MTS Hydraulic Power Unit Model 506-92 - Manual (Page 39)

Theory of Operation 5-2
Main and
supercharge pumps The HPU uses two variable-volume pressure compensated main pumps,
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 pumps returns to the reservoir through check valves
in the low-pressure filter manifold. The supercharge pump is mounted
vertically inside the reservoir.
Filters Hydraulic fluid from the low-pressure filter is routed to the main pumps.
A low-inlet pressure switch monitors the inlet pressure (supercharge
output pressure) to the main pumps.
Fluid from the main pumps 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 pumps 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 compensators are operating
normally, the relief valve does not open.
High/low solenoid
valves High/low pressure operation is controlled by the high/low solenoid
valves. These valves control the main pressure control (pressure
compensator) vent ports and the backup relief valve vent port.
With the high/low solenoid valves de-energized, the main pressure
control vent ports and the back-up relief valve vent port are 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 HPU.
With the high/low solenoid valve energized, the vent ports are 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
water regulating valve. If fluid temperature exceeds a preset limit
(typically 140
°
F), a temperature-sensitive switch turns the HPU off.
Pump protection Pump protection is provided for the main pumps by the main pump low-
inlet pressure switch that turns the supply off if supercharge pressure
drops below the recommended operating pressure. The supercharge
pump and main pumps are protected by a fluid-level switch which turns
the supply off if the level of hydraulic fluid in the reservoir drops below a
preset level.