Shibaura SG280E, Workshop Manual

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79
3) Operation of valves inside the HST unit
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1. Check/relief valve
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A combination valve that functions as both a
balanced type high-pressure relief valve and a
low-pressure supply check valve is used for both
check & relief valves.
[High-pressure relief valve function]
P1 or P2 closed circuit pressure is transmitted via
the orifice to the oil chamber where spring A is
installed.
The oil chamber where poppet B is installed is
released to the tank.
The pressure in P1 or P2 rises, and when the
pressure on the seat area becomes greater than
the set spring force for spring B, the oil in the oil
chamber where spring A is installed is sent to the
tank. Poppet A is lifted, and the oil from P1 or P2
flows to the charge side to control the pressure in
P1 or P2.
[Low-pressure supply check valve function]
If the P1 or P2 pressure drops below the P charge
pressure, poppet A is lifted and P charge oil is
supplied to either P1 or P2.
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2. Low-pressure relief valve
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The low-pressure relief valve maintains the
low-pressure level required for charge oil.
The charge flow rate provides a supply through the
check valve when there is insufficient oil due to
leaks in the channel on the low-pressure side.
The low-pressure relief valve is then opened to
drain the remaining flow to the case.
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3. Unloading valve
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This valve is used to open the hydraulic circuit to
allow operation in the event of damage to the
hydraulic system.
In order for the unloading valve to short circuit port
A and port B, one pump is installed on each.
If it becomes necessary to unload the circuit, turn
the bolt to the left to short circuit ports A and B and
unload the circuit.
Fig.5-3-9
Fig.5-3-11
Fig.5-3-10
Valve body
Case / Drain
Charge pressure