Vestil GLT-4000 - Manual (Page 7)

7
HYDRAULIC OPERATION
When the operator wants to raise the unit, he depresses the UP button. This starts the electric
motor (Item 3) which turns the hydraulic pump (Item 4). Oil from the reservoir (Item 1) is drawn in through
the suction filter (Item 2) and into the pump. The pump delivers the pressurized oil through the check valve
(Item 6) before entering the cylinders.
The function of the check valve is to allow the oil to flow in one direction, i.e. towards the cylinders.
It also prevents the flow of oil back into the pump circuit when the pump stops running. This holds the oil
in the cylinders and maintains the desired elevation.
If the load is excessive, and the UP button is still depressed, pressure will build up in the circuit
between the pump and the cylinders. This forces the relief valve (Item 5) to unseat allowing the pump flow
to return to the reservoir to preventing hydraulic or structural damage.
When the operator desires to lower the units, he depresses the DOWN button. This energizes the
down solenoid valve (Item 7). The poppet in the solenoid valve is unseated and oil now returns from the
cylinders through the return screen (Item 9), solenoid valve, flow control valve (Item 8), oil return hose,
and into the reservoir.
The pressure compensated flow control valve (Item 8) controls the down speed of the table. It is
preset and cannot be adjusted. Releasing the DOWN button will de-energize the solenoid, closing the
valve poppet. This prevents the oil from returning to the reservoir and the cylinders will stop retracting.
The unit is now maintained at that particular elevation.
CARTRIDGE VALVES
The lowering valve, as discussed above, is of cartridge construction and is virtually
maintenance-free. If there is a faulty operation, check Trouble Shooting Section. To clean the
cartridge valve, follow this procedure:
1.) WARNING: remove load and support weight of the table with maintenance stop(s)
before removing cartridge valve.
2.) Use a sharp object and push poppet in from the bottom to open the valve.
3.) Repeat several times while valve is immersed in kerosene or mineral spirits. Blow dry.
4.) Blow compressed air through valve while holding open as descibed in step 2.
5.) Inspect "O" rings and the teflon extrusion washer.
6.) Reinstall. The valve should be tightened to approximately 30 ft. lbs.
VELOCITY FUSE
There is a brass velocity fuse with a stainless steel spring in the base of each cylinder (Item 10). In
the event of a hydraulic hose or fitting failure, the platform starts to lower at a fast rate. As soon as the descent
speed exceeds the preset speed, the Velocity Fuse will shut off the oil flow and the platform will remain nearly
stationary until pressure is re-applied after repairs are done. This safety feature reduces the possibility of
accidental personal injury or damage to the table or contents. If air is introduced into the system, the velocity
fuse can lock up even though no failure has occurred. To reset the velocity fuse just activate pump by
depressing the UP button. Remove the load and cycle the unit several times to purge air.