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Low Cost Automation Tutorial

#270 Ball Screws -8: Preload Measurement and Quality Control

Category : Linear Motion Components
January 6, 2017

"Preloaded ball screws" are selected for uses requiring high precision and rigidity in order to prevent a backlash during direction changes. This volume explains how the preloaded torque is measured as well as the preload control using measurement and quality control data.

(1) How to measure dynamic drag torque

[Fig.1] Dynamic drag torque measurement
  • Dynamic drag torque is the actual torque when a nut for a preloaded ball screw rotates without being affected by the external load. JIS B1192-7.6 defines the measurement method. (See [Fig.1] for details.)
  • In an actual measurement, a load cell is used to measure the force required to stop a nut while the screw shaft is rotating. To calculate the torque, this force F is multiplied by the distance between the screw shaft center and the measuring point (arm length L).

(2) Actual measurement data and quality control

  • For ultra-precision ball screws for positioning and high rigidity ball screws for machine tools, the dynamic drag torque is a required inspection item for quality control in the shipping preparation. [Fig.2] is dynamic drag torque data of ultra-precision & high rigidity ball screws for machine tools at factory shipment.
  • For quality control at factory shipment, the dynamic drag torques in both directions are measured so that both of them meet the acceptance criteria of preloaded torque. Data in [Fig.2] shows the preloaded torque range between 20.6 and 37.2 N・cm.

[Fig.2] Initial value of dynamic drag torque

  • The graph of dynamic drag torque shown in [Fig.3] is the measurement data of the ball screw of [Fig.2] after using it for a long period. The data indicates that the preload has been reduced owing to wear and fatigue of the ball screw. The measurement values are in the range between 9.8 and 24.5 N・cm. Compared to the initial value at factory shipment, the preload has been reduced by about 30 to 40%.
  • This state is referred to as "preload loss", which could cause a backlash that did not exist before, resulting in reduced accuracy or rigidity, as well as servo instability.

[Fig.3] Dynamic drag torque (after usage)