Heavy tooling over a longer distance... that is why we use compressed air to provide the force (through the air actuator) to do the heavy-lifting that is required. What do I mean by that?
A small valve ensures that the poppet or spool that controls the air flow inside that valve is even smaller yet. Being small it is a light weight valve with even less heavy internal components. Small valves poppets or spools only have to move a limited distance to open or close the flow path inside. Those are ideal operating conditions for a direct acting solenoid actuator.
Unfortunately, when valve designers push the limits of the size of an air valve with a direct solenoid actuator, the poppet or spool has to move a correspondingly longer distance, and the valves tend to stick more often.
Compressed air is pretty much filthy. It has water. It has dust. It has pipe scale. Lots of "stuff" to get inside the air valve.
Then, between operations, the valve sits idle. Shut down for an hour or two, overnight, or for days.
What happens is that the guck deposited inside the valve from the air flow dries out and essentially glues the moving parts of the valve together. When the valve is signalled to fire again, the force generated by the small coil cannot overcome the stickiness inside the valve body and bingo, you have a stuck 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) or, perhaps, even installed on a circuit board.
Direct acting solenoid valves can operate very quickly. I recall one time I was giving a demonstration to a group 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 demonstrate that the little direct acting valve I was demonstrating was cycling at over 200 times per second.
That is over 200 hertz! Opening, allowing air to flow, and closing, shutting off the compressed air, more than 200 times every second. Amazing!
If the compressed air supply is 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 the large air valve cannot be operated with a direct acting solenoid coil, what happens when you need to use a solenoid actuator to operate a bigger valve?
That brings us to another design of air valve. It is the solenoid air pilot actuated compressed air valve. You can get information on Solenoid Pilot Operated Air Valves right here.