Vestil CART-23 & -24, ALL MODELS - Manual (Page 9)

11/13/02 08/17/05 CART manual, power unit operation.doc
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T HE P OWER U NIT ’ S O PERATION – CART-23, -24
The electric / hydraulic scissor lift utilizes an electric motor directly coupled to a gear-type hydraulic pump
to produce the needed fluid pressure and flow to allow the cylinders to perform the work of lifting the platform
load.
A hydraulic manifold houses the hydraulic control components, and is bolted directly onto the gear pump.
The power unit’s hydraulic components are all rated for 3,000 psi working pressure.
¬ Important parts of the power unit include:
• The electric motor. Operates on a 12 VDC battery.
• The gear pump. Its shaft is coupled directly to the shaft of the electric motor.
• The check valve. Its purpose is to prevent the backflow of fluid through the pump. In this way it allows the
platform to be held at a given elevation indefinitely.
• The pressure relief valve. Its job is to open a path for fluid to flow back to the reservoir in the event that
the fluid pressure built up by the pump exceeds 3,000 psi.
• The lowering solenoid valve. This is an electrically-operated cartridge valve. It contains a screen to keep
contaminants from entering the valve.
• The pressure-compensated flow control spool. This rests under the lowering valve, and regulates the fluid
flow back to the reservoir when the valve opens. It allows the table to always lower at the same rate
regardless of whether there is a load on the platform or not.
• The hydraulic lift cylinder(s). These are single-acting cylinders.
• The safety velocity fuse. This is a device that is installed in the cylinder’s hose port. It closes quickly in the
event of a catastrophic hose failure to prevent the lift from collapsing down. The platform remains
stationary until pressure is reapplied to the system.
• The hydraulic fluid. The system uses HO150 hydraulic fluid. Any anti-wear hydraulic fluid with an SUS
viscosity of 150 - 200 at 100°F (SAE viscosity grade of 32) such as AW-32 or Dexron transmission fluid are
acceptable.
When the platform is to be raised, press the “UP” pushbutton. The motor turns, which in turning spins the
hydraulic gear pump. Oil is drawn from the reservoir through the suction filter and into the pump. The pump
pushes the then-pressurized oil through the check valve and out to the lift cylinders.
When the platform is to be lowered, press the “DOWN” pushbutton. The lowering valve opens, bypassing the
check valve and allowing the oil in the cylinders to return back to the reservoir through the return hose. The
rate at which the platform lowers is regulated by the internal pressure-compensated flow spool.
¬ In the event that the platform creeps down slowly after releasing the “DOWN” control, it will be necessary
to remove the lowering cartridge valve for inspection and cleaning, as follows:
♦ Remove any load from the platform.
♦ Raise the lift and then lower the maintenance props. Guide the props into the cylinder push assembly as you
lower the platform. Hold the DOWN pushbutton until the lift rests on the props.
♦ Remove the nut holding the solenoid coil on the valve stem, then remove the coil, and then unscrew the
valve from the manifold.
♦ Inspect the valve for contaminants, and the valve’s o-rings and back-up washers for cuts, tears, or other
damage.
♦ With the valve immersed in mineral spirits or kerosene, use a thin tool such as a small screwdriver or a small
hex wrench to push the poppet in and out several times from the bottom end of the valve. The valve should
move freely, about 1/16” from closed to open position. If it sticks in, the valve stem could be bent and will
need to be replaced if it doesn’t free up after cleaning. Blow the valve off with a compressed-air gun while
again pushing the poppet in and out.
♦ Inspect the bottom of the manifold’s valve cavity for contaminants.
♦ Again with the thin tool, press on the middle of the flow control spool located in the bottom of the cavity. It
should move down and back up freely.
♦ Reinstall the valve into the manifold, tightening the valve with approximately 20 lb-ft of torque.
¬ If the platform lowers extremely slowly, or not at all, the cylinder’s velocity fuse could be closing. This can
be caused by air in the hydraulic cylinders. To bleed the air from the system:
♦ Remove any load from the platform.
♦ Raise the lift and then lower the maintenance props. Guide the props into the cylinder push assembly as you
lower the platform. Hold the DOWN pushbutton until the lift rests on the props.
♦ Pull the bottom of the cylinder out of the push assembly and raise the end up higher than any other part of
the hydraulic system. Rotate the cylinder so the hose port is pointing up.
♦ Loosen the hose fitting ½ turn, then “jog” the motor. Oil and air will sputter from the loose fitting – once
no air is observed, tighten the fitting and replace the cylinder into the push assembly.