Helical spline teeth machined on the shaft engage match-
ing splines on the inside diameter of the piston. The out-
side diameter 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 cylinder - while, simultaneously, the splines
cause the shaft to rotate. When the control valve is closed,
oil is trapped inside the housing, preventing piston move-
ment and locking the shaft firmly in position.
The shaft is supported radially by the large upper radial
bearing 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 hydrau-
lic fluid
• Provides a constant controlled rate of rotation in over-
center load conditions
Applying fluid pressure will displace the piston axially
while the helical gearing causes the piston and shaft to
rotate simultaneously. The double helix design com-
pounds 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.
Bars indicate starting positions of piston and shaft. Arrows indicate
direction they will rotate. The housing with integral ring gear
remains stationary. For clarity, the shaft flange, bearings, and end
cap are not shown. Applying fluid pressure will displace the piston axially while the heli-
cal 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 4-27. Actuator Theory of Operation