The lead accuracy of a ball screw refers to the representative deviation and variation of the travel error relative to the effective nut travel or the effective threaded length of the screw shaft. It also includes the variation measured over any 300 mm length of the effective thread and over one revolution (2π rad).
[Travel Error Diagram]
Nominal Travel (l₀) The axial travel for any number of revolutions based on the nominal lead.
Standard Lead (Phs) The lead with corrections applied to account for predicted deformations due to temperature rise and load application.
Target Value of Representative Travel (c) The target value obtained by presetting the standard travel with a positive or negative offset.
Standard Travel (lₛ) The travel for any number of revolutions based on the standard lead.
Actual Travel (lₐ) The actual axial displacement of the nut relative to an arbitrary rotation angle of the screw shaft.
Representative Travel (lₘ) A straight line representing the trend of the actual travel, determined from the actual travel curve using the least-squares method or a similar approximation.
Representative Travel Error (eₚ) The difference between the representative travel and the standard travel corresponding to the effective travel or effective threaded length.
Variation (Vᵤ) The maximum amplitude of the actual travel distance between two parallel lines representing the representative travel.
Variation over 300 mm (V₃₀₀) The maximum amplitude of the actual travel over any 300 mm of the effective threaded section.
Variation over One Revolution (V₂π) The max. amplitude of the actual travel over one revolution (2π rad) of the effective threaded section.
| Effective Thread Length (mm) |
Accuracy Grade | C3 | C5 | |||
|---|---|---|---|---|---|---|
| Over | Up to | ±ep | Vu | ±ep | Vu | |
| - | 100 | 8 | 8 | 18 | 18 | |
| 100 | 200 | 10 | 8 | 20 | 18 | |
| 200 | 315 | 12 | 8 | 23 | 18 | |
| 315 | 400 | 13 | 10 | 25 | 20 | |
| 400 | 500 | 15 | 10 | 27 | 20 | |
| 500 | 630 | 16 | 12 | 30 | 23 | |
| 630 | 800 | 18 | 13 | 35 | 25 | |
| 800 | 1000 | 21 | 13 | 40 | 27 | |
| Precision Grade | C3 | C5 | |||
|---|---|---|---|---|---|
| Item | V300 | V2π | V300 | V2π | |
| Permissible Value | 8 | 6 | 18 | 8 | |
| Precision Grade | C7 | C10 |
|---|---|---|
| V300 | 52 | 210 |
The representative travel error (eₚ) for C7 and C10 is calculated using the following formula:
ep = ± (lu / 300) × V300 lu：Effective thread length (mm)
The standard material, heat treatment, and hardness of DINGS' ball screws are shown in the table below. Values may vary slightly depending on the series and model; please refer to the specifications provided by DINGS'.
| Component | Material | Heat Treatment | Thread Surface Hardness |
|---|---|---|---|
| Screw Shaft | SUJ2 (JIS G 4105) | Induction hardening | HRC 58–62 |
| S55C (JIS G 4105) | Induction hardening | HRC ≥58 | |
| SUS440C | Quenched and tempered | HRC ≥55 | |
| Ball Nut | SCM420H (JIS G 4105) | Carburized and hardened | HRC 58–62 |
| SUS440C | Quenched and tempered | HRC ≥55 |
Note: S55C material is used for rolled ball screws, while SUJ2 material is used for ground ball screws.
In general, a standard single-nut ball screw has a small axial clearance between the screw shaft and the nut.
When an axial load is applied, the sum of this axial clearance and the elastic displacement caused by the load increases the clearance, resulting in backlash.
To eliminate such backlash, the axial clearance of the ball screw must be made negative by applying elastic deformation in advance between the screw shaft and the nut—this method is referred to as preload.
The combinations of axial clearance and accuracy grades for DINGS' ball screws are shown in the table below.
| Accuracy Grade | Axial Clearance | |||
|---|---|---|---|---|
| Z (Preload) | T (≤0.005 mm) | S (≤0.02 mm) | N (≤0.05 mm) | |
| C3 | ● | ● | ● | ● |
| C5 | ● | ● | ● | |
| C7 | ● | ● | ||
| C10 | ● | ● | ||
Applying preload not only eliminates axial clearance in ball screws, but also reduces axial displacement caused by axial loads, thereby increasing stiffness.
The figure below illustrates the difference in elastic displacement under axial load between a clearance-type ball screw and a preloaded (zero-clearance) ball screw (theoretical values). As shown, preload reduces elastic displacement, resulting in improved stiffness.
Elastic Displacement Curves for Clearance-Type and Preloaded Specifications
The preload amount should be determined based on the required stiffness or allowable backlash. However, applying preload may result in the following effects:
Therefore, the preload amount should be set as low as possible while meeting performance requirements.
Ball screws are generally preloaded using the double-nut preload method, in which spacers (shims) are inserted between two nuts. Leveraging the characteristics of miniature ball screws, DINGS' ball screws adopt a large-ball preload method, where steel balls slightly larger than the clearance between the screw shaft and the nut are inserted.
This method completely eliminates clearance using only a single nut, maintaining a compact structure. In addition, spacer balls slightly smaller than the preload balls are alternately used to prevent deterioration of motion performance.
Direct measurement and control of ball screw preload are difficult. Therefore, preload is typically managed by converting it to preload running torque and controlling it through torque measurement. The preload running torque value is specified in the specification drawing.
To ensure proper preload (zero axial clearance), running torque is always measured under defined conditions. As a result, differences in lubrication and operating conditions may cause variations in measured torque. Please also note that starting torque (the torque required to initiate motion) is slightly higher than the running torque.
Torque Measurement Example
Note: The torque variation illustrated is intentionally exaggerated for explanatory purposes.
DINGS' ball screws are coated with rust-preventive oil for long-term storage. Before use, remove the oil with clean refined kerosene and apply lubricating oil or grease. Grease can be applied prior to shipment upon request; however, long-term storage with grease may cause rust.
Note:The rust-preventive oil is for corrosion protection only and provides no lubrication. Using the ball screw without removing this oil may reduce service life and cause increased torque or abnormal heat generation.
Lubrication is essential when using ball screws. Insufficient lubrication may cause increased torque and shortened service life. Proper lubrication suppresses temperature rise due to friction, loss of mechanical efficiency, and accuracy degradation caused by wear. Ball screws can be lubricated with grease or oil.
Selecting the appropriate lubricant according to the application is particularly important. For miniature ball screws, grease churning resistance may increase torque. DINGS' provides proprietary greases optimized for ball screw performance.
Recommended Lubricants
| Lubricant Type | Category | Product Name |
|---|---|---|
| Grease | Lithium-based Grease | AFG Grease |
| Lubricating Oil | Slideway oil or turbine oil | Super Multi 68 |
When using grease lubrication, inspections should be conducted every 2–3 months; when using oil lubrication, inspections should be conducted weekly. During inspection, check the lubricant quantity and contamination, and replenish as necessary. When adding new grease, remove old and discolored grease as thoroughly as possible.
| Lubrication Method | Inspection Interval | Inspection Items | Replenishment / Replacement Interval |
|---|---|---|---|
| Automatic Intermittent Lubrication | Weekly | Oil quantity, contamination | Replenish appropriately at each inspection based on reservoir capacity |
| Grease | Initial operation: 2–3 months | Contamination, chips, foreign matter | Typically replenished once per year; adjust based on inspection results and remove discolored old grease |
| Oil Bath | Before daily operation | Oil level | Adjust appropriately according to consumption |
