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Clamping force calculation

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Neil Wyatt02/05/2020 13:38:07
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19226 forum posts
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I suspect this is a college assignment rather than a practical example... but if it gets the grey matter churning...

Neil

duncan webster02/05/2020 13:44:44
5307 forum posts
83 photos

Well if it is Rama should just put the answer, 'clamping force zero, stupid design' and leave it at that. What's Rama up to getting us to do his homewrok anyway? When I get back from walking the dog (daily exercise, only once promise!) I'll put something up assuming the lever can't move sideways to show the effect at the slopy end

duncan webster02/05/2020 17:43:33
5307 forum posts
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me prob-page-001.jpg

me prob-page-002.jpg

Howard Lewis02/05/2020 18:05:17
7227 forum posts
21 photos

If this is not a theoretical college question, it seems a needlessly complicated way of clamping a bit of bar in to a V block.

Wouldn't be a lot simpler to replace the linkage with a bell crank.

The clamping force, neglecting friction would be the force exerted by the piston multiplied by the lever ratio of the bell crank.

Or have I missed something?

Howard

Steviegtr03/05/2020 00:00:45
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2668 forum posts
352 photos

Have you guys never heard of a mig welder. Job done. Son why have you brought your welding mask into school again. Student. Because your question is flamin stupid.

Well it is Saturday night lock up. Like doing Porrige.

Steve.

Jeff Dayman03/05/2020 00:25:34
2356 forum posts
47 photos

I think it is someone's homework.

+ 1 for powered bellcrank or toggle clamps, used commercially made ones for years in fixture design in industry. No sane tool designer would use a clamp like the OP's sketch. If the V block is supposed to centralize the workpiece as they usually are used, putting a big force vector on it to one side is not a good thing. The part or the V block will likely deform in some sort of bad way if high forces are applied at angles as the OP's sketch.

Good commercial bellcrank / toggle clamp at link below just FYI

**LINK**

Nicholas Farr03/05/2020 08:20:38
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3988 forum posts
1799 photos

Hi, if the link is pivoted at both ends, as the drawing suggests, then the top bar will still move to the left and maybe even bend the piston rod if the cylinder isn't pivoted at either the top or on the base. If the link is fixed to the top bar, then the cylinder will have to be pivoted or the piston rod will bend. It is a bad design/idea. See my rough & ready sketches below assuming the link is pivoted at both ends and the cylinder is fixed.

sketches001.jpg

Regards Nick.

P.S. I also think the cylinder is at a mechanical disadvantage, the "link" would be better if it is closer to the work, but you would need a longer travel for the cylinder piston/rod.

Edited By Nicholas Farr on 03/05/2020 08:38:52

SillyOldDuffer03/05/2020 17:38:11
10668 forum posts
2415 photos

Just out of interest I wondered what Fusion 360 would do with this problem. Created a model clamp, dimensions correct but omitted the cylinder & piston in favour of simulating a 9815N up pull on the end of the lever.

clampmod.jpg

Running the simulation doesn't answer the exam question! At least, not in the local solver. (The paid solver in the cloud might do more, but I'm too mean to try it out of interest.)

Rather than produce actual stress values it colours the model to show where the stress is, going from RED (will break) to BLUE (no problem),

My safety simulation goes red on the axle pin, which I modelled as 10mm diameter mild-steel. (The whole model is mild steel apart from the job, in Brass, cause I like shiny things!

clampsim.jpg

The displacement analysis below shows the lever arm bending but the exciting axle problem is hidden from view. The thin end of the 20 degree arm is going orange, indicating that this too is bending too much for comfort.

clampdisp.jpg

Not sure at all I've modelled this correctly, because the arm has penetrated the job but - without having to do complicated maths - the pretty colours tell me the axle must be beefed up considerably and that the arm needs attention too, especially at the angled end.

So Fusion 360 looks good for checking the strength of designs whilst being no good for doing homework needing numeric answers! Though I'm confident Duncan can do the sums, Fusion doesn't help me confirm or refute his answer.

Dave

duncan webster03/05/2020 18:38:34
5307 forum posts
83 photos

The stress in the pin may well be too high (in fact I think it is if using EN3) but if SOD's FE program is anything like the one we had at work, red just indicates the highest stress, not necessarily that it is too high. Many Chief Engineers could not get their heads round this, red = danger to them

Roger Best17/07/2020 22:48:32
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406 forum posts
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Posted by duncan webster on 03/05/2020 18:38:34:

The stress in the pin may well be too high (in fact I think it is if using EN3) but if SOD's FE program is anything like the one we had at work, red just indicates the highest stress, not necessarily that it is too high. Many Chief Engineers could not get their heads round this, red = danger to them

Good joke, I laughed as this has been my experience too.

The wife asked why I laughed, I said "CAD joke".

That required extensive explanation. laugh

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