The basic dynamic load rating of a ball screw, normally written as Ca, is not the maximum load that the screw can carry and is not the recommended payload of a machine.
Ca is a reference value used to calculate rolling-fatigue life. For a ball screw operating under a constant axial load equal to its Ca value, the reference nominal life is generally one million screw revolutions under standardized conditions.
To use this value correctly, the designer must distinguish the ball screw nut rating from the support-bearing rating, calculate the actual equivalent axial load, and convert the resulting revolutions into travel distance or operating hours.
First Correct the Term: Ball Screw Rating or Support-Bearing Rating?
The phrase "ball screw bearing" is often used loosely, but it may refer to two different components.
| Component | Load Rating | What It Evaluates |
|---|---|---|
| Ball screw and nut assembly | Basic dynamic axial load rating Ca | Rolling-fatigue life of the balls, screw raceway, and nut raceway |
| Fixed-side support bearing | Bearing dynamic load rating C or axial dynamic rating | Fatigue life of the angular-contact or thrust-bearing arrangement |
These ratings are separate. A ball screw nut may satisfy the required life while the support bearing is undersized, or the support bearing may be adequate while the ball nut has insufficient Ca.
Both components must therefore be checked independently using their own catalog data and actual load conditions.
Ball screw and nut assembly: Ca refers to the axial rolling-fatigue rating of the screw-and-nut system.
Different nut structures have different Ca values. Nut appearance alone cannot determine load capacity.
What Does the Basic Dynamic Load Rating Ca Actually Mean?
The basic dynamic load rating Ca is the constant axial load, acting in one direction, under which a group of identical ball screws has a nominal life of one million revolutions.
Nominal life is commonly expressed as L10. It means that 90% of a group of identical ball screws operating under the same conditions are expected to complete the calculated number of revolutions without rolling-fatigue flaking.
| Ca Can Be Used For | Ca Cannot Directly Tell You |
|---|---|
| Comparing the fatigue capacity of different ball screw models | The maximum machine payload |
| Calculating nominal life in revolutions | The maximum motor thrust |
| Estimating operating hours from a known motion cycle | Resistance to a stationary shock load |
| Determining the Ca required for a target service life | Screw buckling, critical speed, or support-bearing capacity |
Dynamic Load Rating Ca vs Static Load Rating Coa
The dynamic and static ratings describe different failure conditions and should not be substituted for each other.
| Parameter | Main Purpose | Typical Design Check |
|---|---|---|
| Ca | Rolling-fatigue life during repeated movement | Calculate L10 from the equivalent dynamic axial load |
| Coa | Resistance to permanent contact deformation | Compare Coa with the maximum stationary, peak, emergency-stop, or impact load |
A ball screw can have an acceptable calculated fatigue life but still be damaged by a single load that is too close to or above its static capacity. Both checks are required.
DLY SFU Ball Screw Load Rating Examples
Ball screws with the same nominal diameter can have different dynamic ratings because ball diameter, number of loaded circuits, nut length, contact geometry, and internal design are not necessarily the same.
| Model | Lead | Ball Circuits | Ca | Coa |
|---|---|---|---|---|
| SFU1204-4 | 4 mm | 4 | 593 kgf / 5.82 kN | 1,129 kgf / 11.07 kN |
| SFU1604-4 | 4 mm | 4 | 629 kgf / 6.17 kN | 1,270 kgf / 12.45 kN |
| SFU1605-3 | 5 mm | 3 | 765 kgf / 7.50 kN | 1,240 kgf / 12.16 kN |
| SFU1605-4 | 5 mm | 4 | 780 kgf / 7.65 kN | 1,790 kgf / 17.55 kN |
| SFU1610-3/2 | 10 mm | 3/2 | 721 kgf / 7.07 kN | 1,249 kgf / 12.25 kN |
The complete suffix matters. For example, SFU1605-3 and SFU1605-4 have the same nominal diameter and lead, but their number of ball circuits and static ratings are different.
More DLY model dimensions and rated-load data are available on the SFU ball screw product page .
How to Calculate Ball Screw L10 Life
Under a constant axial load and ideal operating conditions, nominal ball screw life can be estimated with the following formula:
| Symbol | Meaning | Unit |
|---|---|---|
| L10 | Nominal rolling-fatigue life | Revolutions |
| Ca | Basic dynamic axial load rating | N, kN, or kgf |
| Fa | Constant applied axial load | Same unit as Ca |
Ca and the applied load must use the same force unit. Do not enter machine mass in kilograms directly into the formula.
If the catalog gives Ca in kilogram-force:
Include a Load Factor for Vibration and Impact
Real equipment may experience acceleration, deceleration, vibration, shock, installation error, and load fluctuations. A load factor fw can be applied to obtain a more conservative modified load:
Pm is the equivalent mean axial load. The following ranges are commonly used as initial load-factor references:
| Vibration or Impact | Reference Speed | Load Factor fw |
|---|---|---|
| Faint | V ≤ 0.25 m/s | 1.0–1.2 |
| Weak | 0.25 < V ≤ 1 m/s | 1.2–1.5 |
| Medium | 1 < V ≤ 2 m/s | 1.5–2.0 |
| Strong | V > 2 m/s | 2.0–3.5 |
These values are references rather than universal safety factors. The final factor should reflect the machine structure, motion profile, process force, mounting accuracy, and reliability requirement.
Convert Revolutions into Operating Hours
For a reciprocating axis with constant stroke and cycle frequency:
- Lh = calculated operating life in hours
- Lead = ball screw lead in millimeters per revolution
- Stroke = one-way travel in millimeters
- Cycles per minute = complete forward-and-return cycles per minute
Worked Example: DLY SFU1604-4
Consider an SFU1604-4 ball screw with the following catalog data:
- Dynamic load rating Ca: 629 kgf
- Static load rating Coa: 1,270 kgf
- Lead: 4 mm
- Ball circuits: 4
Assume the calculated equivalent axial load is 150 kgf and the machine uses a load factor of 1.2.
The corrected calculation load is:
The calculated modified nominal life is:
With a 4 mm lead, the corresponding theoretical linear travel is:
If the axis has a 300 mm stroke and completes five forward-and-return cycles per minute:
Calculate the Ca Required for a Target Life
If the target operating life is known, the formula can be rearranged:
For the same 300 mm stroke, five cycles per minute, 4 mm lead, 150 kgf load, and fw = 1.2, a target life of 10,000 operating hours requires approximately:
- Required screw rotations: 450 million revolutions
- Required Ca: approximately 1,379 kgf
- Required Ca in SI units: approximately 13.53 kN
Under those assumptions, SFU1604-4 would not meet the target life. The designer would need to reduce the load or cycle frequency, select a higher-capacity nut structure, or increase the ball screw size.
How to Determine the Equivalent Axial Load
A ball screw should mainly carry axial drive force. Radial loads and moment loads should be carried by the linear guide system rather than by the ball screw nut.
For a horizontal axis, the axial load may include:
- Acceleration and deceleration force
- Cutting, pressing, pushing, or process force
- Friction from the guide and seals
- Transmission resistance and other axial losses
For a vertical axis, the calculation must also include gravity acting on the moving mass:
If the load changes during different parts of the motion cycle, an equivalent mean load can be estimated using the cubic mean:
F1, F2, and other values are the axial load magnitudes during different operating stages. The n values represent the number of revolutions or the proportion of operating time at each load.
Acceleration, constant-speed movement, machining, deceleration, and return travel may all have different axial loads. Using only the maximum load for every part of the cycle may overestimate the required rating, while using only the average arithmetic load may underestimate fatigue.
For more information about how the screw and guide share different loads, see Ball Screw and Linear Guide Coordination .
What Determines the Catalog Ca Value?
Ca is a model-specific product rating. It is mainly determined by the internal structure and manufacturing design of the ball screw.
| Factors That Determine Catalog Ca | Conditions That Affect Actual Service Life |
|---|---|
| Ball diameter | Actual equivalent axial load |
| Number of loaded ball circuits | Vibration and shock |
| Contact angle and raceway geometry | Lubrication condition |
| Effective loaded length of the nut | Contamination and sealing |
| Raceway material and heat treatment | Mounting alignment |
| Internal load distribution | Temperature and operating speed |
Poor lubrication or high temperature does not rewrite the catalog Ca printed in the specification table. Instead, it causes the real operating life to fall below the ideal calculated result.
Similarly, a double-nut design should not automatically be assumed to have twice the Ca of a single nut. Double nuts are often selected to provide preload and reduce axial clearance. Their dynamic rating must still be confirmed from the exact model specification.
Dynamic Load Rating Is Only One Part of Ball Screw Selection
A high Ca value does not guarantee that the complete axis is suitable. The following checks must also be completed:
- Static capacity: Compare Coa with the maximum peak, stationary, impact, and emergency-stop load.
- Screw buckling: A long screw under compression may buckle at a load much lower than the nut's Coa.
- Critical speed: A long rotating screw may vibrate or whip before reaching the required RPM.
- Ball circulation speed: The nut's permissible rotational-speed or DN limit must be checked.
- Support bearings: Fixed-side bearings must carry the axial thrust and achieve their own required life.
- Motor torque: The motor must generate the required acceleration and process thrust without overload.
- Rigidity and accuracy: Elastic deformation, support stiffness, preload, and mounting accuracy affect positioning error.
- Vertical-axis safety: A brake, counterbalance, or other holding method may be required because an efficient ball screw can back-drive.
Information Needed for Ball Screw Load and Life Selection
To calculate a suitable ball screw rather than choosing only by diameter, provide the following application information:
- Horizontal, vertical, or inclined installation
- Moving mass
- Maximum and average axial process force
- Required stroke
- Maximum linear speed
- Acceleration and deceleration
- Cycles per minute or daily operating hours
- Expected service life
- Required accuracy and backlash
- Screw support arrangement
- Working temperature and contamination
- Known shock, impact, or emergency-stop conditions
Frequently Asked Questions
Does a Ca value of 629 kgf mean the machine can move 629 kg?
No. The value is an axial-force rating used for life calculation. Machine payload must be converted into actual axial load and must include orientation, acceleration, friction, process force, cycle frequency, and required life.
Does operating below Ca guarantee a long service life?
Not by itself. Life changes according to the cube of the Ca-to-load ratio, but high cycle frequency can accumulate revolutions quickly. Lubrication, alignment, contamination, vibration, and shock also affect actual life.
Does a double nut automatically have twice the dynamic load rating?
No. A double nut is often used to create preload and reduce backlash. The usable Ca depends on the internal structure and the manufacturer's model-specific rating rather than simply the number of nut bodies.
Does speed change the catalog Ca value?
The published Ca is a product rating. Higher speed does not directly change the printed catalog value, but it may increase heat, vibration, lubrication demand, and total accumulated revolutions, reducing actual operating life.
Which rating limits the system: the ball screw nut or the support bearing?
Either one may become the limiting component. The ball screw nut, fixed-side support bearing, screw shaft, coupling, motor, and mounting structure must be checked as one complete axial-drive system.
Contact DLY
Send DLY your required stroke, moving mass, mounting direction, speed, acceleration, cycle frequency, working force, support arrangement, accuracy requirement, and target service life. We can compare the calculated axial load with the Ca, Coa, rigidity, speed, and structural limits of the appropriate ball screw model.
Email: dlyexport2@dlybearing.com | WhatsApp: +86 16605788856
