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HYDRAULIC SECTION
When the operator wants to raise the platform, he or she presses the "UP" button. This starts the
electric motor, which runs the hydraulic pump. Oil from the reservoir is sucked in through the suc-
tion filter and into the pump. The pump delivers the pressurized oil into the valve block. The oil
flows through the check valve 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 main-
tains the desired elevation.
If the load is excessive, and the ''UP'' button is still pressed, pressure will build up in the circuit
between the pump and the cylinders. This forces the ''ball'' or ''poppet'' in the relief valve to un-
seat and the pump output returns into the reservoir through the return pipe.
When the operator desires to lower the lift, he or she presses the "DOWN" button. This energizes
the down solenoid valve. The poppet in the solenoid valve is unseated and oil now returns from
the cylinders through the flow control valve, the solenoid valve, the pressure-compensated spool
valve, to the oil return pipe, and into the reservoir. The flow control valve controls the down speed
of the lift. The pressure-compensated spool valve ensures that lowering speeds are nearly con-
stant, regardless of load.
Releasing the "DOWN" button will de-energize the solenoid, closing the valve poppet. This pre-
vents the oil from returning to the reservoir and the cylinders will stop retracting. The lift is now
maintained at that particular elevation.
A flow limiter is installed at the base of each cylinder. In the event of a hydraulic hose failure,
the platform lowers at a fast rate. As soon as the descent speed exceeds the preset speed, the
flow limiter will shut off the oil flow and the platform will come down at a very slow speed until
pressure is reapplied. This safety feature reduces the possibility of accidental personal injury or
damage to the lift.
A complete Hydraulic Schematic is shown in Figure 5.
18 HLD-SLD PRESTO-ECOA OWNER'S MANUAL
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