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Specializing in CNC and Manual Precision Machine Tools
Mechanical Alignment
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Laser Calibration
BallBar Analysis

Ball Screw pre-tensioning

On most modern Machine Tools, the ball screw on the X, Y, and Z axis is pre-tensioned or "stretched" when installed to minimize changes in axis positioning due to "thermal growth" of the screw from ambient to normal operating temperature. When the axes are moved, heat is generated by the recirculating balls in the ball nut. The ball screw is therefore subject to "GROWTH" or thermal length deformation, by the coefficient of thermal expansion for steel of 6.5 parts per million per degree Fahrenheit (11.7 ppm/ Celsius)

The average temperature rise of a cold ball screw is 5.4 to 7.2 Fahrenheit (3 to 4 Celsius). Pre-tensioning places a cold ball screw in a state of tensile stress. This stress decreases or normalizes as the screw expands toward its normal operating temperature, minimizing axis position deviation. During normal operation, tension may become relaxed, causing possible end play and positioning error; requiring re-tensioning of the ball screw.

  • .0016" (pretension) = 40" (Travel) X 6F (Temp Rise) X .0000065 (PPM / Fahrenheit)
  • (.041mm (pretension) = 1000 mm (Travel) X 3.5C (Temp Rise) X .0000117 (PPM / Celsius))

If the ball screw preload (tension) has been relaxed for any reason, follow this general procedure for pre-tensioning the ball screw.

BallScrewTension (2)

PRETENSION PROCEDURE

  1. Mount the ball screw.
  2. Install the ball screw support (TAC) bearings on the ball screw [3 & 4] in the proper orientation.
  3. Assemble retainer nuts both ends leaving trailing end (opposite motor) loose.
  4. Tighten the bearing retainer nut [2] on the motor end of the ball screw, to fully seat the bearings [3].
  5. Rotate the ball screw by hand, check and re-tighten the retainer nut [2] and tighten lock nut [1]
  6. Tighten nut [5] (opposite motor end) to seat TAC bearings [4] into the bore.
  7. Loosen nut [5] and run it back in, just to touch.
  8. Place an indicator on each end of the ball screw and set to the indicators to 0 (zero).
    • Be sure to load the indicators at about mid-range, to insure a good reading.
  9. Tighten nut [5] until the indicator equals the pretension amount for that axis.
    • Tension amount usually supplied by machine builder
    • If unavailable it is recommended to measure temperature rise and calculate proper amount of preload
  10. Some pull thru will occur B - A = PRELOAD AMOUNT.
  11. When proper pretension has been achieved, tighten the lock nut to secure the ends.
  12. Mount the servo motor and coupling.
  13. Check the axis reference points and reset if necessary.
  14. Final Laser Calibration of machine positioning required.

We have many years of HANDS-ON experience with ball screw issues, from evaluation only to removal, replacement, and Laser Calibration – sometimes called Alignment; we have even undertaken the On-Site repair of our customers' ball screws. We can help with yours.