Case 580SR, Service Manual

Page: 654 / 1068

64 SECTION 35 - HYDRAULIC SYSTEM
4.2 HYDRAULIC OIL FLOW
Hydraulic feed to the swing cylinders is controlled by
the swing section of the backhoe attachment control
valve, which contains pilot-operated pressure relief
valves with anti-cavitation feature to protect the sys-
tem and the cylinders, should an overload condition
occur.
The diagram illustrates the operation of the swing
circuit when retracting the right-hand cylinder (4)
and extending the left-hand cylinder (5) in order to
swing the backhoe attachment to the right.
When swinging the backhoe to the left the oil flows
are reversed and exactly the same principle of oper-
ation applies.
Oil flow for a right hand swing is as follows:
When the swing control is operated the control valve
directs oil flow to the rod end of the right hand cylin-
der.
The flow of oil at pump pressure lifts the one way re-
strictor (2) of its seat allowing the flow to continue
unrestricted to the inlet port of the piston end of the
left hand cylinder.
System pressure increases causing the right hand
cylinder to retract and the left hand cylinder to ex-
tend.
As the cylinders move, displaced oil on the piston
side of the left hand cylinder flows towards the one
way restrictor (7) in the inlet port on the rod end of
the right hand cylinder.
The oil flow moves the restrictor to the restrict posi-
tion which limits the flow of oil and creates a back
pressure (1st stage restricted return oil) in the right
hand cylinder.
The restricted flow of oil passes through restrictor
(7) into the rod end of the left hand cylinder before
returning to the control valve and back to tank.
The spool in the swing control valve assembly (1) is
designed such that during operation oil can flow
freely through the port directing oil flow to the swing
cylinders but restricts the flow returning oil back to
tank.
This restriction is achieved using metering lands
machined into the lands of the spool and creates a
secondary back pressure (2nd stage restricted re-
turn oil) in the rod end of the cylinder.
By allowing oil to flow unrestricted to the power side
of the cylinder and be restricted on the exhaust side
a smooth controlled swing of the backhoe at opti-
mum speed is achieved.
When the swing system reaches the last 20-25° of
travel the sliding restrictor in the end of the piston
rod touches the outlet port in the cylinder barrel.
This further restricts the flow of oil drained from the
cylinder. As the piston moves towards the end of its
stroke, the sliding restrictor is gently pushed down
towards the centre of the cylinder rod, allowing a
progressive and controlled stop of the swing cycle.
If the swing control valve is suddenly returned to
neutral mid-way through a full power swing the sup-
ply and exhaust ports in the main control valve are
totally blocked.
The momentum of the swinging backhoe assembly
if not controlled will make the machine extremely un-
stable.
To prevent this from occurring, the system pressure
relief valves operate and drain the oil until an excess
pressure is detected. When the relief valve operates
the cylinder rod will move and a void will be created
in the low pressure side of the cylinder. The anti-
cavitation feature in the relief valves prevents the
void from creating by transferring the drain oil from
the side of the system where an excess pressure
occurred to the low pressure side.
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