If looking up a friction coefficient in friction tables, they vary sometimes, or a friction interval is given. So, I am wondering which friction coefficient I have to apply ? Are there any rules of thumb therefore ? Should I take an average or ...
If you need a typical value, say for a homework assignment, take the average. If a surface is being freshly used for "braking" use the higher value, wearing into the lower value. If it is a life-critical application take the value that will cause the most harm.
They are empirical relations, established by Nature, without anything in the way of theoretical support. She likes wiggle room.
When I started my career a wise senior design engineer told me to never count on friction to help you and always assume it will hurt you. In other words, never use the average friction values, use the min or max depending which is most conservative for the problem you are working on. For example, in designing a bolted joint, use the max friction value to determine the minimum pre-load for a given bolt torque. Then use the min friction to determine the maximum tensile stress in the bolt at the same torque. If you can't live with both extremes redesign the joint.
I thank you for the information. So I always have to pick the most harmfull value ... Also thank you verry much for the example, Dave.
Another question : if friction is a critical value in an application, and I can't test it in the application itself, should I rely on a tribometer test ? Or is this a little bit 'exagerated' (is tribotesting just meant for scientific resource ?). In other words, can friction value be that critical that an exact value is needed (since we all use design factors, etc ...) ?
Any investigation you can do into the exact circumstances will pay off in added assurance. But bosses rarely tolerate infinite testing. Since every component in the final assembly, with its full range of surface finishes and manufacturing tolerances, added to all of its neighbor's variations in the assembly, added to every possible contaminant on surfaces, constitute the Universe that you will be investigating.
Where friction is that critical, safety wire is used, or constant supervision (such as applying brakes, or routine re-tightening) is instituted.