Vestil PST series User Manual
Page 6
6
THE POWER UNIT'S OPERATION - EHLT & PST
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:
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The electric motor. Motors are available for operation on single- or three-phase. AC supplies (all are dual-voltage
capable), or on a 12 VDC battery.
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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.
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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.
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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.
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The lowering solenoid valve. This is an electrically-operated cartridge valve. It contains a screen to keep
contaminants from entering the valve.
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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. Several sizes are available.
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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 fro the hydraulic system.
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The safety velocity fuse. This is a devise 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.
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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" push-button 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" push-button 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 cartage valve for inspection and cleaning, as follows:
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Remove any load from the platform.
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Warning: Raise the lift and install the maintenance props. Lower the platform until the lift rests on the props.
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Remove the nut holding the solenoid coil on the valve stem, and then unscrew the valve from the manifold.
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Inspect the valve for contaminants, and the valve's o-rings and back-up washers for cuts, tears, and other damage.
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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.
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Inspect the bottom of the manifold's valve cavity for contaminants.
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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.
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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:
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Remove any load from the platform.
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Warning: Raise the lift and install the maintenance props. Lower the platform until the lift rests on the props.
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Hold a rag over the cylinder's bleeder valve (it looks like a grease zirk) and open 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.