Like gas lubrication, liquids lubricate by keeping the two moving surface apart. The bearing is one of the most common lubricated system. There are three types of bearing: the journal bearing, the thrust bearing, and the slider bearing.
8.3.1 Journal Bearings
This consists of a rotating axel or journal which contacts the bearing for support. Rotations and the small angle between the two surfaces cause a buildup of pressure in the oil layer which supports the load during rotation. This is called hydrodynamic lubrication and is shown in Fig. 8.2. The important characteristic of the bearing is the dimensionless numberCwhere
C¼ZN
P (8.10)
Fig. 8.2 The journal bearing showing the pressure distribution and oil forces supporting the journal
Z¼the viscosity of the lubricant,N¼the speed of the shaft, andP¼the pressure or load divided by the projected area. Values of C>35 imply that hydrodynamic lubrication is effective. When C35, the oil wedge responsible for hydrodynamic lubrication is too thin, leading to an increase in friction, that is, a reduction in lubrication.
The true surface area of a polished surface can be many times the nominal area. Thus, for example, the true area of a polished plate glass surface is about ten times the nominal area. This implies the presence of many hills and valleys which can interact as shown in Fig.8.3. If the speed of rotationN and/or the viscosity of the lubricant is reduced or if the pressure is increased, then although the coefficient of friction may increase, the distance between the journal and the bearing will eventually decrease to less than the height of the largest surface irregularity, and wear will result. There are thus three possible causes of lubrication failure: high pressures, low speeds, and of course, high tempera- ture which reduce the viscosity of the lubricant oil. The effect of temperature on the viscosity of a liquid is discussed in AppendixB.
The variation of viscosity () with temperature (T), in its simplest form, is given by Reynolds relation
¼k eat (8.11)
where k and a are constants depending on the liquid. This can also be expressed as
¼k eEvis=RT (8.12)
A more exact relationship is given
log10log10ðvþ0:8Þ ¼n log10TþC (8.13) wherev¼p(kinematic viscosity with unites of stokes) in whichnandCare constants for a given oil or lubricant andris the density of the liquid.
Fig. 8.3 (a) Fine ground surface showing high spots called asperities. (b) Polished surface showing bumpy contour
134 8 Lubrication and Lubricants
To reduce wear during low speeds, long-chain fatty acids or soaps are added to the oil. These form chemical bonds to the metal surfaces and thus prevent direct metal-metal contact or welding of the two surfaces. This type of lubrication is called boundary lubrication and is illustrated in Fig.8.4.
Two surfaces which are in relative motion with an oil film between them are hydrodynamically lubricated. The viscosity of the oil determines the friction of motion. When the thickness of oil film is less than the surface irregularities, the asperities, then surface wear will eventually bring the two surfaces sufficiently close so as to rely on boundary layer lubrication. This is illustrated in Fig.8.5for a moving piston in a car engine.
8.3.2 Thrust Bearings
Thrust bearings are meant to keep a rotating shaft from motion parallel to the axis of rotation. Various configurations are possible, but the most common is one in which the load is supported by the bearing in a vertical shaft system.
8.3.3 Slider Bearings
Slider bearings are also calledways or guidebearings and are meant for rectilinear and curvilinear motion.
Fig. 8.4 A schematic illustration of a
monomolecular iron stearate boundary film. Cohesive forces between the iron stearate molecules result in a closely packed and difficult to penetrate film (Courtesy of Texaco Inc.)
8.3.4 Ball Bearings
Elastohydrodynamic Lubrication (EHL):In journal bearings or other surface area interactions under lubricating conditions, deformation of the materials or the influence of pressure on viscosity of the lubricant can be ignored. However, in roller and ball bearings, the point of contact is deformed elastically under the high load pressure. This also increases the pressure on the film lubricant which increases in viscosity according to the equation
p¼oeap (8.14)
whereois the viscosity of a liquid at 1 atm pressure pis the viscosity of the liquid at pressureP
ais the pressure coefficient of viscosity
The film thickness is about 10–50mm and increases as the speed is increased. An increase in load has the effect of deforming the metal surfaces more, causing an increase in contact area rather than a decrease in film thickness.
An important characteristic of EHL is the specific film thickness (l) which is the ratio of the film thickness (h) to the composite roughness (s) of the two surfaces, that is,
l¼h
s (8.15)
Fig. 8.5 Schematic diagram describing piston-to-cylinder wall and piston skirt-to-cylinder wall lubrication regime changes during an engine revolution (Courtesy of Texaco Inc.)
136 8 Lubrication and Lubricants
The composite roughness is determined from
s¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s21þs22
q
(8.16) wheres1ands2are the root mean square roughness
h¼B ffiffiffiffiffiffiffiffiffiffiffi 0VR
p (8.17)
where0¼viscosity of oil at atm pressure V¼½(V1+ V2), mean speed
R¼mutual radius of curvature of the two contacting surfaces
and where B is a constant depending on the units used, for example, B¼8106cm for c:g.s units.
The elastic deformation and oil pressure profile is shown in Fig.8.6.