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#132 Mastering Ball Screws - 2 : Ball Screw Preloads

Category : Linear Motion Components
October12, 2012

There is a measure, "Preloading", to increase the feed accuracy and rigidity of ball screws. The conditions of minute clearances between the ball screw's ball grooves and the steel balls largely affects the characteristics of ball screws.

On the "Axial Clearance" column of the ball screw selection chart shown on Pxxx of MISUMI FA Mechanical Standard Components catalog, there are listings such as:

* Preloaded Product* 0.005mm or less* 0.010mm or less* 0.030mm or less

■Preload means (Definition: JIS B 1192)
A force applied internally to ball screws by assembling in a group of steel balls or using a pair of nuts that are axially mutually displaced, in order to reduce backlash and increase the rigidity of the ball screw.

Types of preload structure

■Terminology Explanations
Positional preloadPreloading method using the positional relationship of the screw shaft and the steel balls.
Oversized balls methodPreloading method to obtain a constant force by employing slightly larger sized balls than the screw grooves.
Lead shifted methodThe preload is applied by machining the nut with a groove offset at the center in the amount of desired preload amount.
Spacer methodA preloading method using a spacer between a set of nuts in the amount of the desired preload.
Predetermined pressure preloadThe preload is applied with a spring.

[Fig.1] Preload structure by oversized balls(Source: NSK Catalog),[Fig.2] Preload structure by double nut with a spacer(Source: NSK Catalog),[Fig.3] Double nut structure with predetermined pressure preload methodr(Source: NSK Catalog)