What Is the Dynamic Load Rating of a Ground Ball Screw? Ca and Life Calculation

The basic dynamic load rating of a ground ball screw, normally identified as Ca, is a catalog value used to calculate the screw's nominal rolling-fatigue life under axial load.

It is not the maximum load that the machine can move, and it should not be treated as a recommended continuous operating load. A ball screw operating at a load equal to its Ca rating has a basic nominal life of only one million revolutions under defined reference conditions.

What Does the Dynamic Load Rating Ca Mean?

When a ball screw operates under load, the balls repeatedly contact the raceways of the screw shaft and ball nut. These repeated contact stresses can eventually produce rolling-fatigue damage, usually beginning as surface flaking on a ball or raceway.

The basic dynamic axial load rating Ca provides a standardized reference for estimating when this fatigue may occur. The rating normally assumes:

• A constant axial load
• No change in load direction
• Correct installation and alignment
• Adequate lubrication
• No abnormal shock, contamination, or temperature effect
• A reference life of 10⁶ revolutions

The term "90%" is important. The nominal life is a statistical rating for a group of identical ball screws, not a guarantee that every individual screw will fail at exactly the calculated number of revolutions.

Does a Ground Ball Screw Automatically Have a Higher Ca Rating?

No. "Ground" describes how the screw raceway is manufactured and the level of lead accuracy that can be achieved. It does not define one fixed dynamic load rating.

A precision-ground ball screw may offer better lead accuracy, smoother raceway geometry, tighter preload control, and more consistent motion than a standard rolled ball screw. However, these advantages do not automatically mean that every ground ball screw has a higher Ca value.

The catalog dynamic load rating is mainly determined by the complete internal design, including:

• Nominal screw diameter
• Ball diameter and quantity
• Number of loaded ball circuits
• Effective nut length
• Raceway profile and contact angle
• Ball circulation structure
• Screw, nut, and ball materials
• Heat treatment and raceway hardness

Two ground ball screws with the same diameter and lead can therefore have different dynamic load ratings if they use different nut lengths, ball circuits, ball diameters, or internal structures.

Likewise, two ball screws with the same C3 or C5 accuracy grade do not necessarily have the same Ca value. Accuracy grade and dynamic load rating must be checked as separate specifications.

For more information about lead accuracy, see What Is the Accuracy of a Ground Ball Screw?

Dynamic Rating, Static Rating, and Working Load Are Different

A ball screw can satisfy its static safety requirement but still have insufficient fatigue life. It can also have an acceptable calculated fatigue life while remaining vulnerable to a severe emergency stop, impact load, or shaft buckling.

For an example comparing dynamic and static ratings, read SFU1605 Ball Screw Load Capacity: Ca and Coa Ratings.

How to Calculate Ball Screw Nominal Life

For a simplified basic calculation under constant axial load and suitable operating conditions, the nominal life can be estimated using:

Where:

• L₁₀ = basic nominal life in revolutions
• Ca = basic dynamic axial load rating
• Fm = equivalent mean axial load

Ca and Fm must use the same force unit, such as N, kN, or kgf. Do not divide a value in newtons by a value in kilogram-force without first converting the units.

Why a Small Load Increase Can Greatly Reduce Life

Because the load ratio is raised to the third power, fatigue life falls rapidly as the equivalent operating load approaches Ca.

These values are mathematical rating-life results, not recommended operating-load limits. A real machine still requires margins for shock, preload, lubrication, contamination, alignment, temperature, and application risk.

Worked Example: From Ca to Revolutions, Travel, and Hours

Consider an illustrative ground ball screw with:

• Dynamic load rating Ca = 20 kN
• Equivalent mean axial load Fm = 5 kN
• Lead = 10 mm
• Average screw speed = 300 rpm

Step 1: Calculate Nominal Life in Revolutions

Step 2: Convert Revolutions into Travel Distance

Step 3: Convert Revolutions into Operating Hours

This example is only a basic fatigue-life calculation. It does not include load factors, preload, downtime, acceleration changes, shock, contamination, lubrication deterioration, or other machine limits.

How to Calculate Equivalent Mean Axial Load

Most machines do not operate under one constant load. Acceleration, machining, return travel, holding, and deceleration may each create different axial forces.

For several load stages acting in the same direction, the equivalent mean load can be estimated using a cubic mean:

Here, q represents the revolutions or travel distance completed under each load. If operating speed remains constant, the proportion of operating time may also represent the proportion of revolutions.

For example, assume one cycle contains:

• 2 kN for 70% of the travel
• 6 kN for 30% of the travel

The simple arithmetic average would be only 3.2 kN, which underestimates the fatigue effect of the 6 kN load stage. Peak operating loads matter strongly because fatigue life follows a cubic relationship.

If the axial load changes direction, the positive-direction and negative-direction loads should be evaluated separately according to the manufacturer's calculation method. The more demanding result is then used for life evaluation.

How to Determine the Actual Axial Load

The machine's moving mass is not automatically equal to the axial load on the ball screw. The load path depends on whether the axis is horizontal, vertical, or inclined.

Horizontal Axis

For a horizontal axis, the ball screw normally does not directly support the full weight of the table. The linear guideways support that weight. The screw mainly overcomes:

• Acceleration and deceleration force
• Guideway, seal, and wiper friction
• Machining, pressing, or process force
• Cable-chain and auxiliary resistance
• Misalignment or abnormal assembly resistance

Vertical Axis

For a vertical axis, gravity acts directly along the screw. The moving mass therefore becomes a major part of the axial load.

During downward acceleration, deceleration, emergency stopping, or regenerative motion, the magnitude and direction of the load may change. A complete duty cycle should therefore be calculated rather than checking only the lifting condition.

How Preload Affects Ground Ball Screw Life

Precision-ground ball screws are often preloaded to reduce axial clearance and improve rigidity. However, preload creates internal ball-raceway contact load before the machine applies any external force.

Preload does not mean that the catalog Ca value becomes higher. Instead, it changes the internal loading condition that must be considered during life calculation.

This is especially important when:

• The external operating load is relatively low
• A high preload level is used for rigidity
• The machine operates continuously at high speed
• Temperature rise changes the preload condition
• Misalignment creates additional internal loading

For a preloaded precision ball screw, use the preload and life-calculation method specified for the exact nut structure. Do not simply insert the external machine load into the basic formula and ignore internal preload.

Conditions That Shorten Actual Service Life

The calculated L10 value assumes suitable installation and operating conditions. Actual service life may be shorter when the screw is exposed to:

• Poor alignment: creates uneven ball loading and excessive torque.
• Insufficient lubrication: increases friction, heat, and raceway wear.
• Contamination: introduces abrasive particles into the ball circulation path.
• Shock and vibration: create loads higher than the normal calculated thrust.
• Incorrect preload: increases internal stress and temperature.
• Thermal expansion: may change alignment or increase axial constraint.
• Support-bearing error: can create shaft runout and additional resistance.
• Nut-bracket distortion: forces the nut away from the screw's natural axis.

These conditions normally do not change the Ca number printed in the product catalog. They change how closely the real machine can approach the calculated rating life.

Why Ca Alone Cannot Select a Ground Ball Screw

A screw with an adequate dynamic load rating may still be unsuitable for the machine. The complete selection should also check:

Practical Ground Ball Screw Selection Process

1. Define the complete duty cycle. Include travel, speed, acceleration, process force, dwell time, and direction changes.
2. Calculate each axial load stage. Separate normal motion, acceleration, machining, braking, and emergency conditions.
3. Calculate the equivalent mean load. Use a cubic mean rather than a simple arithmetic average.
4. Set the required service life. Define the required revolutions, travel distance, cycles, or operating hours.
5. Determine the required Ca. Select a screw with sufficient calculated fatigue-life margin.
6. Check Coa and peak loads. Include impact, braking, and emergency-stop forces.
7. Check screw length and speed limits. Verify buckling and critical speed using the actual support arrangement.
8. Select accuracy and preload separately. Do not assume that a higher precision grade automatically provides higher load capacity.
9. Review the complete assembly. Confirm nut dimensions, support units, shaft-end machining, coupling, lubrication, and sealing.

View DLY ball screw products for rolled and precision-ground ball screw configurations, nut types, support units, and customized shaft-end machining.

DLY Ground Ball Screw Selection Information

DLY supplies cold-rolled and precision-ground ball screws for CNC machines, industrial automation, precision positioning equipment, and customized motion assemblies.

Depending on the model and project requirements, available specifications may include C5 and C3 precision grades, customized lengths, ball nut configurations, preload requirements, support units, and shaft-end machining.

For reference, DLY uses approximately:

• C3 lead accuracy: ±0.008 mm per 300 mm
• C5 lead accuracy: ±0.018 mm per 300 mm

These accuracy values do not determine the dynamic load rating. The final Ca and Coa values must be confirmed from the exact screw diameter, lead, nut type, ball circuits, and product specification.

Conclusion

The dynamic load rating Ca of a ground ball screw is a standardized reference used to calculate nominal rolling-fatigue life. It is not the maximum allowable payload and does not mean that the screw should continuously operate at that load.

A ground manufacturing process can improve lead accuracy, motion consistency, and preload control, but the Ca value is mainly determined by the ball screw's internal structure, dimensions, ball circuits, contact geometry, materials, and heat treatment.

Correct selection requires the equivalent mean axial load, required service life, preload, peak load, static rating, screw length, rotational speed, support arrangement, accuracy, lubrication, and installation conditions to be checked together.

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