Ball Screw Load Capacity Explained: From Rated Load to Actual Bearing Power

If you've ever looked at a ball screw datasheet, you might have seen values like "Dynamic Load Rating 629 kgf" and wondered: does that mean this screw can support 629 kilograms of weight? And how does the mounting orientation-horizontal or vertical-affect it?
In this guide, we will explain ball screw load capacity, clarify the difference between dynamic and static load ratings, and show how to calculate actual bearing loads. By the end, you'll confidently choose the right screw for your application.

Ball screws are core components in linear motion systems, widely used in CNC machines, engraving machines, and automation equipment. Understanding their load capacity is crucial for safety, precision, and lifespan.

Many engineers and buyers face questions like:

What does dynamic load rating really mean?
How does static load rating differ from actual load?
Is "629 kgf" equivalent to 629 kg of weight?
How does horizontal vs vertical installation affect capacity?

We'll answer all of these with examples from DLY SFU1604 and SFU1204 ball screws.

Ball screw load capacity mainly includes three parameters:

Dynamic Load Rating (Ca)

The axial force a ball screw can handle during continuous operation.

Determines the lifespan: operating near Ca gives ~1 million rotations life.

Static Load Rating (Coa)

Maximum axial force a stationary screw can withstand.

Exceeding Coa may cause permanent deformation in the balls or raceway.

Actual Working Load (P)

The real axial force your equipment imposes on the screw, including workpiece weight and motor torque.

Directly affects lifespan and safety.

Tip: Do not assume Ca equals the maximum weight your machine can handle.

Many buyers mistake kgf for kg:

kg: mass, tells you how heavy something is.
kgf: force, tells you how much force an object produces under gravity.
Conversion: 1 kgf ≈ 1 kg × 9.81 m/s² ≈ 9.81 N

Quick understanding:

1 kg weight produces roughly 1 kgf of axial force.
Seeing "Dynamic Load 629 kgf" means the screw can handle the axial force equivalent to ~629 kg weight-not that it supports 629 kg mass directly.

Rated load is a reference for lifespan, not an absolute maximum:

Dynamic Load Rating (Ca): ensures lifespan under continuous operation.
Static Load Rating (Coa): ensures safety limit; exceeding may damage the screw.

Ignoring this distinction can lead to premature failure.

Unit	Meaning	Conversion	Notes
N	Newton (force)	1 N = 0.10197 kgf	SI unit
kgf	Kilogram-force	1 kgf = 9.81 N	Common in machinery
kg	Mass	-	Not force

Remember: kgf is axial force, not the mass of the object.

How you mount a screw changes its bearing capacity:

Horizontal: load mainly supported by bearings, higher capacity.
Vertical: must bear the screw's own weight; load capacity decreases, anti-backdrive measures needed.
Support type matters:

Support Type	Feature	Use Case
Fixed-Free	One end rigid, one end free	Short stroke, low load
Fixed-Supported	Both ends supported	Medium-long strokes
Fixed-Fixed	Both ends rigid	High precision, high load

Mounting and support type directly affect lifespan and actual load.

DLY SFU1604 (16mm diameter, 4mm lead):

Dynamic Load Ca = 629 kgf
Static Load Coa = 1270 kgf

Safe Load Calculation:

Actual Safe Load=Ca×Safety Factor

Horizontal, safety factor 0.6–0.7 → recommended 377–440 kgf
Vertical, safety factor 0.3–0.5 → recommended 188–315 kgf

For long-term operation, keep load below 50–70% of Ca to ensure safety and lifespan.

Model	Orientation	Safety Factor	Recommended Long-term Load (kgf)
SFU1204	Horizontal	0.6–0.7	356–415
SFU1204	Vertical	0.3–0.5	169–282
SFU1604	Horizontal	0.6–0.7	377–440
SFU1604	Vertical	0.3–0.5	188–315

Vertical loads are significantly lower; always consider anti-backdrive design.

Don't confuse kgf with kg.
Rated load ≠ maximum load; consider safety and lifespan.
Vertical installation requires anti-backdrive measures.
Support type affects actual capacity.
For long screws or high loads, consider larger diameter or preloaded double-nut screws.

To summarize:

Dynamic Load (Ca): lifespan reference
Static Load (Coa): safety limit
Actual Load: depends on mounting, support, and safety factor
kgf ≠ kg: treat it as axial force

With DLY SFU1204 / SFU1604, horizontal long-term load: 356–440 kgf; vertical: 169–315 kgf. Understanding these concepts lets you select screws confidently, avoid premature failure, and maximize machine performance.

Bottom line: knowing load ratings-from rated to actual-ensures your ball screws work safely and last longer.

Contact now