Journal Bearing

TECHNICAL STUDIES & RESEARCH CENTER MECHANICAL TECHNOLOGY BRANCH MECHATRONICS GRADUATE PROGRAM MACHINE ELEMENT, COURSE FALL 2008-09

PRESENTATION ON JOURNAL BEARINGS Prepared by :FOUZI K. SHTAWI SEFEDDIN A. ALFURJANI : Supervised by: Dr. HUSSAM EDDIN ASHIKH

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 12/2008

other, the friction is greatly journal bearings reduced

2

Photo courtesyThe Timken Company Roller thrust bearing

•

12/2008

journal bearings

3

The bearings were classified to many ways depending on the type of the load and the type of contact as the :followings Depending upon the direction of the load to be )1 :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 .moving element b)Thrust 12/2008

Bearings in this type load acts along the of the journalthe bearings

.rotation axis

4

12/2008

journal bearings

5

Depending of the nature of contact , and the bearing here were classified as: • Sliding contact bearing, the sliding takes place along the surfaces of the contact between the moving element and the fixed element, and known as Journal Bearing ( our case study) . • Rolling contact bearings, the steel balls or the rollers are interposed between the moving and fixed elements. 2)

12/2008

journal bearings

6

After the brief of the introduction of the bearing, we would like to talk 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 grease separating the two parts through fluid dynamic effects. The shaft and bearing are generally both simple polished cylinders with lubricant filling the gap. Rather than the lubricant just "reducing friction" between the surfaces, letting one surface slide more easily against the other, the lubricant is thick enough that once rotating, 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. journal bearings 12/2008 7 In the other hand, The Journal is the part of a shaft that

12/2008

journal bearings

8

Types of the sliding )journal) bearings: The journal bearing classified into three types according to the angle of the contact between the moving element (the journal) and the fixed element (bearing) as the follows: c) Full journal bearings: when the angle of contact of the bearing with the journal is 360˚. b) Partial journal bearings: when the angle of the contact of the bearing with the journal is 120˚ but the diameter of the journal not equal of the bearing diameter. 12/2008

journal bearings

9

c) Fitted journal bearings: when the angle of the contact of the bearing with the journal is 120˚ but the diameter of the journal is equal to the bearing diameter.

12/2008

journal bearings

10

In addition the journal bearings may be classified also according to the thickness layer of the lubricant as: 1) Thick film bearings. The two surface are completely separated by the lubricant and the called hydrodynamic lubricated bearings. 2) Thin film bearings. The two surface are partially contact each other at least part time and this called boundary lubricated bearings 3) Zero film bearings. No lubricant between the surfaces.

12/2008

journal bearings

11

• Properties Bearings:

of

Sliding

Contact

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 must be considered in selecting the best materials as below:

•

Compressive strength : the bearing material should have compressive strength to withstand the maximum pressure so as to prevent extrusion or other permanent deformation of bearing. • Fatigue strength: the bearing material should have sufficient fatigue strength to withstand repeated loads 12/2008 journal bearings 12 without development surface fatigue cracks .

3. Conformability : it is the ability of the bearing material to accommodate shaft deflections and bearing inaccuracies by plastic deformation without excessive wear or heating.

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

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

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

• Thermal conductivity: the bearing material should be of high thermal conductivity to permit the rapid removal of the heat generated by friction.

12/2008

journal bearings

13

8. Thermal expansion:

the bearing material should be of low coefficient of thermal expansion, so that to prevent undue change in the clearances.

12/2008

journal bearings

14

S.N 1 2

Material Tin base babbit lead base babbit

Composition copper 4.5% 0.5%

lead

antimony

0.5% 84%

Tin

5% 9.5%

phosphor

Applications

90% Common use for the bearing

6%

3

Bronze Gun metal

88%

-

-

10%

4

Phosphor bronze

80%

9%

-

10%

5

Cast iron

6

Silver

7

Non metallic

12/2008

zinc

For high grade bearing For high pressure and high speed bearing

2%

1%

For high pressure and high speed bearing For low pressure and low speed bearing For aircraft engines For low pressure and low speed bearing

journal bearings

15

• 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 less wear, heat generation and energy loss – all of which reduce operation costs and downtime. (the lubricant is the thick or thin of the film of oil or grease etc, this lubricant works as supporting the journal during the rotation , The lubricants are used in bearings to reduce friction between the rubbing surfaces and to carry away the heat generated by friction. It also protects the bearing against corrosion. All lubricants are classified into the following three groups: 4. Liquid 2. Semi liquid and 3. Solid 12/2008 journal–bearings 16

• The liquid lubricants usually used in the bearings are mineral oils and synthetic oils. the mineral oils are most commonly used because of their cheapness and stability, the liquid lubricants are usually preferred where they may be retained. • A grease is a semi-liquid lubricant having higher viscosity than oils. The greases are employed where slow speed and heavy pressure exist and where oil drip from the bearing is undesirable.

12/2008

journal bearings

17

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

12/2008

journal bearings

18

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

•

•

Viscosity . It is the measure of the degree of the fluidity of a liquid of which an oil is able to form, retain and offer resistance to shearing a buffer film under heat and pressure. The greater the heat and pressure, the greater viscosity is required of a lubricant to prevent thinning and squeezing out of the film. Oiliness. It is a joint property of the lubricant and the bearing surfaces in contact. It is a measure of lubricating qualities under boundary conditions. There is no absolute measure of oiliness.

12/2008

journal bearings

19

3. Density. This property has no relation to lubricating value, but useful in changing the absolute viscosity to kinematic viscosity. The density of most of the oils at 15.5ºC varies from 860 to 950 kg/m³. also the density can be calculated in any value o temperature ( By formula). 4. Viscosity index. The term of viscosity index is used to denote the degree of variation of viscosity with temperature.

12/2008

journal bearings

20

12/2008

journal bearings

21

Terms used in Hydrodynamic Journal Bearing. The following terms used in hydrodynamic journal bearing are important from the subject of point of view: 3. Diametral clearance it is the difference between the diameters of the bearing and the journal ( c= D- d). 4. Radial clearance : it is the difference between radii of the bearing and the journal ( c/2). 5. Diametral clearance ratio: it is the ration of the diametral clearance and the journal diameter (c/d). 6. Eccentricity. It is the radial distance between the center of the bearing and the displaced center of the bearing 12/2008 journal bearings 22 under load.

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

12/2008

journal bearings

23

Bearing Characteristic Number and Bearing Modulus for journal 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 power due to bearing friction. 12/2008

journal bearings

24

Coefficient of friction for Journal Bearings In order to determine the coefficient of friction for well lubricated full journal bearing the following formula can be used: Coefficient of friction , µ = 33/10Ex 8 {ZN/p}{d/c} + k Where is • µ = Coefficient of friction • Z = Absolute viscosity of the lubricant (in kg). • N = speed of the journal (r.p.m). • P = Bearing pressure on the projected bearing area(N/mm²). • d = diameter o the journal • c = diametral clearance. •12/2008 k = factor of correct for end journal leakage. bearings

25

• Critical pressure of the journal bearing. The pressure at which the oil film breaks down so that metal to metal contact being is known as critical pressure or the mi operating pressure of the bearing it can be obtained by the following formula: p = ZN/4.75x10Ex 6 {d/c}²{1/(d+l)} N/mm² • Heat Generated in Journal Bearing The heat generated in a bearing is due to the fluid friction and friction of the part having relative motion. and can be obtained by: Qg = µ.W.V N-m/s or J/s or watts 12/2008

journal bearings

26

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

12/2008

journal bearings

27

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 can be calculated. • Lubrication in bearings serve a number of functions such as dissipating heat, prevent corrosion and forming a film between moving element and the raceway. • Thrust bearing absorb axial loads. • Self aligned bearing reduce shaft bearing wear.

12/2008

journal bearings

28

References :

2. A text book of Machine design by R.S. KHURMI and J. .2//:http • www.pg.gda.pl/~mwasilcz/files/test_rig.pdf .3 • http://science.howstuffworks.com/bearing3.htm

12/2008

journal bearings

29

THE END THANK YOU FOR YOUR ATTENTION. QUESTIONS??? 12/2008

journal bearings

30