An Introduction To Journal Bearings
Introduction
The term ‘bearing’ typically refers to contacting surfaces through which a load is transmitted.
Bearings may roll or slide or do both simultaneously.The range of bearing types available is
extensive, although they can be broadly split into two categories: sliding bearings also known as plain
surface bearings, where the motion is facilitated by a thin layer or film of lubricant, and rolling element bearings, where the motion is aided by a combination of rolling motion and lubrication. Lubrication is often required in a bearing to reduce friction between surfaces and to remove heat.
illustrates two of the more commonly known bearings: a deep groove ball bearing and a journal
bearing.
The term ‘sliding bearing’ refers to bearings where two surfaces move relative to each other
without the benefit of rolling contact. The two surfaces slide over each other and this motion can
be facilitated by means of a lubricant which gets squeezed by the motion of the components and
can generate sufficient pressure to separate them, thereby reducing frictional contact and wear.
A typical application of sliding bearings is to allow rotation of a load-carrying shaft. The portion of the shaft at the bearing is referred to as the journal and the stationary part, which supports
the load, is called the bearing . For this reason, sliding bearings are often collectively
referred to as journal bearings, although this term ignores the existence of sliding bearings that support linear translation of components. Another common term is ‘plain surface bearings’.This section is principally concerned with bearings for rotary motion and the terms ‘journal’ and ‘sliding’
bearing are used interchangeably.A general classification scheme for the distinction of bearings is given in Figure 2.
The term ‘bearing’ typically refers to contacting surfaces through which a load is transmitted.
Bearings may roll or slide or do both simultaneously.The range of bearing types available is
extensive, although they can be broadly split into two categories: sliding bearings also known as plain
surface bearings, where the motion is facilitated by a thin layer or film of lubricant, and rolling element bearings, where the motion is aided by a combination of rolling motion and lubrication. Lubrication is often required in a bearing to reduce friction between surfaces and to remove heat.
illustrates two of the more commonly known bearings: a deep groove ball bearing and a journal
bearing.
The term ‘sliding bearing’ refers to bearings where two surfaces move relative to each other
without the benefit of rolling contact. The two surfaces slide over each other and this motion can
be facilitated by means of a lubricant which gets squeezed by the motion of the components and
can generate sufficient pressure to separate them, thereby reducing frictional contact and wear.
A typical application of sliding bearings is to allow rotation of a load-carrying shaft. The portion of the shaft at the bearing is referred to as the journal and the stationary part, which supports
the load, is called the bearing . For this reason, sliding bearings are often collectively
referred to as journal bearings, although this term ignores the existence of sliding bearings that support linear translation of components. Another common term is ‘plain surface bearings’.This section is principally concerned with bearings for rotary motion and the terms ‘journal’ and ‘sliding’
bearing are used interchangeably.A general classification scheme for the distinction of bearings is given in Figure 2.
Journal Bearing Types
The types of journal bearings most commonly found in turbo machinery include axial groove bearing, pressure dam bearing, elliptical bearing, offset half bearing, taper-land and multi-lobe bearings and tilt pad bearing.
The types of journal bearings most commonly found in turbo machinery include axial groove bearing, pressure dam bearing, elliptical bearing, offset half bearing, taper-land and multi-lobe bearings and tilt pad bearing.
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| FIG2 |
Bearing classification based on type of load carried
a. Radial bearings
b. Thrust bearings or axial bearings
c. Radial – thrust bearings
Bearing classification based on type of lubrication
The type of lubrication means the extent to which the contacting surfaces are separated in a shaft bearing combination. This classification includes
(a) Thick film lubrication
(b) Thin film lubrication
(c) Boundary lubrication
Bearing classification based on lubrication mechanism
a. Hydrodynamic lubricated bearings
b. Hydrostatic lubricated bearings
c. Elastohydrodynamic lubricated bearings
d. Boundary lubricated bearings
e. Solid film lubricated bearings
1-Radial or Journal
These bearings carry only radial loads.
Plain radial, or journal, bearings also are referred to as sleeve or Babbit bearings. The most common type is the full journal bearing, which has 360-degree contact with its mating journal. The partial journal bearing has less than 180-degree contact and is used when the load direction is constant.
Plain Cylindrical Bearing
The plain cylindrical journal bearing (Figure 3 ) is the simplest of all journal bearing types. The performance characteristics of cylindrical bearings are well established, and extensive design information is available. Practically, use of the unmodified cylindrical bearing is generally limited
to gas-lubricated bearings and low-speed machinery.
Axial Groove Bearing
Axial groove bearings have a cylindrical bore with typically 2 to 4 axial oil feed grooves. (Figure 5) illustrates a two axial groove design. These bearings are very popular in relatively low speed equipment. The stability characteristics of axial groove bearings are primarily controlled by the bearing clearance.
Tight clearances produce higher instability thresholds but tight bearings present other problems that make them undesirable. Due to these, as clearance decreases, the bearing’s operating oil temperature increases.
Furthermore the Babbitt wear during repeated start-ups will increase the bearing clearance thereby degrading stability. In fact, many bearing induced instabilities in the field are caused by bearing clearances that have increased due to wear from oil contamination, repeated starts or slow rolling with boundary lubrication.
Pressure Dam Bearing
The Pressure dam or step journal bearings have long been used to improve the stability of turbo machinery as a replacement for axial groove bearings. In many cases, these bearings provide a quick and inexpensive fix for machines operating at high speeds near or above the stability threshold.
Milling a step in the top pad of the proper size and location may be all that is necessary to eliminate the stability problem. This is much less expensive and faster than installing tilting pad bearings that may require a change in the bearing housing.
The pocket is cut in the upper half of the bearing with one end of the step located in the second quadrant for counter-clockwise shaft rotation.
The pocket has side lands to hold the pressure and flow. A circumferential relief groove or track is sometimes grooved in the bottom half of the bearing.
Both of these dam and relief track effects combine to increase the operating eccentricity of the bearing compared to an axial groove cylindrical bearing.
At high speeds and light loads, the step creates a loading that maintains a minimum operating eccentricity.
Elliptical Bearing
The elliptical or lemon bore bearing is a two-pad-fixed geometry bearing which is preloaded in the vertical direction, as shown in Figure 7 . This bearing can be manufactured by inserting a shim in the
split line before boring. When the shim is removed, the vertical clearance will be less than the horizontal clearance. The centers of curvature of the top and bottom halves are not coincident with the true bearing center.
Offset Half Bearing
This is a unique type of plain journal bearing which is a very simple and yet very effective bearing generally used in some applications. It is a 2-pad multi-lobe bearing with 100% offset. The pads are generally always preloaded. It is easy to make this bearing by boring a hole in plain bushing. It
should be noted that this type of bearing will not tolerate reverse rotation. While more stable than the axial groove journal bearing, there is still a tendency for instability. An offset half bearing is shown in Figure 8
Taper-land and Multi-lobe bearings
The taper land bearing is shown in Figure 9 and multi-lobe bearing is shown in Figure 10 . These are very popular sleeve bearings that are successful in increasing the instability threshold speed when compared to cylindrical sleeve bearings. The taper land bearing has side lands similar to a pressure dam bearing. Care must be taken when using bearings with this design because they are not suited for heavy load applications. Taper land bearings are very frequently utilized in small, light rotors operating at high speeds such as small turbo-expanders and turbochargers.
Tilt Pad Bearing
In the tilt pad design, the cylindrical bearing element is divided into a number of pad arc segments, depending on the shaft load. Each pad segment is supported and held in circumferential position by an outer housing.
This bearing includes pad load orientation, pivot offset, pad preload and pad axial length. The load between pivot configurations is shown in Figure 10. The load between pads provides more symmetric stiffness and damping coefficients.
The offset pivots are very popular with thrust bearings as offsetting the pivot increases the operating film thickness thereby decreasing the operating temperature. Further, offset pivots increase bearing stiffness, especially when compared to centrally pivoted pads.
For this tilt pad bearing it can be seen that as the preload decreases, the bearing damping increases while bearing stiffness remains constant. Both of these trends help in increasing the bearing effective damping. Normally, as bearing damping increases, bearing stiffness also increases. However, there is
a disadvantage to low preload pads. Light preload is the loss of damping
2- Thrust bearings or axial bearings
The thrust load is transferred through lubricant film between thrust collar on rotor and thrust collar on
housing.
a. Radial bearings
b. Thrust bearings or axial bearings
c. Radial – thrust bearings
Bearing classification based on type of lubrication
The type of lubrication means the extent to which the contacting surfaces are separated in a shaft bearing combination. This classification includes
(a) Thick film lubrication
(b) Thin film lubrication
(c) Boundary lubrication
Bearing classification based on lubrication mechanism
a. Hydrodynamic lubricated bearings
b. Hydrostatic lubricated bearings
c. Elastohydrodynamic lubricated bearings
d. Boundary lubricated bearings
e. Solid film lubricated bearings
1-Radial or Journal
These bearings carry only radial loads.
Plain radial, or journal, bearings also are referred to as sleeve or Babbit bearings. The most common type is the full journal bearing, which has 360-degree contact with its mating journal. The partial journal bearing has less than 180-degree contact and is used when the load direction is constant.
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| FIG 3 |
Plain Cylindrical Bearing
The plain cylindrical journal bearing (Figure 3 ) is the simplest of all journal bearing types. The performance characteristics of cylindrical bearings are well established, and extensive design information is available. Practically, use of the unmodified cylindrical bearing is generally limited
to gas-lubricated bearings and low-speed machinery.
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FIG 4
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Axial groove bearings have a cylindrical bore with typically 2 to 4 axial oil feed grooves. (Figure 5) illustrates a two axial groove design. These bearings are very popular in relatively low speed equipment. The stability characteristics of axial groove bearings are primarily controlled by the bearing clearance.
Tight clearances produce higher instability thresholds but tight bearings present other problems that make them undesirable. Due to these, as clearance decreases, the bearing’s operating oil temperature increases.
Furthermore the Babbitt wear during repeated start-ups will increase the bearing clearance thereby degrading stability. In fact, many bearing induced instabilities in the field are caused by bearing clearances that have increased due to wear from oil contamination, repeated starts or slow rolling with boundary lubrication.
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| FIG 5 |
Pressure Dam Bearing
The Pressure dam or step journal bearings have long been used to improve the stability of turbo machinery as a replacement for axial groove bearings. In many cases, these bearings provide a quick and inexpensive fix for machines operating at high speeds near or above the stability threshold.
Milling a step in the top pad of the proper size and location may be all that is necessary to eliminate the stability problem. This is much less expensive and faster than installing tilting pad bearings that may require a change in the bearing housing.
The pocket is cut in the upper half of the bearing with one end of the step located in the second quadrant for counter-clockwise shaft rotation.
The pocket has side lands to hold the pressure and flow. A circumferential relief groove or track is sometimes grooved in the bottom half of the bearing.
Both of these dam and relief track effects combine to increase the operating eccentricity of the bearing compared to an axial groove cylindrical bearing.
At high speeds and light loads, the step creates a loading that maintains a minimum operating eccentricity.
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| FIG 6 |
The elliptical or lemon bore bearing is a two-pad-fixed geometry bearing which is preloaded in the vertical direction, as shown in Figure 7 . This bearing can be manufactured by inserting a shim in the
split line before boring. When the shim is removed, the vertical clearance will be less than the horizontal clearance. The centers of curvature of the top and bottom halves are not coincident with the true bearing center.
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| FIG 7 |
This is a unique type of plain journal bearing which is a very simple and yet very effective bearing generally used in some applications. It is a 2-pad multi-lobe bearing with 100% offset. The pads are generally always preloaded. It is easy to make this bearing by boring a hole in plain bushing. It
should be noted that this type of bearing will not tolerate reverse rotation. While more stable than the axial groove journal bearing, there is still a tendency for instability. An offset half bearing is shown in Figure 8
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| FIG 8 |
The taper land bearing is shown in Figure 9 and multi-lobe bearing is shown in Figure 10 . These are very popular sleeve bearings that are successful in increasing the instability threshold speed when compared to cylindrical sleeve bearings. The taper land bearing has side lands similar to a pressure dam bearing. Care must be taken when using bearings with this design because they are not suited for heavy load applications. Taper land bearings are very frequently utilized in small, light rotors operating at high speeds such as small turbo-expanders and turbochargers.
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| FIG 9 |
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| FIG 10 |
Multilobe bearings are similar in concept to lemon bore bearings. (i.e., there is a separate pad machined bore and bearing set bore). The centers of the pad arc for each of the lobes form a circle, referred to as a preload circle. Therefore, a wide range of preloads is possible by changing the pad
and set bores. Under certain conditions, the cross-coupling can cause the bearing to be unstable and an oil whirl will result.Tilt Pad Bearing
In the tilt pad design, the cylindrical bearing element is divided into a number of pad arc segments, depending on the shaft load. Each pad segment is supported and held in circumferential position by an outer housing.
This bearing includes pad load orientation, pivot offset, pad preload and pad axial length. The load between pivot configurations is shown in Figure 10. The load between pads provides more symmetric stiffness and damping coefficients.
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| FIG 11 |
For this tilt pad bearing it can be seen that as the preload decreases, the bearing damping increases while bearing stiffness remains constant. Both of these trends help in increasing the bearing effective damping. Normally, as bearing damping increases, bearing stiffness also increases. However, there is
a disadvantage to low preload pads. Light preload is the loss of damping
2- Thrust bearings or axial bearings
The thrust load is transferred through lubricant film between thrust collar on rotor and thrust collar on
housing.
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| FIG 12 |
3- Radial thrust bearings
Radial thrust bearings are subjected to combined radial and thrust loads. These bearings carry both radial and thrust loads.
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| FIG 13 |
Bearing classification based on type of lubrication
There are three regimes of lubrication for sliding bearings (figure 14):1. boundary lubrication
2. mixed film lubrication
3. full film lubrication
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| FIG 14 |
1- Boundary lubrication
Here the surface contact is continuous and extensive. The lubricant is continuously smeared over the
surfaces and provides a continuously renewed adsorbed surface film which reduces the friction and wear. The typical coefficient of friction is 0.05 to 0.20.
2- Thin film lubrication (Mixed lubrication)
Here even though the surfaces are separated by thin film of lubricant, at some high spots Metal-to-metal contact does exist. Because of this intermittent contacts, it also known as mixed film lubrication. Surface wear is mild. The coefficient of friction commonly ranges from 0.004 to 0.10.
3- Thick film lubrication (Hydrodynamic lubrication)
The surfaces are separated by thick film of lubricant and there will not be any metal-to-metal contact. The film thickness is anywhere from 8 to 20 μm. Typical values of coefficient of friction are 0.002 to 0.010. Hydrodynamic lubrication is coming under this category. Wear is the minimum in this case.
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| FIG 15 |
Bearing classification based on lubrication mechanism
1- Hydrodynamic lubricated bearings
In these bearings the load-carrying surfaces are separated by a stable thick film of lubricant that prevents the metal-to-metal contact. The film pressure generated by the moving surfaces that force the lubricant through a wedge shaped zone. At sufficiently high speed the pressure developed around the journal sustains the load.
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| FIG 16 |
2- Hydrostatic lubricated bearings
In these bearings, externally pressurized lubricant is fed into the bearings to separate the surfaces with thick film of lubricant. These types of bearings do not require the motion of the surfaces to generate the lubricant film. Hence they can operate from very low speed to high speed.
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| FIG 17 |
3- Elasto hydrodynamic lubricated bearings
Rolling contact bearings come under this category. The oil film thickness is very small. The contact
pressures are going to be very high. Hence to prevent the metal-to-metal contact, surface finishes are to be of high quality. Such a type of lubrication can be seen in gears, rolling contact bearings, cams etc.
4- Boundary lubricated bearings
When the speed of the bearing is inadequate, less quantity of lubricant is delivered to the bearing, an
increase in the bearing load, or an increase in the lubricant temperature resulting in drop in viscosity any one of these may prevent the formation of thick film lubrication and establish continuous metal-to-metal contact extensively. Often bearings operating in such situations are called boundary lubricated bearings.
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| FIG 18 |
5- Solid film lubricated bearings
For extreme temperature operations ordinary mineral oils are not satisfactory. Solid film lubricants such as graphite, molybdenum disulfide or their combinations which withstand high operating temperature are used. These types of bearings are common in furnace applications, or trunnion bearings of liquid metal handling systems, hot drawing mills etc
SLIDING CONTACT BEARINGS - ADVANTAGES AND DISADVANTAGES
These bearings have certain advantages over the rolling contact bearings. They are:
1.The design of the bearing and housing is simple.
2.They occupy less radial space and are more compact.
3.They cost less.
4.The design of shaft is simple.
5.They operate more silently.
6.They have good shock load capacity.
7.They are ideally suited for medium and high speed operation.
The disadvantages are:
1.The frictional power loss is more.
2.They required good attention to lubrication.
3.They are normally designed to carry radial load or axial load only
Decide lubricant for the bearing
The performance of a sliding bearing differs markedly depending on which type of lubricationis physically occurring.This is illustrated in Figure 19, which shows the variation of the coefficient
of friction with a group of variables called the ‘bearing parameter’ which is defined by:
The performance of a sliding bearing differs markedly depending on which type of lubricationis physically occurring.This is illustrated in Figure 19, which shows the variation of the coefficient
of friction with a group of variables called the ‘bearing parameter’ which is defined by:
K =𝛍 ×N/P
where𝛍 viscosity of lubricant (Pa s)
N speed (for this definition normally in rpm)
P, load capacity (N/m²) given by:
P = W/(LD)
where
W, applied load (N);
L, bearing length (m,mm );
D, journal diameter (m, mm).
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| FIG 19 |
The bearing parameter, 𝛍N/P, groups several of the bearing design variables into one number
Normally, of course, a low coefficient of friction is desirable. In general, boundary lubrication is
used for slow speed applications where the surface speed is less than approximately 1.5 m/s. Mixed
film lubrication is rarely used because it is difficult to quantify the actual value of the coefficient of
friction (note the steep gradient in Figure .19 for this zone).
Lubricants
As can be seen from Figure 19 bearing performance is dependent on the type of lubrication occurring
and the viscosity of the lubricant.The viscosity is a measure of a fluid’s resistance to shear.
Lubricants can be solid, liquid or gaseous, although the most commonly known are oils and greases.
The principal classes of liquid lubricants are mineral oils and synthetic oils.Their viscosity is highly
dependent on temperature as illustrated in Figure 20.They are typically solid at 35°C, thin as paraffin at 100°C and burn above 240°C. Many additives are used to affect their performance. For
example, EP (extreme pressure) additives add fatty acids and other compounds to the oil, which attack the metal surfaces to form ‘contaminant’ layers, which protect the surfaces and reduce friction even when the oil film is squeezed out by high contact loads. Greases are oils mixed with soaps to form a thicker lubricant that can be retained on surfaces. The viscosity variation with temperature of oils has been standardized and oils are available with a class number, for example SAE 10, SAE 20, SAE 30, SAE 40, SAE 5W, SAE 10W, etc.The origin of this identification system developed by the Society of Automotive Engineers was to class oils for general purpose use and winter use, the ‘W’ signifying the latter.The lower the numerical value, the thinner or less viscous the oil. Multigrade oil, e.g. SAE 10W/40, is formulated to meet the viscosity requirements of two oils giving some of the benefits of the constituent parts.An equivalent identification system is also available from the International Organisation for Standardisation (ISO 3448).
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| FIG 20 |
Journal Bearing Design
The simplest form of journal bearing is a plain circular bushing with an internal diameter that is slightly larger than the shaft outer diameter. The following parameters are considered for the design of a journal bearing.
Length to diameter ratio
The length to diameter ratio is one of the first things a bearing designer considers. This factor is tuned to give a sufficiently steady and alternating load capacity.An L/D ratio less than 0.3 have
poor damping and an L/D ratio greater than 0.75 usually shows a little gain in the effective damping. Heavily loaded bearings such as gears often exceed an L/D ratio of 1.0 and may need special care in aligning the bores to the shafting.
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| FIG 21 |
Bearing Pressure
The pressure over the projected area in the bearing is assumed uniform for the purpose of expressing the load capacity.
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| FIG 22 |
In steadily loaded bearings, the load is mainly due to the weight of the rotating element, in addition to the dynamic loading imparted by the unbalance in the rotating components. The bearing pressure is given by the Equation :
Where,
P – Bearing pressure in N/mm²
L- Length of the bearing in mm
D- Diameter of the bearing in mm
Bearing clearance
Bearing clearance is one of the most important parameters in the operation of a bearing. Therefore, it is important to determine the installed clearance along with the bore contour and concentricity to the outside fit diameter.
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| FIG 23 |
The final bearing clearance is influenced by the contour of the housing into which it is installed and also by the amount of interference or crush between the bearing shell and the support housing. Proper crush is crucial in the operation of high speed and critical machinery. Improper crush can lead to either a hot bearing or a loose bearing fit. A loose bearing may contribute to a synchronous or a sub synchronous type of vibration. It is critical that a metal to metal fit to 0.002 inches of crush on diameter be maintained for a proper bearing installation. This criterion applies to most thick shell bearing and linings. For thin shell bearings, and in situations where the rigidity of the housing is much greater than that of the bearing liner, the values for the crush should be arrived at by referring with the bearing and equipment manufacturer. Excessive crush or a contact stress could cause local yielding of the material. This might lead to the collapse of the liner and the loss of bearing clearance.
Sommerfield number
Dynamic stiffness and damping coefficients are generally presented as a function of dimensionless parameter known as the Sommerfield number.
The more common way of expressing is by the Equation
Where,
μ is the viscosity in (pa.s)
N is the running speed in revolutions/second
L is the pad length in (inches,mm)
R is the journal radius in( inches ,mm)
W is the bearing load in pounds
C is the pad bore radial clearance in (inches.mm)
Higher loads and lower speeds will result in a lower Sommerfield number for a given bearing configuration. On the other hand, lighter loads and higher speeds result in a higher Sommerfield number.
Materials Used for sliding contact bearings
1. Metallic bearings
. Metallic bearings
. Babbitt metal
. Bronze – Alloy of copper, tin and zinc
. Cast iron
. Silver
2. Non metallic bearings
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