# Avoid snapping an electric cable from too much rotation

I recently built the basic construction for a crane out of lightweight material. Basically the head of the crane contains an electric servo motor which spins the head of the crane by pushing itself away from the tower (the rotor is fixed to the axis which is fixed to the tower. The axis isn't fixed to the head in any way).

All electronics required to operate the motor and more are inside the head. The only thing I should extend to the rest is a 240 V cable.

Since the head is allowed to make multiple rotations (preferably limitless), how do I avoid snapping my cable by rotating it too often? I'm aware such solutions exist, but I have no experience in this area so I have no idea what to ask for specifically.

/---------------------\
|                     |
|        motor        |
\----------|----------/
|
___________|___________
\          |          /
\         |         /


The device you are looking for is called a slip ring such as the one shown below. These devices use internal components which maintain contact as they spin (old ones used brushes) allowing them to turn by an unlimited amount in either direction.

• That would be perfect. Is a slip ring the only solution or just the most common one?
– Mast
Jun 17, 2015 at 12:54
• As far as I know, that's the only solution for unlimited rotation. Note that there do exist slip rings that are doughnut shaped, if you need to run the shaft through the center. For limited rotations, there are a few other options, but they all require a system to keep track of the net number of rotations from 'zero' to avoid breaking the cable. Jun 17, 2015 at 13:03