Newby question: force of attraction calculation

[jalsina]

I am building a low-friction linkage, and want to use pairs of attracting and repelling permanent magnets to provide a calibrated amount of preload to the linkage. As the linkage moves, the distance between the magnet faces will change a total of 3mm. I need the difference in force between the "close" position and the "far" position to be roughly 35 grams. I'm thinking about using small neodymium magnets glued on the ends of adjusting screws. What data do I need to calculate force at a given distance, and what is the formula? What difference will it make if the screws are plain steel instead of non-magnetic material? Thanks.

 

[MagMike]

There is no straightforward formula for calculating what you are looking for. There are approximations, but they are hard to write in a text based forum such as this. I suggest checking the websites of some permanent magnet manufacturers. They oftentimes have a web based calculator for calculating forces produced by a magnet to a piece of steel (for varying air gaps).

I would suggest that as a start. What you are looking for is going to require a lot of iterations.

Regarding your question about plain steel instead of non-magnetic screws, I'd recommend using plain steel. The magnets will operate a bit more efficiently.

 

[MagMike]

jalsina,

I made a mistake when I wrote my reply above. There are no simple calculations when an air gap is involved. I'm not aware of any web based calculators for this.

Instead, I'd suggest looking at one of the freeware/shareware packages in the FAQ section of this forum. As long as you are dealing with cylindrical shapes, you can get a good approximation with FEA software that incorporates rotational symmetry.