How to estimate compression force to cause a certain amount of compression in neoprene

[Theozz]

Hi ALL,

Could any of you please throw some light on this? I'm trying to estimate clamping force. Initially, I need to know how much force is required to compress the neoprene between my clamp and tubular section. I have properties of the neoprene however do not know how to compute the compression force required to cause the maximum compression on the neoprene.

preporties of neoprene
Hardness = 70shore
Elongation = 300%
Compression set % = 35
Tear strength = 20kg/cm
Tensile MPa = 5
Neoprene thickness = 25mm

The OD of my tubular section is 406mm

Many thanks.

 

[SlideRuleEra]

Hardness = 70shore

If you know whether this durometer value is Shore A or Shore D, find Young's Modulus by using the math from "How to Convert a Durometer to Young's Modulus".

Then, find the spring constant by doing the math for "Young's Modulus as a Spring Constant"

In compression, Neoprene behaves pretty much like a metal spring. Use the calculated spring constant to calculate clamp force needed to obtain desired deflection.

http://www.SlideRuleEra.net

 

[Theozz]

Thanks SlideRuleEra

I derived the stiffness and computed the force required to cause 5mm compression of the neoprene. What I'm not sure about is the bearing bearing area, see attached my calc. Also, would you expect the 5mm depression to occur all round the tubular?

Is it then right to divide the derived force by the number bolts?

Thanks again

 

[Theozz]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1560241775/tips/Estimation_of_Neoprene_Compression_ifhe35.pdf[/url]

 

[3DDave]

The problem is that elastomers aren't compressible. They are elastic. Putting a large amount of material with a small leak path makes the resistance to deformation depend on the friction that resists extruding it out the ends of the confinement.

 

[Theozz]

Thanks 3DDave,

Your post contradicts SlideRuleEra's. Would you recommend that I assume that neoprene is infinitly stiff in compression? In design of a clamp, a small gap need to be allowed between the flanges of the clamp to ensure that the clamping force is effect to cause enough grip. Where neoprene is used and assumed to infinitly stiff, should it comppress and the gap closes and the flanges touches each other the grip force will not be adequate.

All, your thought on the above is welcome.

Many thanks.

 

[rb1957]

are you compressing the neoprene against a flat flange (easy to calc), or compressing it around a tube with a belt clamp (and trying to figure the load on the belt clamp screw to compress the neoprene … not so easy to calc) ?

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

 

[Theozz]

https://res.cloudinary.com/engineer...Estimation_of_Neoprene_Compression_ifhe35.pdf

Thanks rb1957
The neoprene is being compressed around a tubular with a circular clamp - see calcs in link above

 

[SlideRuleEra]

Theozz - Compressing Neoprene by 20% (5 mm deflection for material 25 mm thick) is probably too much for simplified calcs like those in the links I referenced.

Also, the shape of the Neoprene sleeve will complicate any calcs. Neoprene does not really compress like a spring. It's volume remains constant, but for simple shapes, like a bridge pad, it bulges at the edges... sleeve can't really do that.

Also, agree with rb1957's comment about how clamping force is applied.
The way I interpret your sketch, clamping force reduces the circumference of the Neoprene pad, but compresses the radius.
In other words, to get 5 mm of compression the diameter will have to be reduced by 10 mm (5 mm on each side of the pipe).
This means the clamp will have to move (reduce circumference) by 31.4 mm (10 mm x pi). The clamping should have a mechanical advantage about equal to pi.
IMHO, the calcs needed are too specialized and complicated for a simple answer

For general info on simple compression see: Link

http://www.SlideRuleEra.net

 

[Theozz]

Thanks SlideRuleEra,

Given the fact that the neoprene will not compress excessively, I have allowed 10mm gap between the flanges of the clamp with the assumption that full grip between the tubular and the neoprene would have been achieved before the 10mm gap closes. I have derived how much torque on the bolts is required to cause adequate clamping force on the tubular based on friction between the neoprene and the tubular.

I see my approach as a test case for the mechanism.

Thanks again for your comments, mcuh appreciated; I'll update all on how I get on with it and share some pictures too.

Theoz