Vestil EHLT, PST - Manual (Page 13)

02/06 24-126-120
V ESTIL M FG . C O . 13
T HE P OWER U NIT ’ S O PERATION – EHLT (1-4K) & PST (1-3K)
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. Motors are available for operation on single- or three-phase AC supplies (all are dual-voltage),
or a 12 VDC battery (portable units).
• The gear pump. Its shaft is coupled directly to the shaft of the electric motor. Several displacements are
available, depending on the motor horsepower used.
• 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. Thus the pump cannot generate more than 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 is installed in a port beside 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. Several flow rates are available.
• The hydraulic lift cylinder(s). These are displacement-style cylinders. They have a bleeder valve located at their
top end to allow air to be bled from the hydraulic system.
• 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 cylinder.
• The hydraulic fluid. The system uses HO150 hydraulic fluid. Any anti-wear hydraulic oil with a viscosity grade of
150 SUS at 100°F (ISO 32 at 40°C) such as AW 32 or a non-synthetic transmission fluid is acceptable.
When the platform is to be raised, press the “UP” pushbutton or foot switch. The motor turns, and in turning it
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 or foot switch. 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.
♦ Warning: Raise the lift and install the maintenance props. Lower the platform 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.
♦ Remove the plug located in the corner of the manifold next to the lowering valve. Inspect for contaminants that
might be plugging up the flow control located at the bottom of the cavity.
♦ With the thin tool, press down on the center of the flow control spool. It should move down and back up freely.
♦ Reinstall the plug, then the valve, 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.
♦ Warning: Raise the lift and install the safety maintenance props. Lower the platform until the rollers nearly rest
on the props.
Hold a rag over the cylinder’s bleeder valve (it looks like a grease zirk) and open the valve about 1/2 turn with a 1/4”
wrench. Oil and air will sputter from the valve – once no air is observed, close the valve.