Solenoid valve actuators page two which is continued from this page.
In order to have a coil actuator that would do lots of physical work, moving tooling over a distance of 18" for example, the solenoid coil would dwarf the tooling and even maybe the machine it was situated on. Not a very practical solution at all.
And that is why we use compressed air to provide the force (through the air actuator) to do the heavy-lifting.
A small valve body means that the poppet or spool that controls the air flow inside that body is smaller yet. It is light weight, and it only has to move a short distance to open or close a flow path. Those are ideal operating conditions for a direct acting solenoid actuator.
Unfortunately, when air valve designers push the limits of the size of an air valve that is using a direct solenoid actuator, valves tend to stick more often.
If that air valve sits idle for some hours between operations, that guck inside the valve dries out and sticks the parts together. When the valve is signalled to fire again, there is not enough force in the small coil to overcome the stickiness inside the valve body and bingo, you have a stuck valve. Air valve.
Anybody have a hammer??? :-)
Direct acting solenoid valves can be very, very small, with extremely low electrical demands. This is ideal for circuits that need the solenoid valves powered right from a PLC (Programmable Logic Controller).
Given that the solenoid actuator is only moving a very small device inside the valve a very short distance, direct acting solenoid valves can operate very quickly. I recall one time I was giving a demonstration to a bunch of maintenance engineers on the relative merits of a particular brand of air valve. I used an oscilloscope that could generate a wave on the screen to visibly demonstrate that the little direct acting valve was cycling at over 200 times per second.
If the compressed air supply is typically marginal, full of water, water vapor, compressor oils, rust and scale from black pipe, then small direct acting solenoid valves may tend stick more often than their larger, more powerful, cousins.
If the application calls out for a physically large valve with higher compressed air flow, and as we have already discussed, you cannot drive a large air valve with a direct acting solenoid coil, what happens when you need to solenoid operate a bigger valve?
That brings us to another design of air valve, a solenoid air pilot actuated compressed air valve, and you can get information on Solenoid Pilot Operated Air Valves right here.