Ball Screw Technical Definitions

Ball Screw Selection

The selection of the correct ball screw and nut for a particular application involves five interrelated factors. Before attempting to determine the ball screw and nut combination, the following values must be known:

  • Load measured in newtons
  • Speed measured in millimeters per minute
  • Length between bearings measured in millimeters
  • Life expectancy
  • End fixity type

Load - The loads that need to be considered are the static loads, dynamic loads, reaction forces and any external forces affecting the screw.

Speed - The travel rate (linear speed) is the rpm at which the screw or nut is rotating multiplied by the lead of the screw.

Length - Unsupported length of the screw.

Static Load - The maximum thrust load - including shock - that can be applied to the ball nut without damaging the assembly.

Dynamic Load - Metric screw designs are per ISO 3408 and show the load ratings in newtons for 1 million revolutions.

Tension Load - A load that tends to "stretch" the screw.

Compression Load - A load that tends to "squeeze" the screw.

Overturning Load - A load that tends to rotate the nut radially around the longitudinal axis of the screw.

Side Load - A load that is applied radially to the nut.

CAUTION - Although a side load will not prevent the ball screw from operating, the nut is not designed to operate with a side load, such as those generated from pulleys, drive belts, misalignment, etc.

Thrust Load - A load parallel to and concentric with the axis of the screw.

Accuracy Grade

The grade of accuracy of Nook Precision Metric Ball Screws conforms to ISO 3408 and is prescribed by ISO Standard Tolerance Grades for Positioning (0, 1, 3, 5) and Transport (0, 1, 3, 5, 7, 10).

Driving Efficiency

Lead Accuracy

Lead Accuracy for Nook Precision Metric Ball Screws conforms to ISO 3408. ISO 3408 prescribes the tolerance on specified travel and travel variation in respect to the effective length of travel of nut or to the effective length of the threaded portion of the screw shaft. It also prescribes travel variation in respect to a length of 300mm taken arbitrarily within the effective length of the screw shaft and on travel variation in respect to arbitrary one revolution (2π rad) within the effective length of threaded portion.

ISO Lead Accuracy

ν300 T3 T5 T7 T10
Permissible value 12μm* 23μm 52μm 210μm

Travel variation in respect to 300mm taken arbitrarily within the effective length of threaded portion of screw shaft.

* Precision ground ball screws

Lo = Nominal Travel

Nominal Travel is the product of the nominal lead times the number of revolutions.

La = Actual Travel

Actual Travel is the actual displacement of the ball nut relative to the ball screw shaft, or vice versa, for a given number of revolutions.

Lm = Actual Mean Travel

Actual mean travel is the straight line which gives the minimum straightness deviation determined for the actual travel.

Lu = Useful Travel

Useful travel is the portion of the travel to which the specified accuracy (stroke plus ball nut body length) is applicable.

ep = Tolerance On Nominal Travel

The tolerance on nominal travel is half the difference between the maximum and minimum value of the permissible actual mean travel, 2ep.

Standard Tolerance Grade T3 T5 T7 T10
Permissible value

Travel variation in respect to the effective travelling distance of nut or to the effective length of threaded portion of screw shaft.

ν = Travel variation

Travel variation is the corresponding band width of the travel deviation parallel to the actual mean travel for a specified travel interval.

  • νu = Corresponds to the effective travel distance of nut or effective length of threaded portion of the screw shaft. (For positioning screws only. Please contact factory for availability.)
  • ν300 = Corresponds to a length of 300mm which is arbitrarily taken within the effective threaded portion of the screw shaft.
  • ν - Corresponds to one arbitrary revolution (2π rad) within the effective threaded portion of screw shaft. For positioning screw only. Contact factory for availability.
Figure 1