A positioning axis may use an accurate servo motor and a rigid machine frame, yet still fail to achieve the required repeatability or surface quality. In many cases, the limitation comes from the mechanical transmission system between the motor and the moving platform.
A ball screw converts rotary motion into controlled linear movement through rolling contact between the screw shaft, balls and nut. Its practical performance, however, depends on much more than the presence of recirculating balls. Lead accuracy, axial clearance, preload, rigidity, support bearings, installation alignment and lubrication all affect the final positioning result.
Understanding these factors helps engineers select a ball screw as part of a complete motion system rather than treating diameter and lead as the only selection parameters.
How a Ball Screw Transmits Motion
Helical grooves are formed on the screw shaft and inside the nut. The loaded balls travel between these grooves and transfer force through rolling contact. When the screw or nut rotates, the other component moves axially according to the screw lead. A return system circulates the balls back into the loaded zone so that continuous motion is possible.
The theoretical relationship between rotational speed and linear speed is:
v = n × Ph / 60
Where v is linear speed in mm/s, n is rotational speed in r/min and Ph is screw lead in mm.
For example, a 10 mm lead ball screw rotating at 1,500 r/min produces a theoretical linear speed of 250 mm/s. Actual axis performance must also account for acceleration, load, friction, screw deformation and control-system settings.
Rolling Contact and Transmission Efficiency
Conventional sliding screws generate friction across sliding contact surfaces. In a ball screw, rolling elements substantially reduce this friction. Under suitable operating conditions, ball screw transmission efficiency can commonly reach approximately 90% or higher, although the actual value depends on lead angle, preload, lubrication, seals, speed and load.
Lower friction reduces the motor torque required to move a given load and limits heat generation during repeated operation. It also makes rapid acceleration and frequent direction changes more practical. However, high efficiency means that some ball screws can be back-driven by an external axial load. A brake or other holding device may therefore be necessary on vertical axes.
Key Technologies Behind Precision Transmission
| Technical factor | What it controls | System-level effect |
|---|---|---|
| Lead accuracy | Difference between commanded travel and actual travel over a specified stroke | Positioning accuracy and travel consistency |
| Axial clearance | Free axial movement between the screw and nut | Lost motion during direction reversal |
| Preload | Internal loading applied to reduce clearance and increase rigidity | Reversal response, stiffness, heat and running torque |
| Axial rigidity | Resistance to elastic displacement under axial force | Load-position deviation and machining stability |
| Support configuration | Constraint at the machined shaft ends | Critical speed, buckling capacity and total axis rigidity |
| Lubrication and sealing | Condition of the rolling contact surfaces | Wear, temperature, noise and service life |
Lead Accuracy Is Not the Same as Repeatability
Lead accuracy describes how closely the actual axial travel follows the specified travel over a defined distance. It is mainly related to screw manufacturing accuracy and accumulated lead deviation.
Repeatability describes the ability of an axis to return to the same position under the same conditions. A system can return consistently to one position while still having a predictable travel error at another position. Repeatability is influenced by clearance, preload, bearing rigidity, temperature, servo tuning and feedback resolution as well as screw accuracy.
For this reason, selecting a higher accuracy grade alone does not automatically eliminate all positioning errors. The required travel accuracy and repeatability should be specified separately.
Preload, Clearance and Axial Rigidity
When an axis reverses direction, axial clearance can appear as lost motion before the load begins to move in the opposite direction. Preload is used to reduce or eliminate this clearance and improve the contact stiffness between the nut, balls and screw grooves.
Preload may be produced by a double-nut arrangement, an offset-pitch nut or selected ball dimensions, depending on the design. A double-nut ball screw, for example, can apply preload between two nut sections and is often considered where low axial clearance and higher rigidity are required.
More preload is not always better. Excessive preload increases running torque, frictional heat and internal contact stress. It may reduce attainable speed and shorten service life. The correct preload level should match the applied load, duty cycle, positioning requirement and thermal conditions.
Rigidity Must Be Evaluated as a System
The total axial displacement of a feed axis is not determined by the nut alone. The screw shaft stretches or compresses under load, while the nut, support bearings, bearing housing, coupling, mounting surfaces and machine frame also deform.
The weakest part of this mechanical chain can limit the overall rigidity. Increasing nut preload will therefore provide limited improvement if the screw is too slender, the unsupported length is excessive or the bearing housing lacks stiffness.
Thermal expansion must also be considered on long or high-speed axes. Heat generated by the nut, bearings and adjacent equipment can change the effective screw length. Depending on the machine design, thermal effects may be controlled through lubrication, cooling, bearing arrangement, temperature compensation or a tensioned screw installation.
Support Bearings and End Machining
Precision support bearings locate the screw axially and help withstand thrust in one or both directions. The end-support arrangement affects shaft rigidity, critical speed and resistance to compressive buckling.
Common arrangements include fixed–free, fixed–supported and fixed–fixed configurations. Fixed–free mounting is structurally simple but provides lower critical-speed and buckling capability. Fixed–supported and fixed–fixed arrangements are generally more suitable as screw length, rotational speed or axial load increases.
End machining must correspond to the selected bearing units, locknut, coupling and mounting dimensions. Incorrect bearing fits, shoulder geometry or shaft-end concentricity can introduce vibration and positioning error even when the screw itself has adequate accuracy.
Speed Limits and Compressive Load
A ball screw should not be selected by linear speed alone. Rotational speed rises as the requested linear speed increases or the lead decreases. A long rotating screw may approach its critical speed and begin to vibrate. The allowable operating speed also depends on screw diameter, unsupported length, end-support condition, nut circulation design and lubrication.
When the screw shaft is subjected to compression, buckling capacity must also be checked. A smaller root diameter and longer unsupported length reduce the permissible compressive load. Critical speed and buckling calculations should both include an appropriate safety margin rather than using theoretical limiting values as continuous operating conditions.
Load Capacity and Service Life
Dynamic load rating is used when estimating fatigue life under repeated motion, while static load rating relates to the risk of permanent deformation under maximum or impact load. Selection should be based on the actual operating cycle rather than the maximum load alone.
A realistic duty cycle includes acceleration, constant-speed travel, deceleration, dwell time and any changes in load direction. Shock, vibration, poor alignment and contamination can reduce practical life even when a basic rating calculation appears sufficient.
Lubrication, Sealing and Installation
Lubricant separates the rolling contact surfaces, reduces wear and helps control frictional heat. The correct lubricant type and replenishment interval depend on speed, load, stroke, operating temperature and environmental conditions.
Wipers and seals help limit the entry of dust and chips, but they do not replace external protection in heavily contaminated environments. Bellows, covers or enclosed axis structures may be required around machining debris, abrasive particles or liquid contamination.
Installation alignment is equally important. Forcing the nut or support bearings into a misaligned position creates abnormal internal loading. Mounting surfaces should be clean and flat, bearing housings should be aligned with the screw axis, and the nut should be moved through the full stroke before final tightening.
Application Requirements
| Application | Typical priority | Points to confirm |
|---|---|---|
| CNC feed axis | Rigidity and positioning accuracy | Accuracy grade, preload, cutting load and bearing arrangement |
| Automation equipment | Cycle speed and repeatability | Lead, acceleration, stroke, duty cycle and lubrication |
| Inspection equipment | Smooth motion and low position variation | Travel accuracy, thermal stability and installation alignment |
| Vertical lifting axis | Load holding and safety | Buckling, back-driving, motor brake and maximum axial load |
| Long-stroke axis | Critical speed and thermal behavior | Root diameter, unsupported length, supports and operating speed |
Information Required for Selection
Before confirming a ball screw specification, prepare the following operating information:
- Required stroke and overall installation length
- Maximum and continuous linear speed
- Acceleration, deceleration and cycle frequency
- Moving mass, external axial force and impact load
- Required positioning accuracy and repeatability
- Horizontal, vertical or inclined installation
- Expected service life and daily operating time
- Temperature, dust, chips, moisture and other environmental conditions
- Available support-bearing and shaft-end dimensions
These conditions are used to determine screw diameter, lead, nut type, accuracy, preload, load rating, support arrangement and lubrication requirements. For standard industrial positioning, a rolled SFU ball screw may be suitable, while applications requiring different rigidity, clearance or installation characteristics may need another nut configuration.
FAQ
Does a higher accuracy grade guarantee better repeatability?Conclusion
The value of a ball screw in precision transmission comes from the coordinated control of rolling contact, lead accuracy, clearance, preload, rigidity and support conditions. Its final performance is also shaped by lubrication, installation alignment, thermal behavior and the machine control system.
A suitable ball screw is therefore not simply the model with the smallest lead error or highest load rating. It is the specification that meets the required speed, accuracy, rigidity and service-life targets under the actual operating cycle.
References
- ISO 3408 series - Ball screws
- Rolling screw technical guidance covering load rating, rigidity, critical speed and buckling calculations
Need help confirming a ball screw specification?
Send the stroke, speed, load, installation direction and accuracy requirements to DLY for technical confirmation.
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