Free-wheeling ball screws eliminate jammingKen Thayer | April 24, 2019
Free-wheeling ball screws offer an alternative to traditional ball screws with soft stops or limit switches. All ball screws operate on the same principle: The ball screw moves linearly relative to a nut as it is rotated. The nut contains balls that ride in helical raceways rolled or machined into the screw.
The difference between free-wheeling ball screws and traditional ball screws lies with the way the balls are arranged and contained in the nut. Most ball screws have a nut with helical grooves and recirculating balls. As the screw rotates, the balls are recirculated or fed back to the opposite side of the nut, either internally to the nut through passageways or externally through ball return tubes. In these designs, the balls continually recirculate through the nut assembly, as shown in the video below.
Instead of a standard nut with recirculating balls, free-wheeling ball screws feature a ball cage inside the nut. The nut features annular grooves as opposed to the helical thread found in standard ball screw nuts. The ball cage and balls revolve around the screw in a planetary motion as the screw rotates. The helical threads in the screw cause it to move linearly relative to the nut as it rotates.
When the nut reaches the end of travel, a pin through the screw contacts a flat cut into the ball cage, stopping linear motion. Planetary slipping of the nut in relation to the screw ensues, letting it spin (free-wheel) without linear movement. When the screw rotation is reversed the pin disengages and the end of the screw moves away from the stop pin until the stop pin at the opposite end of the screw is engaged.
In recirculating ball screws, limit switches or stops are required to prevent jamming in the event of overtravel. Free-wheeling ball screws eliminate the need for these safety measures due to the planetary slipping of the nut when it reaches the end of travel.
Efficiencies of free-wheeling ball screws are in excess of 90%, the same as standard ball screws. Screw leads between the two ball screw types are different, however. In a standard ball screw, the amount the screw moves through one full revolution is referred to as the screw lead.
Due to the planetary slipping of the nut relative to the screw as it rotates, free-wheeling ball screws exhibit what is referred to as an effective lead. The linear advancement of free-wheeling ball screws is proportional to and less than the lead of the thread in the screw and varies slightly with the load. Due to the inexactness of screw lead, free-wheeling ball screws are not recommended for precision motion control and are better suited for applications with fast travel rates, abrupt stops or with fixed repeatable stops. Some applications include bed and chair actuators or machine drives with positive stops.
Since free-wheeling ball screws rely on a minimum amount of friction to prevent slipping prior to the stop pin engagement, greases that promote low sliding friction should be avoided.
For additional information on free-wheeling screws, visit the following manufacturers: