To align ball screw support bearings correctly, the fixed-end bearing seat, supported-end bearing seat and machined screw journals must share a compatible centreline. The support units should locate the screw without pulling it sideways, bending it or preventing the supported end from accommodating normal axial expansion.
Ball screw bearing alignment is not exactly the same as ball screw parallelism. Bearing alignment mainly concerns the coaxial relationship between the two screw supports, while parallelism describes the relationship between the complete screw axis and the machine's linear guideway travel.
Why Ball Screw Bearing Alignment Matters
Support bearings locate and stabilise the rotating screw shaft. If the fixed and supported bearing units are not properly aligned, the screw may still rotate, but unwanted radial force and bending stress can be introduced into the assembly.
Common results of poor bearing alignment include:
- Uneven or excessive running torque
- Bearing heating during operation
- Vibration or abnormal noise
- Premature support-bearing wear
- Local wear inside the ball nut
- Unstable positioning and reversal response
- Binding at one or more positions along the stroke
Tightening the support units more firmly does not correct misalignment. It may instead force the housings or screw shaft into a distorted position.
Understand the Fixed and Supported Ends
A common ball screw installation uses one fixed end and one supported, or floating, end.
| Position | Main function | Installation concern |
|---|---|---|
| Fixed end | Controls axial position and supports axial load | Bearing seating, locknut setting and housing location |
| Supported or floating end | Provides radial support and stabilises rotation | Must not unintentionally restrain thermal axial movement |
The fixed end commonly uses an angular-contact bearing arrangement, while the supported end generally uses a simpler radial bearing arrangement. The exact structure depends on the support-unit model, screw size, speed, axial load and required rigidity.
For additional information about support structures, see What Is a Ball Screw End Support?.
Check the Mounting Surfaces Before Installation
Alignment begins with the machine structure. Before installing the support units, inspect the bearing-seat mounting surfaces and locating shoulders.
- Remove burrs, paint, chips, rust-preventive residue and raised edges.
- Confirm that the support-unit mounting faces are flat and properly located.
- Check whether the fixed and supported mounting faces are square to the intended screw axis.
- Inspect bolt holes and locating shoulders for machining errors or damage.
- Do not use bolt tightening force to pull a housing against an uneven surface.
A small chip or burr trapped under a support housing can change its height or angle. This may be difficult to see after tightening but can produce a noticeable change in running resistance.
Inspect the Screw-End Machining
The machined journals at both ends of the ball screw connect the screw shaft to the support bearings. Their diameter, shoulder position, concentricity and surface condition directly affect installation.
Before assembly, check:
- Bearing-seat journal diameter
- Shoulder position and squareness
- Locknut thread and usable thread length
- Surface damage, dents or corrosion
- Concentricity between the machined journal and screw axis
- Compatibility with the selected support-unit model
Forcing an incorrectly machined journal into a bearing can damage the bearing raceway or introduce runout. The bearing fit should follow the confirmed drawing and support-unit specification.
Recommended Alignment Sequence
1. Install the Linear Guideway First
In most machine structures, the linear guideways establish the table's direction of travel. They should be installed and checked before the ball screw is finally aligned.
The ball screw should not be used to pull a table or nut housing into a position that conflicts with the guideway travel.
2. Assemble the Fixed-End Bearing
Install the fixed-end bearing arrangement on the machined screw journal according to the specified bearing orientation. Seat the bearing against the correct shoulder and install the locknut without striking through the bearing raceways.
Bearing preload or locknut tightening should follow the selected support structure. Excessive tightening can raise torque and bearing temperature, while insufficient tightening may permit axial movement.
3. Position the Fixed Support Unit
Mount the fixed support housing to its prepared machine surface. Tighten the bolts lightly at first so that small position corrections remain possible.
Because the fixed end establishes the axial reference of the screw, its mounting face and locating structure should be confirmed carefully before using it as the installation reference.
4. Position the Supported-End Unit
Install the supported-end bearing and housing without forcing the screw sideways or vertically. Keep the support-housing bolts lightly tightened while checking the screw's natural position.
The supported end should provide radial support without unintentionally becoming a second rigid axial location unless the machine was specifically designed as a fixed-fixed arrangement.
5. Connect the Ball Nut Housing Loosely
Attach the ball nut flange to the nut housing or bracket, but do not fully tighten the table connection immediately. Support the screw so that the nut is not required to carry the full weight of a long shaft during installation.
Leaving the mounting bolts lightly snug allows the nut housing to settle into a position compatible with the screw axis and guideway travel.
6. Move the Table Through the Full Stroke
Move the axis slowly by hand or at the lowest safe drive speed. Check the fixed end, middle and supported end of the stroke.
The required torque should remain reasonably consistent. A position where the resistance rises sharply may indicate:
- Support housings that are not coaxial
- Incorrect nut-housing height or lateral position
- Screw runout or local deformation
- Contamination on the mounting surface
- Guideway and screw axes that are not parallel
7. Tighten Progressively and Recheck
After the components settle into an acceptable position, tighten the fixed support, supported support and nut-housing bolts progressively in the specified sequence.
Move the table through the complete stroke again after each tightening stage. Bolt torque can shift a housing or distort a mounting surface, so alignment should always be rechecked after final tightening.
How to Check Bearing-Seat Coaxiality
A dial indicator can be used to inspect shaft-journal runout and movement near the support positions. The correct setup depends on the machine structure and available reference surfaces.
Useful checks include:
- Radial runout of the machined screw journal
- Axial movement at the fixed end
- Horizontal and vertical position of both support housings
- Change in running torque along the full stroke
- Movement of a lightly secured nut housing during table travel
A feeler gauge between the bearing bore and screw journal is not a suitable method for setting bearing alignment. Alignment should be established through the machined locating surfaces, journal dimensions and housing positions.
Bearing Alignment and Guideway Parallelism Are Different
The fixed and supported bearings can be coaxial with each other while the complete ball screw is still not parallel to the linear guideways. Conversely, the screw may appear parallel from above while the two support units differ in height.
A complete installation should therefore confirm:
- Coaxiality between the fixed and supported bearing seats
- Horizontal parallelism with the guideway travel
- Vertical height relative to the nut housing and guideway
- Alignment of the motor shaft, coupling and screw journal
For the complete screw-to-guideway inspection procedure, see How to Align a Ball Screw During Installation.
Common Installation Mistakes
- Fully tightening both support housings before checking the complete stroke
- Using the bolts to pull a misaligned housing into place
- Hammering the screw shaft or transmitting impact through the bearing
- Locking the supported end axially when it should remain floating
- Using the ball screw to correct a misaligned table or guideway
- Ignoring increased torque after final bolt tightening
- Assuming that smooth movement at one end means the full stroke is aligned
DLY Ball Screw Support and End-Machining Options
DLY supplies ball screws with end machining for commonly used BK/BF, FK/FF and EK/EF support arrangements. Screw-end dimensions, bearing-seat journals, shoulders, locknut threads and coupling sections can be processed according to confirmed drawings.
DLY supplies C7 rolled ball screws for general automation and can produce C5 ground ball screws for applications requiring tighter lead accuracy. However, screw accuracy cannot compensate for incorrectly positioned bearing seats; manufacturing accuracy and installation alignment must be controlled separately.
Available ball screw and assembly options can be viewed on the DLY ball screw page.
Need help matching a ball screw with its support units?
Send DLY the ball screw model, end-machining drawing, support-unit models, installation layout and operating requirements for checking.
Email: export@dlybearing.com
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