Coefficient of friction for rubber conveyor belt against steel idlers

A. Exact figures depend on the type of rubber and temperature, also, the steel finish - obviously if well used it will be "worn in" or "smooth".

Dry and 20 to 50 °C it will be 0.5 to 0.6.

Dry and cold it will be lower by a near linear factor depending on temperature -- colder = lower COF. At most 20% less.

Wet - water and certain liquids are often part of the system in conveying units due to local conditions - anti dust wetting or process chemicals.

The liquid generally acts as a lubricant and makes the steel more slippery down to about below 0.2 on rubber.

Actually as long as the bearings are fine on the idler you want grip as it gives you better support. When the bearings get fouled up you would want a very low COF so the belt slides and does not get damaged or impart heat into the steel idler.

This whole discussion is the subject of tribology -- for more info, tribologists can help.

Hope that helps!