Cracked bearing inner rings can become a serious reliability problem in paper machine dryer sections, potentially resulting in unexpected production stops and expensive maintenance work.

SKF identifies thermal stress during dryer start-up as a primary cause of inner-ring cracking in these applications.

The problem develops because the dryer cylinder journal and bearing inner ring do not necessarily heat and expand at the same rate. During start-up, the journal can expand more rapidly than the bearing inner ring, increasing the interference between the shaft and bearing.

Repeated exposure to these thermal cycles can eventually initiate cracks in the inner ring and lead to bearing failure.

For mills operating continuously, understanding this failure mechanism can help maintenance teams distinguish thermal cracking from other bearing problems and address operating conditions before a failure interrupts production.

Why Dryer Cylinder Bearings Face Thermal Stress

Paper machine dryer sections operate under elevated temperatures as heated cylinders remove moisture from the paper web.

During start-up, temperatures throughout the dryer assembly do not increase uniformly.

According to SKF, the dryer cylinder journal can expand faster than the bearing inner ring. Because the bearing is already mounted with interference on the journal, this differential thermal expansion increases the effective interference.

That generates additional stress in the bearing inner ring.

A single start-up does not necessarily cause immediate failure. The concern is repeated thermal cycling over extended operating periods.

As the machine repeatedly heats and cools, cyclic stresses can contribute to crack initiation and propagation until the inner ring can no longer operate reliably.

Other Factors Can Accelerate Bearing Damage

Thermal stress is not the only condition that can affect dryer bearing life.

SKF also identifies inadequate lubrication as a potential contributor. Lubrication is particularly important in high-temperature applications because operating conditions influence lubricant performance and bearing friction.

Contamination presents another risk.

Water, steam and paper dust can enter the bearing environment and interfere with lubrication or damage bearing surfaces.

Other potential contributors include excessive loads, misalignment and incorrect mounting practices.

Long operating periods under high-temperature conditions can further contribute to fatigue.

This means a cracked inner ring should not automatically be attributed to a single cause. Maintenance teams need to consider the bearing, lubrication system, mounting condition, operating temperature and surrounding environment together.

Warning Signs Require Condition Monitoring

A bearing can deteriorate before complete failure occurs, making condition monitoring an important part of dryer-section maintenance.

Changes in vibration, temperature, lubrication condition or operating behavior can provide indications that a bearing is no longer performing normally.

Identifying abnormalities early gives maintenance teams an opportunity to investigate the equipment during planned maintenance rather than waiting for a breakdown.

This is particularly important for dryer cylinders because bearing problems can affect equipment that is critical to continuous paper production.

Unplanned Downtime Is the Main Operational Risk

The financial impact of a dryer bearing failure extends beyond the cost of replacing the bearing itself.

An unexpected failure can force a paper machine to stop, reducing production output while maintenance teams diagnose the problem, obtain components and complete repairs.

Depending on the nature of the damage, associated components may also require inspection or repair.

Preventive maintenance therefore needs to address the operating conditions responsible for premature bearing damage rather than relying only on periodic bearing replacement.

Correct mounting, appropriate lubrication, contamination control, alignment and monitoring all contribute to bearing reliability.

Where thermal stress is identified as the primary mechanism, start-up conditions and the interaction between the journal and bearing become particularly important areas for investigation.

Understanding the Failure Mechanism Comes First

Dryer cylinder bearings operate in a demanding combination of heat, load and continuous production.

SKF’s analysis highlights why apparently straightforward bearing failures can originate from the thermal behavior of the surrounding machine.

For paper mills, the key is to identify the actual failure mechanism.

A cracked inner ring caused by excessive thermal interference requires a different corrective approach from damage primarily caused by contamination, poor lubrication or misalignment.

Understanding that distinction can help mills move from reacting to failed bearings toward addressing the conditions that cause those failures in the first place.

Last Updated on 19/08/2026 by Jota Machinery