Positioning accuracy drift in ball screws usually traces back to backlash, the axial clearance between nut and screw. This article explains where backlash comes from (nut clearance, wear, support bearings), how preload restores rigidity, and when a single nut is enough versus when a double nut is required. It includes real Dongfeng product data: the DFU double nut zero-backlash series, the SFE ground C5 assembly with pre-loaded double nut (axial play below 0.005 mm, rigidity up to 280 N/µm), and the SFU rolled C7 series.
Your machine commanded a 100 mm move. The linear encoder says it stopped at 100.03 mm. Next cycle it lands at 100.05 mm. Over a shift, the error wanders. The servo is fine, the coupling is tight, and the rails are clean. The problem is often inside the ball screw assembly, and it usually has a name: backlash.
This article explains what ball screw backlash is, where it comes from, how preload restores positioning accuracy, and when a double nut is the right fix. We use real product data from Dongfeng's DFU, SFE, and SFU series to show the difference.
Key takeaways
- Backlash is the axial play between the ball nut and the screw shaft when direction reverses.
- Main sources: nut-to-screw clearance, worn balls or raceways, and loose support bearings.
- Preload removes internal clearance by applying a controlled axial force between two nut halves or a single nut and the screw.
- A double nut with preload can hold axial play below 0.005 mm and rigidity up to 280 N/µm (SFE ground C5 series).
- Single nut is acceptable for low-precision, low-reversal applications; double nut is needed for high-accuracy positioning, heavy loads, or frequent direction changes.
What this article covers
What ball screw backlash actually is
Backlash is lost motion. When the screw reverses direction, the nut does not move immediately. The screw turns a small amount before the balls re-contact the raceway on the opposite side. That gap is backlash, measured in millimeters or arcminutes at the nut.
In a positioning axis, backlash shows up as a consistent offset that changes direction with the load. If you command a move forward and then back to the same point, the two positions differ. The difference is the backlash. Over time, wear can increase it, so the error drifts.
Backlash is not the same as lead error or thermal growth, but all three can coexist. A screw with zero backlash can still have lead error. That is why preload and accuracy grade are separate specifications.
Where backlash comes from
Backlash in a ball screw assembly has three common sources. Identifying the source matters because the fix is different for each.
Nut-to-screw clearance
A standard ball nut is manufactured with a small internal clearance so the balls can roll freely. That clearance is necessary for smooth operation but becomes axial play under reversing loads. Rolled screws with higher lead error often need more clearance, which increases backlash.
Wear on balls and raceways
Over millions of cycles, balls and raceways wear. The contact geometry changes, and the original clearance grows. Contamination accelerates this. A screw that held 0.01 mm backlash when new may drift to 0.05 mm or more after extended service.
Support bearing clearance
Backlash is not only in the nut. The fixed-side support bearing (often an angular contact pair) can have axial clearance. If the bearing housing or locknut loosens, the entire screw shaft moves axially. This is often misdiagnosed as nut backlash. Before replacing the nut, check the support bearings.
How preload removes backlash
Preload applies a controlled axial force inside the nut to eliminate clearance. The balls are pushed into contact with both raceways under load, so there is no free play when direction reverses. The screw and nut act as a rigid unit.
There are two common methods:
- Double nut preload: Two nut halves are separated by a shim or spacer, then locked together. The offset creates opposing contact angles. This is the most common method for high-rigidity, zero-backlash applications.
- Single nut with oversized balls: The nut is assembled with balls slightly larger than the raceway gap, creating internal preload. This is simpler and more compact but offers lower rigidity and is harder to adjust after wear.
Preload does not eliminate lead error. It eliminates axial play. A preloaded screw still needs a correct accuracy grade for the application.
Preload classes and what they mean for accuracy
Preload is specified as a force or as a percentage of dynamic load. Higher preload increases rigidity and reduces elastic deformation under load, but it also increases friction and heat. The right class depends on the load and duty cycle.
| Preload class | Typical application | Effect on backlash |
|---|---|---|
| No preload (standard clearance) | Manual axes, low-speed transfer | Backlash present; not suitable for reversing positioning |
| Light preload (2-3% of dynamic load) | General automation, light cutting | Reduces backlash; some elastic deflection under load |
| Medium preload (5-7%) | CNC machining, pick-and-place | Near-zero backlash; good rigidity |
| Heavy preload (10%+) | Heavy milling, high-rigidity axes | Zero backlash; highest rigidity, higher friction |
For a ground C5 screw with a pre-loaded double nut, axial play can be held below 0.005 mm, and axial rigidity can reach 280 N/µm. That level of rigidity is what keeps positioning accuracy stable under varying cutting forces.
Single nut vs. double nut: when each is right
Not every application needs a double nut. The decision depends on three factors: required positioning accuracy, reversal frequency, and load direction.
When a single nut is sufficient
- Unidirectional loads where the screw always pushes in one direction.
- Low-speed manual adjustment or non-critical positioning.
- Budget-sensitive projects where some backlash is acceptable.
- Rolled screws with C7 accuracy for general automation.
When a double nut is necessary
- Bidirectional positioning with frequent direction changes.
- High-accuracy CNC, EDM, or measuring equipment.
- Heavy cutting loads that would compress a single nut's preload.
- Applications where backlash must stay below 0.01 mm over the machine's life.
A double nut costs more and takes more axial space, but it is the only way to maintain zero backlash under reversing loads without relying on oversized balls that wear quickly.
Dongfeng ball screw options for zero-backlash positioning
Dongfeng manufactures three series that cover the range from general automation to high-precision positioning.
DFU Series Double Nut Zero-Backlash Ball Screw
The DFU Series Double Nut Zero-Backlash Ball Screw is designed for applications that require zero axial play. The double nut is preloaded and locked, eliminating clearance between the balls and raceways. It is suitable for CNC machines, robotics, and any axis where positioning accuracy must not drift with direction changes.
SFE Series Ground C5 with Pre-Loaded Double Nut
The SFE Series Ground Ball Screw Assembly uses a ground screw with C5 accuracy grade and a pre-loaded double nut. Axial play is held below 0.005 mm, and axial rigidity reaches up to 280 N/µm. This series is intended for high-precision machine tools, measuring systems, and semiconductor equipment where thermal stability and rigidity are critical.
SFU Series Rolled C7
The SFU Series Rolled C7 Ball Screw is a cost-effective option for general automation, transport axes, and applications where C7 lead error is acceptable. It can be supplied with a single nut or a preloaded double nut depending on the required backlash level.
FAQ
Can backlash be adjusted after the screw is installed?
On a double nut, yes. The shim or spacer between the two nut halves can be changed to increase or decrease preload. On a single nut with oversized balls, adjustment is not possible without replacing the ball set. For this reason, double nuts are preferred in applications where wear compensation may be needed.
Does preload reduce the life of the ball screw?
Preload increases contact force, which can reduce fatigue life if the preload is too high for the load. The correct preload class balances rigidity against life. For most CNC applications, medium preload (5-7% of dynamic load) provides a good balance.
How do I check if backlash is in the nut or the support bearing?
With the screw disconnected from the drive, apply a small axial force by hand to the screw shaft and measure movement at the nut. If the nut moves relative to the screw, the backlash is in the nut. If the screw shaft moves axially in its housing, the backlash is in the support bearings. A dial indicator on the screw end will show axial movement if the bearing is loose.
What is the difference between backlash and lead error?
Backlash is lost motion when direction reverses. Lead error is the difference between the commanded travel and the actual travel over a distance. A screw can have zero backlash but significant lead error. Both affect positioning accuracy, but they are corrected differently: backlash by preload, lead error by choosing a higher accuracy grade or by compensation in the control.
Next step
If your positioning accuracy drifts and you have ruled out the servo and coupling, measure the backlash at the nut and check the support bearings. If the nut is the source, a preloaded double nut will restore rigidity. Dongfeng can help you select the right series and preload class for your load, speed, and accuracy requirement. Contact us with your application details.