The most common application of a lubricant is in bearings that are included in an endless variety of mechanical equipment. Two types are considered: plain bearings and rolling bearings.
Plain Or Journal And Bush Bearings
When a journal rotates in a plain bearing that is receiving oil in adequate quantity, a fluid film of oil is built up by virtue of the viscosity of the oil as a result of rotation of the journal in the bearing. Under suitable conditions this oil film prevents metallic contact between the journal and the bearing, while frictional resistance to the movement of the journal depends mainly on the speed of rotation of the journal and viscosity of the oil under the conditions existing in the film. Friction may be reduced by using oils of progressively lower viscosities, but in every case, depending on speed and load, there is a lower limit of viscosity below which a fluid film is not fully maintained and metal-to-metal contact is likely to occur. In practice, therefore, it is usual to employ oils of sufficient viscosity to provide a safety margin. The effect of operating temperature on viscosity must not be overlooked.
To ensure fluid film lubrication, the bearing must at all times be flooded with oil, so the oil must be supplied to the bearing at a rate equal to that at which it escapes. The rate of escape depends on the viscosity of the oil (less viscous oils escape more rapidly) and on such factors as bearing clearances and the mechanical condition of the bearing. Speaking generally, the greater the load, the lower the speed; or the larger the clearances, the more viscous the lubricant must be. Conversely, lighter loads, higher speeds and smaller clearance necessitate oils of relatively low viscosity. Viscosity requirement also varies directly with surface specifications and bearing mean pressures.
Cleanliness in handling and applying the lubricant is more important to reduce wear, and every precaution should be taken to prevent impurities and moisture getting into bearings. Correct alignment is also a matter of obvious importance in preventing overheating and wear.
Variation of factors referred above makes it necessary to provide a considerable range of machine oils of different viscosity, so as to give the most effective lubrication under all possible conditions.
Grease-lubricated plain bearings running at normal temperatures require general-purpose cup grease of medium-soft consistency. Moisture-resistant grease is generally preferable. Where operating temperatures are high, special high-temperature grease is required.
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Anti-friction bearings consist of ball and roller type, needle bearings, tapered roller bearings etc.
- The main duties of ball and roller bearing lubricants are:
- To reduce friction between the rolling elements and the separator or cage, and between the rolling elements and the races at any point where true rolling is absent.
- To reduce friction between the ends of rollers and the guiding surfaces which form part of the inner or outer race.
- To assist in dissipating heat generated within the bearing.
- To protect the highly polished working surfaces of the bearing from corrosion and rusting.
- To assist in sealing the bearing against the entry of contaminants such as dust and moisture.
Ball and roller bearings may be lubricated by oil or grease, the choice of lubricant usually being decided by operating conditions and bearing design.
Oil is generally considered to be more effective lubricant, and is to be preferred if the sealing arrangements are adequate, especially where the operating temperature or surface speed of the balls or rollers is high. Oils having viscosities ranging from that of the thinnest spindle oil to that of a heavy steam-cylinder oil are used. The viscosity required for any given set of conditions depends on size speed, load, and temperature of the bearing. Mineral oils of high quality should be chosen, because of their resistance to form resinous products, or acids which might damage the highly polished working surfaces of the bearing.
The method used to apply the oil depends mainly on the running speed of the bearing. Use of proper method is especially important with high-speed bearings, which are liable to overheat through churning of the oil if it is not correctly applied. The following means of application are suitable :
- Oil bath and splash systems - low and medium speeds.
- Circulating systems - medium speeds.
- Spray or mist - high speeds.
Over-lubrication should be avoided. As a general guide, with oil baths, it can be taken that the oil level should not be above the centre of the bottom-rolling element when the bearing is not rotating.
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The field of application of lubricating grease is related to their non-Newtonian characteristics. It is usual to employ grease as a lubricant where temperatures and speeds are not excessive and the sealing arrangements do not offer satisfactory lubrication by oils. In addition to being a lubricant, grease possesses excellent self-sealing properties and protects the bearing against the entry of contaminants, where the neck sealing is inadequate or elementary. The viscosity of grease at low or moderate rates of shear remains much higher than lubricating oil in general and so the bearing reaches hydrodynamic or full-film stage more rapidly than with oil.
However at high speeds frictional losses are relatively high and more frictional heat is also generated. Since the effectiveness of grease as a coolant is limited, grease lubrication of plain bearings is therefore confirmed to units of low or moderate speed often under 200 rpm and rarely over 300 rpm; with journal surface speeds generally 1 to 2 m/s. This speed limit is however for large bearings of 50-cm. dia. or more. With reduction in size of the bearing the maximum journal surface speed upto which losses and heating are tolerable, fall sharply.
Calcium soap greases are mainly used where temperatures are moderate. Calcium-based greases are preferable for bearings that have to run in the presence of excessive moisture. Other types of greases are required where higher working temperatures and more satisfactory sealing properties are required than the conventional 'cup' greases. Such grades include lithium soap and inorganic-base greases and some soda soap greases, but it should be noted that Sodium soap greases are not recommended where the bearing is subjected to excessive washing by water.
Where loading is exceptionally severe for example, in roller-bearing assemblies fitted to the roll necks of a metal rolling mill, it may be necessary to use a grease with extreme pressure properties. Such greases are designed to give effective lubrication under conditions of shock loads and constant high loading. In certain applications they also require resistance to moisture; on a metal-rolling mill, for example, water is frequently used to cool the roll faces.
Whatever type of grease is used, it should have no tendency to separate under the operating conditions. When separation occurs, the oil runs out of the bearing and leaves behind dry soap that hardens and cakes. This interferes with the movement of the rolling elements; overheating and mechanical failure may result. Excessive softening is also undesirable, because the grease might leak out of the bearing and leave the working surfaces unprotected.
The methods used to apply grease are governed by the design of the bearings and by conditions of operation. They are:
- Grease Packing - Applicable where periodic addition of fresh grease is not required.
- Compression Cup or Pressure Gum Application - Employed where the addition of fresh grease is periodically necessary.
- Centralized Pressure Systems - For severe service where the rate of consumption makes it necessary to add fresh grease at frequent and regular intervals (e.g. roller-bearing assemblies of roll necks of metal-rolling mills).
In selecting lubricants for ball and roller bearings, the needs of each application must be studied. Due regard must be paid to the type of bearing, method of lubrication and operating conditions. Thereafter, to ensure satisfactory operation in service, the main points to bear in mind are the exclusion of moisture, dust and other contaminants and regular replenishment or renewal of the lubricant. Care should be taken to avoid over-filling because this may cause operating troubles; for example, a bearing that has been tightly packed with grease is liable to overheat.