Sizing; Limitation - Danfoss PVG 32 Technical Information

Technical Information
PVG 32 Proportional Valve Group
Function
© Danfoss | November 2017
•
Lifting/lowering movement
•
Slewing movement with cylinders
•
For the main lifting/lowering function on a crane it is recommended to fit a "half" pressure control
spool. This means that the spool is designed with a normal flow control on the lifting port and
pressure control connected to the port where the pilot signal to the over-center valve is acting. You
will thus maintain a load-independent lifting movement and achieve a stable but load-depending
lowering movement.
•
As the load pressure on slewing movements is usually steady - irrespective of the crane being loaded
or not – it will be advantageous to use a "full" pressure control spool for A and B port.
In both cases we recommend the use of a basic valve, PVB, with pressure compensator. The pressure
compensator will ensure the individual load-independency between the basic valves.
It is further recommended to use the LS pressure relief valves as not only will they ensure individual
pressure limitation but also make it possible to adjust the maximum oil flow to the function.
It is not recommended to use shock valves as pressure limiting valves in connection with pressure control
spools.

Sizing

The size of "half" (e.g: P - A = flow control P - B pressure control) pressure control spools is determined on
basis of max. flow demand on the lifting port. If e.g. a max. pressure compensated flow of 65 l/min for the
lifting movement, you choose a 65 L/min spool (size D). The metering characteristic has then a given size.
As it is often requested to limit the use of the crane boom for downward push/force mode and the LS
pressure limitation can be used. It will appear from the characteristics enclosed what effect a pressure
limitation, P will have on max. flow on the lowering port.
LS
The size for a "full" pressure control spool is determined on basis of known load pressure, P
requested max. flow.
It will appear from the characteristics enclosed that if the load PLS is low and the pump pressure, P
high as a result of max. stroked spool you will get a large flow.
If P is approaching PLS max. the flow will be reduced and the dead band increased. Max. oil flow can be
LS
reduced by approx. 50% without limiting max. pressure.
The reduction is made by limiting the spool travel from 7 mm to 5.5 mm.

Limitation

If a pressure controlled spool is chosen for stability reasons consideration should be made to features
related to the pressure control principle.
Deadband will change according to the load conditions and the valve section will become load-
dependent and that the pump pressure may exceed the load pressure.
With all of the above in mind, a "pressure controlled spool" will minimize oscillation and obtain a stable
function that can be controlled smooth and precise.
max, and
LS
, is
p
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