Ball Screws vs Lead Screws - Pros and Cons

Christopher Nook by Christopher Nook

Lead screws and ball screws have been the most trusted and cost effective means of translating rotational energy to linear motion.

Image

Lead screws and ball screws have been the most trusted and cost effective means of converting rotary motion to linear motion. Both employ a thread form and nut; however, ball screws utilize balls between the nut and screw threads to reduce friction.

Ball screws as known today, were developed in 1940s by General Motors for automotive steering gear and in World War II to help extend flaps on bombers wings. These extensions helped provide more payload. This proved to be an effective technology and is still used today in most modern aircraft and automobiles. The technology was similiar in concept to radial ball bearings and plain bushings. Further development of ball screws increased accuracy, decreased backlash, improved loading and lowered manufacturing costs, opening up additional opportunities for their application, especially in machine tools.  Ball screws are costly to manufacture due to the number of precision components utilized in their manufacture. They are also bulky and noisy because of the balls whirling around the threads as they transfer from end to end.  Ballscrews require lubrication (grease) which is frowned upon in many medical, semiconductor, instrumentation, food processing, and environments with contamination concerns. 

Lead screws were used in less demanding applications as the early nut materials were limited to cast iron, Babbitt lined and bronze.  Because of metal on metal sliding friction, Acme screws’ efficiency was low and required more power to move the same load as a ball screw.  Additionally, they required copious amounts of lubrication and started wearing as soon as they were placed in service. Babbitt lined Acme nuts were the first attempt to extend life and lower friction by creating an alloy of material that was composed of copper and antimony dispersed through a softer metal tin. These thinly lined Babbitt nuts allowed lubrication access to the internal part of the nut as the tin wore and the other materials supported the load. Early manual machine tools took advantage of the reduced friction and extended life, however the human compensation for nut wear could not be built into the computer driven machines. 

Helix Precision Lead Screws

 

Office automation, especially printers, creates a need for high speed low cost linear motion actuation.  Due to noise in the office environment, ball screws were eliminated early on and engineering efforts were focused on Acme screws. New polymers especially the injection molded type allowed for new designs that have plenty of advantages. Many of them are reinforced with carbon fiber or glass and self-lubricating with PTFE and other lubrication packages. They minimize wear, have high load capacities, can be preloaded, provide corrosion resistance, and are very quiet running. These new screws use 300 series stainless steel that are precision roll formed to cut costs. An important advantage of roll forming is the roller burnishing effect of the screw providing very smooth operation. New technologies in thread rolling also provide a very accurate screw profile, allowing for multiple shallow starts. PTFE coatings for the screws were developed to provide dry lubrication, eliminating messy wet lubrication. High lead screws are rolled to optimize the fast operation of the printers. To enhance print accuracy and the ability to print in both directions, new nut designs were created to facilitate preload. These preloaded nuts, not only helped with print resolution, they automatically adjusted for nut wear, ensuring years of accurate trouble free life. 

The great news is that the technology that was designed to solve an office machine problem has been successfully ported over to other industries. With further advancements in polymers, higher loads are achieved and the efficiencies of operation keep getting better. We now find these types of screw assemblies in entertainment, packaging, transportation, factory automation, material handling, weapons systems, machine tools, aerospace, medical equipment, semiconductor and other industries. 

Learn More in Our Engineers Guide

We would like to offer some more information in the form of our "Precision Miniature Lead Screw Assemblies" catalog. This catalog is a comprehensive guide to selecting the right lead screw assembly for your linear motion technology.