English
Ball screw nuts are often described with terms such as flange nut, internal circulation, external circulation and preloaded nut. These terms sound like competing “nut types,” but they actually describe different design dimensions.
Aflange nutdescribes how the nut body mounts to the carriage or housing.Internal and external circulationdescribe how the balls return from the end of the loaded raceway to the beginning of the next circulation path.Preloaddescribes how axial clearance and stiffness are controlled.
Understanding these distinctions is essential when comparingball screw nut typesfor alinear module. This guide explains the major nut structures, their engineering trade-offs, and how to select a nut based on space, speed, load, stiffness, backlash and maintenance requirements.
Three Different Questions Define a Ball Screw Nut
A useful way to classify a ball nut is to answer three separate questions:
- How is the nut mounted?Flange, cylindrical, square or another housing form.
- How do the balls recirculate?Return tube/pipe, deflector, return piece, end-cap or another circulation system.
- How is axial clearance controlled?Clearance type, oversized-ball preload, offset preload, double-nut preload or another manufacturer-specific arrangement.
Important:a nut can be a flange nut, use internal circulation and also be preloaded at the same time. These labels are not mutually exclusive.
How a Ball Screw Nut Works
Inside a ball screw, rolling balls carry load between the screw shaft groove and the nut groove. As the screw or nut rotates, the balls travel through the loaded raceway.
Because the balls cannot simply exit the nut after reaching the end of the raceway, the nut includes a recirculation path that redirects them back to an earlier groove.
The recirculation system affects:
- Nut outside diameter
- Nut length
- Available lead and ball size
- Permissible rotational speed
- Noise and vibration behavior
- Manufacturing complexity
- Serviceability
Flange Ball Screw Nuts
Aball screw flange nuthas an enlarged mounting flange that bolts to the moving table, nut housing or machine structure.
The flange may be:
- Round
- Cut-flat
- Square-like
- Custom machined
The exact bolt pattern varies by series and manufacturer.
Why flange nuts are common in linear modules
The flange provides a direct mechanical interface between the ball nut and the carriage. This simplifies axial force transfer and makes assembly straightforward.
Typical advantages include:
- Simple mounting
- Clear axial load path
- Convenient replacement
- Easy integration into a carriage block
Flange shape does not determine circulation method
A flange nut may use a return tube, internal deflector, end cap or another circulation mechanism. The flange is therefore an installation feature, not a ball-return classification.
Cylindrical and Compact Nut Bodies
Some ball nuts use a cylindrical or slim body without a large conventional flange. They may be clamped, threaded or mounted inside a dedicated housing.
These configurations can be useful when:
- Radial space is limited
- The machine uses a custom nut housing
- A compact integrated module is required
- The nut must fit inside a narrow profile
Mounting design must still prevent nut rotation and transmit the full axial load safely.
What Does “Internal Circulation” Mean?
In general engineering usage,internal circulation ball nutsreturn the balls through compact return elements incorporated into or closely integrated with the nut body.
Common examples include deflector-type or return-piece-type mechanisms. A deflector redirects a ball from one groove to a neighboring groove so it can re-enter the loaded path.
Depending on manufacturer terminology, some end-cap systems may also use return passages integrated through the nut body. Because naming conventions vary, the exact return structure should be confirmed from the product drawing.
Advantages of Internal Circulation
Internal circulation designs can provide:
- Compact outside dimensions
- No large external return tube protruding from the nut
- Good packaging inside narrow linear modules
- Balanced nut geometry in some high-speed designs
THK, for example, uses deflector circulation in compact ball screw designs specifically to reduce nut dimensions.
Potential limitations
The exact limitations depend on the return geometry, but designers may need to consider:
- Available ball diameter
- Lead range
- Number of circulation circuits
- Manufacturing complexity
- Accessibility of internal return components
Deflector-Type Ball Nuts
A deflector is a compact return element placed in the nut. It guides the balls across the screw thread to an adjacent loaded groove.
This approach is attractive for:
- Compact automation
- Small nut outside diameter
- Integrated linear modules
- Applications where external return tubes would interfere with packaging
Does deflector circulation automatically mean quieter operation?
No. Noise depends on ball speed, return-path geometry, preload, lubrication and manufacturing quality. Some specially optimized internal-return systems are designed for high-speed and low-noise operation, but “internal circulation” alone does not guarantee a specific noise level.
What Does “External Circulation” Mean?
External circulation ball nutstypically use a return tube or return pipe that guides the balls outside the main nut raceway before returning them to the loaded zone.
The return tube is usually attached to the outside of the nut body, creating a clearly visible recirculation path.
Advantages of External Return-Tube Circulation
Return-tube designs are widely used because they can support a broad range of:
- Screw diameters
- Leads
- Ball sizes
- Load capacities
- Preload arrangements
They are common in industrial ball screws, machine tools and larger screw assemblies.
Packaging trade-off
The main visible difference is that the return tube occupies additional radial space around the nut.
This may matter when the ball screw must fit inside a narrow aluminum linear-module profile.
Internal vs External Circulation
| Comparison | Internal / Deflector-Type | External / Return-Tube-Type |
|---|---|---|
| Packaging | Often more compact | Return tube requires external space |
| Visible return path | Mostly integrated into nut | Return tube visible externally |
| Module integration | Useful in compact profiles | Requires clearance around tube |
| Available designs | Depends strongly on manufacturer and series | Widely used across many industrial sizes |
| Speed/noise | Depends on optimized return geometry | Depends on return-tube geometry and operating condition |
Do not select circulation type from one generic comparison table alone.Permissible speed, load rating and noise should be taken from the actual nut series.
End-Cap Circulation
End-cap ball nuts use return components at the ends of the nut together with return passages that bring the balls back to the opposite end of the loaded circuit.
This architecture is often associated with larger leads or designs requiring efficient circulation over a longer return path.
Depending on manufacturer and series, end-cap designs can be optimized for high speed and high load.
Why end-cap designs matter in linear modules
A high-lead screw can provide greater linear travel per revolution, reducing required screw rpm for a target linear speed. End-cap circulation is therefore relevant in some high-speedball screw moduledesigns.
Return Piece and Other Circulation Designs
Manufacturers also use return pieces, bridge-type returns and proprietary circulation structures.
The practical lesson is that “internal vs external” is a useful high-level distinction, but a serious engineering comparison should identify the exact circulation mechanism used in the selected part number.
What Is a Preloaded Ball Screw Nut?
Apreloaded ball screw nutis designed so that the rolling elements or nut arrangement create an internal axial load even before the external process load is applied.
The purpose is to reduce axial clearance and increase rigidity.
Preload can improve:
- Bidirectional positioning
- Reversal response
- Axial stiffness
- Resistance to small external disturbances
Preload Is Not the Same as Accuracy Grade
A high-accuracy screw can have clearance if it is not preloaded, and a lower lead-accuracy screw can have very low axial play if its nut is preloaded.
These specifications control different performance characteristics:
- Accuracy grade:lead/travel accuracy
- Preload/clearance:axial play and stiffness
Precision linear modules often need both to be considered.
Common Preload Methods
Ball screw manufacturers use several preload arrangements.
Double-nut preload
Two nuts are axially separated or loaded against each other, often with a spacer, to remove clearance.
Advantages can include:
- High axial stiffness
- Adjustable or defined preload
- Strong bidirectional positioning performance
The trade-off is greater nut length and mass.
Offset-lead or single-nut preload
The internal groove relationship is manufactured so different portions of one nut generate opposing axial preload.
This can provide preload in a more compact package than a conventional double-nut structure.
Oversized-ball preload
Slightly oversized balls can be used to create interference between the screw and nut raceways.
This can reduce clearance without using two separate nuts, although the achievable preload and application range depend on the specific design.
How Much Preload Is Better?
More preload is not automatically better.
Higher preload can improve stiffness but also increase:
- Running torque
- Heat generation
- Motor RMS torque
- Lubrication demand
- Wear if lubrication or alignment is poor
Preload should match the required stiffness and reversal performance.Using excessive preload can reduce efficiency and thermal stability.
Clearance-Type Ball Nuts
Not every linear module needs a preloaded nut.
A clearance or low-preload nut can be appropriate when:
- The application is mainly unidirectional
- Absolute reversal accuracy is not critical
- Low running torque is important
- Cost is more important than maximum stiffness
- The axis is used for general transport
The allowable axial clearance should still be compared with the machine's positioning requirement.
Single Nut vs Double Nut
| Feature | Single Nut | Double Nut |
|---|---|---|
| Overall length | Shorter | Longer |
| Mass | Lower | Higher |
| Possible preload method | Oversized ball or offset lead depending on design | Opposed nut preload |
| Axial stiffness | Depends on design | Can be very high |
| Module packaging | More compact | Requires more axial space |
Nut Length Affects Available Stroke
In a fixed overall module length, a longer nut or double-nut assembly can reduce usable stroke because more internal space is occupied by the moving nut package and end clearances.
This matters especially in compact machines where the designer wants the maximum possible stroke from a short module body.
Nut Diameter Affects Module Profile Size
A nut with a large flange or external return tube requires more radial space.
This can influence:
- Aluminum profile width
- Carriage height
- Cover geometry
- Lubrication access
- Motor and coupling alignment
Compact internal-circulation nuts can be valuable when module cross-section is a major design constraint.
Load Capacity Is Not Determined by Nut Style Alone
A return-tube nut is not automatically “heavy duty,” and an internal-circulation nut is not automatically “light duty.”
Ball screw load capacity depends on:
- Screw diameter
- Ball diameter
- Number of loaded circuits
- Contact geometry
- Nut length
- Material and heat treatment
Compare actual dynamic and static load ratings from the selected series.
Circulation Circuits Matter
A ball nut may contain one or several circulation circuits.
More loaded circuits can increase the number of balls sharing the axial load, but the effect on capacity depends on the complete contact design.
The number of circuits also influences:
- Nut length
- Load rating
- Rigidity
- Ball-return layout
High-Speed Selection
For a high-speed ball screw module, circulation quality becomes especially important because balls repeatedly enter and exit the loaded raceway at high frequency.
Check:
- Permissible screw rpm
- Manufacturer dN or DmN limit where specified
- Linear feed speed
- Nut temperature
- Noise
- Lubrication method
Some manufacturers develop specialized deflector or end-return systems specifically for higher speed and smoother circulation.
Noise and Vibration
Ball screw noise comes from several sources:
- Ball recirculation
- Ball-to-groove contact
- Return components
- Preload
- Lubrication
- Screw speed
- Support bearings
Circulation architecture matters, but it should not be used as the only predictor of noise.
Lubrication Requirements
All ball screw nut types rely on appropriate lubrication between rolling elements and raceways.
Lubrication selection should consider:
- Speed
- Load
- Preload
- Temperature
- Contamination
- Operating hours
High preload and high speed generally increase the importance of lubricant condition and heat control.
Seals and Wipers
Ball nuts may use seals or wipers to reduce contamination entering the raceway.
Additional sealing can improve contamination protection but may increase running resistance.
For a linear module, consider the complete protection system:
- Nut seals
- Module cover
- Steel strip
- Bellows
- Machine enclosure
Flange Mounting and Alignment
A flange nut must be mounted so its axis is accurately aligned with the screw and linear guide.
Poor alignment can cause:
- Higher running torque
- Uneven ball loading
- Heat
- Noise
- Premature wear
The nut flange should not be forced into position by tightening bolts against a misaligned carriage.
External Side Load Should Be Carried by the Linear Guide
The ball screw nut is primarily designed to carry axial load.
Side forces and overturning moments from the tool, fixture or workpiece should be carried by the module's linear guide system.
Module design rule:use the ball screw for axial drive and the linear guide for transverse load and moment control.
Nut Type for Compact Linear Modules
When profile size is tightly constrained, engineers often prioritize:
- Small nut outside diameter
- Short nut length
- Integrated circulation
- Single-nut preload
- Simple carriage connection
Internal deflector-type nuts can be attractive in this category, depending on the required lead and load.
Nut Type for High-Rigidity Positioning
For a precision axis where reversal stiffness is critical, selection may favor:
- Preloaded nut
- Higher rigidity design
- Precision support bearings
- Stable mounting structure
Double-nut preload or compact offset-preload designs may both be viable depending on available space.
Nut Type for General Automation
General automation may not need the highest preload or most complex circulation system.
A practical solution should balance:
- Required positioning accuracy
- Repeatability
- Cost
- Stroke
- Maintenance
- Available installation space
Nut Type for High-Thrust Applications
High thrust requires more than a “strong nut.”
Check:
- Dynamic load rating
- Static load rating
- Number of circuits
- Screw diameter and lead
- Support-bearing capacity
- Screw buckling
- Required life
The nut circulation method alone does not define maximum thrust.
How Preload Affects Motor Sizing
Preload adds internal friction torque even when the carriage has no external process load.
Motor sizing should therefore include:
- Linear load torque
- Screw rotational acceleration torque
- Nut preload torque
- Guide and seal resistance
This is especially important for small motors and high-precision axes.
How Nut Selection Affects Thermal Behavior
Ball circulation and preload generate heat.
Higher temperatures can cause screw thermal expansion and positioning drift.
High-precision systems may therefore use:
- Optimized preload
- Controlled lubrication
- Nut or screw cooling
- Warm-up routines
- Temperature compensation
Common Nut-Selection Mistakes
Treating flange, internal circulation and preload as competing categories
They describe different design features and can exist together in one nut.
Assuming internal circulation is always more precise
Positioning accuracy is controlled by screw lead accuracy, preload, support bearings and assembly, not circulation type alone.
Assuming external circulation always carries more load
Load rating depends on screw and nut geometry, circuits and ball contact design.
Choosing maximum preload for every precision axis
Excessive preload increases torque and heat.
Ignoring nut envelope
A return tube or large flange may interfere with the module profile or cover.
Ignoring support and guide stiffness
A high-rigidity nut cannot compensate for flexible bearings, guides or machine structure.
A Practical Ball Screw Nut Selection Workflow
- Define required axial load and life.
- Define screw diameter and lead range.
- Define positioning accuracy and repeatability.
- Determine allowable axial clearance and required stiffness.
- Select preload level if needed.
- Check available nut diameter and length.
- Select a circulation system compatible with speed, lead and packaging.
- Select flange or other mounting geometry.
- Check dynamic and static load ratings.
- Check permissible speed and circulation limits.
- Check preload torque and motor capacity.
- Confirm lubrication and sealing.
- Validate alignment with the linear guide and support bearings.
What QRXQ Needs to Select a Ball Screw Nut
- Ball screw diameter
- Lead
- Stroke
- Axial load
- Required speed
- Acceleration
- Positioning accuracy
- Repeatability
- Allowable backlash or axial clearance
- Available nut envelope
- Required service life
- Working environment
The best ball screw nut is selected from several dimensions at once.Mounting shape, ball circulation and preload solve different engineering problems and should not be collapsed into one simple “nut type” ranking.
QRXQ evaluates ball screw nut selection from load, speed, precision, stiffness and module packaging, then matches flange geometry, circulation architecture and preload to the complete linear-axis requirement.
