The main difference between a single-nut and double-nut ball screw is how the nut assembly controls axial clearance and preload. A single-nut design uses one nut body, while a double-nut design uses two nut sections loaded against each other, usually through a spacer or another adjustment structure.
Double-nut ball screws are commonly selected when higher axial rigidity and more controlled preload are required. Single-nut ball screws remain a practical choice when compact dimensions, lower mass, lower running resistance and cost are more important.
Important: Nut quantity alone does not determine lead accuracy, load capacity or service life. The exact model, preload, number of loaded ball circuits, screw diameter, lead and accuracy grade must also be compared.
Single-Nut Ball Screw Structure
A single-nut ball screw contains one nut body with one or more recirculating ball circuits. The balls roll between the screw-shaft raceway and the corresponding raceway inside the nut.
Single-nut designs are available in several structures, including standard flange nuts, cylindrical nuts, internal-circulation nuts and end-cap circulation nuts. DLY series such as SFU, SFA, SFE and SFI use different single-nut structures for different installation and operating requirements.
A single nut may be supplied with:
- Positive axial clearance
- Reduced or controlled clearance
- Light preload achieved through ball selection
- Internal offset preload, depending on the nut design
Therefore, it is not correct to assume that every single-nut ball screw has large backlash. The actual axial clearance or preload specification should be confirmed for the selected model.
Double-Nut Ball Screw Structure
A double-nut ball screw uses two nut sections on the same screw shaft. The two nuts are loaded in opposite axial directions so that both sides of the ball-to-raceway contact remain engaged.
A common design places a spacer between the two nut sections. Changing the effective spacer thickness changes their relative axial position and therefore the preload applied to the ball contacts.
DLY DFU and DFI series use double-nut structures. Compared with a standard single nut of a similar diameter and lead, they generally require more axial installation space.
Single Nut vs Double Nut: Main Differences
| Item | Single nut | Double nut |
|---|---|---|
| Structure | One nut body | Two opposed nut sections |
| Axial length | Usually shorter | Usually longer |
| Weight | Usually lower | Usually higher |
| Preload method | Ball selection or an internal offset structure, depending on model | Opposed nuts with a spacer or adjustment mechanism |
| Axial clearance | Depends on the selected clearance or preload specification | More readily controlled through the opposed-nut preload structure |
| Axial rigidity | Suitable for many general motion systems; varies with preload and nut design | Generally selected when higher bidirectional rigidity is required |
| Running torque | Usually lower when little or no preload is applied | May be higher because of preload and additional ball contacts |
| Heat generation | Usually lower under comparable conditions | Can be higher if substantial preload is used |
| Cost | Generally lower | Generally higher |
| Installation space | Better for compact assemblies | Requires more space along the screw axis |
Difference in Backlash and Axial Clearance
Backlash in a ball screw is the lost axial motion that may occur when the direction of thrust reverses. It is influenced by the clearance between the balls and raceways, but the measured lost motion of a complete axis can also include:
- Support-bearing displacement
- Nut-housing movement
- Coupling clearance
- Screw-shaft elastic deformation
- Machine-frame and bracket deformation
- Control-system response
A preloaded double nut reduces internal axial clearance by maintaining contact on opposite sides of the raceways. However, "zero backlash" should not be treated as an absolute guarantee for the complete machine axis.
A correctly designed preloaded single nut can also provide low axial clearance and stable positioning. The final selection should therefore compare the specified clearance and rigidity, not only the number of nut bodies.
Difference in Axial Rigidity
Preload increases axial rigidity by reducing displacement when the external load changes direction. A double-nut structure provides a practical way to apply preload between two opposed ball-contact groups.
This can benefit machine axes that experience:
- Frequent bidirectional positioning
- Reversing cutting force
- High rigidity requirements
- Small allowable elastic displacement
- Changing axial loads during operation
The rigidity of the complete axis also depends on the screw-shaft length and diameter, support bearings, bearing span, nut bracket and machine frame. Installing a double nut cannot compensate for a flexible support structure.
Does a Double Nut Carry Twice the Load?
No. A double-nut ball screw should not be assumed to have twice the load capacity of a single-nut model.
The basic dynamic and static load ratings depend on:
- Screw diameter and lead
- Ball diameter
- Number of loaded circuits
- Internal contact geometry
- Nut design
- Material and heat treatment
- Applied preload
Preload is itself an internal load. When calculating fatigue life, the preload condition must be considered together with the external operating load. An excessive preload can reduce life even when it improves rigidity.
Always compare the load-rating data of the exact single-nut and double-nut models rather than estimating capacity from the number of nuts.
Does a Double Nut Improve Lead Accuracy?
Not by itself. Lead accuracy describes how closely the actual axial travel follows the specified travel over the screw length. It is determined mainly by the manufacturing process and accuracy grade.
For example, a C5 single-nut ball screw can have better lead accuracy than a C7 double-nut ball screw. The double nut may provide higher axial rigidity and lower clearance, but it does not automatically change the lead accuracy of the screw shaft.
When positioning performance is important, evaluate both:
- Accuracy grade: controls travel deviation.
- Clearance or preload: influences reversal behavior and axial rigidity.
Running Torque, Heat and Speed
A non-preloaded or lightly loaded single nut generally has lower starting and running torque than a substantially preloaded double nut of a comparable size.
As preload increases:
- Axial rigidity generally increases.
- Lost motion generally decreases.
- Friction torque increases.
- Heat generation may increase.
- Motor torque requirements may increase.
- Fatigue life may decrease if the preload is excessive.
For high-speed axes, preload must be balanced against temperature rise and running resistance. A double-nut design should not automatically be selected only because the machine moves quickly.
Installation and Maintenance Differences
A single nut is shorter and usually easier to package into a compact machine. The nut housing and surrounding structure can also be smaller.
A double nut requires enough axial space for both nut sections and the preload structure. Installation must avoid twisting or distorting the assembly. The nut bracket should support the complete mounting face without forcing the two nuts out of alignment.
Do not casually grind, replace or modify the preload spacer. A small change in spacer thickness can significantly alter preload torque and internal loading. Preload adjustment should follow the manufacturer's procedure and be verified through torque or displacement measurement.
Neither type should be removed from the screw shaft without a suitable transfer tube or retaining tool. Otherwise, the balls may leave their circulation path.
When to Choose a Single Nut
A single-nut ball screw is generally suitable when:
- Installation space is limited.
- Lower moving mass is preferred.
- Moderate axial rigidity is sufficient.
- The application can accept the specified axial clearance.
- A preloaded single-nut design already meets the reversal requirement.
- Lower running torque is important.
- Cost control is a priority.
Common examples include compact automation equipment, general CNC auxiliary axes, 3D printers, packaging machinery and positioning mechanisms with moderate rigidity requirements.
DLY SFA nuts are one example of a compact single-nut structure.
When to Choose a Double Nut
A double-nut ball screw may be more appropriate when:
- The axis frequently reverses direction.
- Low axial clearance is required.
- Higher bidirectional rigidity is needed.
- The application is sensitive to elastic displacement under changing load.
- The machine has sufficient axial installation space.
- The motor can accommodate the specified preload torque.
- The additional cost and assembly complexity are justified by the performance requirement.
DLY supplies double ball nuts for applications requiring a double-nut preload structure.
Information to Confirm Before Selection
Before deciding between a single and double nut, provide:
- Screw diameter and lead
- Required stroke and overall length
- Maximum and average axial load
- Speed and acceleration
- Required lead accuracy
- Permissible axial clearance
- Required axial rigidity
- Available nut installation space
- Duty cycle and required life
- Mounting direction
- Nut and shaft-end drawings
Conclusion
Single-nut ball screws are compact, lighter and generally more economical. They are available with different clearance and preload conditions and can meet the requirements of many automation and machine-tool axes.
Double-nut ball screws use two opposed nut sections to provide more controllable preload, reduced axial clearance and higher bidirectional rigidity. Their trade-offs include greater length, weight, running torque, heat generation and cost.
The selection should not be based on the assumption that a double nut is always more accurate or has twice the load capacity. Compare the actual load rating, preload, accuracy grade, rigidity and dimensional data of the specific models.
View DLY's complete ball nut range or send us your application parameters and installation drawing for model confirmation.
