Bearing Load Calculator

What Is a Bearing Load Calculator?

A bearing load calculator is an engineering tool that:

• Combines radial and axial loads
• Applies shock or application factors
• Uses the manufacturer’s dynamic load rating (C)
• Calculates L10 life based on ISO 281 standards

The result shows how long 90% of identical bearings are expected to last before fatigue begins.

That 90% reliability level is important. It means if you install 100 identical bearings under the same conditions, at least 90 of them should reach the calculated life.

Understanding the Key Inputs

To use a bearing life calculator correctly, you need to understand the main variables.

1. Radial Load (Fr)

Radial load acts perpendicular to the shaft.

Examples:

• Belt tension on a pulley
• Gear mesh forces
• Weight of rotating components

This is often the dominant load in electric motors and gearboxes.

2. Axial Load (Fa)

Axial load, also called thrust load, acts parallel to the shaft.

Examples:

• Helical gears
• Pumps generating thrust
• Vertical shaft loads

If your system has both radial and axial forces, you must calculate a combined load.

3. Bearing Type

Different bearings handle loads differently:

• Ball bearings use point contact
• Roller bearings use line contact

This difference changes the life exponent value in the ISO formula:

• Ball bearing exponent: p = 3
• Roller bearing exponent: p = 10/3 (≈3.33)

Even small changes in load can dramatically change life because the load is raised to a power.

How Equivalent Dynamic Load (P) Is Calculated

If there is only radial load:

P = Fr

If both radial and axial loads exist:

P = X·Fr + Y·Fa

Where:

• X = radial factor
• Y = axial factor

These factors depend on bearing geometry.

Typical values:

• Deep groove ball bearing: X = 0.56, Y = 1.5
• Angular contact bearing: X = 0.39, Y = 0.76

After that, the calculator applies the application factor (Ka) to account for shock and vibration:

P_eq = P × Ka

This adjusted value is what the life calculation uses.

The ISO 281 L10 Bearing Life Formula

The core formula is:

L10 (million revolutions) = (C / P)^p

Where:

• C = Basic dynamic load rating
• P = Equivalent dynamic load
• p = Life exponent (3 or 3.33)

To convert to hours:

L10h = (1,000,000 / (60 × RPM)) × L10

This tells you operating life at a specific speed.

What the C/P Ratio Means

The calculator also shows the C/P ratio. This is one of the most important indicators in bearing selection.

• C/P < 1 → Bearing will fail immediately
• C/P < 3 → Heavy load, short life
• C/P between 3 and 15 → Ideal range
• C/P > 15 → Load may be too light

Too light? Yes.

When load is extremely low, rollers can skid instead of roll. This damages raceways and reduces bearing life. Many engineers overlook this.

The calculator visually shows this using a load bar so you can quickly see where your design stands.

Real Example Calculation

Let’s say:

• Radial load = 5,000 N
• Axial load = 1,500 N
• Bearing C rating = 35,000 N
• Speed = 1750 RPM
• Application factor = 1.2
• Deep groove ball bearing

Step 1:
P = (0.56 × 5000) + (1.5 × 1500)
P = 2800 + 2250 = 5050 N

Step 2:
P_eq = 5050 × 1.2 = 6060 N

Step 3:
C/P = 35000 / 6060 ≈ 5.78

This falls within the ideal operating range.

From there, the L10 life can be calculated in hours and years. The calculator handles all this instantly.

Why Application Factor (Ka) Matters

Machines rarely operate under perfect lab conditions.

Different equipment produces different shock levels:

• Electric motors → 1.0
• Belt drives → 1.2
• Pumps and machining → 1.5
• Crushers → 2.5

Ignoring shock loads can reduce actual life by half or more.

The calculator automatically multiplies the load by Ka to give a more realistic result.

When to Use a Bearing Load Calculator

You should use a bearing life calculator when:

• Selecting a new bearing
• Upgrading equipment
• Diagnosing premature bearing failure
• Comparing ball vs roller bearings
• Verifying manufacturer recommendations

It helps prevent both undersizing and oversizing.

Oversizing may seem safe, but it increases cost and can cause low-load skidding problems.

Design Status Output Explained

The calculator classifies the result into clear categories:

OVERLOADED
The equivalent load exceeds capacity. Immediate redesign required.

HEAVY LOAD
Life is short. Improve lubrication, cooling, or choose larger bearing.

OPTIMAL
Operating within proper dynamic range.

VERY LIGHT LOAD
Check minimum load requirements.

This gives you engineering feedback, not just numbers.

Common Bearing Selection Mistakes

1. Ignoring axial load
2. Not applying shock factor
3. Using incorrect X and Y values
4. Choosing bearings based only on size
5. Forgetting minimum load limits

A proper bearing load calculator prevents these errors.