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 TM 55-1925-273-10-1
0039 00
RELIEF VALVES
Two relief valves are installed in the towing machine hydraulic system. The first relief valve is the system relief (figure 5,
sheet 1, item 3). This valve is set to open at 3300 PSI (228 bar) to prevent overpressurization of the towing machine
hydraulic system. The second relief valve (figure 5, sheet 1, item 4) is set to open at 2800 PSI (193 bar). Under heavy
system loading, this valve opens and permits the flow from the lower volume vane section to flow unrestricted back to the
reservoir. This action effectively unloads the lower volume vane section, preventing the hydraulic pump from overloading
the diesel engine.
RESERVOIR
A 350 gallon (1325 liter) reservoir (figure 5, sheets 1 and 2, item 5) contains the hydraulic fluid for the towing machine
hydraulic system. The reservoir also acts as a mounting point for the heat exchanger, the filters, and the gauges and indica-
tors for the towing machine hydraulic system.
HEAT EXCHANGER
A heat exchanger (figure 5, sheet 2, item 6) is provided to cool the hydraulic fluid. As the hydraulic fluid returns to the
reservoir (figure 5, sheets 1 and 2, item 5), it flows through the heat exchanger where its heat is dissipated to the raw water
that is pumped through the heat exchanger. A solenoid actuated valve controls raw water flow to the heat exchanger. The
valve permits raw water to flow through the heat exchanger when the hydraulic fluid is hot and secures the flow when it is
cold. Additional hydraulic fluid cooling is also provided by dissipation of heat through the walls of the reservoir to the air
in AMS 1.
SUCTION AND RETURN FILTERS
A suction strainer (figure 5, sheet 1, item 7) is located in the hydraulic pump (figure 5, sheet 1, item 1) suction line.
This suction strainer removes the larger particulate contaminants from the hydraulic fluid before it enters the hydrau-
lic pump. Two return filters (figure 5, sheet 2, item 8) remove smaller particulate contaminants, down to 10 microns,
from the hydraulic fluid before it is returned to the reservoir (figure 5, sheets 1 and 2, item 5). Each return filter is
fitted with a restriction gauge (figure 5, sheet 2, item 9) and a bypass valve (figure 5, sheet 2, item 10). If the filter
restriction rises above 25 PSI (1.7 bar), the bypass valve opens, permitting hydraulic fluid to bypass the filters and
return to the reservoir. Because permitting unfiltered hydraulic fluid to return to the reservoir opens the door to
eventual system failure, the restriction gauge is provided to enable timely return filter changes. Performing regular
return filter changes helps to ensure that the central hydraulic system and its components enjoy a long service life.
GAUGES AND INDICATORS
Pressure gauges (figure 5, sheet 1, item 11) are installed to provide an indication of the system operating pressure. The gauges
measure pressure from both dual vane sections of the hydraulic pump (figure 5, sheet 1, item 1) as well as drain pressure. A
combination hydraulic fluid level/hydraulic fluid temperature indicator (figure 5, sheet 1, item 12) is also installed to permit
monitoring of hydraulic fluid level and temperature.
SYSTEM OPERATION
When the diesel engine is started, the hydraulic pump (figure 5, sheets 1 and 2, item 1) begins turning. As it turns, it
draws hydraulic fluid from the reservoir (figure 5, sheets 1 and 2, item 5) through the suction strainer (figure 5, sheet
1, item 7) and into the vane sections' inlets. The hydraulic pump pressurizes the hydraulic fluid in response to the
amount of restriction on the system. If the restriction is low (no load applied), system pressure is very low. If the
restriction is high (load applied), the system pressure will vary in response to the load. The pressurized hydraulic
fluid exits the pump and flows through the supply piping (figure 5, sheet 1, item 13) to the towing machines (figure
5, sheet 1, item 14; figure 5, sheet 2, item 15) and their controls (figure 5, sheet 1, item 16; figure 5, sheet 2, items 17
and 18) where the hydraulic pressure and flow are transformed into physical work. Hydraulic fluid exiting the
downstream components returns to the reservoir through the return piping (figure 5, sheet 2, item 19) and the return
filters (figure 5, sheet 2, item 8).
0039 00-9
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