Bush Hog CBH80 - Manual (Page 43)

Hydraulic Hook-Up (CBH 80) For
Tractors With Closed-Center Hydraulic
Systems (Figure 21)
There are two basic types of hydraulic systems,
open-center and closed-center. The open-center
system constantly pumps oil through the hydraulic
cylinder operating valves and returns the oil to the
reservoir. The closed-center system does not pump
oil until there is a demand for oil from one of the
hydraulic cylinders.
In this case, CLOSED-CENTER means that when
the operating valves which control the hydraulic
cylinders are in the neutral position, there is no flow
of oil through the valves. As long as the engine and
pump are running, a constant stand-by oil of pres-
sure approximately 2000PSI or more is maintained
in the system. Pressure oil is available instantly to
go to work when any one of the operating valves is
opened.
Because there must be no flow through the backhoe
control valve when the levers are in neutral position,
the backhoe must be converted for closed-center
operation.
Hook-Up Requirements
The hook-up of the CBH 80 Backhoe to the closed-
center system requires more than connecting it to
the remote couplers. The required devices are:
1. A flow restrictor in the pressure line to keep the
speed of the backhoe operation within safe and
acceptable limits.
2. The main relief valve in the backhoe control valve
must be replaced with plug, to allow tractor system
relief to control pressure. System pressure must not
exceed 2800 PSI.
3. A closed-center plug in the backhoe control valve
to make it a non-circulating or a demand type sys-
tem.
4. A low pressure surge valve in the return line to
keep the tractor back pressure surges from dam-
aging the backhoe control valve seals or tractor
system. A surplus flow line from the relief valve car-
ries only the excess surge oil directly back to the
tractor hydraulic reservoir so that it bypasses all
other functions.
Backhoe Operation
Fot the most part, a backhoe will function about the
same whether it is powered by an open-center or
closed-center hydraulic system. However there are
some differences which the operator should be
aware of when utilizing a closed-Center hydraulic
system.
As a backhoe function (lifting, actuating the bucket to
fill it, etc.) becomes increasingly difficult and the
hydraulic system pressure is rising closer to its maxi-
mum setting, there will be a noticeable slow down
before stoppage occurs. As the system pressure is
approaching maximum, the pump is also beginning
to stroke back for less and less fluid delivery. At
maximum pressure, the pump is completely
destroyed and there is no delivery.
Therefore, when working various backhoe functions
at or near maximum ability, a closed center system
will give the operator the impression that the back-
hoe is sluggish and somewhat unresponsive.
Learning to ease up slight at the first indication of
slow down will permit the backhoe to perform at con-
tinuing maximum speed and efficiency.
41
Figure 21
Replace Open-Center Plug
with Closed-Center Plug,
Shown.
Replace Main Relief with
Closed-Center Plug.
To Tractor
Reservoir
Surge Relief
Valve
Couplers
Hoses Supplied
by Customer