solenoid design how to make this concept work

[spert]

I am working on an idea and would need to design a hold solenoid. Conceptually I think this is possible and would like some guidance before aimlessly attemting. solenoid diameter would vary 5-15". rod would not be solid but tubular(iron?) corresponding to diameter of solenoid. Length could vary from 5" to 15". Holding force would need to be min 20lbs. Would need low dc voltage, Heat build up would be an issure because windings would be incased. Duration would need to be max 24hrs. So far in my research it seems I could vary voltage to initially seat the solenoid and reduce to hold. Is this possible what guage would I need, how many volts is low voltage possible(would a battery weighing ~6oz -12v be possible?)approximate windings to generate force? I know there is alot of varibles but looking to see if this is conceptually possible. For rough principles how will force change when diameter is varied, length adjusted, windings increased, spacing increased assuming one variable adjusted others constant. how much will changing the thublar shaft material matter and would thickness of the tube matter. if I can figure out a workable model I can adjust it from there to the possible varibles

 

[waross]

The length of the magnetic circuit is important. Other than that, look for a textbook on magnetism.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Henneshoe]

Sorry, I do not have any hard equations for you, but here are some general thoughts concerning your concept:
1) The pull-in strength of a solenoid is proportional to the square of the length it is extended. Therefore, if you are looking for a long stroke you will need a large magnetic field.
2) The magnetic field strength increases when the number of windings increases, the distance those windings are from the shaft decreases, and the current flowing through the windings increases. For a large magnetic field you want a lot of windings close to the arm and a large gauge wire to handle the current (especially at the 100% duty cycle as you stated).
3) I believe the magnetic field strength will be less for a tubular shaft than a similar sized solid shaft.
4) Running this system on a battery will be difficult as the solenoid will be drawing current continually while pulled-in.

A alternative design that tends to work better for long stroke lengths is a actuator. Attach an electric motor to a shaft with a worm gear. As an added plus, there is a lot of friction in the system, so you might get your 20lbs. holding strength without needing any external voltage. Then you would only need to drain the battery when changing the position of the shaft.

 

[itsmoked]

I totally agree with Henneshoe. Reach for a "electric linear actuator"

It will weight far less.
It will take far less current.
They come in 12VDC.
It will actually work!
They won't cost a fraction of what you are going to spend in wasted time and money.
Your required force is at the very bottom of the pile, being almost trivial. This means cheap and light.
You could easily do this with a small battery. You will not do the solenoid with a small battery.

Rational stroke lengths for solenoids are usually under about an inch.

Yahoo!: "electric linear actuator"

And you will see what we mean.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[spert]

for this project I need to work with a custom device. The pull force is not very important it is more the holding power as the tube could be positioned prior to powering. I guess functionally it would be electromagnet. Where should I start with wire guage and # of turns?

 

[roydm]

If you just want the solenoid to hold something in position I suggest you look at the type of solenoid used to hold office doors closed, they only draw a few mA A typical 1200 lb unit draws 250 mA @ 12 VDC.
Regards
Roy

 

[waross]

When you work a few real world examples you will find that for DC magnets the gage of the wire is more important than the number of turns.
The relation between the number of turns and the strength of A.C.magnets get much more complicated.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

Spert,
I re-read your post several times, it's hard to visualize what you are trying to do exactly as both length and diameter vary. Can you add a sketch?
Thanks
Roy