Page 374 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
352
Lubricating oils include natural oils and synthetic oils. Natural oils cost less and can
meet general lubrication demands. Chemical additives are frequently added to enhance
viscosity, to reduce friction, wear and corrosion or to retard oxidation. Synthetic oils are
specially designed chemical formulations with better performance yet higher cost than
natural oils [3].
Greases are essentially a mixture of natural or synthetic oils and thickeners. Thicken-
ers act as carriers for the oil, which are metallic soaps formed by the reaction of animal or
vegetable fats with alkaline substances such as lithium, calcium, an aluminium complex
and others [3]. Unlike oils that circulate and serve a cooling and cleaning function, after
being applied to the interface between moving elements, the grease remains in place
to provide lubrication and to prevent harmful contaminants from entering contact sur-
faces. Therefore, periodically replenishing greases is required to discharge contaminated
or oxidized grease [2, 3].
A solid lubricant is a thin solid film that adheres to mating surfaces to reduce fric-
tion and wear. Solid lubricants blended with binders are applied to critical surfaces by
brushing, spraying or dipping [3]. Molybdenum disulfide (MoS ), graphite, powdered
2
metal and Teflon coating are commonly used solid lubricants. They are usually used in
applications at excessively high or low temperatures or operation in a vacuum.

12.4.3.2 Lubricant Properties and Their Selection
Lubricant properties contribute to satisfactory performance of sliding bearings. Lubri-
cants should have adequate viscosity and good lubricity at operation temperature and
pressure. Their chemical and thermal properties should be stable within operating tem-
perature for a reasonable service period. Besides, lubricant should be compatible with
surrounding elements, such as bearings and seals, with regard to corrosion resistance
and degradation [3].
The selection of lubricant depends on a number of factors, such as lubrication type,
load, speed, operating temperature and working environment. Viscosity grade number,
viscosity index (VI), and corrosion protection are the main properties need to be con-
sidered [3]. Viscosity grade number discussed in Section 12.2.3 affects oil film forming
capability. High viscosity facilitates film formation and is recommended for heavy load,
low speed and high temperature application. However, too high a viscosity may result
in a waste of power in overcoming the internal friction of oil. Lower viscosities may be
used, on the other hand, for light load, high speed or low temperature operation.
Viscosity index (VI) is a measure of temperature effect on viscosity. Generally, a high
viscosity index is desirable as it exhibit a more uniform performance as temperature
varies [3]. Corrosion protection can be improved by adding additives to base oils. For
example, corrosion inhibitors can form a protective shield against water on ferrous met-
als and bronze bearing metals. Oxidation inhibitors are used to prolong the life of oils.
The selection of lubricant with suitable additives are determined by specific application
demands and can be referred to in relevant references [17].

12.4.3.3 Lubricant Supply
To maintain hydrodynamic lubrication, sufficient lubricant must be supplied to sliding
bearing surfaces. Continuous lubricant can be supplied by oil holes and grooves, oil
rings, splash, oil bath and oil pump [2]. In addition, appropriate oil filters or shaft seals
are required to keep lubricant free from contaminants to avoid abrasive wear.