Friction of rubber band on ptfe rollers

Hi,

I would like to know when my elastic will slip. This is because I want a roller track with 26 axles to be driven by rubber bands. The centre distance between the axles is 80 mm and the axles have a diameter of 50 mm. The elastic has a diameter of 8mm and will be made of the material rubber. When relaxed, the rubber bands are 348 mm long.

Now I want to know the force required to keep the rubber band from slipping. The friction coefficient of rubber on plastics is about 0.5.

Now how can I calculate the force at which the rubber elastic does not slip. This is because I want to take a rubber band of relaxed length 275 mm, you then have to stretch it extra to put over the axles, this increases the contact pressure as the rubber band wants to return to its original shape. Can anyone help me?