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chriscad (Industrial) (OP)
1 Jun 07 10:15
I need a value for the coefficient of friction between ABS and stainless steel please, even a ballpark figure would be enough to get me started. I have had a good look on the net and several plastics site but the mix of plastic metal seems alien.

Any help and sources would be appreciated.

Thankyou
CoryPad (Materials)
1 Jun 07 10:33
Ballpark: 0.25 ± 0.15

Decent site:

http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm

Suggestion: contact ABS manufacturers (BASF, GE Plastics) or conduct your own testing.  All you need is a fish scale or a method to determine the angle of a ramp.

Regards,

Cory

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Demon3 (Materials)
1 Jun 07 10:43
This link shows that the dynamic coefficient of friction for many common thermoplastics is about 0.05.

http://www.machinist-materials.com/comparison_table_for_plastics.htm

http://www.machinist-materials.com/images/Coefficient_of_Friction.gif

This page specifically gives a value for ABS

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=POMNIA41

COF 0.5 on dry ground steel; load =0.05MPa; speed =0.6 m/s

There will be no accurate value for ABS because ABS is not one material but is very different depending on grade and manufacturer. For example it is made in very different ways by a totally different process if you compare Dow and BASF ABS. In addition to that ABS made by emulsion polymerisation will contain plenty of residual surfactant (such as stearate) that will act as a lubricant and change the coefficient of friction. Additionally, exposure to water would tend to leach out the surfactant leading to a changes in coefficient of friction over time.

As ABS is rubber in SAN you might look for values for SAN and assume ABS is similar.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

Demon3 (Materials)
1 Jun 07 10:46
Nice idea Cory. I have something like this from Bosch that will do the job nicely!

http://www.amazon.com/Bosch-DWM40LK-Digital-Protractor-Extension/dp/B000BD7ADQ

A digital spirit level. Just increase the angle until it slides.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

CoryPad (Materials)
1 Jun 07 10:54
That is a nice product - I have never seen one like that as an off-the-shelf item.  I have used a custom made one that had a motor to change the angle from zero to theta, where theta was the angle reached when sliding was detected by an integral sensor.

Regards,

Cory

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chriscad (Industrial) (OP)
1 Jun 07 10:56
Thankyou CoryPad for such a quick response. The range is a bit broad. You obviously have a better grasp of the table than I do. I was thinking about maybe swapping the s/steel for chrome plating. The table was a bit odd as I expected chromium to be a lot lower. I was always told that chromium had one of the lowest coef of metals.

What would you estimate the coef to be in that combination.

CoryPad (Materials)
1 Jun 07 11:21
I don't think a single number will do you much good.  Friction varies widely based on materials and conditions, so you should ensure that your design can handle varying friction levels.  I really think if this is critical, you need to perform testing.  If it isn't critical, then aim for something in the middle (like 0.25) and then go do something else.

Regards,

Cory

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chriscad (Industrial) (OP)
1 Jun 07 11:43
Its a small plastic pendulum mounted on a turned and polished shaft. It just hangs in space but it does need to hang freely like a plumb bob. At small angular changes the starting torque is too high and we cannot lubricate the assembly so we have to work with dry static coef's.

We can change the injection moulding and we are considering UHMW polyethylene (0.12). PTFE (0.04)and PFA (0.06) are too expensive and bring other problems. The shaft can be specified with chrome plating (0.13). I expect that combination would give us a static coef in the area of 0.15 which is our next comparison.

We can also consider turning the shaft in PTFE or similar but metal is preferred because of the vendor.

Can you suggest other combinations that wont break the bank.



Demon3 (Materials)
1 Jun 07 11:52
You said you can't lubricate it. Does that include "dry" lubrication like PTFE particles applied from a spray can?

http://www.lubricants.co.uk/ptfeandgraphitespray.html

The PTFE is carried in a solvent so that you can spray it. So if you use ABS be careful to test it to make sure the solvent doesn't induce environmental stress cracking in the ABS. A test would be to spray it on the ABS, let it dry, then hit it and see if its impact resistance is retained after exposure to the solvent.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

chriscad (Industrial) (OP)
1 Jun 07 13:27
The solvent issue was expected to be a potential long term problem and so the decision was made to look at what we thought were more familiar solutions. It does set me thinking though and I will follow that up next week.

Many thanks to you all. That was really useful and I do appreciate your help.

Chriscad
patprimmer (Publican)
1 Jun 07 19:07
Use Acetal or POM.

It is a lot harder and a bit heavier than the materials you have mentioned.

It is relatively cheap, easy to mould, has very low static friction (your problem will be static, not dynamic friction) and has a self lubricating surface. It is also resistant to virtually all solvents.

It can be improved by the addition of PTFE powder and silicon oil to the compound, but at a cost of course.

It performs particularly well with nylon as the other surface. Molybdenum disulphide and graphite works well at improving bearing qualities of nylon.

As nylons swell a little on absorption of moisture, this should be taken into account with the design if nylon is used for one component.

DSM Engineering Plastics, LNP, and RTP Company all have tribology data on a large range of polymers in contact with other materials. Reading the data will reinforce the need to test as the numbers change a lot with variations in conditions

Regards

eng-tips, by professional engineers for professional engineers
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Demon3 (Materials)
2 Jun 07 5:00
I totally forgot to mention there is a masterbatch available for ABS that gives low coefficient of friction. It's composed of 50% ultra high molecular weigh silicone in ABS (other polymers available too). Pat's suggestion of POM is a good one too and if you need to improve further on the neat POM you can get the silicone in POM too.

I have tried it in PP, SAN and ABS and it works very well at low addition levels. Because it's UHMW you don't get migration issues like you do with low molecular weight silicones. Wacker has some analogous products.

http://www.multibase.com/siloxane_masterbatches.htm

If you don't find the ABS masterbatch then e-mail to Multibase, I know they have it.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

chriscad (Industrial) (OP)
2 Jun 07 5:14
Guys, this is superb. Thankyou very much.
IRstuff (Aerospace)
2 Jun 07 13:34
What about some other materials like graphalloy, which is supposedly self lubricating, or some specific plastic bearings?

http://www.graphalloy.com/html/bushlist.htm

or:
http://www.oilitebearings.com/calculator.php?measure=metric&type=plain

TTFN

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