A lubricant is an organic or synthetic substance (it can occur in any physical state: liquid, solid, gaseous and even semi-solid or viscous) which has the property of reducing the friction between surfaces in contact under any operating condition, dissipating the heat generated during the relative movement between the surfaces, maintaining its chemical stability, protecting the mechanical parts from corrosive attacks, cushioning possible shocks, guaranteeing the seal. Its function is to create a film of lubricant between the solid surfaces in relative motion in order to reduce friction and wear, depending on the specific properties of the surfaces. These lubricants are called intermediate materials.
Lubricant is an essential element in modern mechanics. Each mechanism, from the most simplest to the most complex, which has moving parts, needs to be lubricated. Depending on the relative motion of the two members in contact, the lubrication function is radically different. In the case of sliding between dry bodies, friction mainly depends on the mechanical action of surface roughness in the areas where contact is made, as well as by intermolecular electrical actions. By partially, or totally, separating the surfaces with an interposed fluid film, the extension and pressure of the contacts are reduced or eliminated, replacing (in whole or in part) the sliding of solids by sliding between solids, with consequent very minor dissipative actions.
In rolling motion, on the other hand, the resistance to motion between the surfaces in contact depends on the elasticity and elastic hysteresis of the materials as well as on shocks and small local sliding. In this case, therefore, lubrication does not limit the resistance to motion, which would, in any case, be impossible because, due to the small area of contact and the consequent very high local pressure, the fluid film of lubricant intended to support the load cannot be formed. In this case, lubrication is mainly used to extend the life of working members, protecting them from abrasive, erosive and corrosive agents, and limiting wear. In both motions, the effect of lubrication on heat dissipation is essential.
Characteristics of lubrication
The characteristics of lubrication depend on many parameters and conditions:
- extent of the transmitted load;
- nature and speed of the relative motion;
- geometry and state of the surfaces in contact;
- chemical-physical properties of both the lubricant and the materials of the kinematic pair.
Consequently, various types of lubrication are distinguished. In a type of classification, the characterizing and discriminating element of the various types of contact friction is the thickness of the lubricant meatus, linked to the number of molecules, or molecular layers in the case of stratified fluid, present between the two surfaces.
The hydrodynamic lubrication is the one adopted in the revolute and prismatic sliding pairs (in the presence of not very high loads but with high relative speeds) made with a lubricant of suitable viscosity and in considerable quantity. The contact between the coupled members is prevented by the constitution of a layer (film or meatus) of lubricant of relatively large thickness which, interposed between the surfaces of the pair, separates them transmitting the mutually exchanged force. The fluid layer has two extreme layers, fixed with respect to the walls, consisting of layers of adsorbed molecules, and a central area active from the hydrodynamic point of view. The contact between solids thus creates an action between flowing layers flowing one on top of the other, characterized by considerably lower tangential actions.
The semi-fluid lubrication is used when the load transmitted is very high, or the relative speed is low; in this case if a slightly viscous lubricant is used or if the facing walls are parallel, the characters of the phenomenon change substantially and there can be two varieties of contacts, with lubricating conditions of different nature, generally coexisting. In some regions of the contact zone, the surface asperities of the conjugated members come to touch, the more widely, the greater the mutual pressure; in other regions of the space between the separate members, fluid layers of very thin thickness are insinuated. The thickness of the fluid layer is generally too small and ineffective from the hydrodynamic point of view, and yet its presence considerably lowers the friction coefficient for a phenomenon of a fundamentally chemical nature, linked to a property (greasiness) possessed especially by some organic oils by virtue of the shape and chemical-physical properties of the molecules.
In some regions of the contact zone, the superficial asperities of the conjugated members come to touch, the more widely, the greater the mutual pressure; in other regions of the space between the separate members, fluid layers of very thin thickness are insinuated. The thickness of the fluid layer is generally too small and ineffective from the hydrodynamic point of view, and yet its presence considerably lowers the friction coefficient for a phenomenon of a fundamentally chemical nature, linked to a property (greasiness) possessed especially by some organic oils by virtue of the shape and chemical-physical properties of the molecules.
Hydrostatic lubrication is adopted in cases of extremely modest speeds and extremely high loads, in which hydrodynamic lubrication is impossible and direct friction is to be avoided. The formation of the supporting layer occurs by forced introduction of oil between the coupled surfaces, with pressure supplied by an external source and therefore not produced by the motion of the fluid layers; lubrication is then independent of the relative motion of the members of the pair.
Unlike the hydrodynamic case, where the forced circulation is carried out due only to the outflow of the vast quantities of oil required, here the external source supplies just the lift pressure that cannot be generated due to the movement of the layer, irrelevant and ineffective.
Types of lubricants
Lubricants can be classified according to their physical state: liquids, solid and semisolid, gaseous; according to their origin: mineral, vegetable, animal, synthetic; according to the conditions of use: lubricants for very high pressures, for low or high temperatures and so on.
Liquid lubricants or lubricating oils are complex mixtures of paraffinic, aromatic and naphthenic hydrocarbons with a molecular weight of between 250 and 1000.
They are widely used for their ease of application, both because they are very suitable for removing the heat developed by friction between the various metal parts, and because with them it is possible to cover the vast range of characteristics required by various uses. Depending on the applications, liquid lubricants can have four different classifications.
Liquid lubricants belong to various classes of chemical compounds: esters, glycols, hydrocarbons, polycarbonates, silicones, fluorine derivatives. A significant percentage of the lubricants used comes from oils and precisely from the atmospheric distillation residue (topping).
This fraction has a boiling point higher than 370 °C approximately and is redistilled under vacuum or in a vapor stream. The operation can be carried out in various ways and generally gives rise to a distillate consisting of gas oil, a residue that can be used for the production of bitumens and intermediate fractions from which lubricating oils are obtained by appropriate refining.
The undesirable components contained in lubricating oils are:
- aromatic hydrocarbons and polycyclic naphthenes, which decrease the viscosity index;
- asphaltic substances, which are responsible for coloring and any carbon deposits;
- linear and high molecular weight paraffins, which raise the pour point.
The aromatic hydrocarbons and polycyclic naphthenes are extracted with suitable solvents (furforol, phenol, nitrobenzene), while the elimination of the asphaltic substances is obtained by using liquefied propane as the solvent. Sometimes a single process is used which uses a double solvent consisting of mutually insoluble mixtures (e.g., propane, which extracts asphaltic substances, and a mixture of cresyl acid and phenol, which extracts the aromatics and naphthenes).
The paraffin, or waxes, contained in oils can be eliminated by cooling at low temperature, so as to crystallize the paraffin, then removing them by filtration or centrifugation, or, when the paraffin solidifies in a practically amorphous form, by treatment with a solvent selective (methyl ethyl ketone, methyl butyl ketone, propane, urea). After these solvent treatments, the lubricating oils are bleached, bringing them into contact with adsorbent substances (activated clays) which also improve resistance to oxidation.
Lubricants obtained from oils meet many of the usually required requirements. However, in general, their characteristics are altered at high temperatures or in particularly reactive environments. To improve their qualities, they are added with various substances (additives) which have the function of protecting them from oxidation (antioxidants), from the formation of gums, to vary their greasiness, to inhibit corrosion, to increase the viscosity index, to exert detergent action, etc.
In many cases, especially for particular uses for which the lubricants obtained from oils are not able to meet the expected requirements, synthetic lubricating oils are used. There are numerous types proposed, which include synthetic hydrocarbons (polymeric oils), polyglycols, aliphatic esters, phosphoric esters, silicones, etc. The synthetic hydrocarbons are made of polymers of ethylene, of propylene, of butylene of molecular weight included, depending on the case, between 250 and 50,000. These have the advantage of more excellent stability, a low pour point, a high viscosity index and are used in electrical engineering and lubrication at low temperatures. Polyglycols include polyethylene glycol and polypropylene glycol which have the characteristic of being soluble in water, a property exploited in special lubricating mixtures (for rubber products, non-flammable hydraulic liquids, etc.).
Solid and semi-solid lubricants
Solid lubricants (graphite, molybdenum sulfide, talc, boron azide, etc.), characterized by a lamellar crystal structure, are used in extremely severe operating conditions in which liquid lubricants do not provide adequate performance (for example, high temperatures, vacuum, etc.). Solid organic lubricants (powders of organic compounds with a high melting point, such as phthalocyanines; polytetrafluoroethylene coatings) are also used.
Intermediates between solid and liquid lubricants are the semi-solid lubricants, called lubricating greases: with this designation, we refer to products, from semi-fluid to solid, used to lubricate in cases where it is not possible to use oils (bearings, bushings, etc.). Greases are substances added with gelling agents and chemical compounds to prevent corrosion and improve the resistance to atmospheric agents of the lubricated parts. They must not escape from the organs within which they are applied either by gravity or by mechanical or thermal stresses. These lubricating greases are made up of lubricating oils, natural or synthetic, more or less thickened, depending on the requirements imposed by the use, with the addition of different substances and also of various additives.
For their preparation, it is possible to start from lubricating oils of petroleum origin, or vegetable oils, or synthetic oils; as thickening substances are added metallic soaps (alkali metal salts, alkaline-earth or heavy metals of stearic acid, palmitic acid, oleic acid, and naphthenic acids). The additives may have different functions: antioxidants, anti-rust, passivating, water-repellent, anti-wear, etc. In addition to these substances, clays, talcum powder, pigments, etc. can be added. The rheological behavior of lubricating greases is mainly related to the length and the shape (elongated, twisted) of thickener fibers dispersed within the oil.
The gaseous lubricants (hydrogen, helium, nitrogen, air, etc.) have the advantage of keeping their viscosity constant as the temperature increases and allowing very low wear of the lubricated members even for a long time and at a high relative speed of the surfaces in contact. Often they consist of the same operating fluids (air, steam, etc.) of the plant to which the organs to be lubricated belong.