Qn.
1 Explain the Importance of lubrication in bearings
The primary purpose of lubrication in a rolling bearing is to separate the contacting surfaces, both rolling and sliding. This purpose is rarely achieved, and boundary lubrication or partial metal-to metal contact frequently occurs. By far the most common lubricants are petroleum products in the form of grease or liquid oil. Synthetics are, however, finding more use in high-temperature applications. Since the lubricant affects bearing life and operation, selecting the proper lubricant is an important design function. The purpose of lubrication in bearing applications is to: 1. Minimize friction at points of contact within the bearings 2. Protect the highly finished bearing surfaces from corrosion 3. Dissipate heat generated within the bearings 4. Remove foreign matter or prevent its entry into the bearings Two basic types of lubricants used with antifriction bearings are oils and greases. Each has its advantages and limitations. Since oil is a liquid, it lubricates all surfaces and is able to dissipate heat from these surfaces more readily. Because oil retains its physical characteristics over a wider range of temperatures, it may be used for highspeed and high-temperature applications. The quantity of oil supplied to the bearing may be accurately controlled. Oil lubricants can be circulated, cleaned, and cooled for more effective lubrication. Grease, which is easier to retain in the bearing housing, aids as a sealant against foreign matter and corrosive fumes. Oil Lubrication 1. Oil is a better lubricant for high speeds or high temperatures. It can be cooled to help reduce bearing temperature. 2. Oil is easier to handle, and with oil it is easier to control the amount of lubricant reaching the bearing. It is harder to retain in the bearing. Lubricant losses may be higher than with grease. 3. As a liquid, oil can be introduced into the bearing in many ways, such as drip feed, wick feed, pressurized circulating systems, oil bath, or air-oil mist. Each is suited to certain types of applications. 4. Oil is easier to keep clean for recirculation systems. Grease Lubrication 1. This type of lubrication is restricted to lower-speed applications within the operating temperature limits of the grease. 2. Grease is easily confined in the housing. This is important in the food, textile, and chemical industries. 3. Bearing enclosure and seal design is simplified. 4. Grease improves the efficiency of mechanical seals to give better protection to the bearing.
Qn. 2 Selection of lubricants - Oil or grease?
The best choice of lubricant for any component depends on the service conditions. If leakage is a problem, or if, during shutdown, the maintenance of a thick film of lubricant is necessary to inhibit corrosion and prevent dry-state operation, greases show to advantage. If generation of heat and consequent excessive temperatures is a problem, users will find it better to use liquid lubricants, which have the advantage of high rates of heat transfer.
Qn.3 Explain Viscosity index (VI) The viscosity index (VI) is an empirical measure of oils change in viscosity with temperature. This is a way of expressing the rate of change of viscosity with temperature. All oils become less viscous as the temperature increases. The rate of change of viscosity varies with different oils and is mainly dependent on the type of crude from which the oil is derived and the refining method. The greater the value of the viscosity index, the less the oil viscosity will change with temperature. Originally ranging from 0 to 100, viscosity indexes greater than 100 are now achieved through better techniques of refining petroleum, through the use of certain synthetic oils, and through the use of additives. The higher the number, the smaller the relative change in viscosity with temperature. The VI of oil is an important property in applications where the operating temperature is subject to considerable change. Paraffinic lubricants have higher viscosity indexes than naphthenic lubricants. Qn. 4 Conditions under which solid lubrication is preferred The solid lubricants are used under the condition where the speed is low and pressure is high. A common and most suitable solid lubricant is graphite. It has a black soft mass in the form of carbon. It is used either in the natural flake form or suspended in oil, grease or distilled water in a finely divided colloidal form. It is more suitable under high pressure condition where film-lubrication is not feasible. Sometimes soapstone is also used as a lubricant, because it is soft and greasy in touch. It also has the ability to fill up the pores of the metal surfaces and make them frictionless.
Solid lubricants, such as graphite, molybdenum disulfide (moly) and polytetrafluoroethylene (PTFE), are not only used by themselves but are also frequently added to oils and greases to improve their performance under boundary lubrication conditions.
