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# Testing the coefficient of friction of materials.(5)

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 ryanmcarthy (Mechanical) 7 Aug 12 6:34
 Hi, Do you think it would be practical to DIY a test rig to calculate the coefficient of friction for certain materials? Mainly food packaging. (Foil bags). For example if you clamped the foil material to a flat horizontal surface, sat a weight of 5Kg on top of it (metal block?), and used a dial force gauge to measure the force needed to move the weight, could you then calculate the coefficient of friction using the formula Mu = F/N? Or any other pointers/suggestions would be greatly appreciated. Thanks a lot
 ryanmcarthy (Mechanical) 7 Aug 12 6:49
 Also, another issue I'm not sure about. I've said about pulling a 5Kg metal block across the surface of the foil. Does this have any implications of working out the coefficient? Or would it be better having the metal block with the surface touching the foil actually covered in foil too? I was half thinking of trying to make someting like this Link, but I am unsure of how to get the most accurate readings from it? Thanks
 TomDOT (Materials) 7 Aug 12 8:45
 At its basics, a friction test typically is basically a horizontal drag test using a force gauge. Getting as close to horizontal as possible is quite important - you don't want to pull your drag sled up or down. While probably the most common facing material is an ASTM spec rubber of particular durometer hardness (which I don't recall offhand) your contact surfaces should represent the friction situation(s) you expect in the field. Will your food bag be sliding off other food bags? Will it be sliding down a polished steel chute? Will it be sliding off a rubber conveyor belt? When I did deck surface friction testing for a Navy project, we were required to have the testing performed with sled facings of shoe leather and also rubber. The tests were performed dry, wet with seawater and wet with specified oil. You also might consider whether static vs dynamic friction matters in your case.
 ryanmcarthy (Mechanical) 7 Aug 12 8:55
 Thank you for your reply. The food bag would be going over a rippled stainless steel tube, so I guess the test block should be of the same material! Which wouldn't be an issue making it to the same spec. The test would only need to be done dry luckily. The test I described above would be static, but as the film does move constantly, I guess a dynamic test would be needed. How would I achieve a dynamic test, as pulling the block by hand would always give me a different velocity really? Thanks
 TomDOT (Materials) 7 Aug 12 9:09
 I agree, replicating that stainless steel tube's surface for your sled (test block) would be ideal. You probably want both static and dynamic friction, for "bags moving" and "oops, bags got stuck/blocked/whatever" When I performed friction testing, we had a load frame take care of both the pulling and force measurement, so we had a very consistent pull rate. If you're into "rolling your own" - the print head out of an inkjet printer is typically moved at a constant rate. You might be able to hack something up cheaply that way. If you need a cert - well, there are commercial labs which perform friction testing.
 ryanmcarthy (Mechanical) 7 Aug 12 9:15
 Hi, We wouldn't require a cert luckily. We just want something in-house to test the packaging, as it should come from the manufacturer with a certain coefficient of friction, but occasionally we have problems with the foil not running through the machine, and you can feel that the foil is a lot more 'sticky' if you rub it together. If we query this with the manufacturer we get told it is in spec. I have looked at test machines, but they cost thousands to buy which we can't afford. Do you know of any companies which could provide/make something cheap? Thanks a lot
 TomDOT (Materials) 7 Aug 12 10:08
 You will probably do well enough with a force gauge and pulling your sled carefully by hand.
 MintJulep (Mechanical) 7 Aug 12 10:12

#### Quote:

you can feel that the foil is a lot more 'sticky' if you rub it together

That may be due to static charge.

The simplest possible friction test machine is an inclined plane.

This is a fancy one.. But you could make your own,
 ryanmcarthy (Mechanical) 7 Aug 12 10:47
 Thanks again Tom. MintJulep, I can see how that works, as in the greater the angle needed, the higher the friction. But how do you calculate the coefficient of friction from it? I presume there must be a formula which includes the angle? Thanks a lot
 MintJulep (Mechanical) 7 Aug 12 11:02
 Resolve the weight vector into components normal to and parallel with the inclined plane.
 ryanmcarthy (Mechanical) 7 Aug 12 11:15
 As in: Fparallel = m • g • sin (XX degrees) = xxN Fperpendicular = m • g • cos (XX degrees) = xxN m = mass of block g = gravity Then that can be applied to Mu = F/N? Thanks a lot
 MintJulep (Mechanical) 7 Aug 12 11:28
 Right. And if you remember that Sin(α)/Cos(α) = Tan(α) you'll see that it resolves to μ=Tan(α)
 TomDOT (Materials) 7 Aug 12 11:39
 The link to the expensive inclined plane reminded me - is humidity constant in your plant? Changes in humidity can alter friction.
 KENAT (Mechanical) 7 Aug 12 11:58
 Doesn't the inclined approach do a better job of measuring static friction though? May be good enough for your application though. If you want dynamic then pulling on a horizontal might be simplest approach. You could always pull using a 'string' over a 'pulley' loaded by a mass. Obviously to directly measure dynamic friction you'd have to 'nudge' the sample to get it going. Or you could put a newton meter in the line of the 'string', once it got going it would obviously accelerate but at least you'd be getting a constant pull force which may make the meter easier to read. Funny, this reminds me of experiments we did in higs chool physics, in fact I'm pretty sure my then girlfriend did the inclined plane version. Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you) What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
 MintJulep (Mechanical) 7 Aug 12 12:31
 To find dynamic friction using the inclined plane just needs a bit of trial and error. Smallest angle at which the mass continues to slide after given an initial nudge.
 KENAT (Mechanical) 7 Aug 12 14:02
 Yep, the nudge would work just as well on the slope. I was thinking about the constant force from mass over pully with force gauge when I wrote that line - doh. Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you) What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
 ryanmcarthy (Mechanical) 8 Aug 12 3:27
 Thanks MintJulep, it is very easy to calculate static μ on the machine really then! Tom, the humidity will be 'about' the same as it is just stored in a dry warehouse. Kenat, I wish I had done that at school, and I wouldn't have to be learning about these things now :) Can I ask, which might sound stupid, is Static friction totally related to kinect/dynamic friction? I.e If the static friction is high, the kinetic friction will also be high? (and vice versa). Thanks for all the help.
 patprimmer (Publican) 8 Aug 12 3:49
 They tend to follow a trend, but they certainly are not tied to a direct relationship. For instance, PTFE (Teflon) has an exceptionally low dynamic friction but only a fairly low static friction, especially after laying in place under compressive load as its soft surface conforms to the mico irregularities of even a highly polished opposing surface and it takes a bit of extra energy to break it free or to jump out of the surface pores. Regards Pat See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers & http://eng-tips.com/market.cfm for site rules
 ryanmcarthy (Mechanical) 8 Aug 12 7:45
 Thanks for that Pat :) The incline tester which MintJulep posted, is there something available like this in the UK do you know? I've tried searching, but cannot really find anything. Thanks
 Tmoose (Mechanical) 13 Aug 12 7:48
 I think applying specific load (psi, etc) similar to operating conditions during testing can be important
 TomDOT (Materials) 13 Aug 12 8:16
 Ryan, Since this is an informal test, I don't think you really need to buy that commercial incline tester. About all you really need is a protractor to find the angle, a plane constructed of whatever surface your bags are sliding on, a few clamps and a handy post/pole/chairleg to attach one end of your slope. Alternately, just a protractor, plane constructed of your surface and a big stack of books (varying thicknesses.)

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