Description
SHAC DFNU Double Ball Screw Nuts
Preloaded double-nut ball screw assemblies for rigid, low-backlash CNC positioning
The SHAC DFNU Double Ball Screw Nut range is designed for machine axes that require greater axial rigidity and more controlled backlash than a conventional single nut. Two matched nut bodies are arranged as one preloaded assembly so axial clearance can be reduced while maintaining smooth recirculating-ball motion. This makes the DFNU family suitable for CNC routers, milling machines, precision linear stages, automation equipment and other bidirectional positioning systems where repeatability and stiffness are important.
Select the complete DFNU model and the corresponding SHAC ball screw code rather than choosing by diameter and lead alone. This product contains one selected double ball screw nut; the matching ball screw is ordered separately.
Recommended purchase:
Whenever possible, order the DFNU nut together with its matching SHAC ball screw and choose installation by Vallder before dispatch. Raceway geometry, ball diameter, axial clearance and preload must match the screw correctly; nominal diameter and lead alone are not sufficient to confirm compatibility.
DFNU Models, Matching Ball Screws and Selection Data
| DFNU Model | Product SKU | Matching Ball Screw | Ball Screw SKU | Nominal Diameter | Lead |
| DFNUR1605-4 | 1261001 | SFU / SCR1605 | 1259001 | 16 mm | 5 mm |
| DFNUR2005-4 | 1261002 | SFU / SCR2005 | 1259003 | 20 mm | 5 mm |
| DFNUR2505-4 | 1261003 | SFU / SCR2505 | 1259004 | 25 mm | 5 mm |
| DFNUR2510-4 | 1261004 | SFU / SCR2510 | 1259005 | 25 mm | 10 mm |
| All product data (pictures, dimensions, tables, drawings) are provided as received from the supplier; we exclude liability for their accuracy or completeness. | |||||
Ball Screw & Stepper Motor Calculator
Use the calculator to match ball screw lead and speed with stepper motor torque, current, voltage and step resolution for your CNC axis.
Why Use a Double Ball Screw Nut?
- Higher axial rigidity:
The preloaded double-nut arrangement resists axial deflection more effectively than a comparable non-preloaded single nut. - Reduced axial clearance:
Preload is used to minimise backlash and improve bidirectional positioning behaviour. - Smooth recirculating-ball motion:
Rolling contact provides low friction and efficient rotary-to-linear motion. - Stable reversing accuracy:
The design is suited to axes that frequently change direction and must return consistently to commanded positions. - Machine-ready flange format:
The DFNU geometry integrates into CNC carriages, nut housings and industrial linear-axis structures.
Application:
Suitable for CNC milling and routing axes, engraving machines, precision positioning stages, automation systems, pick-and-place equipment and machine retrofits where low backlash and high stiffness are required.
Operating Principle and Preload
A ball screw converts rotary motion into linear travel through balls circulating between the screw raceway and the nut. In a DFNU assembly, two nut sections are preloaded against one another. This arrangement reduces free axial movement and increases the force required to deflect the axis, improving stiffness and repeatability during changes of direction.
Important:
Preload also increases internal contact force and operating torque. Correct alignment, lubrication and support-bearing setup are essential; excessive misalignment or unsuitable preload can generate heat, noise and premature wear.
Installation and Handling
- Confirm the exact pair:
Verify the complete DFNU model, matching SCR ball screw, diameter, lead and right-hand direction before assembly. - Use a transfer sleeve:
Keep the sleeve firmly against the screw thread while transferring the nut onto or off the ball screw. - Protect the ball circuit:
Never allow the nut to run off the threaded raceway without support because the circulating balls may dislodge. - Align the complete axis:
The nut housing, ball screw, support bearings and linear guides must be aligned to avoid radial or eccentric loading. - Test manually first:
Rotate the assembly by hand and confirm smooth travel before powered operation.
Lubrication and Environmental Protection
Use oil or grease intended for recirculating ball screws and define the maintenance interval according to speed, load, stroke, duty cycle, temperature and contamination. Keep chips, abrasive dust and coolant away from the screw raceway, and add bellows, spiral covers or other protection where the standard seals are not sufficient.
FAQ frequently asked questions
What is the main advantage of a DFNU double nut?
The double-nut arrangement allows preload to reduce axial clearance and increase rigidity, which improves bidirectional positioning behaviour.
Is the matching ball screw included?
No. This product contains one selected DFNU nut assembly. The matching SCR ball screw is ordered separately.
Can I choose by diameter and lead only?
No. Always confirm the complete nut model and matching ball screw code because raceway geometry, ball diameter, clearance and preload must match.
Does preload increase operating torque?
Yes. Preload increases internal contact force, so the axis may require more drive torque than a comparable non-preloaded single-nut assembly.
Can the nut be removed from the screw?
Yes, but only with a suitable transfer sleeve. Unsupported removal can allow the recirculating balls to leave the nut.
When should I use a double nut instead of a single nut?
Use a double nut when the machine requires lower backlash, greater axial rigidity and more stable positioning during direction changes.
