Bearings play an important role in the proper functioning of mechanical systems; in other words, they allow machines to run smoothly and safely. The purpose of a bearing is to reduce friction between moving parts and restrict their motion to the preferred direction.
Based on their mechanism, mode of operation and purpose, bearings can be classified into several types and sub-types. Before we get started, you should be familiar with bearing loads, more specifically, kinds of loading.
The two most common types of bearing loads are radial and axial (thrust), and depending on where it is being used, a combination of both kinds of loading can be seen.
4. Plain Bearing
A plain bearing on a 1906 S-Motor locomotive | Image Courtesy: Wikimedia Commons
Plain bearing, also known as slide bearing, is one of the most widely used bearing types in the world. As the name suggests, they are the simplest of all bearings featuring just the bearing surface. They usually are lightweight and have a high load-carrying capacity.
Based on the design and type of motion it provides, plain bearings can be distinguished into several sub-types including, journal, linear, thrust, and composite bearing.
Journal bearing is widely used in railroad car axles and locomotives. In North America, railroad car wheels feature rolling-elements bearing instead of a plain bearing.
To make them more effective (low friction) and durable, plain bearings are often made using different materials. In mechanical watches, where precision and low friction are of utmost necessity, the bearings are made of jewels (synthetic sapphire) also known as jewel bearing.
3. Rolling-element Bearing
A rolling-element bearing is any bearing that contains rolling components, i.e balls or rollers, between two bearing rings or races. These bearings have a design advantage over plain bearings or most other types of bearings for that matter.
The relative motion of the races causes the rolling elements to roll with very little rolling resistance and with little sliding.
There are two main types of rolling-element bearing; ball bearing and roller bearing.
3.1 Ball Bearing
Ball-bearing for skateboard wheels | Image Courtesy: Wikimedia Commons/Mr. PIM
In this type of rolling-element bearing, balls (made either of steel or hybrid ceramic) is placed between two bearing races. Usually, one bearing race rotates (causing the balls to rotate as well) and the other remains stationary.
Ball bearings produce a low coefficient of friction as the inner and outer race surfaces of the bearing are separated by smooth-rolling spherical balls and don’t slide against each other. Only a small portion of the ball comes in contact with both race surfaces. They, however, have a low load capacity for their size.
While such bearings are usually designed to support radial loads (perpendicular to the axle) more than axial loads, some are made specifically to support the latter, for example, thrust bearing.
Today, ball bearings are used in skateboard wheels, computer fans, and centrifugal pumps among other things. Some older hard disk drives also feature such bearings.
3.2 Roller Bearing
Unlike ball bearings, the rollers used in this type of bearing are cylindrical in shape; thus the contact between the lower and upper race surfaces is linear in nature (and not point). Roller bearings are designed to support greater radial loads than ball bearings but are not capable of handling larger thrust loading.
Roller bearing can be broadly classified into five sub-types; cylindrical, spherical, gear, tapered and needle roller.
3.2.1 Cylindrical Roller
Cylindrical roller is the simplest of roller bearings and has high radial load capacity (but low axial load support). They are prone to misalignment and in such cases, the total bearing load capacity is drastically reduced.
In cylindrical rollers, the outer load is unevenly distributed or constantly re-distributed among the rollers. At a given time only a few rollers carry most of the load.
3.2.2 Spherical Roller Bearing
A cut-through view of double row spherical roller bearing | Image Courtesy: Wikimedia Commons
Despite the name, spherical roller bearings feature cylindrical rolling elements that are slightly enlarged or thicker in the middle. Such bearings allow misalignment (angular), the trade-off being they cannot operate at very high speeds. Furthermore, they are expensive and generate more friction than other types of roller bearings.
Spherical roller bearing comes in two design variants; single row rollers and double row rollers. Single row rollers have low load carrying capacity and are also known as “barrel roller bearings”. On the other hand, the double row variant (featuring two rows of cylindrical rolling elements) can withstand heavy loads.
3.2.3 Gear Bearing
A cutaway view of gear bearing
Another type of roller-element bearing is gear bearing. It is comprised of numerous smaller gears on both race surfaces (insides of the outer ring and outside of the internal ring) as well as on the cylindrical rolling component.
Similar to chain-driven bicycles, gear bearings perform axial meshing in which the individual gears or teeth must engage properly to function. The very design of gear bearing ensures minimum sliding friction and it also prevents misalignment.
Perhaps the biggest downside of gear bearing is its complex manufacturing process and high-cost maintenance.
3.2.4 Tapered Roller Bearing
A cutaway view of tapered roller bearing
Unlike most other types of roller-element bearings, tapered roller bearings support large loads of both types (axial and radial). A tapered roller bearing feature conical (or tapered) rollers as well as conical raceways.
The conical nature of the tapered roller bearing allows them to support larger loads than ball bearings (line contact instead of point contact). The rollers are stabilized by an internal rim, which prevents them from dislodging.
Tapered rollers are usually preferred for heavy-duty applications such as in mining and construction equipment, automobile wheels, wind turbines, railroad car axle-box, and propeller shaft. They are deployed in a back-to-back arrangement in many modern applications.
3.2.5. Needle Roller Bearing
Needle roller bearing
A needle roller bearing can be identified by its uniquely thin but longer cylindrical rollers. Needle bearings, as compared to other roller bearing types, have a larger area of contact with the race surfaces, meaning they can support larger loads. Another upside of needle roller bearings is they can be easily fitted in compact spaces due to their thin stature.
The needle roller bearing is predominantly used in the automobile industry from drive shafts to transmissions and compressors.
2. Fluid Bearing
Fluid film bearing | Image Courtesy: Waukesha bearings
Instead of balls or rollers, fluid bearings utilize pressurized liquid/gas to operate. There are two types of fluid bearings; hydrodynamic bearings, which depends on race speeds, and hydrostatic bearings, which are pressurized by either oil or water and sometimes even air.
Fluid bearings are increasingly used in applications where very high speed, precision, and durability is needed. Since there is no metal contact between the races, fluid bearings experience lower friction and vibration.
Due to their superior capabilities, fluid bearings have largely replaced ordinary ball bearings from hard disk drives.
1. Magnetic Bearing
A magnetic bearing
As the name suggests, such bearing uses magnetic levitation to support a load or function. They basically work on the principle of electromagnetic suspension. Since magnetic bearings operate without physical exposure, they have extremely low friction and vibration. They can also work in a vacuum and without lubrication.
Magnetic bearings have the highest speed supporting capacity of all bearings, including fluid bearing. They are being used in high-speed compressors, generators, turbines, and other specialized machines.
One popular application of such bearings is the electrodynamic magnetic levitation system (inductrack) used in maglev trains to control excessive noise.