Manitowoc National Crane NBT30H-2, Руководство по обслуживанию

Страница: 51 / 194

2
National Crane
03-20-2019 Control # 613-06
2-11
NBT30H-2 SERVICE MANUAL HYDRAULIC SYSTEM
• Trapped air can be removed from cylinders having wet
rods by cycling. On certain cylinders, a plugged port is
provided on the rod end to bleed off trapped air.
• In the event that trapped air persists, bleed off the air by
loosening various clamp and screw type fittings.
• If the above procedures fail to eliminate trapped air,
contact your authorized National Crane distributor.
HYDRAULIC PUMP
The axial piston hydraulic pump converts input torque into
hydraulic power. Rotational force is transmitted through a
input shaft to a cylinder block. The spinning cylinder block
contains nine reciprocating pistons. Each piston has a brass
sl i pp er co nn ec te d at o ne e nd b y a ba ll jo in t. T he
reciprocating movement of the pistons occurs as the slippers
slide against an inclined swashplate during rotation. One half
of the cylinder block is connected to pump inlet and the other
half to pump outlet. As each piston cycles in and out of its
bore, fluid is drawn from the inlet and displaced to the outlet
to supply power into the system circuit. A small amount of
fluid is allowed to “leak” from the cylinder block/valve plate
and slipper/swashplate interfaces for lubrication and cooling.
Case drain ports are provided to return this fluid to the
reservoir.
The volume of fluid displaced into the system circuit is
controlled by the angle of the swashplate. The swashplate is
forced into an inclined position (into stroke) by the bias piston
and spring. The servo piston opposes the action of the bias
piston and spring forcing the swashplate out of stroke when
hydraulic pressure in the control circuit rises above the
spring force.
Load Sense Control
T he Load Sense (LS) control matches pump flow with
system demand. The LS control senses the flow demand of
the system as a pressure drop across an External Control
Valve (ECV). As the ECV opens and closes, the pressure
delta across the valve changes. Opening the ECV decreases
the delta and closing the ECV increases the delta. The LS
control then increases or decreases pump flow to the system
until the pressure delta becomes equal to the LS setting. The
advantage of the load sense control system is only the
amount of flow required to do work in pumped through the
system. This increases efficiency and reduces system
temperature.
System Circuit Flow
The pump input receives fluid directly from the hydraulic
reservoir. The output of the pump directs flow to the load
sensing Directional Control Valve (DCV). The DCV directs
fluid to the hydraulic components in the system. The speed
of the hydraulic components in the system depends on the
volume of fluid being provided by the pump. The operating
pressure varies depending on the hydraulic component load,
but is limited to:
• An adjustable maximum setting by the Pressure
Compensator (PC) section of the pump control.
• A system relief valve integrated into the side module of
the directional control valve.
The position of the DCV spool sets the demand for flow in the
system and communicates this to the pump control by
means of a hydraulic signal (load sense signal). The pump
provides as much flow to the system as it demands while
limiting the maximum pressure. Therefore flow and pressure
in the system are compensated to meet requirements.
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.
WARNING
High Pressure Hazard!
Do not attempt to loosen fittings in pressurized lines or
while the hydraulic pumps are in operation. Serious injury
or death may result
!
!
FIGURE 2-1
Case
Drain Load
Sense
Pressure
Compensator
Axial Piston
Pump