2
$\begingroup$

Attached is an example picture of a clamp that will be attached to a pipe made of Aluminium 6061-T4. The clamp should have very tight grip on the pipe and prevent it from slipping or rotating but the Aluminium pipe is fragile and so the clamp cannot be tightened too much. Thats why I am looking for a material for the inner surface of the clamp that will provide a firm grip on the Al pipe without requiring too much tightening force.

The clamp-pipe interface material (shown here as the black rubber-like thing) should have a very high coefficient of static friction with Al6061 in order to provive a tight grip. I want to know what materials are best suited for this purpose? Which material has the highest coefficient of static friction with Al6061?

My research so far:

  1. The best material I could find is rubber that supposedly has a coeff of static friction of 0.8 with Aluminium. (source: http://atc.sjf.stuba.sk/files/mechanika_vms_ADAMS/Contact_Table.pdf)

  2. Surprisingly the coeff of static friction between Aluminium-Aluminium is 1.05 !! So does it mean I should use aluminium in the inner surface of the clamp instead of rubber? sounds counter intuitive. (source: http://www.engineershandbook.com/Tables/frictioncoefficients.htm)

  3. If I use a material like rubber or silicone, should I consider any texture on the surface? Are treads better than a smooth surface?

I am not an expert in these things and so I would greatly appreciate if someone can either suggest me a meterial or educate me on how to select the right material that will have the best grip. Clamp with rubber-like material for grip

Edit 1: To visualize, imagine the pipe is held vertically by the clamps and a weight is attached to the bottom of the pipe, what material at the pipe-clamp interface will allow me to hang the heaviest weight? enter image description here

Edit 2: The surface of the pipe is smooth to touch with matt finish, not mirror polish. Its actually anodized. Any damage to the pipe like denting or clamping marks is absolutely not acceptable. Also the pipe needs to be re-positioned occasionally, to do so, we loosen the clamp, slide the pipe to position and then tighten the clamp again

$\endgroup$
  • 1
    $\begingroup$ What is the surface finish of the pipe? Rough or smooth or mirror polish finish? Is any damage or evidence of clamping acceptable? What ae you doing to the pipe? One possibility is to insert an expanding collet to clamp it internally... $\endgroup$ – Solar Mike Nov 18 '18 at 13:32
  • $\begingroup$ @SolarMike, The surface of the pipe is smooth to touch with matt finish, not mirror polish. Its actually anodized. Any damage to the pipe like denting or clamping marks is absolutely not acceptable. Also the pipe needs to be re-positioned occasionally, to do so, we loosen the clamp, slide the pipe to position and then tighten the clamp again. $\endgroup$ – Phaser Nov 18 '18 at 13:49
  • $\begingroup$ Then add that detail to your original question so it is clearer. $\endgroup$ – Solar Mike Nov 18 '18 at 13:51
  • 1
    $\begingroup$ A taper lock collet may work... $\endgroup$ – Solar Mike Nov 18 '18 at 14:30
  • $\begingroup$ You can use anything you like that is softer than anodized 6061, you just have to calculate how much surface area you need to make it work. The range of frictions among materials intended to grip isn't worth worrying about. A pinch valve might work for you as well. $\endgroup$ – Phil Sweet Nov 19 '18 at 3:55
1
$\begingroup$

The anodising is for corrosion prevention, usually. In this case of have the 2 supports show free movement of the Al and clamp a collar around the bar above the top support. To increase the anoint of rich tin without damaging the surface would require 1 of 2 options:

1) Greater force; 2) Greater surface area.

Since you stated greater forces not possible you're left with option two.

Having three of those clamps that you have a picture of side by side above the collar would certainly help since the overall friction would effectively be tripled: 0.8×A

Remembering that stress is force over area and force in this case needs to be limited while maintaining sufficient stress (friction) to prevent slippage.

enterprising of clamp

| improve this answer | |
$\endgroup$
  • $\begingroup$ Multiplying the area over which friction force is applied increases the resistance to movement proportionally. $\endgroup$ – Rhodie Nov 20 '18 at 17:50
  • $\begingroup$ Thanks for the explanation. I was actually looking for a material for the inner lining of the clamps. Lets say there are 5 clamps with silicone rubber as inner lining. Lets say this can hang 25 kg. Now, what material can replace the silicone rubber to hold more than 25 kg? $\endgroup$ – Phaser Nov 20 '18 at 21:15
  • $\begingroup$ @Phaser friction coeficients are hard to come by as numbers as they apply by indiviual pair of surfaces, not by material but by the total sum of everything. They are very bad predictors as you can get different results just by changing manufacturing method. So its quite hard to come by the best of times since we would need the same pipe to verify assumptions. $\endgroup$ – joojaa Nov 21 '18 at 7:35
  • $\begingroup$ A material bonded to itself is the strongest bond but you are trying to avoid this as it would be a weld. Friction occurs at molecular level. In materials it is determined by the shape of molecules working against reach other in the elastic range. Rubber has a very high elasticity index but Al has a very low index comparatively. $\endgroup$ – Rhodie Nov 23 '18 at 1:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.