Pressure Control Spools With Counterbalance Valves - Danfoss PVG 32 Application Manual

Application Guide
Using flow or pressure control spools

Pressure control spools with counterbalance valves

10 | © Danfoss | Feb 2017
The main purpose of a PC spool is to control CB valves. In general, the basic functionality of a CB valve is
to ensure non-moving actuator in case of hose break. See
10.
Lowering a load through a CB valve
A Pilot connections to force open CB
Lowering a load with a PC spool
A PC spool is more applicable when lowering loads in applications with CB valves.
A CB valve can be opened either by a pilot pressure or when the relief setting of the CB valve is reached
(shown as an adjustable spring in the previous figure). Normally, the relief setting and pilot pressure have
a ratio around 5:13. As a result, if the relief setting is 250 bar, the pilot pressure needed to open the valve
is a maximum of 50 bar.
In Lowering a load through a CB valve
pressure on the A-port is used as pilot pressure to open the CB valve on the B-port. The load is held by the
CB valve and, with the pressure build-up on A-port, the opening of the CB valve is controlled. PC is
proportional to spool position and as a result the opening of the CB valve is proportional to the spool
position.
If the pressure is lowered with a FC spool, then there is no control of the pressure. So when the pressure
has increased to a level that opens the pilot for B-Port, flow is allowed back to tank. It also means that the
pressure in the A-Port will decrease with a risk of getting below the opening pressure for pilot control. As
a result, the system becomes unstable and tends to jump when lowering the pressure because the CB
valve switches between opened and closed.
Typically, a spool with FC in one direction (lifting) and PC in the other direction (lowering) is used for
applications with CB valves to provide the best controllability in both cases.
Lowering a load through a CB valve
on page 10, a PC spool builds up pressure at the A-port. The
on page
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