In precision mechanical transmission, ball screws are key components that convert rotational motion into linear motion. Their performance directly affects positioning accuracy, rigidity, and service life. Single nut ball screws (SFU Ball Screw) and double nut ball screws (DFU Ball Screw) are the two main types. Although they work on the same principle, they differ significantly in structure, performance, and application.
Structural Differences
The core of a ball screw is: screw shaft + nut + circulating balls. The main difference between single and double nut designs is the number of nuts and how they interact.
Single Nut Ball Screw (SFU Ball Screw):
Contains one screw shaft and one nut with internal ball circulation. Balls circulate inside a single nut groove. The structure is compact. Installation is simple, no adjustment between nuts is required.
Double Nut Ball Screw (DFU Ball Screw):
Contains one screw shaft and two independent nuts. The nuts are combined by spacers, thread preload, or spring preload. Balls circulate in both nuts. By adjusting the relative positions of the nuts, axial clearance can be eliminated.
Key Performance Comparison
Performance is the core difference. It affects actual machine operation. We can compare them in four aspects:
Axial Clearance: Zero Backlash vs Inherent Gap
Axial clearance is the maximum axial movement of the nut when the screw is fixed. It determines positioning accuracy.
Single Nut Ball Screw:
Due to machining tolerances, there is inherent axial clearance (usually 0.01–0.05 mm). Lower accuracy grades have larger gaps. During direction reversal, clearance causes "lost motion," affecting precision.
Double Nut Ball Screw:
Preload is applied between the two nuts (e.g., using spacers to slightly offset them). Balls are always slightly compressed in the grooves, eliminating clearance. High-precision DFU screws can achieve <0.001 mm clearance, even negative clearance, increasing rigidity.
Rigidity: Higher Support vs Single Support
Rigidity is the ability to resist deformation under load. Insufficient rigidity causes small deflections and reduces motion accuracy.
Single Nut Ball Screw:
Rigidity depends on the nut-screw fit, ball number, and material hardness. Under high axial load, elastic displacement may occur, causing precision fluctuations.
Double Nut Ball Screw:
Both nuts contact the screw simultaneously, creating double support. Preload increases ball-groove contact pressure. Rigidity can be 30–50% higher than SFU; some models even double. In CNC cutting, higher rigidity reduces tool vibration and improves surface finish.
Load Capacity: Higher vs Moderate
Load capacity depends on ball contact area and force distribution.
Single Nut Ball Screw:
Balls circulate in one nut only. Max dynamic and static loads are limited by nut size. Suitable for light to medium loads, e.g., small automation or office equipment.
Double Nut Ball Screw:
Balls in both nuts share the load, doubling contact area. Dynamic load can increase by 20–40%. Can handle higher axial loads and overturning moments. Ideal for heavy CNC tables or robot joints.
Service Life: Longer vs Limited
Service life relates to load distribution and wear speed.
Single Nut Ball Screw:
Operating near rated load accelerates wear. Clearance-induced impact during reversal further reduces life, 20–30% shorter than DFU.
Double Nut Ball Screw:
Preload distributes load evenly across two nuts. Reduced individual ball stress and zero backlash lower wear. Lifespan is significantly longer, especially in continuous industrial operations like semiconductor wafer handling.
Suitable Applications
There is no absolute "better" type, only a "more suitable" type.
Single Nut Ball Screw (SFU):
Best for low-precision, light-load applications.
Examples: non-industrial 3D printers, small conveyors, office printers, household appliances.
Reason: Lower precision requirement (0.1 mm sufficient) and smaller cost (30–50% lower than DFU).
Double Nut Ball Screw (DFU):
Best for high-precision, heavy-load, frequent reversal.
Examples: CNC milling or turning, industrial robots (≥10 kg payload), semiconductor equipment (micron-level positioning), precision CMM.
Reason: Zero backlash and high rigidity improve machine performance and reduce scrapped products.
Summary
To choose between single and double nut ball screws, ask three questions:
Is positioning accuracy requirement below 0.01 mm? If yes → DFU.
Does the machine require frequent direction reversals? If yes → DFU.
Is the axial load higher than the SFU's dynamic load rating? If yes → DFU.
In short:
SFU Ball Screw: economical, suitable for basic transmission.
DFU Ball Screw: high precision, heavy load, industrial-grade performance.
The choice reflects a balance between cost and performance, a core principle in mechanical design.
We provide ball screws including SFU single nut, DFU double nut, SFS silent type, SFE E type nut, SFY Y type nut with screw length customizable.
Email:export@dlybearing.com
