Solenoid Actuator ...2


This is solenoid actuators ...2. This page is a continuation from the Solenoid Actuators page which is here. In order to have a solenoid actuator that could perform heavy physical work, moving tooling over a distance of 18" for example, the solenoid coil would dwarf the tooling and maybe the machine it was situated on. Not a very practical solution at all. What is the solution? Read on!

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?

Small Air Valves


If it is tree, and it is, that compressed air valves that are actuated by a direct acting solenoid must be fairly small by the fact that solenoids cannot generate heavy force over a long distance, what can we use to accomplish that? A small air valve, with a small solenoid, of course.

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.

Direct Acting Solenoid Actuators Have Limits!


Direct acting solenoid valves are relatively inexpensive to manufacture which is why air valve engineers are always trying to maximize flow through the small, direct acting valve. They keep trying to increase the air flow capacity through without enlarging the body of the valve. If the valve body is small, the coil only has to move the pole piece a very short distance and can generate maximum force over that distance.

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.

Why Direct Acting Solenoid
         Valves Stick


Here's a page on this site about the water and crud that comes down the air line and how that can deposit crud inside the valve body.

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??? :-)

Benefits of Direct Acting Valves


Direct acting solenoid valves are the first choice when what is being controlled by that valve uses very low pressure (under 25 PSI), or is controlling a vacuum. Valves for these applications do not require compressed air assist in the shifting of the valve spool or poppet, and as such, can be relied on to control minimal pressure or vacuum.

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!

Negatives of Direct Acting Valves


Since direct acting solenoid valves have, by nature of how they operate, to be quite small, that means flow paths are small too. That means they have limited air flow.

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.