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TECHNICAL STUDIES & RESEARCH CENTER

MECHANICAL TECHNOLOGY BRANCHMECHATRONICS GRADUATE PROGRAM

MACHINE ELEMENT, COURSE FALL 2008-09

PRESENTATION ON JOURNAL BEARINGSPrepared by :FOUZI K. SHTAWI

: SEFEDDIN A. ALFURJANI

Supervised by: Dr. HUSSAM EDDIN ASHIKH

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What is the bearings?

Bearing is a member designed to support a load while permitting

relative motion between two elements of a machine.

The concept behind a bearing is very simple: Things roll better than they

slide. The wheels on your car are like big bearings. If you had

something like skis instead of wheels, your car would be a lot more

difficult to push down the road. That is because when things slide, the

friction between them causes a force that tends to slow them down. But

if the two surfaces can roll over each other, the friction is greatly

reduced.

12/2008 2journal bearings

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The Timken CompanyPhoto courtesy

Roller thrust bearing

http://www.howstuffworks.com/framed.htm?parent=bearing.htm&url=http://www.timken.comhttp://www.howstuffworks.com/framed.htm?parent=bearing.htm&url=http://www.timken.com

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The bearings were classified to many ways depending on the type

of the load and the type of contact as the followings:

1) Depending upon the direction of the load to be supported.

The bearings are classified to:

a) Radial Bearings, in the radial bearings the load acts

perpendicular to the direction of the motion of the movingelement.

b) Thrust Bearings in this type the load acts along the of the rotation axis.12/2008 4journal bearings

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2) Depending of the nature of contact , and the bearing herewere classified as:

a) Sliding contact bearing, the sliding takes place along thesurfaces of the contact between the moving element and thefixed element, and known as Journal Bearing ( our case study) .

b) Rolling contact bearings, the steel balls or the rollers areinterposed between the moving and fixed elements.


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After the brief of the introduction of the bearing, we would like totalk about journal bearing, What is the journal bearing ?

A journal bearing is a simple bearing in which a shaft, or

"journal", rotates in the bearing with a layer of oil or greaseseparating the two parts through fluid dynamic effects. The shaftand bearing are generally both simple polished cylinders withlubricant filling the gap. Rather than the lubricant just "reducingfriction" between the surfaces, letting one surface slide more

easily against the other, the lubricant is thick enough that oncerotating, the surfaces do not come in contact at all. If oil is used,it is generally fed into a hole in the bearing under high pressure.

In the other hand, The Journal is the part of a shaft that rotatesinside the bearing.


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Types of the sliding (journal) bearings:

The journal bearing classified into three types according to theangle of the contact between the moving element (the journal)

and the fixed element (bearing) as the follows:a) Full journal bearings:

when the angle of contact of the bearing with the journal is360.

b) Partial journal bearings:

when the angle of the contact of the bearing with thejournal is 120 but the diameter of the journal not equal of thebearing diameter.


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c) Fitted journal bearings:

when the angle of the contact of the bearing with the journal is120 but the diameter of the journal is equal to the bearing

diameter.


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In addition the journal bearings may be classified also accordingto the thickness layer of the lubricant as:

1) Thick film bearings. The two surface are completely separatedby the lubricant and the called hydrodynamic lubricatedbearings.

2) Thin film bearings. The two surface are partially contact eachother at least part time and this called boundary lubricated

bearings

3) Zero film bearings. No lubricant between the surfaces.


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Properties of Sliding Contact Bearings:The material of the bearings should have sufficient strength and rigidity ,

however, the conditions under which bearing must be operate in

service are generally far from ideal and thus the other properties mustbe considered in selecting the best materials as below:

1. Compressive strength : the bearing material should havecompressive strength to withstand the maximum pressure so asto prevent extrusion or other permanent deformation of bearing.

2. Fatigue strength: the bearing material should have sufficientfatigue strength to withstand repeated loads withoutdevelopment surface fatigue cracks .


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3. Conformability : it is the ability of the bearing material toaccommodate shaft deflections and bearing inaccuracies by plasticdeformation without excessive wear or heating.

4. Embeddability: it is the ability of the bearing material toaccommodate the small particles of dust, grit etc, without scoring thematerial of the journal.

5. Bondability: many high capacity bearing are made by bonding one ormore thin layer of bearing material to high strength steel shell.

6. Corrosion resistance: the bearing should not corrode away underthe action of the lubricating oil.

7. Thermal conductivity: the bearing material should be of highthermal conductivity to permit the rapid removal of the heat generatedby friction.


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8. Thermal expansion: the bearing material should be of lowcoefficient of thermal expansion, so that to prevent undue change in the

clearances.


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ApplicationsComposition

MaterialS.NphosphorzincTinantimonyleadcopper

Common use for thebearing

90%5%0.5%4.5%Tin base babbit1

6%9.5%84%0.5%lead basebabbit

2

For high grade bearingFor high pressure andhigh speed bearing

2%10%--88%BronzeGun metal

3

For high pressure andhigh speed bearing

1%10%-9%80%Phosphorbronze

4

For low pressure andlow speed bearing

Cast iron5

For aircraft enginesSilver6

For low pressure andlow speed bearing

Nonmetallic

7


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Lubricants

What is the lubricant?

As , we know that adding a lubricant to a solid-solid contact will

significantly reduce friction. The reduced friction leads to lesswear, heat generation and energy loss all of which reduceoperation costs and downtime. (the lubricant is the thick or thinof the film of oil or grease etc, this lubricant works as supportingthe journal during the rotation ,The lubricants are used in bearings

to reduce friction between the rubbing surfaces and to carry away theheat generated by friction. It also protects the bearing againstcorrosion. All lubricants are classified into the following three groups:

1. Liquid 2. Semi liquid and 3. Solid


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The liquid lubricants usually used in the bearings are mineraloils and synthetic oils. the mineral oils are most commonly used

because of their cheapness and stability, the liquid lubricantsare usually preferred where they may be retained.

A grease is a semi-liquid lubricant having higher viscosity thanoils. The greases are employed where slow speed and heavypressure exist and where oil drip from the bearing is

undesirable.

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The solid lubricants are useful in reducing friction where oil films cannot bemaintained because of pressures or temperatures. They should besofter than materials being lubricated. A graphite is the most commonof the solid lubricants either alone or mixed with oil or grease.


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There are some important factor should be taken in to consideration for thelubricant properties as:

1. Viscosity. It is the measure of the degree of the fluidity of a liquid ofwhich an oil is able to form, retain and offer resistance to shearing abuffer film under heat and pressure. The greater the heat andpressure, the greater viscosity is required of a lubricant to preventthinning and squeezing out of the film.

2. Oiliness. It is a joint property of the lubricant and the bearing surfacesin contact. It is a measure of lubricating qualities under boundary

conditions. There is no absolute measure of oiliness.


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3. Density. This property has no relation to lubricating value, butuseful in changing the absolute viscosity to kinematic viscosity.

The density of most of the oils at 15.5C varies from 860 to 950

kg/m. also the density can be calculated in any value otemperature ( By formula).

4. Viscosity index. The term of viscosity index is used to denotethe degree of variation of viscosity with temperature.


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Terms used in Hydrodynamic Journal Bearing.

The following terms used in hydrodynamic journal bearing areimportant from the subject of point of view:

1. Diametral clearance it is the difference between the diametersof the bearing and the journal ( c= D- d).

2. Radial clearance : it is the difference between radii of thebearing and the journal ( c/2).

3. Diametral clearance ratio: it is the ration of the diametral

clearance and the journal diameter (c/d).

4. Eccentricity. It is the radial distance between the center of thebearing and the displaced center of the bearing under load.


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5. Minimum oil film thickness. It is the min distance between thebearing and the journal under complete lubrication condition( c/4).


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Bearing Characteristic Number and Bearing Modulus forjournal bearing:

Where is the BCN= ZN/p

BM= min bearing modulus (K). Z= viscosity of the lubricant.

N= speed of journal in r.p.m.

p = pressure on the projected area of the bearing.

The coefficient of friction in design of bearings is great importance,because it affords a means for determining the loss of powerdue to bearing friction.


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Critical pressure of the journal bearing.

The pressure at which the oil film breaks down so that metal tometal contact being is known as critical pressure or the mi

operating pressure of the bearing it can be obtained by thefollowing formula:

p = ZN/4.75x10Ex 6 {d/c}{1/(d+l)} N/mm

Heat Generated in Journal Bearing

The heat generated in a bearing is due to the fluid friction andfriction of the part having relative motion. and can be obtainedby:

Qg =.W.V N-m/s or J/s or watts


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Advantages Journal bearings or Plain surface bearings

Run quietly

Have a longer life span

Are less sensitive to contamination Are less costly

Can better sustain shock loads

Requires less precise mounting

Are available in split halves


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Summary.

Journal bearing is a sliding contact bearing.

Friction is always present even with lubrication.

Bearings fail because of fatigue. Load capacity of a bearing canbe calculated.

Lubrication in bearings serve a number of functions such asdissipating heat, prevent corrosion and forming a film betweenmoving element and the raceway.

Thrust bearing absorb axial loads.

Self aligned bearing reduce shaft bearing wear.


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References :1. A text book of Machine design by R.S. KHURMI and

J.K. GUPTA.http://www.pg.gda.pl/~mwasilcz/files/test_rig.pdf.2

.htm3http://science.howstuffworks.com/bearing.3

http://www.pg.gda.pl/~mwasilcz/files/test_rig.pdfhttp://www.pg.gda.pl/~mwasilcz/files/test_rig.pdfhttp://www.pg.gda.pl/~mwasilcz/files/test_rig.pdfhttp://www.pg.gda.pl/~mwasilcz/files/test_rig.pdfhttp://science.howstuffworks.com/bearing3.htmhttp://science.howstuffworks.com/bearing3.htmhttp://science.howstuffworks.com/bearing3.htmhttp://science.howstuffworks.com/bearing3.htmhttp://science.howstuffworks.com/bearing3.htmhttp://science.howstuffworks.com/bearing3.htmhttp://www.pg.gda.pl/~mwasilcz/files/test_rig.pdfhttp://www.pg.gda.pl/~mwasilcz/files/test_rig.pdfhttp://www.pg.gda.pl/~mwasilcz/files/test_rig.pdfhttp://www.pg.gda.pl/~mwasilcz/files/test_rig.pdfhttp://www.pg.gda.pl/~mwasilcz/files/test_rig.pdfhttp://www.pg.gda.pl/~mwasilcz/files/test_rig.pdf

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THE END


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11621189 journal bearing 2

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