Manitowoc National Crane NBT30H-2, Service Manual

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2
National Crane
03-20-2019 Control # 613-06
2-15
NBT30H-2 SERVICE MANUAL HYDRAULIC SYSTEM
RELIEF PRESSURES
Description
Relief valves protect hydraulic systems from over
pressurization and ensure the components get the proper
pressure and flow. The relief valves for the swing and
telescope circuits are integral to the directional control valve
and are not adjustable. The hoist and lift circuits are set to
the load sense relief valve pressure.
All relief valves are preset at the factory. Certain relief valves
can be adjusted and certain valves are not adjustable, see
the following table for pressure settings and adjustability.
Maintenance
Relief pressures are checked by causing a given circuit to
reach its pressure limit (stall). At the stall point the relief valve
opens and the hydraulic oil is returned to the reservoir.
Hydraulic circuits can be stalled by plugging the work hoses
to the motor or cylinder prior to actuating the control valve. A
pressure gauge placed in the proper line shows the pressure
that the relief valve opens. Release the control lever after
taking each reading.
It is only necessary to hold hydraulic pressure long enough
(usually a few seconds) in the circuit to gain an accurate
reading. Do not overload the hydraulic circuits for long
periods of time.
Use an accurate 0 to 34,500 kPa (0 to 5000 psi) pressure
gauge when adjusting relief valves. To adjust a relief valve,
turn the adjustment screw (in to increase or out to decrease)
until the proper setting is reached. Reservoir oil temperature
is to be 60° - 71°C (140° - 160°F).
CHECKING RELIEF PRESSURE
Preparation
• Start and warm up the engine until the hydraulic oil
temperature reaches a minimum of 60° - 71°C (140° -
160°F).
• Shut down the engine.
Relief pressure for the hoist hydraulic circuits is controlled by
the load sense relief valve located on the directional control
valve (Figure 2-2); this relief valve is not adjustable.
The load sense relief and maximum pressure can be
checked by performing the following steps.
Hydraulic Circuit
Pressure
MPa (PSI)
Tolerance
kPa (PSI)
Location
Telescope In Relief (Not Adjustable) 19.65 (2850) ±689 (±100) Directional Control Valve (Preset)
Telescope Out Relief (Not Adjustable) 19.65 (2850) ±689 (±100) Directional Control Valve (Preset)
Outrigger Circuit Maximum Pressure 20.68 (3000) ±344 (±50) Relief Valve On Pump
Outrigger Circuit Beam Extension 6.89 (1000) ±344 (±50) Front Outrigger Control Manifold
Swing Circuit (Not Adjustable) 20.5 (3100) ±689 (±100) Directional Control Valve (Preset)
Load Sense Relief 26.2 (3800) +2.06 (+300) Relief Valve On Pump
Load Sense Standby 2.48 (360) ±172 (±25) Relief Valve On Pump
Pump Compensator 25.15 (3650) ±689 (±100) Relief Valve On Pump
Options (If Equipped) 17.2 (2500) ±689 (±100) Directional Control Valve (Preset)
CAUTION
Do not hold the relief valve open for more than one minute
at a time. Damage to the hydraulic system may result.
WARNING
High Pressure Hazard!
Pressurized hydraulic fluid can cause serious injury. Be
careful when removing plugs or restrictions from
pressurized hydraulic systems suspected to have
entrapped air.
Fluid injected into skin must be surgically removed within
a few hours by a doctor familiar with this type of injury or
gangrene will result.
!