Start With What SFU1605 Actually Means
The model is fairly straightforward:
- 16 mm — nominal screw diameter
- 05 — 5 mm lead
A 5 mm lead means the nut moves approximately 5 mm for every revolution of the screw.
That is one reason SFU1605 is useful for smaller CNC and automation axes. It provides a reasonable amount of linear thrust without requiring a large screw assembly.
But the 16 mm diameter does not tell us the usable machine load by itself.
The ball nut, number of circuits, preload, screw length, support bearings, operating speed, acceleration, and installation all affect the result.
Don't Turn the Catalog Load Rating Into a Payload Number
This is probably the most common misunderstanding.
A ball screw's dynamic load rating is used when calculating fatigue life. It is not a recommended payload for the machine.
For example, different SFU1605 manufacturers and nut configurations can publish different dynamic and static load ratings. One specification may show around 6.8 kN dynamic capacity, while another configuration may be closer to 7.5–7.8 kN.
So there is no single “SFU1605 load capacity” that applies to every screw sold under that model.
More importantly, even if the catalog says 7 kN, that does not mean the machine should be designed around a 7 kN working load.
The load rating is part of the calculation. It is not the answer to the whole calculation.
A Horizontal CNC Axis Is a Good Example
Let's say we have a CNC axis with a 40 kg moving assembly.
It is easy to look at the 40 kg figure and think the ball screw has to support 40 kg.
On a horizontal axis, that is not really how the load is distributed.
The linear guides carry the weight of the table. The ball screw mainly provides the force needed to accelerate the moving mass, overcome friction, and resist the cutting force.
For acceleration, the basic relationship is:
F = ma
With a 40 kg moving mass and 1 m/s² acceleration:
F = 40 N
Increase the acceleration to 5 m/s²:
F = 200 N
The actual axis will also have guide friction, cutting forces and other losses, but this example shows why “40 kg payload” and “40 kg axial load on the screw” are not the same thing.
This is also why I would not select a ball screw from payload alone.
Vertical Z-Axis Is a Different Situation
Put the same 40 kg assembly on a vertical axis and the calculation changes immediately.
Now gravity acts along the screw axis.
The gravitational force is approximately:
40 × 9.81 = 392 N
If the axis is also accelerating upward at 1 m/s²:
F = m(g + a)
So the required upward force becomes approximately:
40 × (9.81 + 1) = 432 N
And that is still before considering friction, cutting force, acceleration margins, and the rest of the mechanical system.
This is one reason a ball screw that works comfortably on a horizontal axis may need a much closer check when used vertically.
The motor matters here too. A vertical axis needs enough torque to hold and move the load, and the machine should have an appropriate safety arrangement rather than relying on the ball screw alone to prevent the load from dropping.
The 5 mm Lead Matters Too
SFU1605 has a relatively small lead, which gives the motor useful mechanical advantage.
A simplified thrust relationship is:
F ≈ 2πTη / L
where:
- F is axial thrust
- T is motor torque
- η is ball screw efficiency
- L is lead
Because the lead is only 5 mm, the screw can generate useful axial force without requiring extremely high motor torque.
The trade-off is speed.
At 2,000 rpm, a 5 mm lead gives:
5 × 2,000 = 10,000 mm/min
or about:
10 m/min
That may be perfectly reasonable for a compact CNC axis, but if the machine needs much higher linear speed, the screw RPM rises quickly.
At that point, I would start looking at screw length, critical speed, vibration and motor capability rather than focusing only on load capacity.
Screw Length Can Become the Real Limitation
This is easy to overlook when someone specifies an SFU1605.
A short 16 mm screw and a long 16 mm screw are not the same mechanical problem.
As the unsupported length increases, screw stiffness, deflection and critical-speed limitations become more important.
For a longer SFU1605, I would want to know:
- Actual screw length
- Supported or fixed-end arrangement
- Maximum RPM
- Operating speed
- Required positioning performance
- Whether the screw is rotating continuously or only intermittently
If the screw is long and the machine is expected to run at high speed, the question may stop being “Can the screw carry the load?” and become “Can the screw rotate at the required speed without excessive vibration?”
Those are two different problems.
What About Static Load?
Static load rating is useful, but I would not use it as a machine payload specification either.
Static capacity is related to the load level at which permanent deformation of the balls or raceways becomes a concern.
It is useful when checking whether the screw can tolerate a particular static or peak load.
It does not mean that the machine should operate continuously at that value.
A CNC axis can experience short peaks from acceleration, cutting forces or impact that are very different from its normal operating load.
That is why the complete load cycle matters.
Service Life Is Where the Load Rating Becomes Useful
For basic ball screw life calculations, the commonly used relationship is:
L₁₀ = (C/P)³ × 10⁶
where:
- L₁₀ is the basic rated life in revolutions
- C is the dynamic load rating
- P is the equivalent axial load
The useful point here is the cube relationship.
If the actual working load is well below the dynamic rating, the calculated basic fatigue life can be much longer.
But that does not mean the screw will actually last that long in a machine.
Lubrication, contamination, alignment, preload, installation quality and operating conditions can all affect real service life.
In other words, the calculation tells you something about fatigue life. It does not replace mechanical inspection or good machine design.
Can SFU1605 Handle a 100 kg Load?
It depends on what “100 kg load” means.
Suppose a 100 kg table is moving horizontally on properly sized linear guides.
The ball screw does not necessarily carry the full 100 kg as axial load. Most of that weight is supported by the guide system.
Now take the same 100 kg and put it on a vertical Z-axis.
The screw has to work against gravity continuously, and the motor has to provide enough torque to hold and accelerate the load.
Same mass.
Very different ball screw requirement.
This is why I would be cautious about answering an inquiry with a simple statement such as:
“SFU1605 can handle 100 kg.”
Without knowing the installation direction, acceleration, speed and screw length, that answer doesn't tell the customer much.
Preload Changes the Behavior of the Nut
The ball screw size is not the whole story.
Nut preload also affects the machine.
A properly preloaded nut can reduce internal clearance and improve axial rigidity and reversal behavior.
But increasing preload also increases contact force between the balls and raceways.
That means higher friction and potentially higher running torque and temperature.
For a precision machining axis, that additional rigidity may be useful.
For a small CNC router where speed and low motor load are more important, excessive preload may not provide enough benefit to justify the extra resistance.
I would therefore treat preload as part of the machine design, not simply something to increase whenever backlash appears.
Don't Forget the Linear Guides
A ball screw is not supposed to carry every mechanical load in the axis.
The linear guides normally take the table or carriage load and react to side forces and moments.
If the guide system is undersized or badly aligned, increasing the ball screw size will not necessarily solve the problem.
This is especially important when the machine has a large overhung load.
The ball screw may have enough axial capacity while the guide blocks, mounting plate or machine structure are actually the limiting components.
When Does SFU2005 Make More Sense?
If the SFU1605 is getting close to its mechanical limits, moving to a larger screw such as SFU2005 can be a reasonable option.
A larger screw can provide greater load capacity and stiffness, but there are trade-offs:
- Larger nut and support components
- More installation space
- Higher rotating mass
- Different motor inertia
- Different critical-speed behavior
- Higher overall cost
So I would not automatically replace SFU1605 with SFU2005 just because the larger screw has a higher load rating.
If the machine only needs a few hundred newtons of axial force, the larger screw may add little practical benefit.
If the machine is carrying a heavy vertical load, running a long screw at high speed, or needs greater stiffness, then the larger diameter becomes more interesting.
What I Would Ask Before Quoting an SFU1605
If a customer sends an inquiry saying:
“SFU1605, 800 mm, please quote.”
I can quote the part, but I cannot really tell whether it is the right part yet.
For an application check, I would want to know:
- Screw diameter
- Lead
- Travel or screw length
- Moving mass
- Horizontal or vertical installation
- Maximum speed
- Acceleration
- Cutting or external force
- Required positioning accuracy
- Required repeatability
- Motor power and type
- Duty cycle
- Required service life
- Nut configuration
- Preload requirement
For a replacement application, I would also ask about the existing screw:
- Current backlash
- Operating hours
- Noise or vibration
- Running temperature
- Whether resistance changes along the travel
- Existing nut type and preload
That information often tells us more than the payload number.
So, How Much Load Can SFU1605 Handle?
There isn't a single useful kilogram figure.
SFU1605 is a 16 mm × 5 mm ball screw, and the actual load capacity depends on the specific ball nut, manufacturer, load rating, installation and operating conditions.
For a compact horizontal CNC axis with properly sized linear guides, moderate acceleration and a reasonably short screw, SFU1605 can be a practical choice even when the moving assembly weighs tens of kilograms.
For a vertical axis, long travel, high acceleration or heavy cutting loads, the selection needs more calculation.
The important thing is to calculate the axial force that the screw actually sees, then check that against the ball screw's dynamic and static ratings, required life, critical speed, motor torque and mechanical support.
That is a much more useful way to answer the SFU1605 load question than simply converting a catalog number into kilograms.
The ball screw should be selected for the machine's actual load cycle, not just the weight of the table.
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