- Understanding Four-Row Cylindrical Roller Bearings for Rolling Mills
The main types are FC (single inner ring), FCD (double inner ring), and FCDP (double inner ring with flat retaining rings). They operate under severe conditions involving heavy loads, high unit pressure, and substantial heat generation. Their service life is therefore relatively short, and their behavior differs significantly from that of general-purpose bearings.
In application, roll-neck bearings require effective lubrication and cooling, and their radial envelope should be as small as possible. Operating conditions and bearing selection differ substantially between roll positions, such as backup rolls and work rolls. Key conditions include:
Speed: both maximum speed and the ability to form an oil film at low speed must be considered;
Heavy load: load may be distributed unevenly among the roller rows, leaving some rows unloaded;
Inner- and outer-ring eccentricity: this changes clearance and consequently affects strip and plate accuracy;
Axial force: these bearings do not carry axial load and must be used with a separate thrust bearing;
High temperature: lubrication failure is possible and requires particular attention.
Bearing materials must provide high load capacity, a low coefficient of friction, impact resistance, and good tolerance of foreign particles. Large bearings impose special material and process requirements. Domestic capability still has limitations in large premium bearings, and bearings for some critical equipment remain dependent on imports.
From the standpoint of roll-system design, a suitable bearing arrangement is essential to mill performance and reliable bearing operation. As aluminum strip mills move toward higher speed, load, precision, and reliability, the combination of four-row cylindrical roller bearings and thrust bearings is increasingly common. Its advantages include:
Maximum radial load capacity for a given cross-sectional envelope;
Separate bearings carry radial and axial loads, enabling higher rolling speeds;
More precise axial-clearance control, improving product accuracy;
High bearing accuracy and reliability;
Separable inner and outer rings for easier roll assembly and removal;
An interference fit between the inner ring and roll neck prevents ring creep and burn damage;
Simple inspection and maintenance with low total operating cost.
Under JB/T 5389.1-2005, mill-bearing components are made from carburizing steel G20Cr2Ni4(A) or high-carbon chromium bearing steel GCr15(SiMn) and must meet the applicable heat-treatment requirements. Because of rolling-mill operating characteristics, these bearings generally have no fixed service-life rating. On high-precision strip mills, the outer diameter of the inner ring is often supplied with grinding allowance. After installation on the roll, it is ground together with the roll body to the required size, ensuring radial clearance and concentricity.
- Correct Use of Four-Row Cylindrical Roller Bearings
Deficiencies in use and management shorten bearing life, reduce production stability, and compromise product accuracy. Although mill bearings have no fixed service life, operators should extend effective operating time to reduce production cost. Besides normal fatigue spalling, common damage includes wear, burn damage, and grease failure, resulting in rough rotation, rapid temperature rise, abnormal vibration and noise, or seizure.
2.1 Failure Modes and Cause Analysis
(1) Contact-fatigue spalling
Aluminum strip mills often operate reversibly, exposing bearings to heavy loads, severe impacts, and alternating stress. Fatigue spalling commonly develops on roller surfaces.
(2) Wear
Both rolling and sliding friction occur inside the bearing. Poor lubrication accelerates wear, increases clearance, and reduces accuracy. Minor wear may permit continued operation, but the bearing must be replaced once strip accuracy is affected.
(3) Burn damage
A rapid increase in frictional heat causes lubricant failure, microstructural changes at the surface, adhesion, and possible seizure. Key preventive measures include improved bearing design, self-lubricating cages, and stronger cooling and lubrication.
(4) Grease failure
Grease gradually degrades through shearing, oxidation, contamination, and leakage, leading to higher frictional temperatures and surface burn damage.
The principal factors affecting bearing life include:
Dynamic loading: impact loads are a major cause of abnormal failure. Standardized operation is essential to avoid cobbles, double rolling, and other abnormal conditions;
Bearing material: composition and heat treatment significantly affect life and should be optimized jointly with the manufacturer for actual operating conditions;
Installation and sealing: good concentricity, effective sealing, and appropriate clearance are fundamental to long life;
Matched relationships: preserve the assigned pairing between each bearing, roll, and chock; avoid arbitrary regrinding of inner rings and maintain even load distribution;
Lubrication: select the correct lubricant and delivery method, avoid both excessive and insufficient supply, and prevent contamination.
2.2 Key Practices for Correct Use
2.2.1 Establish a Proper Understanding
Mill bearings operate under loads two to five times those of general bearings, across a wide speed range, in harsh environments, with frequent roll changes and stringent accuracy requirements. Their low friction, compact radial dimensions, high precision, and easy assembly make them suitable for work and backup rolls in four-high mills. A separate thrust bearing must carry axial load.
2.2.2 Set Clearance Scientifically and Enforce Matching Control
Radial clearance can be designed using either a basic-shaft or basic-hole system. JB/T 5389.1-2005 recommends the basic-hole system because it supports volume production and quality control. Clearance directly affects internal load distribution and maximum rolling-element load and must account for fits, temperature rise, deformation, and other factors. Radial- and axial-bearing clearances must also be coordinated to prevent interference.
During roll changes, rotate the bearing outer rings by 90° at regular intervals to equalize wear and extend life. Maintain a bearing service record and preserve the assigned pairing with the roll and chock. Clean and inspect bearings regularly, assess replacement against clearance limits, and preferably use complete original matched sets.
2.2.3 Standardize Assembly
Many plants still use overhead cranes to install and remove chocks, making alignment difficult, reducing efficiency, and risking damage to roll necks and bearings. Dedicated assembly equipment is increasingly used in the steel industry and should gradually be adopted by copper processors. Although the initial investment is higher, it enables safer, faster, and more accurate work with substantial long-term benefits.
2.2.4 Standardize Lubrication Management
Lubrication separates metal surfaces with an oil film and reduces wear. Common problems include damaged seals that allow grease to mix with rolling emulsion, emulsion ingress that destroys lubrication, and water contamination that causes corrosion and hydrogen embrittlement. Principal methods are grease lubrication and oil lubrication, including oil-mist and oil-air systems:
Grease lubrication: suitable for small, simple mills. Use water-resistant grease such as lithium-based grease. Select penetration and fill quantity according to speed, temperature, and environment, generally filling one-third to one-half of the available space. Replenish and replace it regularly, and never mix different grades.
Oil-mist lubrication: compressed air atomizes oil and carries it to the lubrication point. Benefits include uniform lubrication, effective cooling, low oil consumption, and some sealing effect. Disadvantages include the need to treat oil-laden exhaust, dependence on compressed air, suitability only for low-viscosity oils, and relatively low utilization.
Oil-air lubrication: oil droplets are carried by compressed air without atomization. Suitable for high-speed, high-temperature, heavy-load, and contaminated environments, it works with oils of various viscosities. Oil and air flow are adjustable, environmental impact is low, and positive internal pressure resists contamination, although capital cost is higher.
2.2.5 Strengthen Roll-System Installation Management
Mill operating conditions and roll-system assembly quality directly affect bearing life. The following points require attention:
Regularly inspect and adjust the parallelism of roll axes; dedicate chocks to specific positions and avoid interchange;
Replace worn drive components promptly to reduce additional axial forces caused by excessive clearance;
Use chocks with self-aligning capability to prevent edge loading;
Inspect the dimensions, geometric tolerances, and fits of housing windows, chocks, and related components;
Control roll deflection to prevent overload and uneven loading among roller rows;
Standardize bearing-to-chock assembly:
Inner-ring installation: use induction or oil-bath heating at no more than 120°C; never use flame heating. Hold the ring firmly in position until it cools;
Outer-ring installation: tap gently into position, observe end-face markings, preserve the original sequence, and record the loaded zone;
Seal assembly: select appropriate seals, such as FKM-cased seals, install the sealing lips in the correct direction, and prevent coolant and contaminants from entering.
Systematic management, standardized operations, and continuous improvement can significantly increase the service life and reliability of four-row cylindrical roller bearings and provide dependable support for high-quality copper strip production.