[Justin Stiltner]

OK,
a bit of terminology first

Back drive: forcing the motor to turn in a manner that it would not normally, for example when you push your robot across the floor without powering the motors, you are back driving the drive train motors.

Now, it sounds like you want the motor to pull the claw closed, and the springs to pull it open. Keep in mind that all the motors in your kit (except the servo, which is different) are reversible, meaning you can control the speed, and the direction of rotation. So in this case you would simply have the motor rotate say forward to pull the string onto the drum(shaft in your case) to pull the claw closed. Then to open the claw the motor simply changes its direction and un-spools some rope from the drum.

Now to use the servo in that same operation you would need to pull enough rope in 180 degrees of rotation to close your claw, I am assuming you are using this claw to hold the tubes, in which case I suggest you look at a different motor.

For an example of the motor reversing I was speaking of, take a drill and put a pencil in it where the drill bit would go. Now wrap a string around the pencil a few times and slowly pull the trigger to have the drill spin, the string will wind around the pencil. Now hit the reverse switch on your drill and spin the drill as you pull on the string, now your claw would be opening.

For further visuals do a search for winch on Google or you tube.