At a Glance:
- Bearing types and functions—from plain bearings to roller-element bearings
- Bearing maintenance; the importance of proper lubrication
- Bearing specialist certification and common manufacturer standards
Bearings are a staple of machine design. In fact, they are among the most commonly used machine components due to their essential function: to reduce friction and enable movement between specific parts.
A bearing, like the name suggests, bears or supports the load that one machine part puts on another, facilitating a desired motion—e.g., sliding, turning or rolling. The bearing can transmit force as a radial rotation, a linear motion, a spherical rotation or a hinge motion.
While most bearings are made of metal—typically steel, copper alloy or aluminum alloy—they can also be made of wood, glass, ceramic and plastic.
Bearing Types
There are two primary types of bearings: a plain bearing and a rolling-element bearing.
A plain bearing is a simple design for high-load and low-speed applications. The bearing fits into a housing to support and stabilize a shaft, allowing it to move while reducing friction with lubrication.
READ MORE: Getting Your Bearings
There are three categories of plain bearings:
- Radial bearing, also known as journal bearing: designed to support the radial load, a force perpendicular to the shaft.
- Thrust bearing: designed to support the thrust load, a force parallel to the shaft.
- Linear bearing: designed to allow linear, sliding motion.
Rolling-element bearings are designed for high-speed applications and precise motion control. Core elements include something that rolls (rollers, balls, cones, spheres or needles); an inner and outer ring with precisely machined raceways; and a support, retainer or cage for keeping rolling elements evenly spaced within the rings. A load applied to the bearing is transmitted from the stationary outer ring to the rolling element to the inner ring that is free to rotate.
Rolling-element bearings are categorized as: ball bearings, roller bearings, tapered roller bearings, spherical roller bearings, needle roller bearings and thrust bearings.
Other, less common bearings would include jewel bearings, fluid bearings, magnetic bearings and flexure bearings.
Bearing Selection
Your choice of bearing comes down to a few key factors, including function, lifespan, reliability and cost versus performance.
Plain bearings are generally the least expensive type of bearing and are often lightweight, compact and impact resistant. Plain bearings are highly customizable and have a high load-bearing capacity.
While most are metal-made, plastic is an increasingly popular option for basic bearings with lower load and friction applications. Plastic bearings have some key advantages, including flexibility to better handle edge loads, lower weight and self-lubricating options. Extreme loads, temperatures and speeds are all limiting factors for plastic.
READ MORE: Plastic Bearings: The Pros and Cons
Roller-element bearings are an often larger, higher-cost option, but are needed for high-speed rotation applications. With the vast majority made of metal to withstand extreme conditions, their design dictates their function as follows:
- Ball bearing: The most common type of bearing, the ball bearing handles radial and thrust forces in low-load, higher-friction applications. A row of spheres is separated and guided by a cage within the rings, which allows them to spin freely in the raceway. The sphere shape provides a small point of contact, which greatly reduces friction, but is easily overloaded.
- Roller bearing: Using a cylindrical roller in place of a ball, roller bearings allow both low-friction contact and a higher radial load and speed. With a line of contact, rather than a point from a ball, rollers distribute the load on a larger area than a ball bearing.
- Tapered roller bearing: As the name suggests, a tapered roller bearing has tapered rings and rollers. The resulting tilt of the rollers allows the bearing to handle an axial load, with the higher the angle the greater the capacity.
- Spherical roller bearing: These bearings typically have two rows of barrel-shaped rollers, each with a separate speedway and facing opposite directions. By supporting the load in both directions, these bearings allow for some misalignment to occur. The grooves in the rings match the barrel profile of the spheres, which enables a high-load capacity.
- Needle roller bearing: The longer, thinner version of the roller bearing, needle roller bearings are often chosen for applications with limited radial space. However, their smaller size generates more friction, which means they are generally used at low speeds.
- Thrust bearing: Designed for high axial loads, thrust bearings are commonly either rollers or balls. While roller thrust bearings can handle a higher load than ball thrust bearings, both are limited by speed. With their high-load capacity, thrust bearings are typically used in automotive, marine and aerospace applications.
READ MORE: A Gallery of Common Bearings
Maintenance
Lubrication—typically, oil or grease—is critical to the life of a bearing and is multipurpose, serving to protect against corrosion, seal against contaminants, create a barrier between elements and provide heat transference.
Oil lubrication (synthetic or petroleum) is ideal for heavy load and high—speed applications. The oil is applied by hydrostatic lubrication—pumped through a pressurization system or hydrodynamic lubrication—squeezed or wedged through the bearing by its rolling action. High loads or low speeds can break down hydrodynamic lubrication in what’s known as boundary lubrication, leading to metal-to-metal contact.
Grease, applied either by packing or plating, can offer better adhesion and retention than oil under more moderate speeds and lighter loads. The downside of grease is the manual labor required to replenish, as well as the cost of high-temperature grease.
While all bearings require some form of lubrication, plastic plain bearings can be designed as self-lubricating. These bearings are made with solid lubricants embedded within their material, providing a maintenance-free solution by eliminating the need for external lubrication.
Beyond lubrication maintenance, bearing mounting, temperature, exposure and cleanliness all directly impact a bearing’s service life.
Certification and Standards
According to the Bearing Specialist Association, based in Illinois, they offer the only bearing industry-specific program to certify an industry professional as a bearing specialist. The association identifies nearly 30 companies that employ certified bearing specialists, ranging from applications engineers and bearing technical support to service engineers and sales representatives.
Bearing manufacturers typically hold a range of global standards, from ISO 9001 to those specific to the aerospace, automobile or medical device industries, among others.
