Bearing (mechanical)

Bearing (mechanical)
Name	Bearing
Caption	Cutaway view of a ball bearing.
Classification	Machine element
Industry	Mechanical engineering, Manufacturing
Related	Bushing, Shaft, Lubrication

Bearing (mechanical). A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. The design of the bearing provides for the free linear movement of the moving part or for free rotation around a fixed axis; it may also prevent motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction, and they are found in virtually every type of machinery, from simple devices to complex systems like jet engines and space telescopes.

Types of bearings

The primary classification distinguishes between sliding contact and rolling contact bearings. Plain bearings, also known as bushings or sleeve bearings, are the simplest type, comprising a surface with no moving elements. Rolling-element bearings utilize rolling elements such as balls or rollers to maintain separation between moving parts; common variants include angular contact, thrust, and tapered roller designs. Specialized types include fluid bearings, which use a thin layer of gas or liquid like those in hard drives and turbines, and magnetic bearings, which employ magnetic levitation and are used in applications like flywheel energy storage and advanced centrifugal compressors. Other notable forms are jewel bearings, historically used in precision instruments like those from Patek Philippe, and flexure bearings, which provide motion through the bending of an element.

Design and construction

Bearing design is a critical subset of mechanical engineering focused on tribology. Key components include the inner and outer races, the rolling elements (balls, cylinders, tapers, or needles), and a cage or separator. Materials are selected for durability and low friction; common choices include chrome steel, stainless steel (e.g., 440C), ceramic (like silicon nitride used by SKF), and polymers such as polyoxymethylene and polytetrafluoroethylene. Construction often involves precision grinding and heat treatment processes to achieve the required tolerances and surface finishes. Seals and shields, made from materials like Buna-N, are integrated to retain lubricants and exclude contaminants.

Function and principles of operation

Bearings function by managing loads—radial, axial, or moment—and converting sliding friction into rolling friction or fluid shear. The fundamental principle involves distributing load over an area to reduce contact stress. In rolling-element bearings, Hertzian contact stress theory describes the stress at the contact points. Elastohydrodynamic lubrication is the regime in which most rolling bearings operate, where the lubricant film is generated under high pressure. For fluid bearings, principles of hydrodynamics or aerodynamics create a supporting film, as seen in the hydrostatic bearings of large telescope mounts like the Keck Observatory. Magnetic bearings rely on feedback control systems, often using sensors from companies like Siemens, to maintain position without physical contact.

History and development

The earliest known bearings were wooden rollers used beneath heavy stones during the construction of monuments like the Pyramids of Giza. The Roman Empire saw the use of wood and bronze sleeve bearings in water mills and chariots. A pivotal advancement was the invention of the ball bearing, with a design attributed to Philip Vaughan who patented an arrangement for a mail coach in Carmarthen. The modern era of bearing manufacturing began in the late 19th century with founders like Friedrich Fischer of FAG, who developed the ball grinding machine, and Sven Wingquist of SKF, who invented the self-aligning ball bearing. The Industrial Revolution and subsequent wars, including World War II, drove massive innovation in materials and mass-production techniques.

Selection and maintenance

Bearing selection is governed by factors such as load, speed, misalignment, temperature, and environmental conditions, often guided by standards from the International Organization for Standardization and the American Bearing Manufacturers Association. Key calculations involve bearing life (predicted using the Lundberg-Palmgren theory and standardized in ISO 281), and static load rating. Maintenance practices include proper lubrication with greases or oils, monitoring for vibration and acoustic emission, and preventing contamination. Predictive maintenance in industries like rail transport and wind turbine farms utilizes condition monitoring systems from companies like General Electric and Schaeffler Group.

Applications

Bearings are ubiquitous in mechanical systems. In automotive applications, they are found in wheel hubs, transmissions, and alternators. The aerospace industry uses high-performance bearings in aircraft engines from Pratt & Whitney and landing gear. They are critical in industrial machinery such as the electric motors from ABB Group, CNC machine tools, and conveyor systems in fulfillment centers. Precision instruments like the Large Hadron Collider at CERN and medical devices like dental drills rely on specialized bearings. Everyday consumer products, from hard disk drives and electric fans to skateboards and fishing reels, incorporate various bearing types.

Category:Mechanical engineering Category:Machine parts