BEARINGS

A world without bearings would look far different. Although rarely noticed, bearings are ubiquitous: computers have a few, electric appliances contain several, and an automobile has hundreds. Without bearings, much of the motion that is taken for granted would not be possible.

Since the first wheel was invented, people understood that it takes less effort to move an object on rollers than to simply slide it over a surface. People later discovered that lubrication also reduces the effort to side objects. Bearings combine these two basic findings to provide rolling motion necessary for things as simple as a tiny in-line skate wheel and as complex and large as a steam turbine. Bearings save energy, which is otherwise required to counteract friction arising from any elements related rotation, the better the bearing, the greater the energy savings.

TYPES OF BEARINGS

Bearings can be divided into rolling element bearings and sleeve (or plain) bearings. Sleeve (or plain) bearings consist of many sizes, shapes, and types, each of which functions essentially as a band (or sleeve) of close fitting material that encloses and supports a moving member. The sleeve is usually stationary and is called the bearing. The moving member is generally called the journal. Rolling element bearings are generally constructed with one or two rows of steel balls or rollers positioned between inner and outer rings. A retainer is used to equally space these rolling elements. Grooves, or raceways, are cut into the inner and outer rings to guide the rolling elements.

Rolling bearings have a lot of advantages compared with the sleeve bearings. Just to name a few: lower energy consumption, lower starting moment, lower friction at all speeds, and higher reliability. This is why there is a larger variety of rolling bearings than sleeve bearings. The variety of applications calls for the multitude of the rolling bearing types and designs. There are simple applications such as bicycles, in-line skates and electric motors. There are also complex applications such as aircraft gas turbines, rolling mills, dental drill assemblies, gyroscopes and power transmission products. Better automobile transmissions, containing hundred of bearings and delivering more mechanical energy to the wheels, have resulted in dramatic improvement in fuel economy and performance from 1975 to 2000. In delivering more mechanical energy to the wheels, and much of that improvement can be attributed to better bearings.

Rolling bearings can be further classified into ball and roller bearings. The following are some of the very common types of rolling bearings.

Deep Groove Ball Bearings

Deep groove ball bearings, widely found in automobile applications, are the most popular of all rolling bearings. They are available in single and double row designs. The single row bearings are also available in a sealed version. They are simple in construction as well as easy to operate and maintain. They can run at high speeds and can support both radial and thrust loads imposed by rotating shaft and other moving objects. They are versatile, quiet, lubricated-for-life, and maintenance-free. The bearing cost/performance ratio for deep groove ball bearings is excellent. They are widely found in automobile applications.

Self-Aligning Ball Bearings

Commonly found in vertical pumps, self-aligning ball bearings have two rows of balls with a common spherical outer ring raceway. This feature gives the bearings their self-aligning property, permitting angular misalignment of the shaft with respect to the housing. Self-aligning ball bearings show very low vibration and noise level owing to the high accuracy of form and smoothness of the raceways.

Angular Contact Ball Bearings

Angular contact ball bearings are available in single and double row designs as well as four-point contact ball bearings. They are designed for a combined load and provide stiff bearing arrangements. Angular contact ball bearings have raceways in the inner and outer rings, which are displaced with respect to each other in the direction of the bearing axis. This means that they are particularly suitable for the accommodation of combined loads (i.e., simultaneously acting radial and thrust loads such as for orbital sanders). The benefits are high-load carrying capacity and speed capability, low operating temperatures, long relubrication intervals and quiet operation.

Thrust Ball Bearings

Thrust ball bearings are manufactured in single direction and double direction designs. They are only able to accept thrust loads but can be operated at relatively high speeds. Mounting is simple because the various bearing components (shaft washer, housing washer, ball and cage thrust assembly) can be installed separately. The benefits of using thrust ball bearings derive from their high running accuracy and high load carrying capacity, which is why they are used in automobile clutches and speed reducers.

Cylindrical Roller Bearings

Cylindrical roller bearings, found in tractor and machine tools, can carry heavy radial loads at high speeds because the rollers and raceway are in linear contact. Single row bearings have optimized internal geometry that increases their radial and thrust load carrying capacity, reduces their sensitivity to misalignment, and facilitates their lubrication. Full complement bearings incorporate the maximum number of rollers and have no cage, and are intended for very heavy loads and moderate speeds.

Needle Roller Bearings

Needle roller bearings can support heavy radial load such as clutches. A wide variety of designs, including bearings for combined radial and thrust loads, provide simple, compact and economic bearing arrangements. Their small sectional height makes them suitable for limited radial space of the housing.

Spherical Roller Bearings

Spherical roller bearings are robust, self-aligning bearings that are insensitive to angular misalignment. They offer high reliability and long life even under difficult operating conditions. They are mounted on an adapter assembly or withdrawal sleeve and housed in plummer blocks. They are also available with seals for maintenance-free operation.

Tapered Roller Bearings

Tapered roller bearings are designed for heavy combined loads or impact loading such as freight train locomotives and rail cars. Composed of a cup, a cone and a number of rollers, tapered roller bearings can do a much better job of withstanding sideward forces. These three components have tapered surfaces whose apexes converge at a common point on the bearing axis. Their excellent load carrying capacity/cross section ratios provide an economic bearing arrangement.

APPLICATIONS

High-carbon chrome bearing steel specified in SAE 52100 is used as a general material in bearing rings and rolling elements. The cages can be made of various materials, such as steel sheet, steel, copper alloy and synthetic resins. Once relegated to high-end applications, such as aircraft wing-flap actuators and precision instruments, hybrid bearings with ceramic balls are moving into the mainstream. Hybrid ceramic bearings offer many new options for demanding applications. Benefits include high speed, corrosion resistance, durability, reduced vibration, ability to operate with less lubricant, and electrical insulation.

Today's in-line skate market takes the advantage of the newly available technology in bearing design. For example, the ABEC (Annular Bearing Engineers' Committee) scale classifies different accuracy and tolerance ranges for bearings. A good in-line skate bearing typically adopts a high ABEC rating bearing, chromium steel for rings and balls, 100 percent synthetic speed oil for skate lube, ultrafast self-lubricating synthetic resin retainers, and unique and attractive packaging.

Jiang Long

See also: Materials; Tribology.

BIBLIOGRAPHY

Harris, T. A. (2000). Rolling Bearing Analysis, 4th ed. New

York: John Wiley.