Selasa, 29 Mei 2012

LUBRICANT

LUBRICANT

A wide variety of lubricants are available. The principal types are reviewed here.

Liquid, oily lubricants

Animal and vegetable products were certainly man's first lubricants and were used in large quantities. But, because they lack chemical inertness and because lubrication requirements have become more demanding, they have been largely superseded by petroleum products and by synthetic materials. Some organic substances such as lard oil and sperm oil are still in use as additives because of their special lubricating properties.

Petroleum lubricants are predominantly hydrocarbon products extracted from fluids that occur naturally within the Earth. They are used widely as lubricants because they possess a combination of the following desirable properties:
1. availability in suitable viscosities
2. low volatility
3. inertness (resistance to deterioration of the lubricant)
4. corrosion protection (resistance to deterioration of the sliding surfaces)
5. low cost

However, petroleum lubricants loose their inertness when subjected to elevated temperatures, such as those encountered in modern engines. This causes deterioration of the lubricant by oxidation, and leads to formation of gum, varnish and other insoluble deposits. Therefore in most applications petroleum lubricants have to be frequently changed, if longevity of the equipment is desired.

Synthetic lubricants generally can be characterized as oily, neutral liquid materials not usually obtained directly from petroleum but having some properties similar to petroleum lubricants.
In certain ways they are superior to hydrocarbon products. Some synthetics exhibit greater stability of viscosity with temperature changes, resistance to scuffing and oxidation, and fire resistance. Since the properties of different types of synthetics vary considerably, each synthetic lubricant tends to find a special application. There is NO single synthetic lubricant type that is ideal for all lubricant applications. Commercial synthetic lubricants (Motor Oil, Gear Oil) are therefore a blend of several different types of Synthetics as well as select additives.

Grease
Another form of oily lubricant is grease, a solid or semisolid substance consisting of a liquid lubricant containing a thickening agent.

The liquid lubricant is made from inedible lard, the rendered fat of waste animal parts, or is petroleum-derived or synthetic high viscosity oil.
Soaps of aluminum, barium, calcium, lithium, sodium, and strontium are the major thickening agents. Nonsoap thickeners consist of such inorganic compounds as modified clays or fine silicas, or such organic materials as arylureas or phthalocyanine pigments.

White grease is made from inedible hog fat and has a low content of free fatty acids. Yellow grease is made from darker parts of the hog and may include parts used to make white grease.

Brown grease contains beef and mutton fats as well as hog fats. Fleshing grease is the fatty material trimmed from hides and pelts. Bone grease, hide grease, and garbage grease is named according to their origin. In some factories, food offal is used along with animal carcasses, butcher-shop scraps, and garbage from restaurants for recovery of fats.

Greases of mineral or synthetic origin consist of a thickening agent dispersed in a liquid lubricant such as petroleum oil or a synthetic fluid. The thickening agent may be soap, an inorganic gel, or an organic substance. Other additives inhibit oxidation and corrosion, prevent wear, and change viscosity. The fluid component is the more important lubricant for clearances between parts that are relatively large, but for small clearances the molecular soap layers provide the lubrication.

Synthetic grease may consist of synthetic oils containing standard soaps or may be a mixture of synthetic thickeners, or bases, in petroleum oils. Silicones are greases in which both the base and the oil are synthetic. Synthetic greases are made in water-soluble and water-resistant forms and may be used over a wide temperature range. The synthetics can be used in contact with natural or other rubbers because they do not soften these materials.

Special-purpose greases may contain two or more soap bases or special additives to gain a special characteristic.

Lubrication by grease may prove more desirable than lubrication by oil under conditions
when:
1. less frequent lubricant application is necessary
2. grease acts as a seal against loss of lubricant and ingress of contaminants
3. less dripping or splattering of lubricant is called for
4. less sensitivity to inaccuracies in the mating parts is needed


Solid lubricants

Definition of solid lubricant: A solid lubricant is a material used as powder or thin film to provide protection from damage during relative movement and to reduce friction and wear. Other terms commonly used for solid lubrication include dry lubrication, dry-film lubrication, and solid-film lubrication. Although these terms imply that solid lubrication takes place under dry conditions, fluids are frequently used as a medium or as a lubricant with solid additives.

A solid lubricant is a film of solid material composed of inorganic or organic compounds or of metal. Perhaps the most commonly used solid lubricants are the inorganic compounds graphite and molybdenum disulfide (MoS2 ) and the polymer material polytetrafluoroethylene (PTFE).

There are three general kinds of inorganic compounds that serve as solid lubricants:
  1. Layer-lattice solids: materials such as graphite and molybdenum disulfide, commonly called molysulfide, have a crystal lattice structure arranged in layers. Strong bonds between atoms within a layer and relatively weak bonds between atoms of different layers allow the lamina to slide on one another. Other such materials are tungsten disulfide, mica, boron nitride, borax, silver sulfate, cadmium iodide, and lead iodide. Graphite's low friction is due largely to adsorbed films; in the absence of water vapor, graphite loses its lubricating properties and becomes abrasive. Both graphite and molysulfide are chemically inert and have high thermal stability.
  2. Miscellaneous soft solids: a variety of inorganic solids such as white lead, lime, talc, bentonite, silver iodide, and lead monoxide are used as lubricants.
  3. Chemical conversion coatings: many inorganic compounds can be formed on ametallic surface by chemical reaction. The best known such lubricating coatings are
    sulfide, chloride, oxide, phosphate, and oxalate films.

 Solid organic lubricants are usually divided into two broad classes:

  1. Soaps, waxes, and fats: this class includes metallic soaps of calcium, sodium, lithium; animal waxes (e.g., beeswax and spermaceti wax); fatty acids (e.g., stearic and palmitic acids); and fatty esters (e.g., lard and tallow).
  2. Polymeric films: these are synthetic substances such as polytetrafluoroethylene(PTFE also known as Teflon®) and polychlorofluoroethylene. One major advantageof such film-type lubricants is their resistance to deterioration during exposure tothe elements. For Example: ½" (13mm) thick plates of polymeric film are used inmodern prestressed concrete construction to permit thermal movement of beamsresting atop columns. The long-lived polymeric film plate facilitates suchexpansion and contraction of the structural members.
Thin films of soft metal on a hard substrate can act as effective lubricants, if the adhesion to the substrate is good. Such metals include lead, tin, and indium.

Characteristics of Solid Lubricants

Characteristics: The properties important in determining the suitability of a material for use as a solid lubricant are discussed below.

  1. Crystal structure. Solid lubricants such as graphite and MoS2 possess a lamellar crystal structure with an inherently low shear strength. Although the lamellar structure is very favorable for materials such as lubricants, nonlamellar materials also provide satisfactory lubrication.
  2. Thermal stability. Thermal stability is very important since one of the most significant uses for solid lubricants is in high temperature applications not tolerated by other lubricants. Good thermal stability ensures that the solid lubricant will not undergo undesirable phase or structural changes at high or low temperature extremes.
  3. Oxidation stability. The lubricant should not undergo undesirable oxidative changeswhen used within the applicable temperature range.
4 . Volatility. The lubricant should have a low vapor pressure for the expected application at extreme temperatures and in low-pressure conditions.
5. Chemical reactivity. The lubricant should form a strong, adherent film on the base material.
6. Mobility. The life of solid films can only be maintained if the film remains intact. Mobility of adsorbates on the surfaces promotes self-healing and prolongs the endurance of films.
7. Melting point. If the melting point is exceeded, the atomic bonds that maintain the molecular structure are destroyed, rendering the lubricant ineffective.
8. Hardness. Some materials with suitable characteristics, such as those already noted, have failed as solid lubricants because of excessive hardness. A maximum hardness of 5 on the Mohs’ scale appears to be the practical limit for solid lubricants.
9. Electrical conductivity. Certain applications, such as sliding electric contacts, require high electrical conductivity while other applications, such as insulators making rubbing contact, require low conductivity.

Applications of Solid Lubrication

Applications of Solid Lubrication. Generally, solid lubricants are used in applications not tolerated by more conventional lubricants. The most common conditions requiring use of solid lubricants are:

(1) Extreme temperature and pressure conditions. These are defined as high-temperature applications up to 1926°C ( 3500°F), where other lubricants are prone to degradation or decomposition; extremely low temperatures, down to -212°C (-350°F), where lubricants may solidify or congeal; and high-to-fullvacuum applications, such as space, where lubricants may volatilize.

(2) As additives. Graphite, MoS2 , and zinc oxide are frequently added to fluids and greases. Surface conversion coatings are often used to supplement other lubricants.

(3) Intermittent loading conditions. When equipment is stored or is idle for prolonged periods, solids provide permanent, noncorrosive lubrication.

(4) Inaccessible locations. Where access for servicing is especially difficult, solid lubricants offer a distinct advantage, provided the lubricant is satisfactory for the intended loads and speeds.

(5) High dust and lint areas. Solids are also useful in areas where fluids may tend to pick up dust and lint with liquid lubricants; these contaminants more readily form a grinding paste, causing damage to equipment.

(6) Contamination. Because of their solid consistency, solids may be used in applications where the lubricant must not migrate to other locations and cause contamination of other equipment, parts, or products.

(7) Environmental. Solid lubricants are effective in applications where the lubricated equipment is immersed in water that may be polluted by other lubricants, such as oils and greases.

Advantages of Solid Lubrication
Some advantages of solid lubrication are:
(1) More effective than fluid lubricants at high loads and speeds.
(2) High resistance to deterioration in storage.
(3) Highly stable in extreme temperature, pressure, radiation, and reactive environments.
(4) Permit equipment to be lighter and simpler because lubrication distribution systems and seals are not required.

Disadvantages of Solid Lubrication
(1) Poor self-healing properties. A broken solid film tends to shorten the useful life of the lubricant.
(2) Poor heat dissipation. This condition is especially true with polymers due to their low thermal conductivities.
(3) Higher coefficient of friction and wear than hydrodynamically lubricated bearings.
(4) Color associated with solids may be undesirable.

Graphite has a low friction coefficient and very high thermal stability (2000°C [3632°F] and above). However, practical application is limited to a range of 500 to 600°C (932 to 1112°F) due to oxidation. Furthermore, because graphite relies on adsorbed moisture or vapors to achieve low friction, use may be further limited. At temperatures as low as 100°C (212°F), the amount of water vapor adsorbed may be significantly reduced to the point that low friction cannot be maintained. In some instances sufficient vapors may be extracted from contaminants in the surrounding environment or may be deliberately introduced to maintain low friction. When necessary, additives composed of inorganic compounds may be added to enable use at temperatures to 550°C ( 1022°F). Another concern is that graphite promotes electrolysis. Graphite has a very noble potential of + 0.25V, which can lead to severe galvanic corrosion of copper alloys and stainless steels in saline waters.

Molybdenum disulfide (MoS2 ). Like graphite, MoS2 has a low friction coefficient, but, unlike graphite, it does not rely on adsorbed vapors or moisture. In fact, adsorbed vapors may actually result in a slight, but insignificant, increase in friction. MoS2 also has greater load-carrying capacity and its manufacturing quality is better controlled. Thermal stability in nonoxidizing environments is acceptable to 1100°C (2012°F), but in air it may be reduced to a range of 350 to 400°C (662 to 752°F).

Soft metal films. Many soft metals such as lead, gold, silver, copper, and zinc, possess low shear strengths and can be used as lubricants by depositing them as thin films on hard substrates. Deposition methods include electroplating, evaporating, sputtering, and ion plating. These films are most useful for high temperature applications up to 1000°C (1832°F) and roller bearing applications where sliding is minimal

Surface treatments commonly used as alternatives to surface film depositions include thermal diffusion, ion implantation, and chemical conversion coatings.

(a) Thermal diffusion: This is a process that introduces foreign atoms into a surface for various purposes such as increasing wear-resistance by increasing surface hardness; producing low shear strength to inhibit scuffing or seizure; and in combination with these to enhance corrosion-resistance.

(b) Ion implantation: This is a recently developed method that bombards a surface with ions to increase hardness, which improves wear- and fatigue-resistance.

(c) Chemical conversion coatings: Frequently, solid lubricants will not adhere to the protected metal surface. A conversion coating is a porous nonlubricating film applied to the base metal to enable adherence of the solid lubricant. The conversion coating by itself is not a suitable lubricant.

(d) Polymers: Polymers are used as thin films, as self-lubricating materials, and as binders for lamellar solids. Films are produced by a process combining spraying and sintering. Alternatively, a coating can be produced by bonding the polymer with a resin. Sputtering can also be used to produce films. The most common polymer used for solid lubrication is PTFE The main advantages of PTFE are low friction coefficient, wide application range of -200 to 250°C (-328 to 418°F), and lack of chemical reactivity. Disadvantages include
lower load-carrying capacity and endurance limits than other alternatives. Low thermal conductivity limits use to low speed sliding applications where MoS2 is not satisfactory. Common applications include antistick coatings and self-lubricating composites.

Methods of Applying Solid Lubricants

There are several methods for applying solid lubricants.

Powdered solids. The oldest and simplest methods of applying solid lubricants are noted as follows:

(a) Burnishing: Burnishing is a rubbing process used to apply a thin film of dry powdered solid lubricant such as graphite, MoS2 , etc., to a metal surface. This process produces a highly polished surface that is effective where lubrication requirements and wear-life are not stringent, where clearance requirements must be maintained, and where wear debris from the lubricant must be minimized. Surface roughness of the metal substrate and particle size of the powder are critical to ensure good application.

(b) Hand rubbing: Hand rubbing is a procedure for loosely applying a thin coating of solid lubricant.

 (c) Dusting: Powder is applied without any attempt to evenly spread the lubricant. This method results in a loose and uneven application that is generally unsatisfactory.

(d) Tumbling:. Parts to be lubricated are tumbled in a powdered lubricant. Although adhesion is not very good, the method is satisfactory for noncritical parts such as small threaded fasteners and rivets.

(e) Dispersions: Dispersions are mixtures of solid lubricant in grease or fluid lubricants. The most common solids used are graphite, MoS2 , PTFE, and Teflon®. The grease or fluid provides normal lubrication while the solid lubricant increases lubricity and provides extreme pressure protection. Addition of MoS2 to lubricating oils can increase load-carrying capacity, reduce wear, and increase life in roller bearings, and has also been found to reduce wear and friction in automotive applications. However, caution must be exercised when using these solids with greases and lubricating fluids. Grease and oil may prevent good adhesion of the solid to the protected surface. Detergent additives in some oils can also inhibit the wearreducing ability of MoS2 and graphite, and some antiwear additives may actually increase wear. Solid lubricants can also affect the oxidation stability of oils and greases. Consequently, the concentration of oxidation inhibitors required must be carefully examined and controlled. Aerosol sprays are frequently used to apply solid lubricant in a volatile carrier or in an air-drying organic resin. However, this method should be limited to short-term uses or to light- or moderate-duty applications where thick films are not necessary. Specifications for solid lubricant dispersions are not included in this manual. Before using dispersions, users should become familiar with their applications and should obtain information in addition to that provided in this manual. The information should be based on real-world experiences with similar or comparable applications.

Self-Lubricating Composites

Self-lubricating composites: The primary applications for self-lubricating composites include dry bearings, gears, seals, sliding electrical contacts, and retainers in roller bearings. Composites may be polymer, metal-solid, carbon and graphite, and ceramic and cermets.

Polymer Lubrication

Polymer Lubrication: The low thermal conductivity of polymers inhibits heat dissipation, which causes premature failure due to melting. This condition is exacerbated if the counterface material has the same or similar thermal conductivity. Two polymers in sliding contact will normally operate at significantly reduced speeds than a polymer against a metal surface. The wear rate of polymer composites is highly dependent upon the surface roughness of the metal counterfaces. In the initial operating stages, wear is significant but
can be reduced by providing smooth counterfaces. As the run-in period is completed, the wear rate is reduced due to polymer film transfer or by polishing action between the sliding surfaces. Environmental factors also influence wear rate. Increased relative humidity inhibits transfer film formation in polymer composites such as PTFE, which rely on transfer film formation on counterfaces. The presence of hydrocarbon lubricants may also produce similar effects. Composites such as nylons and acetals, which do not rely on transfer film formation, experience reduced wear in the presence of small amounts of hydrocarbon lubricants.

Metal-Solid Lubrication

Metal-solid Lubrication: Composites containing lamellar solids rely on film transfer to achieve low friction. The significant amount of solids required to improve film transfer produces a weak composite with reduced wear life. Addition of nonlamellar solids to these composites can increase strength and reduce wear. Various manufacturing techniques are used in the production of metal-solid composites. These include powder metallurgy, infiltration of porous metals, plasma spraying, and electrochemical code position. Another fabrication technique requires drilling holes in machine parts and packing the holes with solid lubricants. One of the most common applications for these composites is selflubricating roller bearing retainers used in vacuum or high temperatures up to 400°C (752°F). Another application is in fail-safe operations, where the bearing must continue to operate for a limited time following failure of the normal lubrication system.

Carbon and Graphites

Carbon and graphites: The primary limitations of bulk carbon are low tensile strength and lack of ductility. However, their high thermal and oxidation stabilities at temperatures of 500 to 600°C (932 to 1112°F) (higher with additives) enable use at high temperatures and high sliding speeds. For graphitic carbons in dry conditions, the wear rate increases with temperature. This condition is exacerbated when adsorbed moisture inhibits transfer film formation. Furthermore, dusting may also cause failure at high temperatures and sliding speeds. However, additives are available to inhibit dusting.

Ceramics and Cemets

Ceramics and cermets. Ceramics and cermets can be used in applications where low wear rate is more critical than low friction. These composites can be used at temperatures up to 1000°C (1832°F). Cermets have a distinct advantage over ceramics in terms of toughness and ductility. However, the metal content tends to reduce the maximum temperature limit. Solid lubricant use with bulk ceramics is limited to insertion in machined holes or recesses.

Gaseous lubricants

Lubrication with a gas is analogous in many respects to lubrication with a liquid, since the same principles of fluid-film lubrication apply. Although both gases and liquids are viscous fluids, they differ in two important particulars. The viscosity of gases is much lower and the compressibility much greater than for liquids. Film thicknesses and load capacities therefore are much lower with a gas such as air. In equipment that handles gases of various kinds, it is often desirable to lubricate the sliding surfaces with gas in order to simplify the apparatus and reduce contamination to and from the lubricant. The list of gases used in this manner is extensive and includes air, steam, industrial gases, and liquidmetal vapors.

Tidak ada komentar:

Posting Komentar

Pengikut