friction and roughness

[jeanpaul]

I am working in a company which use an old design of rubber keypad.
I fall on a "little" problem, sometimes the keys are sticking in their "barrels". These cylinders have a surface finish of the type "mirror polished" ie very smooth.
I have found expermimentaly that my rubber become very sticky against a very smooth surface. For example, against a greater roughness ( Charmille 27) the friction is well less.
Do you have some guideline for the choice of roughness because all my previous experiences with friction said me that the greater the roughness, the greater the friction.;-)

 

[Hush]

In general the rougher the finish the greater the friction, however there are some other things to consider.

A finish below about 5 microinch RMS (sorry I don't speak SI so well) can result in the rubber wiping the surface completely clean of any lubricants, i.e. you go from a 'wetted' friction to a dry friction.

Also, too smooth a finish will restrict air (or what ever media you're working with) from moving between the surfaces, in effect you create and pull against a vaccuum as you move the rubber. This can be considerable if you have a large area in contact.

There are a lot of factors in designing a good seal. Pressure, retained fluid, squeeze, stretch, to name a few> In general though if you require a seal in a dynamic operation then something in the neighborhood of 16 microinch RMS is a good finish. If you don't require a seal you probably still want to stay below 63 RMS to avoid excess wear on the rubber. Course, if you don't require a seal why are you using rubber?