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CHAPTER 1 GENERAL INFORMATION AND OPERATING INSTRUCTIONS
NMC-WOLLARD
2021 Truax Blvd. Eau Claire, WI 54703 ● P: 715-835-3151● F: 715-835-6625 ● www.nmc-wollard.com
TML-704H Maintenance Lift Manual No. 310547
1 DESCRIPTION
Actuating either Platform "Up" or "Down" switch energizes solenoid valve
SV1, causing the spool to shift, directing oil to raise or lower the platform as
appropriate.
When raising the platform, oil under pressure free-ows through ow control
FC1 and pilot check PC1 into blind end of cylinder to extend the cylinder.
Escaping oil from the rod end of the cylinder free-ows through check valve
CV3 and returns via solenoid valve SV1 to tank. Pilot check PC2 remains
closed, preventing oil from bypassing cylinder.
When lowering the platform, oil under pressure is trapped by check valve
CV3. Pump pressure rises to relief valve setting and all oil ow from the pump
is diverted through the relief valve to tank. This pressurizes the pilot lines
for pilot checks PC1 and PC2, forcing these valves to open. Platform weight
forces oil out of the cylinder blind end and through the pressure-compensated
ow control FC1, which limits the down speed regardless of platform load.
Movement of the cylinder piston creates a suction in the cylinder rod end,
which causes oil to be pulled from the "extend" line through (open) pilot check
valve PC2. The result is that most of the oil leaving the cylinder blind end is
diverted back to rod end. Excess oil returns to tank via solenoid valve SV1.
Power Steering System
The steering gear is powered by a gimbal-mounted, double acting, hydraulic
cylinder mounted on the front axle. The control unit is actuated directly by the
steering wheel to control ow of hydraulic power to the steering cylinder.
Turning the steering wheel actuates the rotary valve in power steering unit.
As the valve opens, hydraulic oil travels into a metering motor within the
power steering unit; then out from the motor through the valve and out to
the retract side of the steering cylinder. Return oil from the extend side ows
through the valve to the reservoir. Oil owing through the metering motor
causes it to turn, and as it turns, it moves a feedback linkage that returns the
rotary valve to center and locks the steering cylinder in position. The metering
motor therefore ensures that the steering cylinder is precisely controlled by
the command received from the steering wheel.
In event of a power failure, the steering wheel can manually actuate the
steering cylinder. In this case, steering wheel movement turns the metering
motor directly by means of feedback linkage. The motor is then used as a
pump to force oil into the cylinder, while drawing oil into the suction side
(through check valve) from the return line.
Powered steering is only possible when Operate Safety valve SV3 is
de-energized, allowing pump oil to ow through the power steering unit.
Stabilizer System
The four stabilizer assemblies on the chassis are each actuated by a single
hydraulic cylinder. Pilot-operated check valves lock the stabilizers in both
raised and lowered positions. A relief valve limits stabilizer contact force to
prevent the unit's wheels from being raised o the ground.
SEPTEMBER 01, 2015
Property of American Airlines
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