Motor bearings are critical components that support the rotating shaft and keep the rotor running accurately inside the motor.

When selecting or designing motor bearings, engineers must consider two main types of mechanical load:

radial load
and
axial load

These two forces act in different directions and affect bearing life, shaft stability, vibration, temperature, and overall motor reliability.

Understanding the difference between axial and radial loads is especially important for large industrial motors, vertical motors, belt-driven motors, pumps, fans, compressors, and other heavy-duty applications.

What Is Radial Load on a Motor Bearing?

Radial load acts perpendicular to the motor shaft.

In simple terms, it pushes the shaft sideways.

Typical sources of radial load include:

• Belt tension

• Pulley forces

• Gear forces

• Rotor weight

• Coupling misalignment

• External mechanical loads

For example, when a motor drives a machine through belts and pulleys, the belt tension pulls sideways on the motor shaft.

This force is transferred directly to the motor bearings.

If the radial load is too high, it may cause:

• Excessive bearing wear

• Higher bearing temperature

• Increased vibration

• Shaft deflection

• Reduced bearing life

• Premature bearing failure

For this reason, belt-driven applications often require careful checking of allowable radial load.

What Is Axial Load on a Motor Bearing?

Axial load acts parallel to the motor shaft.

It pushes or pulls the shaft along its own axis.

Typical sources of axial load include:

• Vertical rotor weight

• Pump thrust

• Fan thrust

• Helical gears

• Screw compressors

• Process equipment

• Coupling forces

Axial load is especially important in vertical motor applications.

In a vertical motor, the rotor weight itself may create a continuous axial force on the bearing system.

If the driven equipment also produces additional thrust, the bearing must support both loads.

Axial Load vs Radial Load

The difference can be summarized simply:

In real industrial applications, a bearing may experience both radial and axial loads at the same time.

This is called a combined load.

Why Are Motor Bearings Sensitive to Load Direction?

Different bearing types are designed to carry different kinds of loads.

For example:

Deep Groove Ball Bearings

Deep groove ball bearings are very common in electric motors.

They are suitable for:

• Radial loads

• Moderate axial loads

• High-speed operation

They are widely used because they offer low friction, good speed capability, and reliable operation.

Cylindrical Roller Bearings

Cylindrical roller bearings can carry relatively high radial loads.

They are often used in larger motors or applications where radial loading is significant.

However, their axial load capability may be limited depending on bearing design.

Angular Contact Ball Bearings

Angular contact bearings can support both radial and axial loads.

They are often selected where axial force is more significant.

Tapered Roller Bearings

Tapered roller bearings are designed to carry combined radial and axial loads.

They can provide high load-carrying capacity, but they also require proper adjustment and installation.

Thrust Bearings

Thrust bearings are mainly designed to carry axial loads.

They are common in applications where large thrust forces are present.

Why Is Radial Load Important in Belt-Driven Motors?

Belt-driven systems are one of the most common examples of significant radial loading.

The belt must be tensioned to transmit torque.

This tension creates a sideways force on the shaft.

If the pulley diameter is too small or belt tension is too high, the radial load on the motor bearing can increase significantly.

Excessive radial load may shorten bearing life even if the motor power is correctly selected.

For this reason, motor manufacturers may request information such as:

• Pulley diameter

• Belt type

• Belt tension

• Distance from pulley center to bearing

• Motor speed

These parameters allow engineers to calculate the actual radial bearing load.

Why Is Axial Load Important in Vertical Motors?

Axial load becomes especially important when the motor shaft is installed vertically.

In a horizontal motor, rotor weight mainly acts radially on the bearings.

In a vertical motor, rotor weight acts along the shaft axis.

This means the bearing arrangement must support the rotor mass vertically.

The situation becomes more demanding when the driven equipment also generates thrust.

For example, a vertical pump may create substantial hydraulic axial thrust.

The bearing system must therefore carry:

motor rotor weight + external equipment thrust

This is why vertical motors may require a different bearing arrangement from horizontal motors.

Can Ball Bearings Carry Axial Load?

Yes.

Standard deep groove ball bearings can carry both radial and moderate axial loads.

This is one reason they are widely used in electric motors.

However, the allowable axial load depends on:

• Bearing size

• Bearing design

• Motor speed

• Lubrication

• Temperature

• Required bearing life

• Direction of axial force

If axial load becomes too high, a different bearing type or bearing arrangement may be required.

Therefore, using a ball bearing does not automatically mean that the motor cannot be installed vertically or cannot carry axial load.

The actual load must be calculated.

Why Is Bearing Load Important for Bearing Life?

Bearing life is strongly related to the load applied to the bearing.

As bearing load increases, fatigue life decreases.

This means that even a relatively small increase in bearing load can significantly affect expected service life.

If the bearing is overloaded, possible consequences include:

• Raceway fatigue

• Pitting

• Excessive heat

• Noise

• Vibration

• Lubricant degradation

• Bearing seizure

Correct load calculation is therefore essential for reliable motor operation.

What Happens If the Wrong Bearing Is Selected?

If the bearing type is not suitable for the actual radial and axial loads, several problems may occur.

Excessive Vibration

Improper load distribution can create unstable shaft movement.

High Bearing Temperature

Overloaded bearings generate more friction and heat.

Short Bearing Life

Bearing fatigue may occur much earlier than expected.

Shaft Movement

Excessive axial movement may affect coupling alignment, rotor position, or internal motor clearances.

Motor Failure

In severe cases, bearing failure can damage the shaft, rotor, stator, or driven equipment.

For large high-voltage motors, such failures can lead to significant downtime and repair costs.

Fixed Bearing and Free Bearing Arrangement

Many electric motors use a fixed-end and free-end bearing arrangement.

The fixed bearing controls the axial position of the shaft.

The free bearing allows small axial movement caused by thermal expansion.

This is important because the motor shaft expands slightly as temperature increases during operation.

If both bearings rigidly prevent axial movement, thermal expansion may create unwanted internal forces.

A properly designed bearing arrangement allows controlled movement while maintaining shaft stability.

Why Is External Load Information Important During Motor Selection?

When selecting a motor, engineers should not consider only:

• Power

• Voltage

• Speed

• Mounting arrangement

Mechanical load information is also important.

For some applications, the manufacturer may need to know:

• Radial load

• Axial load

• Load direction

• Coupling type

• Pulley dimensions

• Gear arrangement

• Pump thrust

• Vertical or horizontal mounting

• Shaft extension dimensions

Without this information, the bearing arrangement may not be suitable for the actual application.

Which Applications Require Special Attention?

Bearing load should receive particular attention in applications such as:

• Vertical pumps

• Belt-driven fans

• Crushers

• Conveyors

• Compressors

• Gear-driven equipment

• Cooling towers

• Large blowers

• Mills

• Marine equipment

• Heavy industrial machinery

These applications may impose significant external radial or axial loads on the motor shaft.

How to Select the Correct Motor Bearing Arrangement

The correct bearing design depends on the complete mechanical system.

Engineers should evaluate:

• Radial load magnitude

• Axial load magnitude

• Load direction

• Motor speed

• Shaft size

• Mounting arrangement

• Driven equipment

• Operating temperature

• Lubrication method

• Required bearing life

• Starting conditions

For large or special motors, the motor manufacturer should confirm the bearing arrangement according to the actual application.

Conclusion

Motor bearings must consider both axial and radial loads because these forces directly affect shaft stability, bearing life, vibration, temperature, and motor reliability.

Radial loads act perpendicular to the shaft and are commonly caused by belts, pulleys, gears, and rotor weight.

Axial loads act parallel to the shaft and are commonly caused by vertical installation, pump thrust, helical gears, and other driven equipment.

Different bearing types have different load-carrying capabilities.

For this reason, correct bearing selection should always be based on the actual mechanical loads of the application rather than motor power alone.

For large industrial motors, especially vertical motors or motors driving equipment through belts, gears, or high-thrust machinery, radial and axial load information should be confirmed before finalizing the motor design.