86
Hydraulics and Pneumatics
Retract
P
Extend
///
Retract
[
A B
l
'1
Extend
',,I/
!A
i, I
i
I
Iit
:
a
(b) Three position valve
(a) Two position valve
Figure 4.3
Off
Valve control positions
Finite position valves are commonly described as a port/position valve where port is the number of ports and position is the number of positions. Figure 4.3a therefore illustrates a 4/2 valve, and Figure 4.3b shows a 4/3 valve. A simple block/allow valve is a 2/2 valve.
~
Extend
i
Figure 4.4
Off A B
P
,--IA B ....
Retract p [ l ~B
Possible valve action for a 4/3 valve
The numbers of ports and positions does not, however, completely describe the valve. We must also describe its action. Figure 4.4 shows one possible action for the 4/3 valve of Figure 4.3b. Extend and retract connections are similar, but in the off position ports P and T are connected-unloading the pump back to the tank without need of a separate loading valve, while leaving the ram locked in position. (This approach could, of course, only be used where the pump supplies one load). Other possible arrangements may block all four ports in the off position (to maintain pressure), or connect ports A, B and T (to leave the ram free in the off position). A complete valve description thus needs number of ports, number of positions and the control action.