BALL AND ACME LEADSCREWS Machinery's Handbook, 31st Edition
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Contact Area
Ball Screw Assembly
Ball Bearing
Shaft
Fig. 3. Contact Points in a Preloaded Anti-Backlash Ball Screw Assembly
End Supports: End support type, free or fixed, and the number of end supports for a lead screw shaft can have a significant impact on the speed and load capacity of the assembly. Environment: Temperature extremes, dirty, dusty conditions, corrosive compounds, and metal chips can cause serious problems with lead-screw assemblies. For applications in such conditions the use of wipers, boots, or both should be considered. Stainless steel or plated assemblies should be considered for corrosive applications. Corrosive environ ments can also attack lubricants and thus indirectly damage the assemblies. Wipers: Brush type wipers maximize ball screw performance by helping to spread lubri cant over the length of the screw and preventing internal ball nut contamination from foreign materials. Bellows and telescopic wipers are recommended for severely “dirty” environments. Lubrication: Ball screws must be lubricated and kept clean. Failure to lubricate can sub stantially reduce bearing life. Oil and grease can be used but grease is not recommended for low temperature or high speed applications. PV Value: Speed and load limits for acme leadscrew assemblies using plastic nuts are generally limited by heat build-up. The factor which affects heat generation in an applica tion is the product of the pressure (lbs/in 2 ) on the nut, and the surface velocity at the thread major diameter. This product is called the PV value. CNC Considerations: CNC machine tools require the elimination of backlash and the minimization of elastic deflection to achieve high system stiffness, response, and repeat ability of positioning. Ball Screw Selection.— Many types and styles of ball screws and ball screw assemblies are offered by manufacturers. Consult the vendor’s specific data sheets for precise design criteria. The selection of the correct ball screw and nut for a particular application in- volves four factors, all of which are interrelated. These are: the equivalent load measured in pounds, the required life measured in linear inches of travel, travel speed measured in inches per minute, and the length between bearings measured in inches.
Lead
Minor (Root) Dia
Screw Shaft
Major Screw Dia.
Ball Nut
Axial Lash (Back Lash)
Fig. 4. Ball Screw Detail
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