I have a system that has several different sections wich different coefficients of friction. Most rolling. The mechanics go in this order: motor, gearbox, pulleys & wire rope, and rollers that align a vertical door in a track. So I'm going to say my lubricated metal gearbox is ~0.01. My wire rope & pulleys rolling friction is 0.002. and the polymer wheels on steel is 0.002 as well. How do I combine these to get an overall system coefficient of friction? add them? multiple? multiply squares & square root? Please advise... I have a sizing software program that I use, but I have to give it a coefficient of friction for the entire system. Thanks in advance!