Importance of Lubrication
In industrial equipment/machineries, the surface of mechanical parts will have physical contact on neighboring parts to establish a relative motion between them. During the operation of equipment, those contacting surfaces are subjected to friction which depends on the area of material, properties of the material, etc. which is undesirable. It leads to progressive damage thereby resulting in a material loss which is defined as wear. Friction and wear also generate heat and responsible for the overall loss in system efficiency. All of them contribute to significant economic costs due to equipment failure, the cost for replacement and downtime.
To reduce the power loss and also wear and tear of moving parts, a foreign substance called lubricant is introduced in between rubbing surfaces. The lubricant keeps the mating surfaces apart.
Objectives of Lubrication
The primary objective of lubrication is to reduce wear and heat between contacting surfaces in relative motion. Utilizing lubrication, the coefficient of friction (which depends on .area of contact and amount of load acting) could be reduced and in turn heat and wear of the surfaces. Lubrication also aids to
(i) reduce oxidation and rust formation
(ii) provide insulation in transformer application
(iii) transmit mechanical power in hydro fluid power systems
(iv) seal against dust, dirt and water.
Purposes of Lubrication or Functions of Lubrication
Selecting the right lubricant, amount of lubricant and correct application of the lubricant are essential to the successful performance of any friction surfaces in machinery because lubricants serve three purposes.
(i) It reduces friction between moving parts by separating them.
(ii) It reduces wear and tear of the moving parts.
(iii) It minimizes the power loss due to friction.
(iv) It provides the cooling effect: During circulation, it carries the heat from hot moving parts and delivers it to the surrounding.
(v) It provides a cushion effect.
(vi) It provides the cleaning action.
(vii) It provides a sealing action. It helps the piston rings to provide an effective seal against high-pressure gases in the cylinder from leaking out.
(viii) It reduces noise.
Any material used to reduce friction between wearing surfaces with a high coefficient of friction by establishing a low-viscous (shear strength) film is called lubricants. Lubricants are available in liquid, solid and gaseous forms. Solid lubricants (soap, mica, molybdenum- disulfide etc.) are used for industrial applications when oil or grease is not suitable. Graphite is used when the loading at the contact points is heavy.
Methods of Lubrication
Various methods of lubrication normally used for industrial applications are as follows.
a) Hydrostatic lubrication
b) Hydrodynamic or fluid film lubrication
c) Boundary lubrication
d) Elastohydrodynamic lubrication (EHD)
e) Extreme pressure (EP) lubrication.
1. Hydrostatic lubrication:
In hydrostatic lubrication systems, a thin film of lubrication is created between mating surfaces, for example, journal and bearing, by supplying lubricant under pressure with an external source similar to a pump.
2. Hydrodynamic or fluid film lubrication:
In heavily loaded machinery which are thrust bearings and horizontal journal bearings apart from the viscosity of the fluid, higher fluid pressure is also required to support the load until the film is established. If the pressure is generated externally, it is called hydrostatic lubrication and if generated internally i.e. within the bearing by dynamic action and it is referred to as hydrodynamic lubrication. In hydrodynamic lubrication, a fluid wedge is formed by the relative surface motion of journals or the thrust runners over their bearing surfaces.
In hydrodynamic lubrication, the wearing surfaces are completely separated by a film of oil. This type of lubrication is similar to motorized speed in a boat moving on water. When the boat is not moving or begins to move, it experiences a resistance due to the viscosity of water. It causes a slight lift of the leading edge of the boat and it allows a small amount of water between the space due to lifting and supporting water surface. As the velocity of the boat increases, the wedge-shaped water film will also increase until a constant velocity is reached. When the velocity is constant, the amount of water entering the leading edge will equal the amount of water passing outward from the trailing edge. For the boat to remain above the supporting surface, there should exist an upward pressure that is equal to the load. The same principle can be applied to the sliding surface.
The operation of thrust bearing is an example of hydrodynamic lubrication. Thrust bearing assembly used in hydropower industries are also called tilt pad bearings. The pads of these bearings are designed to lift and tilt to provide enough area for lifting the load of the generator. As the thrust runner moves over the thrust shoe, fluid adhering to the runner is drawn between runner and shoe forming a wedge of oil. As the velocity of the thrust runner increases, the pressure of the oil wedge and the runner is lifted as full, the fluid film lubrication takes place. When the load is high, the pressure pumps are used to provide an initial oil film.
The operation of a journal or sleeve bearing is also an example of hydrodynamic lubrication. When the journal is at rest, its weight will squeeze the oil film so that the journal directly rests on the bearing surface. During operation, the journal tends to roll up the side of bearing. So, the fluid adhering to the journal is drawn into the contact area and when the speed increases an oil wedge is formed which is shown in Figure. The pressure of the oil wedge increases until the journal is lifted vertically but it is also pushed to the side by pressure of oil wedge.
3. Boundary lubrication:
When a full fluid film is not developed between rubbing surfaces, the thickness of the film may be reduced so that the dry contact is formed at high points or asperities of mating surfaces. This condition is a characteristic of boundary lubrication. This situation arises when any one of the full film thickness forming factors are missing. The common examples of this type we experience during starting and stopping of bearings in equipment, for example, reciprocating equipment-compressor pistons, turbine wicket gates, gear teeth contact, etc.
4. Extreme pressure lubrication:
Anti-wear agents (chemicals) which are normally used in boundary lubrication will not be effective beyond a certain temperature (250°C). In heavy loading applications, oil temperature raises beyond the anti-wear protection. Under this situation, lubricants containing additives that protect against extreme pressure called EP lubricants are used. EP lubrication can be achieved by chemical compounds of boron, phosphorus, sulfur, chloride or combination of them. These are activated by high temperatures resulting from extreme pressure. At these temperatures, EP molecules become reactive and release derivatives bf phosphorus chloride or sulfur. These derivatives form a solid protective coating that fills the asperities of the exposed surfaces.
5. Elastohydrodynamic (EHD) lubrication:
The lubrication principle applies to rolling bodies such as ball or roller bearings known as Elastohydrodynamic (EHD) Lubrication. The formation of the lubricant film between mating bearing surfaces is called the Elastohydrodynamic (EHD) mechanism of lubrication. The two major considerations in EHD lubrication are the elastic deformation of the contacting bodies under load and hydrodynamic effects forcing the lubricant to separate contacting surfaces while the pressure of the load is deforming them. The contact between the large end of the roller and inner race rib is called elastohydrodynamic contact or a hydrodynamic contact (as elastic deformations are negligible). As
the rib/roller loads are much lower than roller/race loads, the film at the rib/roller end contact is usually twice as thick as on the roller/race contact.
However, scoring and welding may still occur in severe conditions including high speeds, viscosity, load or inadequate lubrication. In these conditions, a lubricant with EP (extreme pressure) additives is to be used to prevent the bearing damage. Even though the lubrication principle of rolling objects is different from sliding objects, the principle of hydrodynamic lubrication can be applied up to certain limits. An oil wedge similar to hydrodynamic lubrication exists at the lower leading edge of bearing. Adhesion of oil to the sliding element and supporting surface increases the pressure and it creates an oil film between two surfaces. Since the area of contact is extremely small in a roller bearing or ball bearing, the force per unit area will be extremely high. Under this pressure, it would appear that the oil could be squeezed from between surfaces.
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