Magnets, Friction, and Lever question. 1

[TSE3]

So I am designing a device, and it utilizes two magnets to hold two simple rectangular pieces togethe side by side. One of the sides will be perfectly verticle to hold the other piece which will be a lever, similar to a see-saw. I want to be able to hold position, either horizontally, vertically, or anywhere in between.

I want one side of the lever to be the handle, so it will be shorter. Obviously, each side of the lever will be different weights. Will this mean the lever won't hold place when it's not being held up? Is there not enough friction?

Any solutions? Attached is to help you guys comprehend what I'm trying to say through words. Thanks!

 

[LittleInch]

The magnet will provide a force affected by strength of magnet and area. No idea how to work that out or what units are used to describe magnetic force, but I'm sure it's out there somewhere. Once you have a force you have friction load based on force x coefficient of friction.

Depending on the weight and moment of your arm it might work, it might not, but you should be bale to work out the worst case (horizontal) fairly easily.

I'm very glad you added the sketch though - I wouldn't have come up with that from the words...

If your dimensions are reasonably to scale, your lever had better be quite light as it doesn't look right to me.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[TSE3]

The arm is about 20cm long and 1.5 whereas the handle is about 5 cm long, and is using 5mm thick DiBond (polypropylene or ethylene?) sandwiched between two sheets of aluminum, so it's fairly light. And the magnets are rated at 900g strength.

Any ideas on how to increase friction at the joint just incase it doesn't work out?

 

[rb1957]

my 2c ... i don't see it working as intended.

Quando Omni Flunkus Moritati

 

[AWloo]

There are two ways to increase the friction force. Increase the coefficient of friction by changing the material (or coating) or increasing the normal force (squeezing the bars together).

The rest of this problem once all your loading is defined becomes a simple static analysis. The horizontal position will have the highest moment by the looks of your diagram. Assuming there is no end loading.

 

[rb1957]

i don't see how the lever can be moved and held in place by a simple magnetic force (as shown) ?

Quando Omni Flunkus Moritati

 

[LittleInch]

I think this idea will hinge (sorry about the pun) on the hinge arrangement and size. The friction surfaces should be as far away from the centre point if indeed there is one. That's what you'll need to concentrate on to get the resistive moment. I can't recall what the moment force is for two plates, but this is a simple clutch arrangement so will be out there somewhere. I would recommend a simple pin through the second plate to force the moment to be in a fixed place and force the movement to be around the pin.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[chicopee]

At point of contact between the two levers, you would need one lever to have a cup like recess so that the second lever with a boss with enough surface area for contact with the bottom of the cup, can spin within the cup recess without sliding out of position. The two magnets on opposite sides of both levers should hold everything together. Check out a physics book on magnetism to calculate the forces based on the given magnetic strength of 900 gauss and the contact surface areas of the boss and cup like recess.

 

[TSE3]

Thanks for the help so far guys.... any suggestions on how to make it work if you foresee it not working? I should be getting the parts this week.

 

[LittleInch]

add a bit more weight to the short end of the lever?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[IRstuff]

Well, I would suggest looking at prior art. Rather than inventing something that might, or might not, work, look to what has been successful. Typical swivel joints are either bolted together with thumbscrews to provide the clamping force, or a ball&socket that has some sort of knurled ring nut to apply the clamping force.

TTFN
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[TSE3]

Thanks for the help guys! I'll update you guys when it's done!

 

[zeusfaber]

You'll need to look a little at the pole arrangement of the magnets. Anything that seeks to improve attraction by bringing both poles out into the same plane is going to give you interesting effects when you try to use it as a hinge.

A.

 

[chicopee]

Wikipedia has formulae about forces due to magnetic field. Not knowing the shape of the magnets, remember that a dipole represents a N-S arrangement between opposite magnets so if you have two round magnets on opposite ends, you would have two dipoles. Also permeability of the material between magnets will have to be researched. The title of the link is Force between magnets - Wikipedia at en.wikipedia.org/wiki/Force_between_magnets

 

[Sparweb]

I don't know what you're really building, but maybe you can prototype it in the "fantastic contraption" (google it)


STF