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
Lowering a load through a CB valve
on page
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
on page 10, a PC spool builds up pressure at the A-port. The
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.
Application Guide
Using flow or pressure control spools
Pressure control spools with counterbalance valves
10 |
©
Danfoss | Feb 2017
AB00000079en-US0101
