Fixed-fixed ball screws restrain the shaft axially at both ends. Fixed-supported arrangements establish axial location at one end while the other support primarily provides radial guidance and accommodates the intended axial movement. Fixed-fixed support can improve shaft restraint, but it requires a more deliberate thermal, bearing and assembly design.
Choose the arrangement from the complete axis requirements: support span, operating speed, load, temperature change and assembly method. A housing bolted firmly to the machine is not automatically an axially fixed screw support.
What “fixed” describes
The terminology describes the shaft’s mechanical boundary condition, not merely whether a bearing housing is present. A fixed end commonly uses a bearing arrangement capable of reacting axial loads and controlling shaft location. A supported or simple end provides a different constraint.
MISUMI’s ball-screw support explanation distinguishes the fixed side from the arrangement that accommodates heat-related expansion. Check the actual bearing and shaft-retention drawing; product names such as “support block” do not establish the boundary condition by themselves.
| Question | Fixed-supported | Fixed-fixed |
|---|---|---|
| Where is axial location established? | Principally at the fixed end | At both ends by the designed arrangement |
| How is thermal growth accommodated? | Through the specified axial freedom | Through a calculated thermal and restraint strategy |
| What changes in shaft-speed calculations? | Use its own support coefficient | Use the applicable fixed-fixed coefficient |
| What needs special assembly attention? | Correct floating function at the supported end | Alignment, axial stack, bearing loads and any specified pretension |
The labels in this table are idealized boundary conditions. A real assembly’s stiffness and behavior depend on its bearings, housing and machine structure.
Critical speed and support span
Greater shaft restraint can raise the calculated critical speed, all else equal. The actual result still depends strongly on the unsupported length and shaft root geometry. NSK’s automation ball-screw brochure presents separate support conditions in its speed assessment.
Do not apply a generic “twice as fast” rule when switching support arrangements. Use the selected shaft’s permitted-speed method and the appropriate support coefficient. Also check nut circulation limits, bearing speed and drive limitations. A higher shaft critical speed does not remove those other limits.
The span for the calculation is the relevant bearing support span, not simply the machine’s usable stroke. Ball screw DN and critical-speed checks explain why both the shaft and the nut need assessment.
Thermal growth can change the bearing load
A steel screw heats during operation and changes length relative to its mounting structure. A fixed-supported design must actually provide the intended expansion freedom. Paint, an incorrect fit or unintended axial capture can turn an ostensibly floating support into a restrained one.
In a fixed-fixed design, the screw, supports and base must be assessed together. Thermal growth can change the assembly’s axial force. Deliberate pretension, if specified, is an engineered condition rather than a value to create by tightening the end supports until the screw feels rigid.
For scale, consider a hypothetical steel shaft with a 1 m reference length, an assumed expansion coefficient of 12 µm/(m·K), and a 10 K rise. Its unconstrained growth would be about 120 µm. This is an illustrative assumption, not a measured temperature rise or a thermal correction for a LIMON product. The bearing force in a restrained assembly cannot be determined from this growth alone without its stiffness and thermal relationships.
Keep three preload concepts separate
Ball-nut preload controls the rolling contact relationship inside the nut. Support-bearing preload concerns the bearing arrangement locating the shaft. Screw pretension is an intentional axial condition imposed on the shaft in some designs.
Changing one does not automatically establish the other two. A tight nut can coexist with an axially loose support bearing, and an apparently rigid shaft support does not prove the nut has the required preload.
A practical selection sequence
Begin with the required travel, maximum speed, acceleration, process force and positioning requirement. Calculate the resulting screw speed from the proposed lead. Then check shaft critical speed and the selected nut’s limits with the candidate support span.
If fixed-supported meets those requirements, document its fixed and floating functions and assess thermal movement. If it does not, evaluate a larger shaft, a different lead, shorter support span, a different drive architecture or a properly designed fixed-fixed arrangement. The right remedy is not necessarily an additional fixed bearing.
Finally, review the end-machining drawing together with the housing stack and assembly instructions. A correct calculation cannot compensate for a support arrangement that is built differently from the calculation model.
Information to include in the support-unit enquiry
Provide a section through both bearing supports, shaft and nut codes, support span, orientation, speed profile and expected thermal environment. State whether pretension is part of the intended design. LIMON ball screw support units y ball screw assemblies should be reviewed as mating parts. Request confirmation against the complete drawing before ordering a repeat OEM build.
