Engineering Senior Design Project Motor Calcuations

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Hey, folks! I'm a junior at UMaine starting my senior design project. I'm an EET major and apparently did not pay close enough attention in my rotating machines class because I'm struggling with some motor sizing calculations. I'm hoping you guys can give me a hand or at the very least point me in the direction of where I can track down the information.

So for my senior project we're building a machine for spooling out user-programmed lengths of wire. Below is a basic cad drawing of one concept:


So, brief explanation: there is a spool on either side. The cable is thread through the three pulls in the middle (those aren't set in stone, but they were seen in my artistic 'mind's eye' so I've put them in there as a way to show the path for the cable.) One of the spools--the one to which the cable is traveling--is motorized. A (stepper/dc motor) turns that spool, raveling the cable around it. A braking mechanism of some kind maintains a bit of tension on the feeder-spool to keep it from unraveling once the receiver stops turning.

So, for the motor that is turning the receiving spool, how do I calcuate the initial torque required to overcome the friction/torque of the spool at rest? Is my question making sense? Part of the this project is the documentation so I need to explain WHY I chose a motor of a certain size. I know I can just grab a big motor and say 'well, I just felt like it would be big enough from past experience' but I really should be able to show the calculations to back up my claim.

Does anyone have any help to offer? Feel free to ask me questions regarding any other data required to answer this inquiry. Thank you for your time!

[jboggs]

Why are you driving more than one motor? Why can't the receiving spool, known as the "take-up" in the industry, be the only driven one? You can still measure the length of the wire with an encoder on one of the rollers in the middle.

By the way, have you reviewed the homework policies of the forum? You should before you expect too much help.