What is the impact of frequent starting and stopping on the fatigue life of deep groove ball bearings used in sports equipment

In sports equipment such as treadmills, exercise bikes, skateboard wheels, inline skates, and skiing simulators, deep groove ball bearings play a critical role in supporting rotating components. Unlike industrial machinery that runs continuously, sports equipment operates under frequent start-stop cycles. This operating pattern has a direct and significant influence on bearing fatigue life, internal stress distribution, and lubrication performance.

Load Characteristics During Frequent Start-Stop Operation

Sports equipment is subjected to highly unstable loads. Each time a user starts pedaling, pushing, jumping, or braking, bearing speed rapidly changes from zero to a high rotational value and then back to zero. These rapid changes generate high acceleration and deceleration forces that act directly on the bearing components.

During every start and stop, the rolling elements experience transient peak loads that are much higher than those under steady rotation. These load peaks become the main source of rolling contact fatigue inside the bearing.

Effect of Starting on Raceway Fatigue

When a bearing starts from rest, the contact between balls and raceways initially occurs under boundary lubrication or partial metal-to-metal contact. A complete lubricant film has not yet been established. Under the combined influence of user weight and inertia, the contact stress on the raceways becomes very high.

This condition promotes the formation of micro-cracks on the raceway surface. With frequent starting, these micro-cracks grow and eventually lead to surface flaking and fatigue spalling.

Inertia Impact During Stopping

When rotation stops, the balls continue moving due to inertia while the raceways decelerate rapidly. This speed difference creates sliding and impact forces. These forces produce local friction, heat, and surface stress on the raceways.

In fitness equipment and training machines where quick braking occurs, these inertia impacts accelerate rolling element fatigue and raceway damage.

Micro-Motion Wear Caused by Start-Stop Cycles

Frequent start-stop operation often keeps bearings operating within very small angular displacements. The rolling elements repeatedly move back and forth over short distances. This prevents stable lubricant film formation.

Such conditions cause fretting wear on the raceway surface. The resulting wear marks act as stress concentration points, accelerating fatigue crack initiation and growth.

Thermal Stress Effects

Friction is highest during acceleration and deceleration. This leads to repeated temperature fluctuations inside the bearing. These thermal cycles create internal stresses within the bearing material.

Repeated heating and cooling gradually reduce material fatigue strength and make the raceways more vulnerable to cracking and surface damage.

Lubrication Instability During Start-Stop Operation

Under steady high-speed rotation, a stable elastohydrodynamic lubricant film forms between balls and raceways. Under frequent start-stop conditions, bearings operate for long periods at low or zero speed, allowing lubricant to flow away from contact zones.

Each restart occurs with incomplete lubrication, increasing metal contact and accelerating wear and fatigue damage.

Residual Stress Accumulation

Repeated changes in load direction and magnitude generate alternating stress cycles inside the bearing material. Over time, residual stresses build up within the raceways and rolling elements.

Once these stresses exceed the material’s fatigue limit, surface flaking and early failure occur even under normal working loads.

Actual Fatigue Life in Sports Equipment

Although deep groove ball bearings show long theoretical life under continuous rotation, real sports equipment conditions produce much shorter service life. Frequent starts, stops, shocks, lubrication interruption, and thermal cycling significantly reduce effective bearing life.

Failures in treadmill rollers, bicycle trainers, and skateboard wheels are often caused by accumulated fatigue damage rather than overload.

Common Failure Modes

Typical failures under frequent start-stop conditions include raceway pitting, surface flaking, ball fatigue cracking, cage wear, and lubricant degradation. All these modes are directly related to repeated acceleration, deceleration, and lubrication breakdown.