Page 337 - Analysis and Design of Machine Elements

(a) (b) Rolling Contact Bearings 315

Figure 11.16 Bearing preloading.

contaminants, protects bearing components from corrosion and prolongs the expected
life of bearings. Lower friction and longer life are obtained by adding a lubricant of
either grease or oil. Operating temperature, load and rotational speed are the main
factors considered while selecting lubricants.
Grease is used at relatively slow speeds under normal ambient temperature. Since
grease is easy to retain in a bearing housing over a long period of time and acts as a
seal against contaminants, most rolling bearings are grease lubricated. Grease can be
delivered to bearings by either hand-packing, or manually lubricated with a grease gun.
When bearings operate at high speeds or at high ambient temperatures, oil lubricants
with a steady supply of a continuous oil flow are required. In oil lubrication, the mainte-
nance of oil film thickness at the contact surface is extremely important, as oil starvation
of the contact area is a main reason for bearing failure. To maintain sufficient oil film
thickness at the contact area between rolling elements and raceways, lubricant oil must
have a minimum viscosity at the operating temperature.
Lubricant oil can be supplied by oil bath, recirculation splash, oil jet or oil-air mist.
In bath lubrication, the height of oil level should not exceed the centre of the lowest
rolling elements to avoid energy loss due to oil churning. In splash lubrication, lubricant
oil is thrown to the housing cover and is directed in its draining to a bearing [14]. As
rotational speed and load increase, oil jet lubrication can be used. Pressurized oil spray
on the rolling elements and recirculate for future use. In even high-speed devices, such
as jet engines, lubricating oil is pumped under pressure to form a fine oil mist on rolling
elements.

11.4.4.8 Sealing
Seals are used to retain lubricant in bearings and also to prevent the entry of contami-
nants and moisture. They are especially helpful when bearings operate in dirty or moist
environments. The selection of a suitable sealing arrangement depends upon lubricant
types, rotational speeds, operating temperature, available space and so on. Two com-
mon types of sealing, that is contact and non-contact sealing, are used for low speeds
and high speeds, respectively.
In contact sealing, seals directly contact rotating shafts. The presence of seals
increases friction and wear; therefore, the rubbing surfaces should have a high polish.
Figure 11.17a shows a felt seal placed in a machined groove for grease lubrication at
low speeds. The commercial seal is an assembly consisting of elastomer and a spring
backing [14], encased in a counterbored hole in a bearing cover. The opening of a
commercial seal facing a bearing is used to retain lubricant, as shown in Figure 11.17b,