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 may. For example, provide for free linear movement of the moving part or for free rotation around a fixed axis. It may prevent a motion by controlling the vectors of normal forces that bear on the moving parts.
Rotary bearings hold rotating components such as shafts or axles within mechanical systems. It transfer axial and radial loads from the source of the load to the structure supporting it. The simplest form of bearing, the plain bearing, consists of a shaft rotating in a hole. Lubrication is used to reduce friction. In the ball bearing and roller bearing, to reduce sliding friction, rolling elements such as rollers or balls with a circular cross-section are located between the races or journals of the bearing assembly.
Read Bearing and Its History, Type, Function by Pritish Kumar Halder:
The invention of the rolling bearing, in the form of wooden rollers supporting, or bearing, an object being moved is of great antiquity. It may predate the invention of a wheel rotating on a plain bearing.
Though it is often claimed that the Egyptians used roller bearings in the form of tree trunks under sleds. This is modern speculation. The Egyptians’ own drawings in the tomb of Djehutihotep show the process of moving massive stone blocks on sledges as using liquid-lubricated runners which would constitute plain bearings. There are also Egyptian drawings of plain bearings used with hand drills.
Wheeled vehicles using plain bearings emerged between about 5000 BC and 3000 BC.
The earliest recovered example of a rolling element bearing is a wooden ball bearing supporting a rotating table from the remains of the Roman Nemi ships in Lake Nemi, Italy. The wrecks were dated to 40 BC.
The first practical caged-roller bearing was invented in the mid-1740s by horologist John Harrison for his H3 marine timekeeper. In this timepiece the caged bearing was only used for a very limited osmovement in a contemporaneous regulator clockcillating motion, but later on Harrison applied a similar bearing design with a true rotational
There are at least 6 common types of bearing, each of which operates on different principles:
Plain bearing: It consisting of a shaft rotating in a hole. There are several specific styles: bushing, journal bearing, sleeve bearing, rifle bearing, composite bearing;
Rolling-element bearing: In which rolling elements placed between the turning and stationary races prevent sliding friction. There are two main types:
Ball bearing: In which the rolling elements are spherical balls;
Roller bearing: In which the rolling elements are cylindrical, taper or spherical rollers;
Jewel bearing: A plain bearing in which one of the bearing surfaces is made of an ultrahard glassy jewel material such as sapphire to reduce friction and wear;
Fluid bearing: A noncontact bearing in which the load is supported by a gas or liquid (i.e. air bearing);
Magnetic bearing: In which the load is supported by a magnetic field;
Flexure bearing: In which the motion is supported by a load element which bends. Movement in a contemporaneous regulator clock
Common motions permitted by bearings are:
Radial rotation e.g. shaft rotation;
linear motion e.g. drawer;
spherical rotation e.g. ball and socket joint;
hinge motion e.g. door, elbow, knee.
Reducing friction in bearings is often important for efficiency, to reduce wear and to facilitate extended use at high speeds and to avoid overheating and premature failure of the bearing. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces or by separating the surfaces with an electromagnetic field.
By shape, gains advantage usually by using spheres or rollers, or by forming flexure bearings.
By material, exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)
By fluid, exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching, or by reducing the normal force between them.
By fields, exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.
Air pressure exploits air pressure to keep solid parts from touching.
Combinations of these can even be employed within the same bearing. An example of this is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.ge motion e.g. door, elbow, knee.