SFU1605 is a commonly used ball screw specification for CNC equipment, compact linear modules, engraving machines and industrial automation axes. The designation generally identifies a 16 mm nominal screw diameter and a 5 mm lead, meaning that the nut travels 5 mm for each complete revolution of the screw.
However, "SFU1605" alone does not determine one fixed load capacity. The basic dynamic load rating and basic static load rating depend on the ball nut structure, number of loaded ball circuits, internal geometry and manufacturing specification. For example, DLY supplies both SFU1605-3 and SFU1605-4 configurations, and their static load ratings are different.
Short answer: DLY's SFU1605-3 has a basic static axial load rating of 1,240 kgf, while SFU1605-4 has a basic static axial load rating of 1,790 kgf. These are reference ratings-not the recommended working payload of the machine.
SFU1605-3 and SFU1605-4 Load Ratings
The following specifications are based on DLY's current 1605 ball screw product data:
| Model | Nominal diameter | Lead | Ball circuits | Dynamic rating Ca | Static rating Coa | Rigidity K |
|---|---|---|---|---|---|---|
| SFU1605-3 | 16 mm | 5 mm | 3 | 765 kgf ≈ 7.50 kN |
1,240 kgf ≈ 12.16 kN |
17 kgf/μm |
| SFU1605-4 | 16 mm | 5 mm | 4 | 780 kgf ≈ 7.65 kN |
1,790 kgf ≈ 17.55 kN |
20 kgf/μm |
kgf means kilogram-force, not kilograms of payload. One kilogram-force equals approximately 9.80665 newtons. Therefore, a static rating of 1,790 kgf is approximately 17.55 kN, but it does not mean that the machine can safely move a 1,790 kg load.
Machine orientation, acceleration, cutting force, shock, support arrangement, screw buckling, mounting accuracy and required service life must all be considered before determining the permissible moving mass.
What Does SFU1605 Mean?
The model designation provides basic dimensional information:
- SFU: A flanged single ball nut series;
- 16: Nominal screw shaft diameter of 16 mm;
- 05: Nominal lead of 5 mm per revolution;
- -3 or -4: The nut configuration, including the number of ball circuits used in DLY's model designation.
The 5 mm lead determines linear travel per revolution. For example, a screw speed of 1,000 rpm theoretically produces a linear speed of 5,000 mm/min, before considering acceleration and the permissible operating speed of the complete assembly.
The complete model must be confirmed when ordering. A replacement 1605 ball nut should also be checked for flange dimensions, overall length, circuit number, preload or clearance and compatibility with the existing screw shaft.
What Is the Basic Static Axial Load Rating?
The basic static axial load rating, identified as Coa or C0a, is a standardized reference value associated with a specified level of permanent deformation at the most heavily loaded contact between the balls and raceways.
It should not be interpreted as a recommended continuous operating load or an absolute failure point. Operating close to the static rating may create permanent indentations in the balls or raceways, especially when shock, vibration, impact or installation error is present.
Permanent raceway indentation can produce:
- Uneven running resistance;
- Repetitive noise at a specific screw position;
- Higher motor torque and temperature;
- Reduced positioning accuracy;
- Accelerated wear and shorter service life.
The static rating is particularly important when the ball screw is stationary or moving very slowly under a high load, or when the system is exposed to impact and emergency-stop forces.
What Is the Basic Dynamic Axial Load Rating?
The basic dynamic axial load rating, identified as Ca, is used to calculate the nominal fatigue life of a ball screw operating under load. It is not the maximum load the screw can carry during one movement.
For ball screws, the basic rating life in revolutions can be estimated using:
Basic rating life:
L10 = (Ca ÷ Fm)³ × 106 revolutions
Where:
- L10 = basic rating life in revolutions;
- Ca = basic dynamic axial load rating;
- Fm = equivalent mean axial load.
Because the load ratio is raised to the third power, even a moderate increase in equivalent operating load can substantially reduce calculated fatigue life.
This formula provides a basic reference under defined conditions. Actual service life can be shortened by contamination, poor lubrication, misalignment, excessive preload, shock loading, temperature and manufacturing or installation variations.
Static Rating and Dynamic Rating Are Not Interchangeable
| Parameter | Basic static rating Coa | Basic dynamic rating Ca |
|---|---|---|
| Main purpose | Evaluate permanent deformation risk | Calculate nominal fatigue life |
| Most relevant condition | Stationary, low-speed, shock or peak-load conditions | Repeated movement under operating load |
| Selection check | Static safety factor | Required travel life or operating life |
| Should it equal working load? | No | No |
Both checks are necessary. A ball screw might satisfy the static safety requirement but still have insufficient fatigue life. Conversely, it might have an acceptable calculated dynamic life but still be vulnerable to a severe peak or impact load.
How to Calculate the Static Safety Factor
The static safety factor can be expressed as:
Static safety factor:
fs = Coa ÷ Fmax
Where:
- fs = static safety factor;
- Coa = basic static axial load rating;
- Fmax = maximum axial load expected in the application.
The maximum axial load should include more than the normal running force. Depending on the application, it may include:
- Weight acting on a vertical axis;
- Acceleration and deceleration force;
- Machining or pressing force;
- Friction from guides and seals;
- Emergency-stop force;
- Impact, vibration or unexpected obstruction.
The required safety factor depends on motion conditions, impact, accuracy requirements and the consequences of failure. It should be selected using the manufacturer's engineering guidance and the application's risk level rather than applying one universal value to every machine.
Why SFU1605-4 Has a Higher Static Rating
DLY's SFU1605-4 uses four ball circuits, compared with three circuits in SFU1605-3. The additional loaded circuit distributes the axial load over more ball-to-raceway contacts.
As a result, SFU1605-4 has:
- A higher basic static load rating: 1,790 kgf versus 1,240 kgf;
- A slightly higher dynamic load rating: 780 kgf versus 765 kgf;
- Higher listed rigidity: 20 kgf/μm versus 17 kgf/μm;
- A longer nut body than the three-circuit version.
SFU1605-4 may be preferred when higher static capacity and rigidity are required and the machine has enough installation space for the longer nut. SFU1605-3 can remain suitable for compact axes with lower load requirements.
The higher rating does not automatically make SFU1605-4 the correct choice. Installation space, required stroke, nut length, expected life, accuracy and cost should also be considered.
Can SFU1605 Carry 1,790 kg?
No. The 1,790 kgf value listed for SFU1605-4 is its basic static axial load rating, not a recommended payload of 1,790 kg.
For a horizontal axis, the ball screw does not normally lift the entire machine mass directly. It mainly overcomes acceleration force, guide friction and external process force. For a vertical axis, gravity acts directly along the screw, so the moving mass becomes a major part of the axial load.
For example, two machines with the same 100 kg moving mass can impose very different loads on the ball screw:
- A slowly moving horizontal table with low-friction linear guides may require relatively little continuous thrust.
- A fast vertical axis must resist gravity and provide additional thrust for acceleration and deceleration.
- A machining or pressing axis may experience external force much higher than the force caused by moving mass alone.
Therefore, the allowable payload cannot be determined from Coa alone.
Other Limits That Must Be Checked
Even when the SFU1605 nut has sufficient load rating, the complete ball screw assembly may be limited by other factors.
Screw Shaft Buckling
A long, slender screw under compressive axial load can buckle before the ball nut reaches its static load rating. The permissible compressive load depends on screw root diameter, unsupported length and end-support arrangement.
Critical Speed
A rotating screw can vibrate or whip when it approaches its critical speed. The limit depends on shaft diameter, mounting length and support configuration. A long SFU1605 screw may therefore have a lower permissible rotational speed than a shorter screw of the same model.
Support-Bearing Capacity
The fixed-end bearings, support units and locknut must withstand the expected axial load and operating speed. Selecting a ball nut with a higher rating does not increase the capacity of an undersized support-bearing arrangement.
Screw Accuracy and Rigidity
Load capacity does not indicate positioning accuracy. A C7 rolled ball screw and a C5 ground ball screw may use the same nominal diameter and lead but serve different accuracy requirements.
Lubrication and Contamination
Insufficient lubrication, chips, dust and coolant can reduce actual service life even when the calculated load is within the rated range. Protection and maintenance should be planned according to the operating environment.
How to Select Between SFU1605-3 and SFU1605-4
Use the following process rather than selecting only from the highest load figure:
- Calculate the maximum axial load, including gravity, acceleration, process force and shock.
- Calculate the equivalent mean axial load over the complete operating cycle.
- Check the static safety factor using Coa and the maximum axial load.
- Calculate the required fatigue life using Ca and the equivalent mean load.
- Verify screw buckling for compressive-load applications.
- Check critical speed using the screw length and end-support arrangement.
- Confirm that the nut length and flange dimensions fit the machine structure.
- Check support bearings, coupling, motor torque and installation accuracy.
DLY's detailed 1605 ball screw specifications include dimensional and load-rating information for SFU, DFU, SFI, DFI and other 1605 nut configurations. These models should not be treated as having identical capacity simply because they share a 16 mm diameter and 5 mm lead.
What Information Should Be Provided for Selection?
To review whether SFU1605 is suitable, provide the following details:
- Horizontal, vertical or inclined installation;
- Moving mass;
- External machining, pressing or operating force;
- Maximum speed and motor speed;
- Acceleration and deceleration time;
- Stroke and total screw length;
- Required service life or operating cycles;
- Accuracy and backlash requirements;
- End-support configuration;
- Operating environment and lubrication method;
- Required end machining or existing drawing.
These values allow the load rating, life, buckling resistance, critical speed and installation dimensions to be checked as one complete system.
Conclusion
DLY's SFU1605-3 has a basic dynamic load rating of 765 kgf and a basic static load rating of 1,240 kgf. SFU1605-4 has a dynamic rating of 780 kgf and a static rating of 1,790 kgf.
These values are engineering reference ratings, not the allowable payload of the machine. Correct selection also requires the actual axial load, safety factor, desired fatigue life, screw length, operating speed, support arrangement and installation direction.
SFU1605-4 provides higher static capacity and rigidity, while SFU1605-3 offers a shorter nut body for applications with lower load requirements. The final model should be selected from the complete operating conditions rather than diameter and lead alone.
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