Ball Bearings are mechanical assemblies that consist of rolling spherical elements that are captured between circular inner and outer races. They provide a means of supporting rotating shafts and minimizing friction between shafts and stationary machine members. Ball bearings are used primarily in machinery that has shafts requiring support for low friction rotation. There are several configurations, most notably shielded or sealed. Ball bearings are standardized to permit interchangeability. Ball bearings are also known as rolling element bearings or anti-friction bearings. Considerations include
Roller Bearings are mechanical assemblies that consist of cylindrical or tapered rolling elements usually captured between inner and outer races. They provide a means of supporting rotating shafts and minimizing friction between shafts and stationary machine members. Roller bearings are used primarily in machinery with rotating shafts that require the support of heavier loads than ball bearings provide. Tapered roller bearings are often used to accommodate higher thrust loads in addition to the radial loads. Types range from cylindrical to spherical rollers. Roller bearings are standardized like ball bearings, albeit to a lesser degree. Considerations include
Mounted Bearings are mechanical assemblies that consist of bearings housed within bolt-on or threaded mounting components and include pillow blocks, flanged units, etc. They provide means of supporting rotating shafts and minimizing friction between shafts and stationary machine members. Mounted bearings are used primarily in machinery with exposed rotating shafting. They are used as take-up devices on the ends of conveyors and as flanged units along intermediate points. The bearings can be rolling element or journal bearing configurations. Mounted bearings are designed for bolt-on mounting and ease of replacement. Other varieties of mounted bearings include rod end bearings and cam followers. Considerations include
Linear Bearings are mechanical assemblies that consist of ball or roller elements captured in housings and used to provide linear movement along shafts. Linear bearings are used primarily in machinery that requires linear movement and positioning along shafts. They also may have
Slide bearings are mechanical assemblies designed to provide free motion in one dimension between structural elements. Slide bearings are used primarily in the structural support of bridges as well as commercial and industrial buildings. These parts accommodate thermal movement, allow for end-beam rotation, and isolate components of the structure against vibration, noise, and shock. Other types of slide bearings include those used on truss base plates, heat exchangers, and process equipment.
Jewel bearings are mechanical devices used in light rotating applications such as watches, meter movements, gyroscopes, etc. where loads are small and the supported rotating shafts are tiny. Jewel bearings are constructed from a range of synthetics, with ruby and sapphire being particularly common.
Frictionless bearings are mechanical or electro-mechanical alternatives to conventional bearings that provide controllable shaft support through air, magnetic fields, etc. for critical, high precision applications.
Bearing applications span across virtually every industry which employs moving components and equipment. For example:
While bearings are used nearly everywhere, there are some industries that use so many or have specific requirements for durability, cleanliness, etc. that they warrant mentioning here. Some of these industries are
When selecting a bearing for a particular application, there are several considerations to keep in mind, including bearing friction, temperature, and lubrication. Along with the specific design and construction of the bearing, these three interacting factors can affect the overall performance.
Radial ball bearings are used primarily for radially loaded shafts and those with light axial loads. Angular contact ball bearings are designed to take higher axial loads in one direction in addition to their radial capacities. Ball thrust bearings are available which are specifically intended to take axial loads alone. The most common configuration for radial ball bearings is the single row version, which could be shielded or sealed depending on whether it is to be used within a housed area—a transmission, say—or in an exposed environment such as on a bicycle wheel. The seals and shields keep lubricant in the bearing and dirt and debris out of it. Ball bearings are usually fitted with retainers which space the balls evenly between and around the perimeters of their outer and inner races. Full capacity bearings dispense with retainers in order to fill as many balls as possible between the races, adding to the bearing’s load handling capacity.
Roller bearings employ a host of different shapes for their rolling elements, including straight rollers, needle rollers, tapered rollers, spherical rollers, etc. Roller bearings are able to take higher radial loads than their ball bearing counterparts due to the higher contact area between the rollers and the races. Some roller bearings are designed to take high thrust loads using tapered elements and races.
Mounted bearings are ball, roller, or sleeve bearings which are furnished in housings, flanges, etc. and usually installed with seals and/or shields for environmental protection. Common mounting styles include pillow blocks, flanges, take-ups, etc. They are often used on conveyors where take-up assemblies provide adjustment for conveyor belt tension.
In selecting rolling element bearings, either ball or roller or as mounted units, designers usually consider a number of factors including loads, both their quantities and directions, the accuracy requirements of the shaft system, misalignment factors, speeds, noise, and friction. Where radial loads are high, a designer may opt for a roller bearing over a ball bearing and might do the same where high axial loads are anticipated. Where the bearing needs to be able to accommodate some shaft misalignment, the designer may elect a ball bearing where loads are normal or go to a spherical roller bearing which is also very capable of handling misalignment. Ball bearings tend to be better at handling high speeds than roller bearings, and in some cases where accuracy and low friction are paramount, such as machine tools, a ball bearing may be the only way of meeting the requirements.
Of particular interest in considering bearings are their static and dynamic load ratings. Bearing that are subject to high loads when they are not rotating can undergo a phenomenon known as brinelling, where the balls dent the races in the same place repeatedly. The same loads applied to the bearing when running may cause less concern because any indentations will distribute around the bearing races and not pile up in the same spots each time.
Bearing makers list bearing rated capacities for their bearings, which for ball bearings are identified as extra-light, light-, medium-duty, and so on, where the dimensions of the bore or shaft requirements increase to accommodate increasing loads. The rated capacity is based on a statistical measure which states that a certain percentage of bearings will complete a stated number of revolutions without failing. These catalog numbers can be massaged to better pick the bearing suited to the actual conditions of use.
Linear bearings are sized according to linear travel, total linear distance, load, precision requirements, etc., with many parameters being analogous to the radial bearing considerations. Linear bearings are used with ground shafting for dimensional accuracy and low-friction sliding.
Slide bearings are used to accommodate expansion and contraction in stationary structures such as bridges and building. Often they consist of two Teflon plates which are sandwich between major structural members. Sometimes stainless steel is used instead of Teflon for one of the two facing bearing surfaces. Of principal concern with slide bearings is the force per unit area they can withstand.
Jewel bearings are used in very light loading applications. Jewel bearings provide very accurate, hard surfaces which can support lightly loaded shafts that see mostly intermittent motion.
Frictionless bearings are bearings that use air or other gases or magnetic fields to support rotating journals and are so-called to distinguish them from anti-friction bearings—another term for rolling element bearings, which in itself was coined to distinguish these from original journal bearings which used friction developed through shaft rotation to create films of fluid for supporting shaft journals.
Frictionless bearings represent a small slice of the bearing world and are usually applied only in very rare situations.
The ABMA (American Bearings Manufacturers Association) provides standards for many bearing types and is affiliated with the so-called ABEC system which rates bearing precision.
For ball bearings, the Conrad, or non-slot fill bearing is the most common, whose design dispenses with a filling slot and instead relies on displacing the inner race to load the balls and a cage to keep them evenly spaced. For roller bearings, bearing type requires a selection of roller type, be it cylindrical, tapered, spherical, etc. Mounted units require a type selection of ball, roller, or spherical, as well, and then an additional selection of style, as defined below. Linear bearing types range from ball bearing cages—essentially bare cages holding balls that are often used as die-post bushings—to recirculating ball designs where the balls roll linearly along the shaft and then return to their starting points through channels on the non-shaft sides of the bearings.
This attribute applies solely to mounted units, where a distinction must be made between the housing for the bearing, among them the choice of pillow blocks, flanges, take-ups, etc.
Material choice for ball and roller bearings is generally limited to a few specialty steel alloys, some plastics, occasionally ceramics, etc. while mounted units have more material choices owing to the additional materials available for housings.
Ball bearings exposed to the environment can be ordered with seals and/or shields where shields provide some protection of the bearing elements from dirt ingress with minimal added friction and seals provide shaft contacting lips that exclude moisture but do add to the friction on the bearing. Seals and shields can be added on both sides, either side, alone or in combination. The image to the right shows a radial bearing in cross section with shields on both sides.
Ball bearings races fall generally into two designs: angular contact and radial. Angular contact bearings (image right) load the balls at angles to the perpendicular radial planes, whereas radial contact bearings (image above) load the balls through the perpendicular planes. Angular contact bearings are generally preferred where axial loading is a consideration. Deep groove bearings are commonly associated with radial contact bearings. Cup and cone bearings are common on bicycle wheels where the bearings are loosely packed between cones and the cones are adjusted for play.
Mounted pillow block units are classified as expandable and non-expandable and in situations where two are more pillow block bearings are installed for shaft support one will ordinarily be specified as non-expandable and the other as expandable which allows the bearing to accommodate slight growth of the shaft. Some units are configured to allow either option.
Bearing loading is based on static and dynamic values and the choice of which governs is a function of the operating conditions the bearing will see.
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