Can crossed roller bearings be used reliably over the long term under frequent start–stop operating conditions?

Crossed roller bearings, owing to their high stiffness, high precision, and composite load-carrying capacity, play a critical supporting role in fields such as industrial robotics, automated rotary tables, semiconductor equipment, and medical machinery. As smart manufacturing places ever‑higher demands on production efficiency, many application scenarios require equipment to complete positioning, machining, or handling operations within very short time frames, with start–stop frequencies rising from the traditional few times per minute to dozens of times per minute or even higher. Such high‑frequency start–stop cycles entail repeated transitions between zero speed and rated rotational speed, each accompanied by drastic fluctuations in friction torque, instantaneous changes in the contact conditions between rollers and raceways, and the formation and rupture of the lubricant film. These non‑steady‑state operating conditions pose severe challenges to bearing material fatigue, lubrication system performance, and dynamic stability. This paper conducts an in-depth analysis of whether crossed roller bearings can withstand long‑term operation under high‑frequency start–stop conditions, explores corresponding mitigation strategies, and assesses the limits of their feasibility for sustained use.

Can crossed roller bearings be used reliably over the long term under frequent start–stop operating conditions?
I. The Core Mechanism of How Frequent Start-Stop Cycles Affect Bearings
1. Roller slippage and micro-slip increase significantly.

When a crossed roller bearing operates under steady conditions, the rollers maintain pure rolling contact with the raceways of both the inner and outer rings, resulting in a uniform stress distribution in the contact zone. During start-up and shutdown, due to inertia, the angular velocity of the rollers lags behind the rotational speed of the rings, leading to relative sliding between the rollers and the raceways. Frequent high‑frequency starts and stops cause this slipping phenomenon to recur repeatedly, inducing micro‑wear and scuffing on the roller surfaces through cyclic slip. Particularly in the initial phase of startup, when the lubricant film has not yet fully formed, direct metal‑to‑metal contact under boundary lubrication conditions increases the coefficient of friction, causes a sharp rise in temperature in the slip zone, and leads to temper softening of the surface material. As the number of cycles accumulates, the rollers gradually lose their cylindrical form and exhibit reduced diameter consistency, ultimately compromising the overall precision of the bearing.

2. Periodic Destruction and Reconstruction of the Lubrication State

Lubricating grease requires a certain amount of time and operating speed to form a stable lubricant film inside the bearing. When the bearing is stationary, gravity causes the grease to settle, leading to a gradual loss of the oil film on the raceway surface. Upon restart, the rollers and raceways must run for several to dozens of revolutions under dry‑friction or boundary‑lubrication conditions before the grease is re‑suspended and evenly distributed. Frequent start–stop cycles subject the bearing to repeated cycles of lubrication failure and film re‑establishment, reducing the proportion of time spent under effective lubrication and significantly increasing the likelihood of direct metal‑to‑metal contact. Moreover, the extreme‑pressure additives in the grease are rapidly depleted during these repeated starts and stops, oxidation of the base oil accelerates, and the grease prematurely ages and hardens, losing its lubricating effectiveness.

3. Accelerated accumulation of fatigue damage

Bearing fatigue life theory is based on calculations using the rated dynamic load and rotational speed, assuming steady loads and constant speeds. However, high-frequency start–stop cycles introduce dynamic impact loads: at startup, the motor torque must overcome static friction and inertial forces, causing the equivalent dynamic load on the bearing to potentially exceed several times its steady-state value. Similarly, the braking phase at shutdown generates reverse impacts. These alternating shocks subject the raceway surface material to asymmetric cyclic stresses, thereby reducing the fatigue crack initiation life. Moreover, slip during start–stop operations induces shear stresses on the raceway surface, which, when superimposed on the normal rolling contact stresses, accelerate the propagation of subsurface cracks.

4. Fluctuations and Relaxation in the Preload State

Crossed roller bearings rely on preload to maintain high stiffness and precision. Temperature fluctuations caused by frequent start–stop cycles induce thermal expansion and contraction of the bearing materials, leading to periodic variations in preload. During initial startup, rapid temperature rise increases the preload, potentially exceeding its design limit and causing a sharp rise in friction; upon shutdown, cooling and contraction reduce the preload, resulting in clearance. Repeated preload fluctuations induce stress relaxation and fretting wear in adjustment components such as spacer rings and shims; over time, this leads to a permanent reduction in preload and a gradual loss of bearing accuracy.

II. Typical Failure Modes Under High-Frequency Start-Stop Conditions
1. Wear on the roller end faces and flanges

Skidding‑induced misalignment causes the roller end faces to repeatedly rub against the inner and outer ring flanges, resulting in mirror‑like scuffing, scratches, and material transfer on the end faces. As the flanges wear, axial positioning of the rollers fails, further exacerbating the misalignment. The clearance between the cage and the rollers increases due to wear, leading to greater rocking of the rollers within the raceway pockets and generating abnormal noise and vibration.

2. Micro‑spalling on the raceway surface and deterioration of waviness

High-frequency impacts and slip cause microcracks to initiate in the raceway surface at regions of stress concentration; as these cracks propagate, they lead to flake‑like spalling. The spalled debris becomes embedded in the lubricant, acting as abrasive particles and inducing secondary damage. The raceway waviness increases due to uneven wear, and as the rollers pass over it, periodic vibrations are generated, with characteristic frequencies appearing in the vibration spectrum that correlate with the location of the damage.

3. Cage fracture and rivet loosening

The cage is subjected to the inertial forces of the rollers during start‑stop impacts, and high‑frequency cycling induces fatigue in the cage material. Cracks initiate at the edges of the window openings due to roller impacts and progressively propagate until fracture occurs. In riveted cages, the rivets loosen under vibration; once the cage fractures, the rollers scatter, causing the bearing to seize instantaneously.

4. Grease Carbonization and Bearing Seizure

Localized high temperatures cause the base oil in the grease to evaporate, while the thickener concentrates and carbonizes, forming a hard, lacquer‑like film on the raceways and rolling elements. When this film flakes off and enters the lubrication system, it exacerbates wear. In severe cases, the grease can completely solidify, leading to a sharp increase in bearing rotational resistance, triggering motor overload protection and shutdown, and causing the bearing to seize due to dry friction.

Crossed roller bearing

III. Selection and Optimization Strategies for Long-Term Use
1. Targeted selection of bearing designs

Preferably use reinforced cages, such as solid brass or engineering‑plastic cages, which offer superior impact resistance and fatigue life compared to stamped steel cages. Increase the number of rollers appropriately to distribute impact loads, while carefully balancing frictional heat generation. Perform ultra‑precision grinding and ion‑implantation surface hardening on the raceways to enhance surface hardness and fatigue resistance. Adopt a tapered‑roller design that leverages slight axial displacement to automatically adjust the contact condition, thereby reducing the tendency for slippage.

2. Dynamic Adaptation of the Preload Scheme

Constant‑pressure preloading is employed in place of positioning‑based preloading, with disc springs or hydraulic actuators applying a steady axial force to automatically compensate for preload variations caused by thermal expansion and wear. Spring stiffness must be carefully selected to ensure stable preload while avoiding resonance. For high‑frequency applications, an active preloading system can be considered, which adjusts the preload in real time based on rotational speed and temperature—increasing preload at low speeds to eliminate clearance and moderately reducing it at high speeds to control temperature rise.

3. Enhanced Design of the Lubrication System

Select a grease with excellent start‑stop performance; its base oil should exhibit low viscosity, high film strength, and rapid distribution to ensure immediate lubrication upon startup. Replace grease lubrication with oil‑mist or oil‑air lubrication, whereby the lubricant is directly injected into the raceways under pressure, thereby preventing grease settling; however, adequate supply pressure must be established promptly with each start‑stop cycle. Implement a lubrication‑circulation cooling system to maintain bearing operating temperatures within an appropriate range, thus slowing grease degradation and material thermal softening.

IV. Operation Control and Maintenance Strategy
1. Optimized control of the start–stop curve

By configuring the servo drive to implement soft‑start and soft‑stop functions, the start-up acceleration is kept within the bearing’s allowable limits, thereby preventing abrupt torque shocks. S‑curve acceleration and deceleration profiles are employed to ensure smooth, continuous changes in acceleration and jerk, minimizing inertial impacts. Where positioning requirements permit, appropriately extending the acceleration/deceleration transition times helps reduce peak dynamic loads.

2. Lubrication maintenance for intermittent operation

During downtime, the minimum‑quantity lubrication system periodically injects a small amount of lubricant into the bearings to maintain an oil film on the raceway surfaces, thereby reducing the duration of dry friction at the next start-up. For extended shutdowns, manually turn the shaft several revolutions at regular intervals to redistribute the grease and prevent localized solidification.

3. Condition Monitoring and Life Prediction

Install vibration and temperature sensors to monitor impact peaks and temperature‑rise rates during start‑stop cycles in real time. Develop a high‑frequency start‑stop–specific life model that incorporates the number of start‑stop cycles, impact amplitudes, and thermal cycling into the damage calculation, replacing the conventional L10 life formula. When cumulative damage reaches the warning threshold, schedule preventive maintenance—replacing the lubricant or adjusting preload—to prevent sudden failures.

4. Regular Maintenance and Relubrication

Shorten the grease‑change interval and determine relubrication intervals based on the number of start–stop cycles. Under high‑frequency operating conditions, it is recommended to replace the grease every 500 to 1,000 operating hours or monthly. During replacement, clean the bearing interior to remove wear debris and aged grease. Inspect the rollers and raceways; address minor damage promptly, and replace the bearing if the damage is severe.

V. Assessment of the Applicable Scope of High-Frequency Start-Stop Operations
1. The Matching Relationship Between Frequency and Load

The permissible limits for high‑frequency start–stop cycles depend on the load magnitude and impact intensity. In light‑load, low‑inertia applications—such as small rotary tables or lightly loaded robot joints—optimized control can sustain long‑term operation at several dozen starts and stops per minute. By contrast, in heavy‑load, high‑inertia scenarios—like the spindles of large machining centers or heavily loaded material‑handling robots—the start–stop frequency must be strictly limited; otherwise, component life will decline sharply. Manufacturers should provide start–stop life‑cycle correction factors for various operating conditions to assist users in selecting appropriate equipment.

2. Trade-off between precision retention and service life

Frequent start–stop cycles inevitably accelerate precision degradation. For applications with stringent accuracy requirements, it is necessary to accept a shorter precision‑retention interval and incorporate bearing replacement into the regular maintenance schedule. For applications with more relaxed accuracy demands, life can be extended—and moderate precision loss tolerated—by reducing preload and optimizing lubrication.

Can crossed roller bearings be used reliably over the long term under high‑frequency start–stop conditions? While long‑term operation under such conditions is feasible, it hinges on a thorough understanding of the challenges and the implementation of systematic mitigation measures. Issues such as roller slippage, lubrication failure, accelerated fatigue, and preload fluctuations—caused by frequent starts and stops—can be effectively addressed through structural optimization, proper preload matching, enhanced lubrication, and adjustments to control strategies. The key lies in integrating bearing selection, system design, operational control, and maintenance management into a cohesive, holistic approach, developing a life‑prediction model based on damage accumulation, and shifting from reactive maintenance to proactive prevention. By clearly defining operating‑condition boundaries and accounting for the predictable degradation of precision, crossed roller bearings can indeed achieve an economically viable service life in high‑frequency start–stop environments, thereby meeting the continuous production demands of smart manufacturing.