In addition to our full line of recirculating ball screws, we also offer a free-wheeling ball screw assembly. The free wheeling screw (also referred to as planetary or epicyclic ball screws) is different from a standard ball screw in that it utilizes a ball cage (retainer) inside the nut. As the cage contacts the stop pins in the screw at the ends of the stroke, the ball nut will stop linear movement but the screw will continue to rotate (free-wheel). When the screw rotation reverses, linear motion occurs away from the stop pin and will travel until the cage contacts the pin at the other end of the stroke.
The advantage of the free wheeling screw is that limit switches or other types of stops are not necessary. This eliminates the possibility of over travel which can cause problems with many applications. The controlled stroke feature is used in many applications such as bed or chair actuations, trim tab actuators and electrical switching devices.
The free wheeling screw operates with the same efficiency (>90%) as a standard ball screw. Due to the planetary slipping of the nut in relation to the screw, there is an effective lead that is different than the actual lead of the screw. The effective lead is always less than the actual lead and varies with the direction and magnitude of the load. Since the lead is a variable, this device is not recommended for applications that rely on rotation of the screw for position feedback.