[rsisk]

We are having a similar problem with our arm. We are adding a disc brake from a bicycle. When the arm should be stopped, we engage the brake. When the arm should be moving, we release the brake.

Obviously there are a lot of implementation details that have to be worked out.

We are using an avid bb5 brake, about $45 on amazon
The disc is fixed to our dead axle using a keyed hub bolted to disc
The calipers are mounted on the arm so they turn around the disc
A cylinder is mounted on the arm to close/open the calipers
And the magic happens in the programming
When the motor is moving the calipers a re released
When the motor is stopped the calers are engaged