Home Page
H834 Catalog Page
Techno Linear Motion Catalog 12 Technical Information Ball  Screws  are  very  similar  to  lead  screws  with  the exception of a ball bearing train riding between the screw and  nut  in  a  recirculating  raceway.    This  raceway  is generally lubricated, which allows for predictable service life.  Due to the increased number of mating and moving parts, matching tolerances becomes more critical.  The screw  threads  have  rounded  shapes  to  conform  to  the shape of the balls.  The function, terminology, and formulas are  the  same  as  found  with  lead  screws,  however  the performance  of  ball  screws  is  far  superior.    The  rolling action of the balls versus the sliding action of the ACME nut  provides  significant  advantages.    Advantages  of ball screw drives are  increased efficiency (typically up to 90 – 95%) which allows required motor torque to be lower, predictable service life, low wear rate and maintenance costs.  Disadvantages include limited material choice, 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 output linear force, there are many factors to consider.  The following equations provide a  practical approach in making force and torque calculations. Force Calculations: FT = FA + FE + FF (1) where:  FT = Total Force FA = Acceleration Force FE = External Force FF = Friction Force    W     a FA = ––– x ––– lb (2)   g     12 where:  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  work requirements (feeding material, stretching material, etc.) Friction  Force  (FF)  required  to  overcome  all  of  the  friction  in  the  load  bearing  system  (with  a  low friction bearing system, this can be negligible) The Total force must be below the compressive (thrust) rating of the screw chosen.  A modest factor of safety should be added to the total force so that unexpected dynamic loads are safely handled by the screw system. Torque Calculations: For most typical applications, rotary inertia, motor rotor inertia, and screw inertia are negligible, therefore left out of the torque calculations.          1 T = FT x ––––– (3)      2ppe where:  FT = Total Force p = screw pitch (revolutions per inch) e = efficiency (no units) NOTE: Balls Ball Screw ––––– Ball Nut ––––– Ball Return Ball Screw System