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Measurement & Machine Technology Know How:
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 it is installed to minimize changes in axis positioning due to "growth" of the screw from cold to normal operating temperature. When the axes are moved, heat is generated by the recirculating balls in the ball nut. The ballscrew is therefore subject to "GROWTH" or thermal length deformation, by the coefficient of thermal expansion for steel of 6.5 parts per million (ppm) /° 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 ballscrew 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 be relaxed released or removed causing end play (possible axial load on the motor) and excessive positioning error, requiring re-tensioning of the ballscrew.
.0016” (pretension) = 40” (Travel) X 6°F (Temp Rise) X .0000065 (PPM /° Fahrenheit)
(.041mm (pretension) = 1000 mm (Travel) X 3.5°C (Temp Rise) X .0000117 (PPM /° Celsius))
If the ball screw tension has been relaxed released or removed for any reason, follow this procedure for pre- tensioning the ball screw.
Before mounting the axis servomotor.
- Mount the ball screw.
- Install the TAC bearings on the ball screw [3 & 4] in the proper orientation.
- Tightening the bearing retainer nut  on the motor end of the ballscrew, to fully seat the bearings .
- Rotate the ball screw by hand, check and retighten the retainer nut  and tighten lock nut 
- Tighten nut  (opposite motor end) to seat TAC bearings  into the bore.
- Loosen nut  and run it back in, just to touch.
- 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.
- Tighten nut  until the indicator equals the pretension amount for that axis.
- Some pull thru will occur B - A = PRETENSION AMOUNT.
- When pretension has been achieved, tighten the lock nut and bend in the tab of retainer washers.
- Mount the servo motor and coupling.
- Check the axis reference points and reset if necessary.
We can when asked, in most cases perform all the necessary mechanical & electrical/electronic adjustments necessary to optimize your machine tool, regardless of the manufacturer. In extreme cases we can even undertake the total geometry rebuild of your machine On-Site with minimal disruption to your operation, allowing you to overlook the entire project.
We are completely portable and can bring to your site the necessary men and equipment to complete our task.
JUST A FEW OF THE MACHINE TOOLS WE HAVE WORKED ON:
- Leblond Makino
- Giddings & Lewis
- Cincinnati (Gilbert & Milacron)
- Niigata CNC Machine Tools
- Kearney Trecker
- Pratt Whitney
THERE ARE FAR TO MANY MORE TO LIST HERE