Rotary solenoids use the same principle as linear solenoids, but the core in linear solenoids exerts a push-and-pull force rather than a rotational force.
How do rotary solenoids work?
When an electrical current is applied, the metal core is drawn backwards into the coil. With the help of ball bearings, the disc rotates towards the positive magnetic field, giving the device rotational force. When the current is switched off, a spring on the disc swiftly rotates it back to its original position, also pulling the core out of the coil.
What are rotary solenoids used for?
Rotary solenoids were originally developed for use in defence, but are nowadays often found in industrial machinery such as lasers and shutters, as they're more robust than other types of solenoids.