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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
v e r s u s t h e s l i d i n g a c t i o n o f t h e AC M E n u t
p r o v i d e s 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 = ––– ––– 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:
L
T = FT –––––
(3)
2 e
where: FT = Total Force (lbs)
L = Lead (inches)
e = efficiency (no units, use 0.9 for Ball screws assemblies.)
Ball Return
Ball Screw System
Ball Nut
3
Balls3
3
Ball Screw
3
3
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