Most actuators move the valve, sluice or penstock by rotating a machined drive nut in the actuator, this, in time can wear out and a specialist replacement part is required, machined to a ACME, Trapazoidal or other thread form.
The picture below shows a typical penstock spindle operated by the Actuator drive nut to open a flood prevention control valve, and examples of different types of blank drive nuts ready for machining for spares.