Updated May 2015 by Lisa Eitel. || Industrial bearings are simple machine elements but are crucial for motion applications. The purpose of a bearing is to reduce frictional forces between two moving parts by giving a surface something to roll on, rather than slide over.
A bearing consists of smooth rollers or metal balls and the smooth inner and outer surfaces, known as races, that the rollers or balls roll against. These rollers or balls act as the load carrier for the device, allowing it to spin freely.
Bearings typically encounter two kinds of load: radial and axial. Radial loads occur perpendicular to the shaft, while axial loads occur parallel to the shaft. Depending on the application the bearing is being used in, some bearings experience both loads simultaneously.
Bearing types abound to satisfy myriad applications.
Ball bearings
One of the most common forms of bearings is the ball bearing. As the name implies, ball bearings use balls to provide a low friction means of motion between two bearing races.
Because the contact area between the balls and races is so small, ball bearings cannot support as large a load as other bearing types and are best suited for light to moderate loads. However, their small surface contact also limits the heat generated by friction, meaning that ball bearings can be used in high-speed applications.
Roller bearings
Possibly the oldest form of bearing, roller bearings can be spherically or cylindrically shaped and are commonly used in applications like conveyor belt rollers. Because of their shape, roller bearings have greater surface contact than ball bearings, and are thus able to handle larger loads without deforming. Their shape also allows for a moderate amount of thrust load since the weight is distributed across cylinders instead of spheres.
Needle roller bearings
When you need to reduce friction between two moving parts but have very limited space to do so, a needle roller bearing may just be what you’re looking for. A needle roller bearing is a roller bearing with rollers whose length is at least four times their diameter. Despite their low cross section, the large surface area of the needle roller bearing allows them to support extremely high radial loads.
They usually consist of a cage, which orients and contains the needle rollers and an outer race, which is sometimes the housing itself. The bearings can often be found in two different arrangements. The first is a radial arrangement, in which the rollers run parallel to the shaft. The second is a thrust arrangement, in which the rollers are placed flat in a radial pattern and run perpendicular to the shaft.
These bearings are often used in automotive applications, such as rocker arm pivots, pumps, compressors and transmissions. The drive shaft of a rear-wheel drive vehicle typically has at least eight needle bearings (four in each U joint) and often more if it is particularly long, or operates on steep slopes.
Thrust ball bearings
Thrust ball bearings are designed for use in applications with primarily axial loads and are capable of handling shaft misalignment. These bearings are also useful in high-speed applications, such as in the aerospace and automotive industries.
Thrust roller bearings
Thrust roller bearings are designed so that the load is transmitted from one raceway to the other, meaning that these bearings can accommodate radial loads. Bearings like these also have a self-aligning capability that makes them immune to shaft deflection and alignment errors.
Tapered roller bearings
Tapered roller bearings feature tapered inner and outer ring raceways with tapered rollers arranged between them, angled so the surface of the rollers converge at the axis of the bearing. These bearings are unique in that, unlike most bearings that can handle either axial or radial loads, they can handle large amounts of load in both directions.
A single-row taper bearing is limited in that it can only take high axial loads from one direction, but if adjusted against a second tapered roller bearing, that axial load is counteracted. This allows the bearings to accept high radial and axial loads from multiple directions.
See the video: What are bearings? at the bottom of this page.
The ability of a tapered roller bearing to accommodate angular misalignment of the inner ring in relation to the outer ring is limited to a few minutes of arc. As with other roller bearings, tapered roller bearings must be given a minimum load, especially in high speed applications where the inertial forces and friction can have a damaging effect between the rollers and raceway.
Linear motion bearings
Linear motion bearings are specifically designed to allow motion in one direction and are typically used to carry a load on a slide or rail. They can be powered by a motor or by hand and experience over turning moments of force instead of radial and axial loads.
Plain Bearings
Plain bearings are the simplest form of bearing available, as they have no moving parts. They are often cylindrical, though the design of the bearing differs depending on the intended motion. Plain bearings are available in three designs: journal, linear and thrust.
Journal bearings support radial motion where a shaft rotates within the bearing.
Linear bearings often go in applications needing slide plates, as these bearings are designed to permit motion in a linear direction.
Plain thrust bearings do the same job as roller bearings, but instead of using cone-shaped rolling elements, the bearing uses pads arranged in a circle around the cylinder. These pads create wedge-shaped regions of oil inside the bearing between the pads and a rotating disk, which supports applied thrust and eliminates metal-on-metal contact.
Out of all the bearing types available, plain bearings tend to be the least expensive. They can be made from a variety of materials including bronze, graphite and plastics, such as Nylon, PTFE and polyacetal. Improvements in material characteristics have made plastic plain bearings increasingly popular in recent years. Plain bearings of all types, however, are lightweight, compact and can carry a substantial load.
Some plain bearings require outside lubrication while others are self-lubricating.
Plain bearings made of bronze or polyacetal, for example, contain lubricant within the walls of the bearing, but require some outside lubrication to maximize performance. For other plain bearings, the material itself acts as the lubricant. Such is the case with bearings made from PTFE or metalized graphite.
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The growing popularity of plain plastic bearings and increasingly stringent industry standards has resulted in more consumers requiring the bearings to meet FDA and RoHS standards. There has even been a call for the bearings to meet the standards of EU directive 10/2011/EC, which also takes the material manufacturing process into account.
Applications
Bearings are all around us in everyday life and most of the time they go unnoticed. But without them, many of the tasks we undertake would move along much less smoothly. The ball bearings’ simple design, ability to operate at high speeds and relatively low-maintenance requirements, makes them one of the most common roller bearings found in a variety of industrial applications.
For example, deep groove ball bearings are often used in small- to medium-sized electric motors because of their ability to accommodate both high speeds and radial and axial loads. Self-aligning ball bearings, on the other hand, are ideal for use in fans. These bearings have two rows of balls with a common raceway in the outer ring. This design allows for angular misalignment while maintaining running accuracy. They are, however, one of the most difficult bearings to install correctly.
Tapered roller bearings are another form of industrial bearing that just about every industry depends on one way or another. They are usually found in applications where support for axial and radial loads is required, such as in a tire hub where the bearing must deal with the radial load from the weight of the vehicle and the axial load experienced while cornering.
These bearings are also commonly found in gearboxes where they are generally mounted with a second bearing of the same type in a face-to-face or back-to-back orientation. They provide rigid shaft support, keeping deflection to a minimum. This reduced shaft deflection minimizes gear backlash.
Tapered bearings also have the advantage of having less mass but high efficiency, however this does limit their overall speed.
In applications where bearings are mounted vertically, they are typically oriented in a face-to-face setup, while horizontal applications use a back-to-back setup. Some pumps use this design because of shaft deflection concerns.
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