A pretensioned ball screw system applies a calculated initial tensile force to the screw shaft through its support-bearing arrangement. A non-pretensioned system installs the screw without this intentional axial stretch and commonly allows thermal expansion toward a floating or supported end.
Shaft pretension can improve axial rigidity and dynamic stability in long, fast or precision machine axes, but it also increases bearing load, installation sensitivity and thermal-design requirements. It should not be confused with ball nut preload, which controls clearance between the balls, nut and screw raceways.
What Ball Screw Pretension Means
In a pretensioned ball screw axis, the screw shaft is normally constrained by fixed bearing groups at both ends. During assembly, one bearing group is displaced or adjusted to stretch the screw by a calculated amount before the final locking operation.
The resulting tensile force remains in the screw shaft after installation. This initial force can increase the system's resistance to axial deformation and help maintain a more stable rotating shaft, provided that the screw, bearings, housings and machine structure are designed for the additional load.
A non-pretensioned ball screw is installed without intentional axial stretching. A common arrangement uses one fixed end to locate the screw axially and one floating or supported end to support it radially while allowing axial thermal movement.
Pretensioned vs Non-Pretensioned Ball Screw Comparison
| Comparison | Pretensioned Ball Screw | Non-Pretensioned Ball Screw |
|---|---|---|
| Initial shaft condition | Screw shaft is intentionally stretched by a calculated amount | No intentional axial stretch is applied |
| Typical support arrangement | Fixed–fixed bearing arrangement | Frequently fixed–floating or fixed–supported |
| Axial rigidity | Generally higher when the complete bearing and housing system is correctly designed | Generally lower but sufficient for many conventional machine axes |
| Thermal behavior | Thermal expansion changes the initial tensile force and must be calculated | Floating end allows axial expansion, although screw elongation can still affect position |
| Dynamic stability | Can improve stability of long or high-speed rotating screws | Suitable for moderate speed and conventional screw lengths |
| Bearing load | Support bearings continuously carry the initial pretension force | No additional bearing load from deliberate shaft pretension |
| Installation | Requires accurate alignment, controlled extension and correct locking | Simpler installation with greater thermal freedom |
| Typical application | Long, fast, high-rigidity or precision feed axes | General CNC, automation and positioning equipment |
How Shaft Pretension Affects Ball Screw Performance
Axial Rigidity
An axially loaded screw shaft stretches or compresses under operating force. Initial tension can help reduce the effect of compressive loading and improve the axial rigidity of a correctly designed fixed–fixed system.
The final rigidity does not depend on the screw alone. Support-bearing stiffness, bearing preload, bearing-housing rigidity, shaft-end dimensions, nut mounting and machine structure must all be considered.
Critical Speed and Vibration
A long rotating ball screw can vibrate as its speed approaches the system's critical speed. Stronger end restraint and controlled shaft tension can improve dynamic stability, but pretension does not remove every speed limit.
Root diameter, unsupported length, support arrangement, straightness, installation alignment, ball circulation speed, bearing speed and lubrication must still be checked before selecting the maximum operating speed.
Thermal Expansion
When the screw temperature rises, its free thermal elongation can be estimated during initial design by:
Where ΔL is the change in length, α is the material's thermal expansion coefficient, L is the effective screw length and ΔT is the temperature change.
In a fixed–floating arrangement, the screw can expand toward the floating end, but its effective lead and position relative to the fixed reference still change as the screw becomes longer.
In a pretensioned fixed–fixed arrangement, heating changes the original tensile force. If the initial pretension and expected temperature rise are properly matched, the arrangement can help control the screw's thermal behavior. If the calculation or installation is incorrect, the bearings and shaft may experience excessive or unstable internal force.
For a detailed thermal calculation, see Ball Screw Thermal Expansion Calculation and Compensation.
Heat and Service Life
Higher internal load is not always better. Excessive shaft pretension or bearing preload can increase running torque, bearing friction, heat generation and fatigue loading. The design should use only the force required to meet the rigidity, speed and thermal-performance targets.
Screw-Shaft Pretension Is Not Ball Nut Preload
The terms are sometimes used interchangeably in general descriptions, but they refer to different parts of the ball screw system.
| Adjustment | Where It Is Applied | Main Purpose |
|---|---|---|
| Screw-shaft pretension | Along the screw shaft through the end-support system | Improve system rigidity and control long-screw dynamic or thermal behavior |
| Ball nut preload | Between the balls and screw/nut raceways | Reduce axial clearance and increase nut rigidity |
| Support-bearing preload | Inside the fixed-side angular-contact bearing group | Reduce bearing clearance and increase support rigidity |
A machine can use a preloaded ball nut without applying shaft pretension. It can also use a pretensioned screw shaft while still requiring the ball nut preload and support-bearing preload to be specified separately.
When to Use Pretensioned or Non-Pretensioned Ball Screws
Consider a Pretensioned System When:
- The screw is relatively long and rotates at high speed.
- High axial rigidity is important to the machine process.
- The axis is sensitive to vibration or dynamic instability.
- The expected screw temperature and thermal elongation can be calculated.
- The machine has accurately aligned fixed-bearing housings at both ends.
- The support bearings can carry the calculated initial and operating loads.
- Installation and maintenance can follow a controlled pretension procedure.
Consider a Non-Pretensioned System When:
- The screw length and operating speed are moderate.
- A fixed–floating arrangement provides sufficient rigidity.
- Simple installation and thermal freedom are priorities.
- The application does not justify the additional bearing and housing requirements.
- Thermal error is acceptable or handled through compensation or external feedback.
- The equipment uses standard BK/BF, FK/FF or EK/EF support arrangements.
Before choosing either system, provide the screw diameter, lead, total and unsupported length, rotational speed, axial load, mounting orientation, accuracy requirement, expected temperature change and proposed bearing arrangement.
For a comparison of fixed–fixed, fixed–floating and fixed–free support methods, see Ball Screw Support Bearing Arrangements.
DLY supplies rolled and ground ball screws with shaft-end machining and matching support-unit options. Send the screw model, length, speed, load, accuracy requirement, operating temperature and installation drawing for confirmation.
Email: export@dlybearing.com
