How does this thingamajig work?

February 9, 2010 11:46 AM

Linear solenoids. Need help understanding how they work and if mine is broken.

I recently purchased an AC and a DC linear solenoid. I was told the DC solenoid would come standard with a spring return. It did not. Neither did the AC. Is there an expectation that the back and forth functionality of the device will be controlled by the current switching direction or some external force on the end of the device? Is this what I have misunderstood? Should I contact my dealer about a flaw in the equipment? (I’m asking here first because it’s a large corporation with a huge product line and sales people tend to guess at an answer that will make the sale and I’d like to be better informed when I contact them.) Another way to answer this question would be to suggest resources on linear solenoids that don’t relate to starting motors but use them as locking or positioning devices would be appreciated.

posted by edbles to Technology (10 answers total)

Best answer: I don't know about big industrial solenoids, but the small ones I've used, whether AC or DC, can only provide force in one direction. There's a coil, and when you apply power to the coil, it attracts the metal core; there's no way to repel the core to move it the other way. Reversing the movement is always done with an internal or external spring, or by gravity.
posted by moonmilk at 12:12 PM on February 9, 2010

Response by poster: Yes it's a small .5" diameter 1" rod small rod not a big industrial guy.
posted by edbles at 12:25 PM on February 9, 2010

Best answer: FWIW, practically all of the linear solenoids I have bought have not come with a built-in return spring. I have been happy that they don't because it saves space on the solenoid itself and allows a return spring (if needed) to be included at only at the right place in the attached mechanism.
posted by buzzv at 12:29 PM on February 9, 2010

Think of the classic, "I'll buzz you in" door. The door latch is spring loaded - it locks when the door is closed. When the solenoid is powered it overcomes the spring and the door is unlocked. The moment power is taken away, the door locks again. This keeps you from having to power the solenoid constantly just to keep the door closed.
posted by Kid Charlemagne at 12:43 PM on February 9, 2010

Response by poster: buzzv: Any recommendations on where to acquire and how to attach a return spring?
posted by edbles at 1:00 PM on February 9, 2010

Response by poster: Also it's a pull style so the return spring would need to be internal I'm guessing.
posted by edbles at 1:11 PM on February 9, 2010

I presume that only the DC version is ordinarily supplied with a return spring because the DC version will become permanently magnetized in operation to the point it doesn't return much more readily than the AC version, due to the fact that switching the direction of the current 60 times a second helps prevent permanent magnetization.
posted by jamjam at 2:58 PM on February 9, 2010

Er, make that switching the direction of the current 120 times a second.
posted by jamjam at 3:16 PM on February 9, 2010

Best answer: I don't know if this will be helpful, but here's an example of a two-way solenoid -- it's really two solenoids in one, with one coil to pull the core one way and another to pull it back again. And this is a solenoid with a built-in return spring. (I've used this particular model - it's only intended to pull for a second or two at most before resting for a while, or it gets quite hot.)
posted by moonmilk at 4:31 PM on February 9, 2010

Here is my solenoid collection. As you can see, all return springs are external, usually on one half of a lever.
posted by fake at 5:39 PM on February 9, 2010