To save the pulley from eventually deforming by being always in contact with the rim, there’s a need for some mechanism to move it out of the way when not in use.
From the start I used a simple vertical hinge consisting of an urethane strip. With power off, a spring pushes the motor / pulley assembly away from the rim. When power is back, a solenoid retracts the spring and gravity pulls the motor down and the pulley touches the rim.
However, the pulley seldom contacts the rim in an absolutely perpendicular “true” way, ending up “riding” slightly the surface of the rim. This causes small speed fluctuations (wow) that sometimes can be heard.
To counteract this i devised a more complex clutch, where the pulley approaches the rim in a completely perpendicular / true way, only applying pressure when needed.
I'm quite pleased with the way it worked out. It looks good and it's noiseless when engaging / disengaging. In fact the pulley is always touching the rim, but pressure is only applied when On.
I still have to measure the final result using PlatterSpeed / Adjust+...
https://restlessengineer.files.wordpress.com/2017/10/20171013_194155.jpg?w=900
https://restlessengineer.files.wordpress.com/2017/10/20171016_093114.jpg?w=900