MTS Model 506-62-72 Hydraulic Power Supply - Manual (Page 36)

5-2 Theory of Operation
Main and supercharge
pumps The HPS uses a variable-volume pressure compensated main pump,
with a pressurized (supercharged) hydraulic fluid inlet to provide
fluid flow. The supercharge pump draws hydraulic fluid from the
reservoir and forces it through a heat exchanger and a low-pressure
filter. Fluid not required by the main pump returns to the reservoir
through check valves in the low-pressure filter manifold.
Filters Hydraulic fluid from the low-pressure filter is routed to the main
pump. A low-inlet pressure switch monitors the inlet pressure
(supercharge output pressure) to the main pump.
Fluid from the main pump outlet passes through a check valve and a
10-micron high-pressure filter. The output pressure is controlled by the
pressure compensator adjustment on the main pump and is monitored by
the output pressure gage. Output volume varies automatically with
external circuit demand.
Back-up relief valve An adjustable back-up relief valve limits output pressure by porting
fluid back to the reservoir. When the pump compensator is operating
normally, the relief valve does not open.
High/low solenoid valve High/low pressure operation is controlled by the high/low solenoid
valve. This valve controls the backup relief valve vent port.
With the high/low solenoid valve de-energized, the back-up relief
valve vent port is open and output volume is ported to the reservoir,
limiting output pressure to approximately 150 psi (1.03 MPa). This
low-pressure condition occurs during supply turn-on for low pressure
start-up and during power-up of the HPS.
With the high/low solenoid valve energized, the vent port is blocked
and system pressure can rise to the preset operating pressure, typically
3000 psi (21 MPa).
Fluid temperature Hydraulic fluid temperature is maintained at a recommended
operating level by a fluid-to-water heat exchanger which passes the
fluid over water-filled tubes. Water flow through the tubes is
automatically regulated by a solenoid valve which is controlled by a
programmable logic controller (PLC). If fluid temperature exceeds a
preset limit (typically 140
°
F), a temperature-sensitive switch turns the
HPS off.
The fluid level and temperature gage on the HPS is used for reference
only. Use a laboratory grade thermometer to set the temperature.