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 TM 55-1925-273-10-1
OPERATOR'S MANUAL
INLAND AND COASTAL LARGE TUG (LT)
THEORY OF OPERATION
DECK MACHINERY
CENTRAL HYDRAULIC POWER UNIT (HPU)
Under usual conditions the central hydraulic system HPU supplies hydraulic power to the anchor windlass, the capstan, the
deck crane, and the tow pins. The central HPU can also be aligned to power the towing machines in an emergency. The
central hydraulic system HPU is comprised of two electric motor-driven hydraulic pumps, a system relief valve, a system
pressure gauge, a reservoir, suction and return filters, safety monitoring and shutoff devices, and a central hydraulic power
unit motor controller. Each component is described in greater detail in the paragraphs below. Following the description of
the components, an overview of the unit's operation is presented.
HYDRAULIC PUMPS
Two hydraulic pumps (figure 1, item 1) provide hydraulic pressure and flow to the central hydraulic system. Each pump is
rated at 32 gal/min (121 L/min) at 2550 PSI (176 bar). Each pump is driven by its own 50 horsepower (37.3 kW) electric
motor (figure 1, item 2). During normal operation, only one pump is operated. Under special circumstances, when hydrau-
lic flow demands are high, both pumps may be operated. However, the hydraulic fluid temperature must be closely moni-
tored during two-pump operation to prevent overheating of the system. Spring loaded check valves (figure 1, item 3) are
provided at the outlet of each pump to prevent backflow through the idle pump when only one pump is in use. The spring
loaded check valves are permanently set to OPEN at 7 PSI (0.5 bar) to provide minimal restriction to hydraulic flow.
DRAIN
14
RETURN
13
SUPPLY
5
15
SUPPLY
RETURN
DRAIN
2700 PSI
17
3
3
16
1
2
4
1
2
M
M
50 HP
50 HP
12
7
10
7
9
8
8
60 MESH
60 MESH
11
6
Figure 1. Central Hydraulic Power Unit Hydraulic Schematic
0039 00-1
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