Spherical-roller bearings graduate to a new class

By Joacim Fogelstrom
Manager Development Center
SKF AB
Gothenburg, Sweden
Daniel R. Snyder
Director of Applications
Engineering
SKF USA Inc.
Kulpsville, Pa.

EDITED BY KENNETH J. KORANE

A new class of spherical-roller bearings represents an important leap forward in bearing technology. The bearings, called the Explorer Series, are the culmination of a decade of research into bearing steels, internal geometry, and the design of each individual component. New production methods were also necessary to transform design theory into tangible products. The end result is a new generation of spherical-roller bearings with high load ratings and a life expectancy up to three times longer than that of conventional roller bearings.

One key to longer life is ever-cleaner steels. Advancements in production methods means the steel used in Explorer bearings is extremely clean and homogeneous with an absolute minimum of inclusions — the starting point of many bearing failures. In fact, cleanliness is far better than the best grades covered by present classification methods, so SKF metallurgists are looking at new methods for classifying cleanliness with an eye toward standardization.

New procedures have also been devised to heat treat the ultraclean steel. This appreciably improves wear resistance compared with traditional SKF spherical-roller bearings, with no loss of temperature resistance or toughness.

Many small improvements incorporated in the manufacturing process have enhanced product quality and tightened tolerances. Refined surface finish of the bearing raceways is just one example. Sophisticated analysis software has also let engineers study internal bearing dynamics to an extent not previously possible. One resulting design refinement, for instance, is enhanced roller self-guidance, which minimizes friction, wear, and noise.

The new bearings let design engineers downsize components or increase power capacity in devices such as pumps, fans, and gearboxes across a range of industries that include paper, mining, and construction.

PREDICTING LIFE
One way to view performance improvements embodied in the new designs is to examine bearing life using the SKF Life Method. This method was first presented as the SKF New Life Theory in 1989, and it is an extension of the fatigue life theory developed by Lundberg and Palmgren.

For roller bearings, adjusted rating life

is

Ln = a1asL10

or

For constant speed applications, life is often expressed in operating hours,

The life-adjustment factor as represents a complex relationship between various influencing factors including contamination and lubrication conditions. The accompanying diagram shows as for different values of viscosity ratio , contamination factor c, fatigue load limit Pu, and equivalent dynamic bearing load P. Explorer's values for c( Pu/ P) are approximately 1.4 times greater than that of standard spherical-roller bearings, representing the life-extending refinements of the development.

The diagram incorporates safety factors commonly used in fatigue-life considerations and is valid for lubricants without EP additives. When lubricants contain such additives, consult the manufacturer or reference information at www.skf.com.

Equivalent dynamic bearing load for spherical-roller bearings is calculated from

P = Fr + Y1Fa when Fa/Fr ≤ e

and

P = 0.67Fr + Y2Fa when Fa /Fr > e.

Approximate values for e, Y1, and Y2 are found in catalog data. The equivalent static bearing load for spherical-roller bearings is found from

P0 = Fr + Y0Fa.

The performance enhancements incorporated into the Explorer spherical-roller bearings are best demonstrated by comparing life to SKF's previous standard version (a 22218 E bearing). For the older design the basic dynamic load rating C = 282 kN and fatigue load limit Pu = 39 kN. For the Explorer, C = 325 kN and Pu = 39 kN. Operating conditions for each are an equivalent dynamic-bearing load P = 28.2 kN, viscosity ratio = 2, and contamination factor c =0.4.

For the earlier version, c(Pu /P) = (0.4)(39/28.2) = 0.55. From the diagram, as = 3.7, and life is calculated at

L10 = 7,970 million revolutions.

For the Explorer, as = 7.1 and life is calculated at

L10 = 24,500 million revolutions.

UPGRADING PERFORMANCE
The enhanced life capability is expected to significantly impact both current applications and new-equipment designs. On existing machines where performance does not change, switching to an equivalent-size Explorer bearing will provide several times the previous service life, greater up-time, and an appreciable reduction in machine-cycle costs. Or manufacturers can increase ratings on existing machines by merely upgrading the bearings.

On new machines, designs can be lighter, more compact, operate at higher speeds, and run quieter with less lubrication, because engineers can significantly reduce the size of bearings without reducing power output. In some applications, for example, bearing widths can be reduced by more than 20% while retaining the same bearing bore and OD.

The bearing's lower cross-sectional height will also allow, for the same bearing OD, a stronger or hollow shaft, a stiffer but less-expensive overall design, and the same or higher operating speeds, as well as lower machine-cycle costs. This provides opportunities to "power up" without increasing bearing size. Compared with conventional bearings of equal size, the new design has greater power density and higher load ratings. Consequently, engineers can increase power in existing applications without costly redesigns.