friction between teflon and delrin

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They compound plastic with various loadings of teflon, and other fillers. There is much technical stuff on the site about the friction between materials, including metals as they work with various plastic compounds.

- Billy Hiebert HIEBERT SCULPTURE WORKS Small part plastic > Does Teflon against Delrin have less friction than Delrin against polished

Does Teflon against Delrin have less friction than Delrin against polished stainless steel?

Thanks

Dear Alex:

Does hydrogen bonding mean anything to you? Polymer to polymer friction, I would expect to be higher than polymer to metal (or really ceramic), for this reason.

For a really low friction coefficient, try an air bearing.

David A. Smith

Why don't you just make a small experiment, that's what a scientist would do. They would just slide the sample down an inclined plane.

I'm sure the differences will either close or far apart. It doesn't make a difference; because if they are far apart it makes it easy to choose.

If they are close it probably doesn't make a difference which one you use. However if it does make a difference you can refine your experiment and repeat it a few times to help make the best selection.

Heck, you might learn something. *Smile

Regards,

Stan-

Stan, I believe your experiment has one flaw. There are sometimes big differences between static friction (object at rest, also called stiction) and dynamic friction (object already moving) The angle which causes the object to start moving is greater than the angle needed to keep it moving. Therefore you only get an estimate of the static friction this way. To measure dynamic friction you'd need to use a horizontal surface and measure the pulling force while moving at a constant speed. In motion control static friction (stiction) is often the most problematic. It takes more force to get an object moving, and the suddenly that resistance is greatly decreased. The control must then deal with a different set of force values than what was required to get the object moving.

Gary H. Lucas

Now decrease the angle until it just slows and stops. The two angles give you both static and sliding coefficient of friction.

Ted

The original poster did not say whether he was concerned with static or dynamic friction.

Dynamic friction is not always related to static friction. Teflon to teflon is an example. It is about the only combination where the static friction is equal to the dynamic friction.

My guess is that Deldrin and teflon will be the lower than stainless and Deldrin in both static and dynamic. But that is just a guess.

Dan

As an aside, I once worked with an interesting example of static friction: In stand-alone servo-hydraulic controllers there is a dither setting. I had to re-create it when programming a servo-hydraulic unit to control position on a filter roller to ~0.001" (measured by LVDT). The dither causes a small percentage fluctuation in the output driving the servo spool valve. I had to play with the percentage until I got a smooth response to a real change in output. The dither overcomes the breakout (or static) friction of the o-ring seals on the spool valve. This allows a real output change to actually move the spool, effecting a change in position. IIRC, +/- 5% dither worked well.

Otherwise, the spool would sit there as the output increased until a significant error had raised the output to a level required to overcome the o-ring friction, causing the valve to overshoot.

The dither worked well. The servo didn't control at all without it.

Regards,

Pete Keillor

Dear Peter T. Keillor III:

And just for completeness, not all servo valves require this much dither (they may not have a "moving" o-ring). Also, as the o-ring abrades and hardens, this much dither may be too much and may be too little.

David A. Smith

Dither is used to supply holding pressure on both sides of the actuator piston. Without dither the piston would drift in or out until the system and valve would correct the drift error. Dither is used in air as well as hydraulic actuat4ed diaphrams as well as piston type actuators. The dither removes the deadband where no valve is open. For precise motion the ratio of the amount or time between the open condition of the two valves determine the movement and rate.

John

I did this once to study low friction material pairs and also found that cleaning made a huge difference from one test to another. I agree that well designed tests will tell you exactly what you need to know. Joe Carter

Did you try a UHMW/Teflon pair?

Is it preffered to minimaze contact surface? I can put the carriage on polished ss balls(not rotating) they will slide on UHMW bar The other option more expencive is two ss rods with UHMW carriage sliding on them.

What's is preffered in terms of friction?

How does the pressure on UHMW/SSteel pair affects friction?

Thanks

Alex, It sounds like you want minimal clearance, so you are providing screw adjustment of the clearance. I believe this design with metal surrounding UHMW is going to give you no end of problems. UHMW is not a real stable material, and plastics of all kinds expand an contract much more than metals. So a clearance that will be just right on a cool low humidity day will be way too tight on a hot summer day. I experience this problem with my own design, and have make the plastic parts sufficiently undersize to avoid binding. It's a real simple mechanism, and a real pain in the ass to get to work properly if the part tolerances aren't held.

Gary H. Lucas

I don't have a formula handy, but I will tell you that I have seen UHMW parts as small as .250 x .250 x .125 thick move .015 from one day to the next. A part the size that you describe most likely will move a lot more. The other problem that you encounter with this material is that the material movement will not be consistent. Some sections may move a little and other sections may move a lot. I believe you would be much better off choosing another material. This material is best used on wear pads where the change in size has a very minimal effect on the application.

Rick

Friction by definition is area-independent. In my experience, though that is an approximation and doesn't apply when pressures are very high for the material (yield point? I'm not really sure, but friction coefficient seems to increase for materials as pressure reaches high levels).

What is usually preferred is friction as low as possible on motion surfaces.

Joe Carter