Expanding the performance of cylindrical roller bearings, Solutions!, Online Exclusives, June 2004

EXPANDING THE PERFORMANCE OF CYLINDRICAL ROLLER BEARINGS

By Rutgerus Heemskerk, SKF

Cylindrical roller bearings are used wherever high radial loads and high speeds are competing with low frictional losses. The growing demands in the field offer a continual development potential. Among those are longer service life, higher power density and longer maintenance intervals.

The latest generation of bearings represents a big step forward in bearing technology, which cannot be described with the standardized calculation methods. Better, extremely clean and homogenous materials, an optimized heat treatment, improved manufacturing processes as well as an optimized internal geometry and surface topography are only a few of the characteristics that result in a new performance class. This performance increase can be calculated with the help of a higher dynamic load rating and the life modification factor. The boundary dimensions of this generation of bearings are interchangeable with existing bearings.

Cylindrical roller bearings in operation
Cylindrical roller bearings are the right choice wherever high radial loads, high rotational speeds and minimal frictional losses must be accommodated (Fig.1). Cylindrical roller bearings are the ideal non-locating bearings. They are used where thermal expansions of shaft and housing must be compensated smoothly inside the rotating bearing. The bearing rings can be mounted separately. This simplifies the mounting, especially if press fits are necessary for both bearing rings in order to reduce the risk for fretting corrosion when load directions are changing.

Cylindrical roller bearings with integrated flange at the loose ring or with loose flange washer or angle rings can also carry axial loads and be used as locating bearings. Cylindrical roller bearings of the "EC"-Generation have an optimally utilized cross-section, and the main dimensions correspond to DIN 5412-1:2000 resp. DIN 616:2000 or ISO 15:1998.

Based on the successful EC-Generation, SKF scientists and engineers have further developed the cylindrical roller bearing. Some improvements are:

• Cleanliness of material
• Heat treatment
• Material hardness
• Logarithmic contact profile
• Roller end/flange geometry
• Surface topography
• Manufacturing quality
• Cage design
• Interchangeability of bearing parts

Material and heat treatment
A key factor to a longer service life in rolling contact is the cleanliness of the bearing steel. Progress in steel production, such as vacuum degassing, could reduce oxygen content of bearing steel to 5 ppm on average and therefore also the amount and size of nonmetallic inclusions.

Heat treatment of the bearing components was chosen to achieve an optimum between material hardness and dimensional stability. SKF cylindrical roller bearings are dimensionally stable up to operating temperatures of 150°C. Permanent temperatures up to 125°C do not have any negative effect on bearing clearance, hardness and fits.

Adjusted heat treatment of rings and rollers make SKF Explorer cylindrical roller bearings considerably less sensitive to contamination and metallic contact due to thin lubricant films.

Internal geometry and manufacturing quality
Surface topography of roller and ring raceways were further improved and therefore provides better lubrication conditions, especially with thin lubricant films. The logarithmic profile developed by SKF optimizes the contact stresses between roller and ring raceways and reduces the risk for edge stresses to a minimum. The raceway profile of SKF Explorer cylindrical rollers could be further improved so that they are less sensitive to misalignment.

"Open" flanges and a special profile and surface finish of roller ends improve the lubrication conditions and reduce the friction in the flange contact. The resultant advantages are lower operating temperatures and a higher axial carrying capacity of cylindrical roller bearings.

New and improved manufacturing processes have enabled a further tightening of tolerances. Production can be maintained at the center of the tolerance spread with the lowest possible variation in each manufacturing step. With this method it is possible to keep tight tolerances for dimensional accuracy without a negative influence on fits. Also, running accuracy could be improved considerably. As a result, tighter tolerances don’t have to be ordered with a special designation.

Bearing parts that are produced with this manufacturing method are completely interchangeable, also for reduced radial clearances. It is therefore not necessary to keep bearing parts together during the assembly process. The results of the reduced tolerances are not visible or audibly noticeable as SKF Explorer cylindrical roller bearings run much quieter and with less vibration.

Cage design and variants
Because of the numerous application possibilities of cylindrical roller bearings, SKF has developed different cage variants so that for each application the optimal cage can be chosen (Fig. 2). The standard recommendation remains the window-type polyamide cage that has shown reliability in many millions of applications. Where high temperatures or aggressive lubricants might attack the polyamide cage, pressed steel (window-type) cages are available for many bearing sizes. For special application conditions, mills can choose between different solid brass cage variants, such as the SKF-developed form-turned, one-piece ML-cage.

The ML-cage is suitable for the highest rotational speeds and accelerations. Two-part machined brass riveted cages as roller-guided execution are especially suitable for grease lubrication. Strong cage bars with integrated trapezoidal-shaped rivets allow high temperatures and vibrations with little risk for rivet-loosening.

Bearing service life and influences
Influences that can prolong bearing service life are considered mathematically in a life equation. The method developed by Harris and Ioannides was introduced by SKF in 1989 and was later standardized in ISO 281:1990/Amd.2:2000. The "advanced rating life" for roller bearings results from: L10m = aSKF (C/P)10/3.

L10m is the "advanced rating life" (in million revolutions). The life modification factor is influenced by various factors; among these are lubrication conditions and contamination in the lubricant. The increased capacity of the cylindrical roller bearings can be shown with a bearing NU 207 ECP as example (Fig. 3). With an equivalent bearing load P = 4.8 kN, a basic dynamic load rating C = 48.4 kN for standard bearing or C = 56 kN for the SKF Explorer bearing, a fatigue load limit Pu = 6.1 kN, a viscosity ratio ? = ?2 and a contamination factor ?c = 0.4, the "rating life" of the previous standard bearing can be calculated to be L10m = 6 700 million revolutions and for the SKF Explorer bearing to be L10m = 20 300 million revolutions, which means a 3 times higher calculated rating life.

More efficient, more compact
The increased capacity of these bearings is of significant interest for existing machines as well as for new machines, for instance to

• extend life for existing machines
• increase power density of existing machines
• downsize new machines with the same power
• increase power density of new machines

In all cases, the use of these bearings can reduce vibration and noise, extend maintenance intervals and reduce frictional heat.

About the author:
Rutgerus Heemskerk is manager, development center, for SKF GmbH in Schweinfurt, Germany. To learn more about cylindrical roller bearings, please contact your local SKF office or distributor, or visit SKF’s website at www.skf.com.