# Clamping pressure question

1. Sep 13, 2015

### remettub

Hi all,

I'm designing a small 3-pronged clamping mechanism, and I'm having trouble figuring out the best way to keep the pressure even between the upper contact surfaces (pads). Here's a picture of my attempt from this afternoon:

It's difficult to tell from the image, but the lower pad is directly under the halfway point between the top pads. I naively assumed that this would give me equal pressure under the top pads, even though the tightening screw is on the left, but of course in actuality the left pad applies considerably more pressure than the right (other design requirements make it impossible to put the tightening screw straight up through the center).

Since it took a good part of the day to design and make this guy, I'd like to have a clear understanding of how to predict the pressure distribution before I make the next version. I'm thinking it's either that there is a small amount of wiggle in the joint, or that the "C" path to the right pad is more flexible than the "\" path to the left.

If kept everything the same but shifted the lower pad to the right, would that balance the pressure more towards the top right? Is there a way to calculate how much to shift it? Or is it all about the relative flexibility on each side (I could give the left side an "S" shape to make it equally flexible). Or do I just need to tilt the bottom pad to compensate for the wiggle in the joint. Obviously my understanding is very limited at this point. Any enlightenment would be greatly appreciated!

Thanks,
-remettub

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2. Sep 14, 2015

### Baluncore

Any eccentric clamp will have problems.

I suspect the problem is maintaining alignment of the sliding joint. Maybe you could consider a tube on the single pad side, that fits over a round pillar on the two pad frame. The round pillar would have an internal threaded hole for the tightening screw.

Alternatively, extend the frame all the way down to, or beyond, the single pad. That will put the tightening screw on the axis of symmetry behind the two pads.

3. Sep 14, 2015

### remettub

Thanks Baluncore, it seems like you are saying there's no reliable way to model the pressure of an eccentric clamp. Is that correct?

I was hoping I could take measurements of the current pressure distribution, arm myself with knowledge, and then make an adjustment to the design to even it out, keeping the eccentric design.

4. Sep 14, 2015

### Nidum

Needs to be a mechanism .

(1) Separate the upper beam and put a pivot point half way between pads . Use screw tightening and bottom single pad arrangement but make an inverted L shape at top with a pivot hole on outboard end of leg . Use a pivot screw to connect the end of L pivot hole to the beam pivot hole .This causes primary line of action of tightening force to go from bottom pad to mid way between top pads and system is balanced .

Use friction or a wire spring to stabilise the two pad beam so that it stays in approximately the right place when not engaged to whatever is being held .

Or

(2) Use the toolmakers clamp principle with an additional screw or a fixed link if whatever is being held is always roughly same size .