BALL AND ACME LEADSCREWS BALL AND ACME LEADSCREWS Leadscrew Overview Machinery's Handbook, 31st Edition
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Leadscrew assemblies convert rotary motion into linear travel and are well suited to precise control using electric motors. Acme leadscrews, much like machine screws, are threaded to allow a mating nut to be pushed or pulled along the axis of the screw as the shaft is rotated. Ball screws are threaded to allow ball bearings to support the attached nut. The nut has raceways that allow the ball bearings to recirculate as the nut travels down the axis of the shaft. Both provide efficient and accurate linear positioning for motion con - trol applications. Each has strengths and weaknesses that makes each type of technology more suitable for different types of applications. Ball Screws.— Ball screws generally require lower-torque motors than similarly sized Acme screws due to their higher efficiency in concerting energy into linear movement. This efficiency is generally above 90 percent compared to 40 percent for Acme screws with plastic nuts and 25 percent for acme screws with bronze-nuts. Ball screws can handle higher loading with high travel rates and lower backlash, though both technologies offer various anti-backlash nut assemblies. Ball screw life is more easily predicted using ball bearing load data. Compared to acme screws, ball screw assemblies exhibit a lower degra dation in postitional accuracy over time due to wear on the contact surfaces. Ball screws are more expensive and can be noisier than acme screws. Ball screws may back drive the shaft in vertical orientations.
Bearings
Fig. 1a. Ball Screw Cross Section Fig. 1b. Ball Screw Detail Acme Screws.— Acme screws with plastic nuts generally have an initial lower cost and lower noise levels compared to ball screws. Bronze-nut acme screws offer the lowest lead screw cost as well as the lowest leadscrew performance characteristics. Due to their low efficiency, bronze-nut acme screws may not back drive in some applications where a ball screw would. Loading and travel rates are lower than ball screws. Also, compared to ball screws, lubrication requirements are higher and predicting travel life of acme screws is more difficult due to higher variations in friction and heat build up.
Nut
Spring Fig. 2a. Acme Leadscrew with Anti-backlash Nut.
Fig. 2b. Acme Screw Detail
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