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Material with high coefficient of friction and good structural integrity for friction washer.

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Charon99

Charon99

Industrial
Nov 10, 2006
22
I’m looking for a material to give me high coefficient of friction (against steel) and good structural integrity in a 20mm o’d ish washer. The washers will be clamped against the steel offering functionality like car brake pads on a disk. But cheap, simple and small.

Any advice the best material would be much appreciated. Ideally the washer shapes would be pressed from sheet.
 
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Aluminum

The problem with sloppy work is that the supply FAR EXCEEDS the demand
 
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The material would be clamped between a steel washer and the sliding face with a nut. More tension on the nut would result in more friction. I don’t want the material to crush or disintegrate over a short time. It will only slide 20 -50mm once a week so long term wear isn’t an issue. Positional placement needs to be fine so no judder in movement. I want high coefficient of fiction because there is a large lever moment it must resist so the nut will be tight.
See sketch attached. The lever length can vary so the tension of the nut would be set to suit and give a good “feel”. Resistance to movement needs to be positive but easily adjusted.
 
 https://files.engineering.com/getfile.aspx?folder=8ffa8774-712b-44b7-9f7d-07fc9e18557c&file=IMG_1545.JPG
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A material like a brake pad will wear becoming thinner. You'll lose your preload. You might need something like a belleville washer to try to maintain preload.
 
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For the application you described you actually want something low friction. The closer the static friction is to the kinetic friction coefficient, the less break-away force you'll need vs the sliding force. Oil impregnated bronze may be a good choice here as well as a few hard plastic options such as UHMWPE.

Also, use the nut to compress a spring against the washer. This will provide much more uniform sliding over a wide range of conditions as well as compensate for wear.
 
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Picking up on TugboatEng spring idea, you could use Bevel Washers.
If you need stack them in opposing directions to gain some travel if you need that.
 
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A colleague of mine at my old company said they experimented with sandblasting materials to get the COF higher. It worked - until they slipped the first time. The slipping smoothed out the roughness. I don't remember if they tried hardening the surface after sandblasting.

You might have to resort to a bicyle type brake and handle actuator to get something that has a big difference between static and dynamic friction. That will only increase your cost by a factor of 10. [bigsmile]
 
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BrianE22 said:
You might have to resort to a bicyle type brake and handle actuator to get something that has a big difference between static and dynamic friction. That will only increase your cost by a factor of 10.
Click to expand...

You could also use a cam lock similar to a bicycle skewer, which would be cheap and very adjustable.
 
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Just regular flat washers work fine for this.

Stacked with some Belleville springs to keep the force sort of consistent.
 
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Thanks all,

The plan was always to use belleville washers and a high friction washer of unknown material. Perhaps my error is proposing a high friction material.

A similar mechanism has a thumb screw and steel washer that never works well. Unable to set the pressure accurately and needs a better friction interface. It's semi ok to lock off, but on the fly sliding adjustment is poor. We will try low friction Pa6 (it's readily available) or a hard urethane.
 
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The problem with just a screw for force adjustment is that when the stack is very stiff then a very small turn of the screw makes a large change in force.

The Belleville washers solve that problem.
 
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Hi Charon99

Could you use a compression spring on the thread/nut adjuster, so that you tighten the nut and compress the spring, which then changes the force on the washer altering the friction?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
 
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Desrtfox, A compression spring would be good theoretically but in this instance it would be too thick. :)
 
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Hi Charon99

It was just a thought, do you need a lot of these washers? If it’s just a few you could design a pre bent single leaf spring, I done that sort of thing for holding a aluminium light reflector in a ceiling lamp.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
 
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