A practical selection guide for buyers specifying ball screw end support bearings. Understand the difference between fixed and supported ends, how each affects critical speed and axial rigidity, and how to match support units to your application.
What Ball Screw End Supports Do
A ball screw works as a system, not a standalone shaft. The screw needs bearings at one or both ends to hold it in position, take the axial load from the nut, and keep the shaft from whipping at speed. A ball screw end support bearing carries that axial load. The related term ball screw support unit usually refers to an assembled housing with a bearing, seals, and mounting features, ready to bolt to a machine frame.
The choice between a fixed end and a floating (supported) end is one of the first decisions a buyer makes when specifying a ball screw assembly. Pick wrong, and you either pay for rigidity you do not need or find out at commissioning that the screw whips, the bearings overheat, or positioning drifts. This guide explains the two configurations, how they affect critical speed and rigidity, and how to decide which one your application actually requires.
- The fixed end takes axial load and controls positioning. The floating end only supports the shaft radially.
- Critical speed depends on shaft diameter, unsupported length, and end fixity. Fixing both ends raises it; fixing one end limits it.
- Axial rigidity is largely set by the fixed-end bearing arrangement, not by the floating end.
- Many applications use a fixed end at the drive motor side and a floating end at the free side. This is the default for moderate lengths.
- Review duty cycle, preload, speed, and thermal growth before locking in a configuration. Ask the supplier to confirm the support unit load rating and permissible speed.
Fixed End vs Floating End: What Changes
The difference is not cosmetic. It changes how the screw is constrained, how load travels into the machine frame, and how much the shaft can move.
The Fixed Side
A fixed (or "fixed side") end uses a bearing arrangement that restrains the screw in both axial directions. Common arrangements include a pair of angular contact bearings, a double-row angular contact bearing, or a preloaded thrust bearing set. This end:
- Takes the axial force generated by the nut as it drives the load.
- Sets the axial reference for the screw, so thermal expansion tends to move the free end, not the driven end.
- Usually sits on the motor side, coupling the screw to the servo or stepper drive.
- Is preloaded to reduce axial play and improve positioning repeatability.
When the application needs bidirectional thrust capacity and tight positioning, the fixed end is where the engineering effort goes.
The Floating Side
A floating end (sometimes called a supported end) uses a bearing that allows axial movement while holding the shaft radially. A single deep groove ball bearing or a cylindrical roller bearing is typical. This end:
- Supports the screw to reduce sag and vibration.
- Allows the shaft to expand and contract with temperature change without adding axial stress.
- Does not carry the main axial load, so it is not the positioning reference.
- Can be omitted entirely on very short screws that do not need radial support at the free end.
Floating the non-drive end is a deliberate choice to let the screw grow. If both ends were fully fixed with no axial compliance, thermal expansion would build stress into the bearings and the frame.
The Three Common Configurations
In practice, most ball screw assemblies fall into one of three support schemes. The names vary by supplier, but the mechanical behavior is consistent.
| Configuration | Fixed end | Floating end | Typical use |
|---|---|---|---|
| FixedâFloating | Motor side, preloaded angular contact bearing | Free side, radial support bearing | Most general positioning applications; moderate stroke and speed |
| FixedâFixed | Both ends axially restrained | Not applicable | High-speed, high-rigidity, or long-stroke screws where buckling and critical speed matter |
| FixedâFree | Motor side only | No support at free end | Short screws, low speed, light load, or where space at the free end is unavailable |
The default for a general-purpose axis is fixedâfloating. It balances cost, assembly simplicity, and performance. Fixedâfixed is chosen when the application needs the highest possible critical speed or stiffness. Fixedâfree is acceptable only when the screw is short enough that sag and whirl are not concerns.
How Support Choice Affects Critical Speed
Critical speed is the shaft speed at which the screw begins to resonate. It is a function of screw diameter, unsupported length, and end fixity. End fixity is expressed as a factor in the critical speed formula; a fixedâfixed arrangement has a higher factor than a fixedâfloating arrangement because both ends resist rotation and translation.
For a given screw diameter and length, moving from fixedâfloating to fixedâfixed raises the critical speed. That can allow a smaller-diameter screw to run at the same speed, or a longer screw to stay below its limit. But fixedâfixed also requires careful alignment and thermal management. If the screw is fixed at both ends with no allowance for expansion, temperature rise can overload the bearings.
When you evaluate a ball screw support unit selection for a high-speed axis, ask the supplier for the permissible speed of the support unit itself. The bearing inside the support has its own speed limit, separate from the screw's critical speed. The lower of the two governs.
How Support Choice Affects Axial Rigidity
Axial rigidity determines how much the screw compresses or stretches under load, which in turn affects positioning accuracy. The fixed end contributes the bearing rigidity; the floating end contributes almost nothing axial. If you need very high axial stiffness:
- Use a preloaded angular contact bearing pair at the fixed end.
- Keep the fixed-end housing rigid and bolted to a stiff machine member.
- Consider fixedâfixed only if the screw can be preloaded and thermal growth is managed.
- Do not rely on the floating end to improve axial stiffness; its bearing is designed for radial support.
For many positioning tasks, the fixed-end bearing is the dominant compliance in the drivetrain. Upgrading the fixed-end support from a simple bearing to a preloaded pair often yields more benefit than changing the floating end.
When Each Configuration Matters
Choose FixedâFloating When
- The stroke is moderate and the screw is not near its critical speed.
- You want a straightforward, cost-effective assembly with one precision end.
- Thermal growth can be accommodated by letting the free end float.
- The machine frame does not provide a convenient mounting face at the free end.
Choose FixedâFixed When
- The screw is long relative to its diameter and must run at higher speed.
- Buckling under compressive load is a concern.
- You need the highest axial rigidity and repeatability.
- You can control thermal growth through preload, cooling, or a controlled temperature environment.
Choose FixedâFree When
- The screw is short and stiff enough that the free end does not sag noticeably.
- Speeds are low and loads are light.
- Space or mounting constraints prevent a free-end support.
Selection Checklist for Buyers
Before you finalize a ball screw support unit selection, gather the following inputs. The answers will narrow the configuration quickly.
- Screw diameter and length: Determines critical speed and buckling risk.
- Maximum speed: Must stay below both screw critical speed and support bearing speed limit.
- Axial load and direction: Continuous thrust, intermittent thrust, or bidirectional.
- Required positioning accuracy and repeatability: Drives the fixed-end bearing preload and rigidity.
- Operating temperature range: Determines whether the floating end is needed for thermal growth.
- Mounting interface: Housing size, bolt pattern, and available space at each end.
- Lubrication and sealing: Matches the environment (dust, coolant, washdown).
If you cannot answer some of these, ask your ball screw or support unit supplier for engineering support. A supplier with application experience can often recommend a standard support unit that fits the duty without a custom housing.
Quality and Verification Points
Support units are precision components. When evaluating a supplier, look for evidence that the bearing bores, mounting faces, and preload are controlled. Dongfeng Bearing, based in Xiamen and founded in 2014, operates under ISO 9001 and provides CMM reports for dimensional verification. The company also holds CE and SGS certifications and offers a 24-month warranty. These are verifiable facts about the company's quality system, not a guarantee of any specific product's performance. Ask for the CMM report for the exact support unit model you intend to purchase, and confirm that the preload and runout values meet your assembly requirements.
FAQ
Can I use two fixed ends on a short screw?
It is possible, but a short screw usually does not need the extra rigidity, and fixing both ends can make assembly sensitive to alignment and thermal growth. Fixedâfloating is simpler and often sufficient.
Does the floating end need a bearing at all?
For short, slow screws, no. For longer screws, a radial support at the free end reduces sag and vibration. The floating bearing should allow axial movement so it does not fight thermal expansion.
How do I know if my screw will whip?
Whipping is a critical-speed phenomenon. Calculate the critical speed using the screw diameter, unsupported length, and end fixity, then compare it to your maximum operating speed. Keep a safety margin. The support unit's own speed limit also applies.
What preload should the fixed-end bearing have?
Preload depends on the required axial rigidity and the expected load. Too little preload allows play; too much increases friction and heat. The bearing manufacturer's catalog and your supplier's application engineering can guide the choice.
Next Step
If you are assembling a ball screw axis and need help matching a support unit to your screw, share your screw diameter, length, maximum speed, axial load, and mounting constraints with a supplier that offers engineering selection support. Dongfeng Bearing provides application support to help buyers confirm the right fixed and floating end configuration before ordering.
This guide is for general selection guidance. Final support unit specifications should be confirmed against your application requirements and the supplier's current data.