Motor twitching and erratic behavior when using Master Prime lenses with follow focus systems

Gear

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Ultra VestWhich vest should I choose for my body size, comfort and operator type?39

2008Follow Focus forum

18 years old. Gear and prices may have changed since.

A follow focus motor mounted on support bars may exhibit violent twitching and rapid oscillation when used with certain lenses, particularly Master Primes, while functioning smoothly when handheld or when used with other lens types.
Applying manual drag to the focus ring can temporarily stop the twitching behavior while the motor is engaged.
The root cause can be a loose potentiometer gear inside the motor that slips on its drive gear, causing uncontrolled acceleration during calibration or operation.
Lenses with higher focus ring resistance, such as Super Speed lenses, may not trigger the twitching because the increased drag prevents the potentiometer gear from slipping.
When a potentiometer gear has loosened, tightening it may resolve the immediate issue, though the potentiometer itself may require eventual replacement.
The problem can occur in multiple motor models and may develop over time with various lens types.

The issue appears specific to certain motor models and may be related to internal wear rather than a design flaw present in all units.
Testing and repair require opening the motor casing, which may not be straightforward on all models.
The long-term reliability after tightening a loose gear is uncertain; potentiometers are consumable components with finite lifespans.
Power supply and cable quality were suggested as potential causes but were not confirmed as the root issue in this case.