Getting a clamping action on a hinged part
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  1. #1
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    Default Getting a clamping action on a hinged part

    I have a clamp that holds two pins as follows:

    two-pin-strut.jpg

    Material is 642 bronze. The planned clamping action is by putting screws through the flat part of the clamp to tighten it up.

    There are a couple of problems. One is that an interference fit is about 0.001" which translates to 0.0015" on the diameter of the horizontal rod, but with EDM, about the narrowest I can get the slot is 0.01", so there will be a gap. In other words, there is no obvious way to machine a slit so narrow that will close up completely when the clamp is screwed down.

    The other problem is that since the clamp is hinged at the round end, my concern is that it will tend to pinch at the square end. In other words, the square end will clamp down, but the rest of the length of the rectangular bar will not be tight. Is this true or not? In other words, let's say we have a #10 screw near the junction of the flat and round parts of the clamp, and tighten it. Will that screw generate enough force to deform the clamp and tighten it down? The goal is to get an even clamping force across the whole length of the rectangular piece (where the horizontal pin goes).

    One Nutty Idea: Taper the Bore

    One kind of nutty idea I had was to taper the horizontal bore slightly, so that it was wider at the far end of the clamp and narrowed as you got nearer the junction with the round end. The purpose of this is that when you screwed it down, it would have to bend more at the far end and that would compensate for the hinging action (in theory). The challenge is how do you compute the taper? Also, it will be a very slight taper, like 0.005" over 4 inches or something like that, so it would be somewhat annoying to machine that.
    Last edited by jscpm; 08-03-2019 at 06:25 AM. Reason: added tapering idea

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    Ever screwed in a pool ladder. They have diagonal hole adjacent to the primary hole, then a tapered nut is driven sideways into the shaft for a tight fit.
    Observe the assembly then miniaturize it.

    4" Ladder Anchor Socket Bronze 1.9" Rail Permacast Hanover Clone (NE1244) (PS-4019-BC) - Handrails & Ladders - Safety Equipment - In-Ground Pools/Spas - Pool Plaza

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    If the screw is located properly it will clamp the vertical pin and the back part of the horizontal one. If the bore for the horizontal pin is long the clamping action may actually spread the front of the horizontal bore.

    If you can tolerate the change I would suggest a "humped" profile in the center with clearance cutout so the part acts more like two curved strap clamps. Picture one of the homemade mill clamps made out of thick wall pipe where the clamping force from the bolt is transferred to the ends.

    Your design may work if the pins and bores are held to close tolerances but as I mentioned the clamping force will tend to spring those flat sides leaving the end of the horizontal pin less well clamped.

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    If the bore is 'exactly' the size of the horizontal pin you are clamping, it will not pinch. As you allow slop in that bore, the more it will taper. The width of the slot is not important other than it has to be wider than the clearance between the object being clamped and the clamp.
    The evenness of the force across the length of the horizontal pin is related to the stiffness of the clamp.

    The bigger issue I see is any variance between the sizes of the two pins will cause one or the other to be loose. Meaning you want to build slop into the system. Meaning it does not work as well. Actually I don't see any way that the vertical pin will tolerate being small. And if it is large you will distort you clamp

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    Cut it completely in half,the clamp screws will hold it together.

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    Putting my engineering hat on, this is not a good design. You are asking too much from one or two clamping screws. You could probably get it to work with tight tolerances and careful placement to the clamping screws along the flat side, but after some use that will probably change. And by "use" I do not necessarily mean disassembly and reassembly. Normal working forces can take things out of balance. And that is only for the prototype. If you have to make more than one, each one may be a struggle in finding the right fit.

    I would try to redesign the part with separate clamping actions for each of the two pins.

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    Another possibility would be to make the piece you have shown *without* the slit, and use "split cotters"* to do the clamping.

    I'm not at all sure that "split cotter" is the correct term - I do *not* mean a "cotter pin," but rather a sleeve that overlaps the bore just slightly, is cut to match the bore, then cut in half. Here's the first thing I found via Google that shows a picture - it's a link to a home-shop project, so my apologies for that: Quorn Split Cotters | Tom's Maker Site

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    Another possibility would be to make the piece you have shown *without* the slit, and use "split cotters"* to do the clamping.

    *I'm not at all sure that "split cotter" is the correct term - I do *not* mean a "cotter pin," but rather a sleeve that overlaps the bore just slightly, is cut to match the bore, then cut in half. Here's the first thing I found via Google that shows a picture - it's a link to a home-shop project, so my apologies for that: Quorn Split Cotters | Tom's Maker Site

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    Another thing I was wondering is if your application could tolerate having one or even both of the pins permanently mounted?

    If so the options might be press fit, use of retaining compound, or even groove the pins and stake in.


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