A plain bearing , or more commonly sliding bearing and shear (in railroading sometimes called dense pads or friction pads ), is the simplest type of cushion, consisting only of bearing surfaces and no rolling elements. Therefore, the journal (that is, the part of the shaft that is in contact with the bearings) glide over the bearing surface. The simplest example of a plain bearing is a rotating shaft in a hole. Simple linear bearings can be a pair of flat surfaces designed to allow for motion; for example, drawers and slides stuck or way over the bed lathe.
Plain bearings, in general, are the cheapest type of bearing. They are also compact and lightweight, and they have a high carrying capacity.
Video Plain bearing
Design
The usual bearing design depends on the type of motion that should be provided by the pads. Three possible types of movement are:
- Journal ( friction , radial or play ) pads : This is the most common type of common bearing; it is just a shaft that spins in a cushion. In locomotive and railway car applications, journal pads specifically refers to the ordinary pads once used on the axle of the set of railroad tracks, which are encircled by journal boxes axleboxes). The axlebox pad is currently no longer a plain bearing but is a rolling element bearing.
- Linear Bearing : This pad provides linear motion; it may be a circular bearing and axle or two other mating surfaces (eg shear plate).
- Push bearings : Thrust bearings provide bearing surfaces for axial-acting force to the shaft. One example is the propeller shaft.
Integral
The integral bearings are not common because the bushing is easier to accommodate and can be replaced if needed. Depending on the material, the integral pads may be cheaper but can not be replaced. If the pad is inseparably discharged, the item can be replaced or reworked to receive the busing. The integral bearings are very common in 19th century machines, but are becoming less common as interchangeable making becomes popular.
For example, a common integral common bearing is a hinge, which is a thrust bearing and a journal bearing.
Bushing
A bushing , also known as bush , is an ordinary independent pad that is inserted into housing to provide bearing surfaces for rotary applications; this is the most common form of plain bearing. Common designs include solid ( sleeve and flanged ), split , and clenched bushings. An arm, a split, or a bushing piece is just the "arm" of a material with an inner diameter (ID), outer diameter (OD), and length. The difference between these three types is solid solid solid bushing all the way around, separate bushing has pieces along its length, and the clenched pads are similar to split bushing but with clench (or clinched) across the pieces connecting parts. Flanged busing is the arm bus with flanges at one end extending radially out of the OD. Flanges are used to place bushing positively when mounted or to provide a thrust bearing surface.
The dimensionally inch sleeve pad is almost exclusively dimensionless using the SAE numbering system. Numbering systems use the -XXYY-ZZ format, where XX is the ID in sixteen inches, YY is OD at sixteen inches, and ZZ is the length in eight inches. Metric sizes also exist.
A linear busing is usually not pressed into the housing, but rather secured with a radial feature. The two examples include two retaining rings, or a ring printed to an OD busing that matches the grooves in the housing. This is usually a more durable way to maintain the busing, because the force acting on the bushing can suppress it.
The push shape of the bushing is conventionally called thrust washer .
Two pieces
Two ordinary bearing cutters, known as full bearing in industrial machines, are commonly used for larger diameters, such as crankshaft bearings. Both sections are called shell . There are various systems used to keep the shell in place. The most common method is the tab on the edge of the parting line that correlates with the notch in the housing to prevent axial movement after installation. For large and thick shells, a button or dowel pin is used. The stop button is screwed into the housing, while the dowel pin unifies the two shells. Another less common method is to use a dowel pin that locks the shell into a housing through a hole or slot in the shell.
The distance from one side of the separation to the other side is slightly larger than the corresponding distance at home so that a small amount of pressure is required to mount the pad. This keeps the bearings firmly in place because both parts of the housing are installed. Finally, the circumference of the shell is also slightly larger than the circumference of the housing so that when the two sections are bolted together the pads crushes slightly. This creates a large number of radial forces around the pads, which hold it from spinning . It also forms a good interface for heat to travel out of the cushion to the housing.
Maps Plain bearing
Gallery
Materials
Plain bearings should be made of durable material, low friction, low wear to bearings and shaft, resistant to high temperature, and corrosion resistant. Often pads consist of at least two constituents, where the soft and the other are difficult. In general, the harder the contact surface the lower the coefficient of friction and the greater the pressure required for the two bile or to seize when the lubrication fails.
Babbitt
Babbitt is usually used in integral bearings. It is bore coated, usually with a thickness of 1 to 100 ye (0.025 to 2.540 mm), depending on the diameter. Babbitt bearings are designed not to damage the journal during direct contact and to collect contaminants in lubrication.
Bi-material
Bi-material bearings consist of two materials, metal shells and plastic bearing surfaces. Common combinations include steel-coated PTFE plated steel and Frelon supported by aluminum. The steel-coated PTFE bronze pads are rated for more load than most other bi-metal bearings and are used for rotary and oscillating motions. Frelon-supported aluminum is commonly used in corrosive environments because Frelon is chemically inert.
Bronze
The usual common bearing design uses a hardened and polished steel shaft as well as a softer bronze bushing. The busing is replaced whenever it is used too much.
Common bronze alloys used for bearings include: SAE 841, SAE 660 (CDA 932), SAE 863, and CDA 954.
Cast iron
Cast iron bearings can be used with hardened steel shafts because the coefficient of friction is relatively low. Cast iron glazes therefore wear to be meaningless.
Graphite
In harsh environments, such as ovens and dryers, copper and graphite alloys, commonly known as graphalloy trademarks, are used. Graphite is a dry lubricant, therefore low friction and low maintenance. Copper adds strength, durability, and provides heat dissipation characteristics.
Uncoated graphite pads are used in special applications, such as locations submerged in water.
Gems
Known as a jewel bearing, these pads use gems, such as sapphire, ruby, and garnet.
Plastic
Plastic solid pads are now increasingly popular due to dry lubrication-free behavior. Conventional polymer density bearings are low-weight, corrosion-resistant, and maintenance-free. After a study spanning decades, an accurate calculation of service life of polymer plain bearings is possible today. Designing with ordinary solid polymer bearings is complicated by wide range, and non-linearity, coefficient of thermal expansion. These materials can heat up quickly when used in applications beyond the recommended pV limit.
The solid polymer type bearings are limited by the injection molding process. Not all forms are possible with this process, and the shape may be limited to what is considered good design practice for injection molding. Plastic bearings are subject to the same design warnings as all other plastic components: creep, high thermal expansion, softening (increased wear life/reduction) at high temperatures, brittle fractures at cold temperatures, and swelling due to water absorption. While most plastics/polymer bearing levels are designed to reduce the prudence of this design, it still exists and should be considered carefully before determining the type of solid (plastic) polymer.
Plastic pads are now fairly common, including the use of copiers, tills, agricultural equipment, textile machinery, medical equipment, food and packaging machinery, car seats and marine equipment.
Common plastics include nylon, polyacetal, polytetrafluoroethylene (PTFE), ultra heavy molecular weight polyethylene (UHMWPE), rulon, PEEK, urethane, and vespel (high performance polymide).
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- at the same time, iglidur Polymer bearing material specially developed with life prediction
- Ceramic pads are so hard that the sand and other sand entering the cushion are only pounded into fine powder that does not inhibit the operation of the pads.
- Rewrite
- Lignum vitae is a self-lubricating wood and in that hour gives a very long life.
- In the piano, various (usually) wooden parts of the keyboard and action are connected together by a central pin usually made of German silver. This relationship usually feels, or more rarely, leather bushing.
Lubrication
The types of lubrication systems can be categorized into three groups:
- Class I - pads requiring lubricant applications from external sources (for example, oil, grease, etc.).
- Class II - Bearings containing lubricants inside bearing walls (eg, bronze, graphite, etc.). Usually these pads require an outer lubricant to achieve maximum performance.
- Class III - pads made of lubricant. These bearings are usually considered "self-lubricating" and can run without an external lubricant.
Examples of these types of bearings are Oilites and plastic pads made of polyacetal; examples of the third type are metalized graphite bearings and PTFE bearings.
Most plain bearings have a flat inner surface; However, some are notched. The groove helps the lubricant get into the pads and cover the entire journal.
Self-lubricating plain bearings have a lubricant inside the bearing wall. There are many forms of self-lubricating bearings. The first, and most commonly, are sintered metal bearing, which has a porous wall. Porous walls pull oil through capillary action and release oil when pressure or heat is applied. Examples of sintered metal pads can be seen in the self-lubricating chain, which requires no additional lubrication during operation. Another form is a solid one-piece metal busing with channel number eight in inner diameter filled with graphite. A similar bearing replaces a figure of eight grooves with a hole embedded in graphite. It lubricates the pads inside and out. The last form is a plastic pad, which has a lubricant formed into the pads. The lubricant is released when the bearings are started.
There are three main types of lubrication: full-film conditions , boundary conditions , and dry conditions . Full-film conditions are when the bearing load is performed solely by a liquid lubricant film and there is no contact between the two bearing surfaces. In mixed or border conditions, the load is carried in part by direct surface contact and partly by the formation of the film between the two. In dry conditions, the full load is brought by surface contact to the surface.
Bearings made of bearing grade material always run in dry conditions. Two other plain bearing classes can run in all three conditions; the conditions under which the bearing is running depend on operating conditions, load, relative surface velocity, clearance in bearings, lubricant quality and quantity, and temperature (affecting the viscosity of the lubricant). If a plain cushion is not designed to run in dry or border conditions, it has a high coefficient of friction and wear. Dry and boundary conditions can be experienced even in fluid bearings when operating outside normal operating conditions; for example, when starting and shutting down.
Liquid lubrication
Liquid lubrication produces full or border film lubrication mode. Well-designed bearing systems reduce friction by removing surface-to-surface contact between journals and bearings through the dynamic effects of liquids.
Fluid pads can be hydrodynamic or hydrodynamic lubricated. The hydraulically lubricated bearings are lubricated by an external pump that maintains a number of static pressure. In hydrodynamic pads, the pressure in the oil film is maintained by the rotation of the journal. The hydrostatic pad enters the hydrodynamic state as the journal revolves. Hydrostatic bearings typically use oil, while hydrodynamic pads may use oil or grease, but bearings can be designed to use whatever liquid is available, and some pump designs use liquids that are pumped as lubricants.
Hydrodynamic pads require better maintenance in design and operation than hydrostatic pads. They are also more prone to use early because lubrication does not occur until there is a shaft rotation. At low rotation speeds the lubrication may not achieve complete separation between the shaft and bushing. Consequently, hydrodynamic bearings may be assisted by secondary bearings that support the shaft during the start and stop periods, protecting the smooth tolerance engine surfaces of the journal bearing. On the other hand, hydrodynamic pads are easier to install and less expensive.
In hydrodynamic conditions a form of "wedge" lubrication, which lifts the journal. The journal also slightly shifts horizontally toward the rotation. The location of the journal is measured by the attitude angle , which is the angle formed between the vertical line and the line across the center of the journal and the center of the bearing, and the eccentricity ratio, which is the ratio of the center distance of the journal from the center of the bearing, to the overall radial clearance. The attitude angle and eccentricity ratio depend on the direction and speed of rotation and load. In hydrostatic bearings, the oil pressure also affects the eccentricity ratio. In electromagnetic equipment such as motors, electromagnetic forces can resist gravity burden, causing the journal to take an unusual position.
One particular disadvantage for the liquid lubricated liquid jelly bearing in high-speed engines is spinning oil - vibrant vibrations from journals. Swirling oils occur when lubricating lubrication becomes unstable: a small disturbance of the journal results in the reaction strength of the oil film, which causes further movement, causing both oil film and journal to "spin" around the bearing sheath. Usually the vortex frequency is about 42% of the speed behind the journal. In extreme cases, oil lubrication leads to direct contact between journals and bearings, which quickly remove the pads. In some cases, the navel frequency coincides with and "locks into" the engine's critical speed of the shaft; this condition is known as "oil whip". Whip oil can be very damaging.
Oil vortex can be prevented by the stabilization force applied to the journal. A number of bearing designs attempt to use bearing geometry to provide resistance to the spinning fluid or to provide stabilization loads to minimize the vortex. One of them is called lemon bore or elliptical bore . In this design, shims are installed between two parts of the housing bearing and then the bore is worked out by size. After the shims are removed, the holes resemble the shape of the lemon, which reduces the clearance in one direction of the hole and increases the pre-load in that direction. The disadvantage of this design is lower load carrying capacity, compared to typical journal bearings. It is also still susceptible to oil spinning at high speed, but the cost is relatively low.
Another design is a pressure dam or a dotted groove , which has a shallow relief piece in the center of the bearing above the top half of the pad. The groove suddenly stops to create power downward to stabilize the journal. This design has a high load capacity and fixes most of the oil breaking situations. The disadvantage is that it only works in one direction. Offsetting the bearing section does the same thing with the pressure dam. The only difference is the load capacity increases as the offset increases.
A more radical design is the tilting-pad design, which uses multiple bearings designed to move by changing the load. It's typically used in very large applications but also finds extensive applications in modern turbomachinery as it almost completely removes spinning oil.
Related components
Other components commonly used with plain bearings include:
- Pillow block : This is a standard bearing pad designed to accept plain bearing. They are designed to be mounted onto a flat surface.
- Ring oiler : The lubricating mechanism used in the first half of the 20th century for medium speed applications.
- Filling Box : The sealing system is used to keep the liquid from leaking out of the pressurized system through a plain bearing.
See also
- Hot box
- Pillow block bearings
- Plastigauge
References
Bibliography
- Bearing and metal bearing , The Industrial Press, 1921 Ã, Neale, Michael John (1995), The tribology handbook (2nd ed.), Butterworth-Heinemann, ISBNÃ, 978-0 -7506 -1.198-5
External links
- Kinematic Model for Design Digital Library (KMODDL) - Film and photos from hundreds of mechanical system models working at Cornell University. Also includes a classic text book e-book on mechanical design and engineering.
- Dictionary CSX J
Source of the article : Wikipedia
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