Combined Load Bearing: 9 Proven Ways to Calculate Load & Capacity

A combined load bearing is a bearing that operates under two or more types of loads at the same time, most commonly radial load and axial load. In real-world machinery, bearings rarely experience only one type of force. Rotating shafts, gears, pumps, motors, conveyors, and automotive components often generate forces in different directions.

Understanding combined loading is important because selecting a bearing only on the basis of radial capacity or axial capacity can lead to overheating, excessive wear, vibration, premature failure, or reduced service life.

The basic principle is simple:

Combined Load = Radial Load + Axial Load acting simultaneously

For bearing calculations, these loads are generally converted into an equivalent bearing load, which can then be used to estimate bearing life and select an appropriate bearing.

What Is Combined Load?

A combined load occurs when a bearing is subjected to forces acting in different directions at the same time.

The two most common components are:

• Radial Load (Fr): Force acting perpendicular to the shaft axis.
• Axial Load (Fa): Force acting parallel to the shaft axis.

For example, consider a shaft supported by a bearing. A gear mounted on the shaft may produce a radial force while the gear’s helical angle simultaneously produces an axial force. The bearing must withstand both forces.

Therefore:

Combined Bearing Load = Radial Load + Axial Load

However, these loads cannot simply be added numerically because they act in different directions. Bearing manufacturers therefore use factors to calculate an equivalent dynamic bearing load.

This guide explains 9 proven methods and calculation approaches for evaluating combined bearing load and capacity.

1. Calculate the Radial Load Bearing

The first step is to determine the total radial load (Fr) acting on the bearing. Radial load acts perpendicular to the shaft axis.

Common sources include:

• Shaft weight
• Pulley forces
• Belt tension
• Gear forces
• Chain forces
• External machine loads

If several radial forces act on the bearing, their resultant must be determined according to their directions.

For simple perpendicular forces:

Fr = √(F₁² + F₂²)

Example Of How To Calculate the Radial Load

If two radial forces are:

Fr = √(1,500² + 1,000²)

Fr ≈ 1,803 N

Therefore, the resultant radial load is approximately:

Fr = 1.80 kN

2. Calculate the Axial Load Bearing

The next step is to determine the axial load (Fa). Axial load acts parallel to the shaft axis and is also called thrust load.

Typical sources include:

• Helical gears
• Screw mechanisms
• Pump impellers
• Propellers
• Thrust forces
• Axial acceleration
• Thermal expansion

If multiple axial forces act in opposite directions, the net axial force can be calculated as:

Fa = |F₁ − F₂|

For forces acting in the same direction:

Fa = F₁ + F₂

Example Of How To Calculate the Radial Load Bearing

Suppose a shaft experiences:

F₁ = 1,200 N

F₂ = 400 N

in opposite directions.

Then:

Fa = 1,200 − 400

Fa = 800 N

Therefore:

Fa = 0.8 kN

3. Calculate the Radial-to-Axial Load Ratio

The relationship between radial load and axial load is important when determining whether a bearing is primarily radially loaded or heavily affected by axial loading.

The basic ratio is: Fa / Fr

Where:

• Fa = Axial load
• Fr = Radial load

Example Of How To Calculate the Radial-to-Axial Load Ratio

If:

Fr = 2,000 N

Fa = 800 N

Then:

Fa / Fr = 800 / 2,000

Fa / Fr = 0.40

This ratio can help determine the appropriate bearing calculation method and whether the axial load is significant.

For actual bearing selection, the manufacturer’s catalogue should be consulted because the limiting values and factors depend on bearing design.

4. Calculate the Equivalent Dynamic Bearing Load

This is one of the most important methods for combined loading. When a bearing experiences both radial load and axial load, the equivalent dynamic bearing load is commonly calculated using:

P = XFr + YFa

Where:

• P = Equivalent dynamic bearing load
• Fr = Radial load
• Fa = Axial load
• X = Radial load factor
• Y = Axial load factor

The values of X and Y are not universal. They depend on the bearing type, internal geometry, contact angle, and manufacturer’s Bearing Load calculation method.

For Example Of Equivalent Dynamic Bearing Load

Assume:

Fr = 2,000 N

Fa = 800 N

For bearing loads illustration, assume:

X = 0.56

Y = 1.50

Then:

P = (0.56 × 2,000) + (1.50 × 800)

P = 1,120 + 1,200

P = 2,320 N

Therefore:

Equivalent Dynamic Load = 2.32 kN

The X and Y values in this example are illustrative only and must be replaced with the actual values specified for the selected bearing.

5. Calculate Basic Dynamic Load Capacity

The basic dynamic load rating (C) is one of the most important bearing specifications. It represents the bearing’s capacity for calculating fatigue life under dynamic operating conditions. Once the equivalent load P is known, bearing life can be estimated.

• For ball bearings: L₁₀ = (C/P)³
• For roller bearings: L₁₀ = (C/P)^(10/3)

Where:

• L₁₀ = Basic rating life in million revolutions
• C = Basic dynamic load rating
• P = Equivalent dynamic bearing load

For Example How To Calculate Basic Dynamic Load Capacity For Ball Bearing

Suppose:

C = 20 kN

P = 2.32 kN

Then:

L₁₀ = (20 / 2.32)³

L₁₀ ≈ 640 million revolutions

This demonstrates why reducing the equivalent bearing load can have a major effect on theoretical bearing life.

6. Calculate Static Load Capacity

Dynamic loading is not the only consideration. A bearing may also experience heavy loads while stationary or rotating slowly.

The basic static load rating is represented by: C₀

The equivalent static load can be determined according to the bearing manufacturer’s method.

A basic static safety factor can then be expressed as: S₀ = C₀ / P₀

Where:

• S₀ = Static safety factor
• C₀ = Basic static load rating
• P₀ = Equivalent static bearing load

A higher static safety factor generally provides greater resistance to permanent deformation under heavy or shock loading. The required safety factor depends on the application and operating conditions.

7. Calculate Bearing Capacity for Shock and Peak Loads

A machine may experience loads much higher than its normal operating load.

Examples include:

• Sudden starting and stopping
• Gear impacts
• Machine vibration
• Heavy material handling
• Accidental overload
• Intermittent shock

For such applications, the design should not consider only the average load.

A practical approach is to identify:

• Normal Load
• Peak Load
• Shock Load

The maximum expected load should be checked against the bearing’s static and dynamic capabilities.

For example, if the normal combined load is:

P = 3 kN

but a machine can experience a short-duration peak load of:

Pmax = 8 kN

the bearing must be checked for both operating life and static safety.

8. Calculate Bearing Capacity Based on Speed and Operating Conditions

Bearing capacity is not determined by load alone.

Rotational speed, temperature, lubrication, alignment, and operating environment can significantly affect performance.

Important factors include:

• Speed : Higher RPM increases frictional heat and may reduce the permissible load or require a different bearing arrangement.
• Lubrication : Insufficient or incorrect lubrication increases friction and wear.
• Temperature : High temperatures can reduce lubricant performance and affect bearing clearance.
• Alignment : Misalignment can introduce additional loads and uneven stress distribution.
• Contamination : Dust, water, and foreign particles can damage raceways and rolling elements.

Therefore, a bearing with an adequate load rating may still fail if the operating conditions exceed its permissible limits.

9. Select the Correct Bearing Based on Combined Capacity

The final step is selecting a bearing that can safely handle the calculated combined load.

A simplified selection process is:

Example Of Combined Load Bearing Calculation

Consider a bearing subjected to:

Radial Load (Fr) = 3,000 N

Axial Load (Fa) = 1,000 N

Assume, for illustration:

X = 0.56

Y = 1.50

Step 1: Equivalent Dynamic Load

P = XFr + YFa

P = (0.56 × 3,000) + (1.50 × 1,000)

P = 1,680 + 1,500

P = 3,180 N

Therefore:

P = 3.18 kN

Step 2: Bearing Life

Suppose the bearing has:

C = 25 kN

For a ball bearing:

L₁₀ = (C/P)³

L₁₀ = (25/3.18)³

L₁₀ ≈ 486 million revolutions

This is a theoretical basic rating life and does not account for every real-world factor such as contamination, lubrication, misalignment, installation quality, or material effects.

Advantages of Proper Combined Load Bearing Calculation

Correct calculation provides several benefits:

• Longer bearing life
• Better machine reliability
• Reduced unexpected downtime
• Improved bearing selection
• Lower maintenance costs
• Better energy efficiency
• Reduced overheating and wear
• Improved mechanical safety

Common Mistakes in Combined Load Calculation

• Mistake 1 : Simply Adding Radial and Axial Loads : Use the equivalent dynamic bearing load (P) calculated according to the bearing manufacturer’s recommended method. The calculation typically considers the radial load (Fr), axial load (Fa), and appropriate bearing-specific factors.
• Mistake 2 : Using Incorrect X and Y Values : Always obtain the X and Y factors from the bearing manufacturer’s technical catalog or calculation data for the specific bearing model and operating conditions.
• Mistake 3 : Ignoring Static Capacity : In addition to calculating bearing fatigue life, check the bearing’s static load capacity (C₀) against the maximum or peak load expected during operation.
• Mistake 4 : Ignoring Speed : Consider the applied load, rotational speed, lubrication method, operating temperature, and bearing type together when selecting a bearing.
• Mistake 5 : Ignoring Misalignment : Consider the expected shaft or housing misalignment, mounting accuracy, bearing type, and operating conditions when selecting a bearing. Where misalignment is unavoidable, choose a bearing designed to accommodate it.

Quick Formula Reference

Calculation	Formula
Resultant radial load	Fr = √(F₁² + F₂²)
Net axial load	Fa = |F₁ − F₂|
Load ratio	Fa/Fr
Equivalent dynamic load	P = XFr + YFa
Ball-bearing life	L₁₀ = (C/P)³
Roller-bearing life	L₁₀ = (C/P)^(10/3)
Static safety factor	S₀ = C₀/P₀

How to Select a Bearing for Combined Load ?

A practical selection process is:

Confirm that the bearing is suitable for the required rotational speed, temperature, and lubrication method.

Which Bearings Suitable for Combined Loads ?

Different bearing designs have different capabilities.

1. Deep groove ball bearings : Deep groove ball bearings are versatile bearings that can accommodate radial loads, moderate axial loads, and combined loads. Their simple design, low friction, and ability to handle loads in both directions make them widely used in electric motors, pumps, fans, conveyors, gearboxes, and other industrial machinery. They are particularly suitable when the application involves a significant radial load along with a moderate axial load.
2. Angular Contact Ball Bearings : Angular contact ball bearings are particularly suitable for applications where both radial and axial loads are significant. Their raceway design allows them to efficiently support combined loads while maintaining good rotational accuracy and speed performance. They are commonly used in machine tools, pumps, spindles, compressors, and high-speed machinery, where reliable handling of radial and axial forces is essential.
3. Tapered Roller Bearings : Tapered roller bearings are excellent for combined radial and axial loading because their tapered raceways and rollers are designed to handle both types of forces efficiently. They are particularly suitable for applications involving heavy loads, shock loads, and demanding operating conditions. Tapered roller bearings are commonly used in automotive wheel bearings, gearboxes, heavy machinery, and industrial transmissions, where high load-carrying capacity and durability are required.
4. Spherical Roller Bearings : Spherical roller bearings are primarily designed to handle high radial loads, while certain designs can also accommodate axial loads. Their self-aligning capability makes them particularly useful in applications where shaft or housing misalignment may occur. They are commonly selected for demanding industrial applications where heavy loads, shaft misalignment, and harsh operating conditions are present, such as conveyors, crushers, mining equipment, and heavy-duty machinery.

Advantages of Combined Load Bearing

Combined-load-capable bearings provide several important benefits:

1. Handle multiple forces
   One bearing can support radial and axial loads simultaneously.
2. Compact machine design
   Multiple separate bearing arrangements may not always be necessary.
3. Improved reliability
   Proper bearing selection can reduce premature failure.
4. Better shaft support
   Combined-load bearings can provide stable shaft positioning.
5. Wide industrial application
   They are suitable for motors, gearboxes, pumps, vehicles, and machine tools.

Disadvantages and Limitations of Combined Load Bearing

Combined loading also creates additional design challenges.

• Bearing selection becomes more complex.
• Equivalent load calculations are required.
• Excessive axial load can significantly reduce bearing life.
• Misalignment can increase internal stresses.
• Poor lubrication can accelerate wear.
• Shock and vibration may require additional safety margins.

Therefore, the bearing should not be selected solely by comparing its radial load rating with the radial force.

Practical Applications

Combined load bearings are used extensively in industrial and mechanical systems.

Electric Motors :Motor bearings may experience radial forces from the rotor and axial forces from the machine arrangement.

Gearboxes :Gears can generate both radial and axial forces, especially helical gears.

Pumps :Pump bearings may experience radial loads from shaft rotation and axial thrust generated by the impeller.

Machine Tools :Machine-tool spindles often require precise bearings capable of handling radial and axial forces at high speeds.

Conclusion

Calculating combined load bearing capacity is essential when a bearing simultaneously experiences radial and axial forces. The most important calculation is the equivalent dynamic bearing load:

P = XFr + YFa

The complete evaluation should also consider dynamic load rating, static load rating, bearing life, speed, lubrication, temperature, alignment, and shock loads.

For an actual engineering application, always use the specific calculation factors and bearing load ratings supplied by the bearing manufacturer.

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