A Closer Look At Bearings
by Daniel R. Snyder
Sunday, December 03, 2006
Daniel R. Snyder
Director, Applications Engineering
SKF Bearings, Inc.
Kulpsville, Penn.
Rolling bearing reliability is particularly critical in electric motors, fans, blowers, compressors, and pumps. They reduce friction, support shaft loads, locate shafts, and make systems more rigid. Each rolling bearing type has properties that make it most appropriate for a particular application. For example, deep-groove ball bearings handle some radial as well as axial loads. They are high-precision bearings with low friction properties and run quietly. For these reasons, they often are specified for small and medium-size electric motors.
Coatings can impart additional features for specific application conditions. One of the most important is the use of a thin aluminum oxide layer to form a barrier against electric arcing, which leads to premature bearing failure.
Spherical and toroidal roller bearings are self-aligning and can carry extremely heavy radial loads. These properties also help moderate shaft deflections and misaligned components. The appropriate bearing depends on loading, misalignment, precision, speed, system rigidity, and axial location. Materials used for rolling bearing rings, rolling elements, and cages can affect their reliability and service life.
Loading
The magnitude of the load is one of the most critical factors to consider when sizing bearings. Generally, roller bearings can support heavier loads than ball bearings and caged bearings of similar size. Ball bearings typically are used where loads are relatively light or moderate. Roller bearings should be used for heavy loads and large shaft diameters.
Certain cylindrical roller, needle roller, and toroidal roller bearings can support only radial loads, while all other radial bearings also handle some axial loads. Thrust ball bearings and four-point contact ball bearings handle only light to moderate axial loads. A bearing’s ability to carry an axial load is determined by the angle of contact or load action internal to the bearing. Angular contact ball bearings can support some axial loads at relatively high speeds. For other axial loads acting in one direction, spherical roller thrust bearings or taper roller bearings are the best choice.
Radial and axial loads acting simultaneously are called combined loads. The most common bearings used here include single and double-row, angular contact ball bearings, and single-row, tapered ball bearings. Deep-groove ball bearings depend on the load magnitude and ratio of axial to radial loading.
When the axial component of combined loads is especially large, a separate bearing may
support it independently from the radial load. In addition to thrust bearings, some radial
bearings, such as deep-groove ball bearings or four-point contact ball bearings, can
handle this task.
A load acting eccentrically on a bearing produces a tilting moment. For these loads,
consider double-row bearings. Suitable options include deep groove, angular-contact ball
bearings; spherical roller bearings; paired single-row angular contact ball bearings; or
tapered roller bearings arranged face-to-face or back-to-back.
Bearings must always have a minimum load for the elements to move properly and
sufficient lubricant film in rolling contact areas to minimize friction. Otherwise, the
elements will skid, degrade the lubricant, and generate high operating temperatures.
A general rule of thumb: roller bearing loads should be about 0.02 times the dynamic
radial load rating, and ball bearing loads should be about 0.01 times the dynamic radial
load rating. These ratings are particularly critical for high accelerations and speeds that
are about 75% of the maximum ratings.
Misalignment
Shafts that bend and flex under operating loads, bearing seats that are not the same
height, and shafts supported by widely spaced bearing housings can go out of alignment.
Rigid bearings such as deep-groove ball bearings and cylindrical roller bearings can
counteract only minor misalignments.
Various types of self-aligning rolling bearings, including spherical roller bearings,
toroidal roller bearings, and self-aligning ball bearings work well when operating loads or
errors in machining or mounting produce the misalignment. Tolerable misalignment
values for most bearing types should be available from the manufacturer.
Precision and speed
When machinery needs high accuracy at high speeds such as machine tool spindles,
specify high-precision bearings. Dimensional and running accuracy tolerances of rolling
bearings have been standardized internationally. Tolerance classes offer a reliable guide
for specifying the proper high-precision bearings for the application.
Rolling bearings have a top-speed rating. Generally, the maximum lubricant operating
temperature or the specific design and material of the bearing components govern the
permissible speed. First, check out thermal reference speeds for a given bearing type. But
bearing type, size, internal design, precision, load, cooling, cage design, accuracy, and
internal clearance, also influence the speed capability of a bearing.
Bearings may operate at speeds above the reference speed only when bearing friction is
reduced by using lubrication systems that dispense small, accurately measured quantities
of lubricant, or when heat is removed, typically by using circulating oil lubrication either
with cooling ribs on the housing or with directed cooling air streams. In some cases,
changes in component designs and materials to suit the application can boost permissible
operating speeds.
Stiffness
The stiffness of a rolling bearing is similar to the stiffness of a spring and is characterized
by the magnitude of the elastic deformation (resilience) in the bearing under load. In
general, this deformation is extremely small and can be neglected. In some cases,
however, stiffness is critical, such as in spindle bearings for machine tools or pinion-
bearings in automotive axle drives.
In general, the contact integrity between rolling elements and raceways in roller bearings
provide a higher degree of stiffness compared with ball bearings, but applying a preload
can increase bearing stiffness.
Depending on the application, the components of a bearing can be made of different
materials. For example, the ball rolling elements can be made from a ceramic while the
remainder of the bearing is composed of steel materials.
Preload
In addition to increasing system stiffness, a bearing preload can reduce running noise,
improve shaft guidance accuracy, compensate for wear and settling, and extend service
life because it basically imposes a negative operational clearance in a bearing.
Preload also should come into play when bearings operate without any load or under very
light load at high speeds. In these cases, the minimum preload prevents roller elements
from sliding and damaging the bearings.
Depending on bearing type, the preload may be either radial or axial. Cylindrical roller
bearings can only be preloaded radially. Thrust ball bearings can only be preloaded
axially. Single row angular contact ball bearings and tapered roller
bearings are also preloaded axially. Springs, washers, friction torque, direct force, and
certain kinds of adjustments are a few common techniques for applying preloads.
Materials
The materials used for bearings directly affect their performance and reliability. For
bearing rings and rolling elements, typical material properties to specify include hardness
for load-carrying capacity, fatigue resistance under rolling contact conditions, and the
dimensional stability of bearing components. For cages, consider friction, strain, inertia
forces, and, in some cases, the chemical effects of lubricants, solvents, coolants, and
refrigerants.
A variety of materials, processes, and coatings have been engineered to address these
factors.
• Materials for Bearing Rings and Rolling Elements: Through-hardened bearing steels,
such as carbon chromium steel, normally receive a martensitic or bainitic heat treatment
for hardening to 58 to 65 HRC. Within the past few years, process developments for this
steel have reduced oxygen and non-metallic elements, improved properties, and
contributed to an entirely new class of bearings. Through-hardened steels are used for
almost all ball bearings and extensively in smaller roller bearings.
Case-hardening bearing steels consist of chromium-nickel and manganese-chromium
alloyed steel and constitute the most commonly used conventional material for standard
rolling bearings that require high toughness or shock loading. In applications with high
tensile hoop stress from interference fits and high shock loads, bearings with carburized
rings and/or rolling elements also should be considered.
For operating temperatures to 250°C, a special heat treatment for bearings made from
hardened or surface-hardened steel can be applied. However, this can reduce hardness
and load-carrying capacity.
For bearings operating at temperatures higher than 250°C for extended periods, highly
alloyed high-temperature bearing steels retain hardness and bearing performance
characteristics.Bearing grade ceramics, such as silicon nitride, have gained favor in some applications due to high hardness, low density, low thermal expansion, high electric resistivity, low dielectric constant, and no response to magnetic fields.
For bearings where corrosion may develop, stainless bearing steels, such as chromium or
chromium/molybdenum stainless, provide a measure of protection. Their reduced
hardness, however, lowers load-carrying capability compared with conventional steels. In
addition, corrosion resistance is only effective when the entire surface is perfectly
polished and not roughened or damaged during mounting.
• Cage Materials: Most pressed sheet steel cages are made from continuously hot-rolled
low carbon steel. These lightweight cages have relatively high strength and can be
surface-treated to reduce friction and wear.
Machined steel cages are normally manufactured from non-alloyed structural steel and
can be surface-treated to improve sliding and wear resistance. These cages are usually
specified for large-size bearings or as substitutes for bronze cages in applications where
there is a potential for season cracking caused by a chemical reaction. They perform at
operating temperatures to 300°C and are unaffected by rolling bearing minerals, synthetic
oil-based lubricants, or the organic solvents used to clean the bearings.
Most brass cages are robust and machined from a cast or wrought brass that withstands
most common bearing lubricants and normal organic solvents. These cages, however, are
not for applications where temperatures may exceed 250°C.
Polyamide polymer cages, with or without glass fiber reinforcement, deliver strength and
elasticity, but cage life may decrease over time with increasing temperatures and
aggressive lubricants, depending on the particular grade of polyamide material.
Polyether either ketone (PEEK) glass fiber-reinforced cages are able to stand up to high
speeds, chemical attack, or high temperatures.
Certain types of coatings let designers upgrade bearing materials for additional features for
specific application conditions within designated parameters. Noteworthy examples include a
low-friction coating applied on a bearing’s inner surfaces. Compared with standard uncoated types, such bearings become harder, generate less friction and heat, and can better tolerate minor damage from contamination and marginal lubrication. They are better equipped to resist
wear, operate at higher speeds, accommodate higher loads, and perform even during
periods of insufficient lubrication.
Another coating can be applied to the exterior of a bearing’s outer or inner ring to resist
potential damage from electric current through the bearing. A thin aluminum oxide layer
forms a barrier against electric arcing, which can cause fluting damage to rollers and
raceways or oxidation to grease.
The CARB toroidal roller bearings act as non-locating, self-aligning bearings in industrial
fans. They compensate internally for angular misalignment and accommodate axial shaft
expansion, eliminating outer ring loose fits or axial slide housings. They are quiet, cool-
running bearings and are less-sensitive to high-temperature shaft expansions and flat
mounting surfaces.
Exploring the ranges of bearing capability
Perhaps as a logical extension of all thetechnologies and engineering available to
designers and users, a new performance class of rolling bearings has emerged. The
Explorer class of bearings may last up to 300% longer in applications than standard
bearing products.
The Explorer offers most sizes and types of spherical roller bearings, angular contact ball
bearings, cylindrical roller bearings, spherical roller thrust bearings, taper roller bearings,
deep groove ball bearings, and four-point contact ball bearings are included. More are on
the way. All are dimensionally interchangeable with standard products.
The features and benefits of this class of bearings are: ground transition
raceways/shoulders that deliver higher thrust load capability, lower overall contact
stresses, and reduced edge stresses for increased power density, load rating, and safety.
Modified polyamide/brass cage geometry promotes higher speed ratings and
acceleration, lower heat generation, and reduced vibration and noise.
Upgraded balls and rollers offer improved running accuracy. A unique heat-treating
process minimizes dimensional changes on balls made from steel with low oxygen
content. State-of-the-art techniques extend life and reduce fatigue failure.
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