bearing technology - plain bearing

A plain bearing, also known as a plane bearing, is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore the journal (i.e., the part of the shaft in contact with the bearing) slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a lathe.

Plain bearings, in general, are the least expensive type of bearing. They are also compact, light weight, and have a high load-carrying capacity.

TYPES BASED ON LOAD DIRECTION

The design of a plain bearing firstly depends on the type of motion the bearing must provide. The three types of motions possible are:
> Journal (friction, radial or rotary) bearing: This is the most common type of plain bearing; it is simply a shaft rotating in a bearing. 

> Linear bearing: This bearing provides linear motion; it may take the form of a circular bearing and shaft or two matching surfaces (e.g., a slide plate). 

> Thrust bearing: A thrust bearing provides a bearing surface for forces acting axial to the shaft.

TYPES BASED ON MOUNTING METHOD

And then , plain bearings are differentiated by it's mounting method and design
>> Integral

Integral plain bearings are built into the object of use. It is a hole that has been prepared into a bearing surface. Industrial integral bearings are usually made from cast iron or babbitt and a hardened steel shaft is used in the bearing.

Integral bearings are not as common because bushings are easy to accommodate and if they wear out then they are just replaced. Depending on the material an integral bearing may be less expensive but it cannot be replaced. If an integral bearing wears out then the item may be replaced or reworked to accept a bushing. Integral bearings were very common in 19th-century machinery but became progressively less common as interchangeable manufacture permeated the industry.
An example of a common integral plain bearing is the hinge, which is both a thrust bearing and a journal bearing.

>> Bushing
A bushing, also known as a bush, is an independent plain bearing that is inserted into a housing to provide a bearing surface for rotary applications; this is the most common form of a plain bearing.

Common designs include solid (sleeve and flanged), split, and clenched bushings. A sleeve, split, or clenched bushing is only a "sleeve" of material with an inner diameter (ID), outer diameter (OD), and length. The difference between the three types is that a solid sleeved bushing is solid all the way around, a split bushing has a cut along its length, and a clenched bearing is similar to a split bushing but with a clench across the cut. A flanged bushing is a sleeve bushing with a flange extending radially outward from the ID. The flange is used to positively locate the bushing when it is installed or to provide a thrust bearing surface. 

A linear bushing is not usually pressed into a housing, but rather secured with a radial feature. Two such examples include two retaining rings, or a ring that is molded onto the OD of the bushing that matches with a groove in the housing. This is usually a more durable way to retain the bushing, because the forces acting on the bushing could press it out.
The thrust form of a bushing is conventionally called a thrust washer.

>> Two-piece
Two-piece plain bearings, known as full bearings in industrial machinery, are commonly used for larger diameters, such as crankshaft bearings. The two halves are called shells. There are various system used to keep the shells located. The most common method is a tab on the parting line edge that correlates with a notch in the housing to prevent axial movement after installation. For large, thick shells a button stop or dowel pin is used. The button stop is screwed to the housing, while the dowel pin keys the two shells together. Another less common method uses a dowel pin that keys the shell to the housing through a hole or slot in the shell.

The distance from one parting edge to the other is slightly larger than the corresponding distance in the housing so that a light amount of pressure is required to install the bearing. This keeps the bearing in place as the two halves of the housing are installed. Finally, the shell's circumference is also slightly larger than the housing circumference so that when the two halves are bolted together the bearing crushes slightly. This creates a large amount of radial force around the entire bearing which keeps it from spinning. It also forms a good interface for heat to travel out of the bearings into the housing

MATERIALS

There are several material that are well-known being used as bearing such as:

> Bronze
A common plain bearing design utilizes a hardened and polished steel shaft and a softer bronze bushing. The bushing is replaced whenever it has worn too much. 

Common bronze alloys used for bearings include: SAE 841, SAE 660 (CDA 932), SAE 863, and CDA 954

> Cast Iron
A cast iron bearing is commonly used with a hardened steel shaft because the coefficient of friction is relatively low. The cast iron glazes over therefore wear becomes negligible

> Graphite
In harsh environments, such as ovens and dryers, a copper and graphite alloy, commonly known by the trademarked name graphalloy, is used. The graphite is a dry lubricant, therefore it is low friction and low maintenance. The copper adds strength, durability, and provides heat dissipation characteristics.

Unalloyed graphite bearings are used in special applications, such as locations that are submerged in water

> Plastic
Solid plastic plain bearings are now increasingly popular due to dry-running lubrication-free behavior. Solid polymer plain bearings are low weight, corrosion resistant, and maintenance free. After research spanning decades, an accurate calculation of the service life of polymer plain bearings is possible today. Designing with solid polymer plain bearings is complicated by the wide range, and non-linearity, of coefficient of thermal expansion. These materials can heat rapidly when loaded.
Solid polymer type bearings are limited by the injection molding process. Not all shapes are possible with this process and the shapes which are possible are limited to what is considered good design practice for injection molding. Plastic bearings are subject to the same design cautions as all other plastic parts: creep, high thermal expansion, softening (increased wear/reduced life) at elevated temperature, brittle fractures at cold temperatures, swelling due to moisture absorption. While most bearing-grade plastics/polymers are designed to reduce these design cautions, they still exist and should be carefully considered before specifying an a solid polymer type.
Plastic bearings are now everywhere from photocopy machines to the tills in the supermarket. Other applications include farm equipment, textile machinery, medical devices, food and packaging machines, car seating, marine equipment and many more.
Common plastics include nylon, polyacetal, polytetrafluoroethylene (PTFE), ultra high molecular weight polyethylene (UHMWPE), rulon, PEEK, urethane and vespel (a high-performance polyimide)

> Babbit
Babbitt is usually used in integral bearings. It is coated over the bore, usually to a thickness of 0.025 to 2.5 mm, depending on the diameter. Babbitt bearings are designed to not damage the journal during direct contact and to collect any contaminants in the lubrication