Techno Linear Motion Catalog12Technical InformationBall Screwsare very similar to lead screws with theexception of a ball bearing train riding between the screwand nut in a recirculating raceway. This raceway isgenerally lubricated, which allows for predictable servicelife. Due to the increased number of mating and movingparts, matching tolerances becomes more critical. Thescrew threads have rounded shapes to conform to theshape of the balls. The function, terminology, and formulasare the same as found with lead screws, however theperformance of ball screws is far superior. The rollingaction of the balls versus the sliding action of the ACMEnut provides significant advantages. Advantages ofball screw drives are increased efficiency (typically up to 90 95%) which allows required motor torque to belower, predictable service life, low wear rate and maintenance costs. Disadvantages include limited materialchoice, higher initial cost, and an auxiliary brake is required to prevent back driving with vertical applications.Helpful Formulas: When determining the amount of input torque required to produce an amount of outputlinear force, there are many factors to consider. The following equations provide a practical approach inmaking force and torque calculations.Force Calculations:FT= FA+ FE+ FF(1)where: FT = Total ForceFA= Acceleration ForceFE= External ForceFF= Friction ForceWaFA= x lb(2)g12where: W = total weight to accelerate (lb)a= linear acceleration (in/sec2)g= acceleration from gravity (ft/sec2)External Force (FE) may be due to gravity in vertical applications, or may be from external workrequirements (feeding material, stretching material, etc.)Friction Force (FF) required to overcome all of the friction in the load bearing system (with a lowfriction bearing system, this can be negligible)The Total force must be below the compressive (thrust) rating of the screw chosen. A modestfactor of safety should be added to the total force so that unexpected dynamic loads are safely handledby the screw system.Torque Calculations:For most typical applications, rotary inertia, motor rotor inertia, and screw inertia are negligible, thereforeleft out of the torque calculations.1T= FTx (3)2ppewhere: FT = Total Forcep= screw pitch (revolutions per inch)e= efficiency (no units)NOTE:BallsBall Screw Ball Nut Ball ReturnBall Screw System