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Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

(OP)
I understand that this may not be structural engineering but I cannot find the general engineering section here. Structural Engineering can deal with similar issues to this on a bigger scale so I would really appreciate your feedback! Thanks.


After a wrist injury I had to stop lifting weights. This was annoying so I decided to make an arm brace that would transfer the weight of a dumbbell through my forearm rather than through my hand and wrist (with the help from my local metal works). I've now got the brace and its perfect, aside from one flaw. One point on the brace acts as a pivot point concentrating the pressure in one area of my forearm. This makes lifting dumbbells very painful!

I've uploaded pictures detailing the issue with the help of some annotations and very sophisticated arrows (lol).

I was wondering if any of you guys would be able to throw a couple tips my way that might help me get past this point? Thanks a lot for any help, its very appreciated. I'll be online all of this evening to reply and work through any suggestions you have.













RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

Maybe another plate on the opposite side of your forearm, tightening the 2 plates together with the straps.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

(OP)
Hey sbisteel, I didn't think of that. Thats an interesting idea. I can't visualise how it would reduce the stress on the pivot point though - but I could be misunderstanding

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

It would really add a second "pivot" point, with a location of high stress/load at the far end of the outside of your forarm, but I think it would help spread the load out over your arm.

The plate in the picture is pushing down at your wrist, and pulling up near your elbow. The second plate would engage your forearm near the elbow side.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

Hi RickDeckard

You need to speak with someone on a medical level regarding sporting injuries who could probably help you better.
I can see why the all the load is transferred to the point you indicate, its the load can react and that's because the top and lower plate are rigidly connected, from an engineering point of view and not medical you could shorten the lower plate so that it contacted your arm further up away from the wrist.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

I think it is a fit problem that is causing the force to be concentrated on your arm. You could consider making a fiberglass casting to fit your arm tightly. Casting the fiberglass is not too difficult so long as you use a good bond breaker. The fiberglass could be easily bonded to the existing steel plate you have.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

RickDeckard:
Keep the “C” shape for the dumbbell and the top bar that it is attached to. Eliminate the bottom bar which touches your arm, and replace it with a fiberglass mold (half cast) that is approx. a “C” shape with the opening downward to fit your forearm, and actually molded to the shape of your arm. The point that you say is the problem will be the location where the primary reaction (loading downward) always takes place. You might take this molded shape clear out (to the right) to match your palm and the heel of your hand, and be strapped to your hand. The molded fiberglass brace will bridge the bending load over your wrist, from the dumbbell “C” shape up into a couple straps on your forearm. This form should have some fairly stiff, but conforming, padding. This assumes you can tolerate some load on the heel of your hand. A prosthetics shop, artificial limbs maker, should be able to help you with this.

I assume you are primarily working your biceps. Keep everything I’ve suggested above in the way of the molded brace. But, shorten the top bar, or eliminate it, so that the “C” shape for the dumbbell is 3-4" above your wrist on the brace. If you remember your Statics course from college, you will need more weight to make your biceps do the same work, because of the shorter lever arm, but the same work nonetheless, and the same range of motion. But, now the load will be more evenly distributed to your forearm, under the dumbbell. This eliminates the high reaction force, right where you say it hurts, from your current cantilever arrangement. You need the brace to go out onto your hand so you have some control over the dumbbell, in terms of it rotating on you forearm and whacking you up-side the head or on the thigh.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

(OP)
Thanks for your ideas.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

Your commitment to staying fit post-injury is inspiring. The pre-molded ideas suggested by others would surely yield the best result. Baring that, I might try something similar to what I've shown in the sketch below. Maybe in combination with a hawking glove or something...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

The issue is as described above by several posters. Your arm is not flat but the plate is. Make the plate fit the shape of your arm. I'd also be sure to avoid the carpal tunnel area - make the bar go up to the beginning of your palm. I think you could use a load bar from a backpack - some are 1/8" thick x 1" aluminum of an alloy that you can relatively easily bend to the contour needed.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

Have you tried using it with a thick foam board or similar? the softer material will distribute the load more evenly over your arm (custom fit as others mentioned) and will help eliminate the concentrated point load effect. Simple enough to try out a few options without any fabrication.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

Why not just hang a weight from a protective pad on your wrist (or forearm) and leave your hand and related out of the action altogether? What muscles are you working on?

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

I like, FWIW !?, the last idea ... instead of holding the weight in your hand, make a rest of the weight in the middle of your fore-arm.

another idea would be to make a mechanical fore-arm ... plates both sides of your arm, strapped together, with a holder for your weight; and take your hand out of the equation altogether. I think this is what was meant by the first response ?

without meaning to sound like a snit (though probably doing a great job of sounding just like that !?) I have trouble seeing how you arrived at your present design ...

another day in paradise, or is paradise one day closer ?

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

You could use the idea that I proposed above but with nylon straps instead of pipes. More comfortable that way. I own a hamstring stretching apparatus that works on the same principle.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

I like the strapping idea a lot more than using metal bars & plates to connect to your body.

I am concerned about adding the second 'lower' bar as sibisteel mentioned as it will just distribute the load to two point loads instead of one, and the double bar idea in Kootks first post accomplishes the same thing.

Have you tried using a rigid wrist brace? Good ones will extend from the palm all the way to your elbow, and do a very good job of distributing the load throughout your forearm. They usually have light aluminum inserts, but when I was playing football I copied the inserts in some scrap steel for very good results (played half a season at centre with a broken wrist with very little discomfort).

I will crack open my old ergonomics text tonight to see what it has for suggestions!

RE: Limiting the effects of a pivot point? Some help with an homemade arm brace (with pics!)

Neat idea. I used to wear a Rollerblade wrist brace for curling when my wrist was acting up.

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