Image courtesy of Steinmeyer Inc.
Ball-Screw Design: The Advantages of Internal Ball Returns Exterior tube returns create noise, slow the balls down, and cause jams. They are also a source of lubrication leaks. Internal returns solve all these problems and much more. Lead screws are one of the most common linear-motion devices, thanks to their low cost and reliability. And recent advances in modern machining can make them extremely precise with lead errors below 1 micron. That’s why they can be found in linear drives for machine tools, aerospace controls, precision stages, and a host of industrial devices.
However, lead screws are fundamentally limited. The sliding between the threads of the shaft and nut generates high frictional forces. As a result, lead screws struggle to reach even 50% in efficiency. Of course, the friction also generates heat, which limits the rotational speed to a few hundred rpm. Furthermore, there is backlash because of play between the threads.
For some applications, such as actuation and coarse, low-load positioning, efficiency is not that important and backlash can be tolerated. But for many others, particularly high-load CNC machining, these limitations are critical. Low efficiency requires much larger drive motors, and backlash interferes with precision servo control. Any attempt to eliminate backlash by pre-loading causes even higher friction and lowers efficiency to below 30%.
Placing ball bearings inside the nut assembly and pre-loading them to maintain contact overcomes these problems; efficiency soars to over 90% and backlash is eliminated. Adding those bearings gave rise to the ball screw.
For centuries, the conventional lead screw (threaded shaft and nut) was the primary mechanism for converting rotary motion to linear motion. They were inexpensive to manufacture and reliable.
Ball screws date all the way back to the late 19th century, when some ingenious engineer placed ball bearings inside the nut threads to reduce friction. This eventually led to efficiencies of over 90% and the elimination of backlash.
In 1898, The Practical Machinist published what may have been the first publicly-documented ball screw. The Cleveland Machine Screw Co. is credited with the design. However, the performance of this device was probably quite limited. Ball bearings were just emerging around that time, so the most likely limitation was inconsistently sized balls, making recurring jams in the return tubes a problem. It was not until the post-war era that ball-bearing fabrication caught up, which partially took care of the problem. But return tubes were still a problem.
One of the first commercial ball screws was designed into the famous Saginaw steering gear. This gear was used on many vehicles as diverse as the Ford Fairlane (1956-78) and Chevrolet Corvette (1962-82). The Saginaw design used a ball screw to transform rotation of the steering wheel into perpendicular linear motion of a rack. It featured a tube return. Actually, two tubes were employed to recirculate the several dozen balls on separate circuits. No pre-loading was necessary due to the coarse positioning requirements. However, the tubes were still subject to jamming. To mitigate this problem, some designs used alternating balls, with every other one 0.001 in. larger than its neighbors.
Written By: Bruce Gretz is National Key Accounts Manager for Steinmeyer Inc.