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Mech Eng question

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Wed Nov 25, 2015 1:08 pm PostPost subject: Mech Eng question
jockthedog
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A question for a mechanical engineer.....

I want to apply 200N of pressure between two steel plates, using a bolt through the plates.

Now I know thanks to Dr Newton that 200N would be (approx) equivalent to sitting 20kg on top of the top plate.

I looked at this http://www.engineersedge.com/calculators/torque_calc.htm
and input 200N, 0.2 coeff of friction for steel, 10mm bolt diameter (M10) and I get 400Nmm as an answer.

But that doesn't seem enough. 400Nmm is just 0.4Nm, which is not even finger-tight!
https://en.wiktionary.org/wiki/finger_tight
Is this really enough to exert 200N of force between the plates?

What am I missing?
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Wed Nov 25, 2015 2:47 pm PostPost subject:
alistairolsen
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I suspect it's about right, but that calculator is using a simplified formula T=cDP which wont work well at very low torques

The proper formulae look more like this: http://www.wtools.com.tw/Relation-Formula.shtml

But the real issue is in what you're trying to do, as you wont get a nice even clamping force using a single bolt and you wont get proper response from something as big as an M10 with that little tension either in terms of fastener design.

What are you trying to clamp and how?
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Wed Nov 25, 2015 2:53 pm PostPost subject:
jockthedog
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alistairolsen wrote:
I suspect it's about right, but that calculator is using a simplified formula T=cDP which wont work well at very low torques

The proper formulae look more like this: http://www.wtools.com.tw/Relation-Formula.shtml

But the real issue is in what you're trying to do, as you wont get a nice even clamping force using a single bolt and you wont get proper response from something as big as an M10 with that little tension either in terms of fastener design.

What are you trying to clamp and how?


Ta for that. I know it's not a simple one.

I'm compressing clutch packs to measure their depth under specific clamping force.

Spec says to compress to 200N, but it is using a special tool that I don't have. That special tool does just have a knob to tighten - no bolt as such - and I've seen other graphs etc which support the 0.something of a Nm to get a few 100N.
Just seems very little clamping indeed!
I don't have a torque wrench that goes under 9Nm, so I'll probably just find 20kg of steel to stack on top of the plate.
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Wed Nov 25, 2015 2:59 pm PostPost subject:
alistairolsen
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Its not really reliable anyway, even if it was an accurate calc, as the friction can vary so much. Personally Id be using something like an old drill press and some sprung bathroom scales as a load cell
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Wed Nov 25, 2015 3:34 pm PostPost subject:
jockthedog
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alistairolsen wrote:
Its not really reliable anyway, even if it was an accurate calc, as the friction can vary so much. Personally Id be using something like an old drill press and some sprung bathroom scales as a load cell


yes, the alt to 20kg of steel is to sit the whole thing on scales and use my press to exert the force.

Either way it's more complex than a few bolts but I reckon it'll be loads (sorry!) more accurate Smile
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Wed Nov 25, 2015 4:20 pm PostPost subject:
alistairolsen
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Aye, you can see here: http://www.trfastenings.com/pages/Pre-Load+and+Tightening+Torques+for+Steel+Fasteners+Standard+Metric+Threads

That even M1.8 would do for 200N with a torque of 7Ncm.

Even something small in the mainstream sizes like a grade 8.8 M5 is normally torqued to about 6Nm and good for a clamping load of 6500N or 2/3 tonne
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