A 

 PROJECT REPORT ON   

DESIGN OF TWO-LOBE BEARING      FOR  TURBOGENERATORBy

  Mr. B . SIVA PRADEEP

BHEL - AN OVER VIEWBHEL is the largest engineering and manufacturing enterprise in India in the energy related in fracture sector today. BHEL was established more than 40 years ago ,when its first plant was set up in Bhopal ushering in the indigenous Heavy Electrical Equipment industry in India, a dream that has been more than realized with a well-recognized tract record of performance. It has been earning profits continuously since 1971-72 and achieved a sales turnover of Rs. 7482.3 crores with a profit before tax of Rs. 802.4 crores in 2002 – 2003.

BHEL caters to core sectors of the Indian Economy Viz., Power

Generation & Transmission, Industry, Transportation, Telecommunication, Renewable Energy, Defence etc., The wide network of BHEL’S 14 manufacturing divisions, four Power Sector regional centers, eight service centers and 18 regional offices and a large number of project sites spread all over India and abroad enables the company to promptly serve its customers and provide them with suitable products, systems and services efficiently and at competitive prices. BHEL has already attained ISO 9000 and all the major units/divisions of BHEL have been upgraded to the latest ISO-9001: 2000 version quality standard version quality standard certification for quality management.

B.H.E.L….HYDERABADCurrently there are nine important products

being manufactured at R.C.PURAM.1)Gas turbines 8)heat

exchangers2)Steam turbines 9)oil filled

equipment.3)Compressors4)generators and exciters5)pumps6)pulverizers7)switch gears*Bearings for all these products are also

designed and manufactured by BHEL.

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Bearings and their necessity.. Bearings are used to support a rotating shaft in a machine. The

relative motion between the rotating shaft (journal) & stationary bearing generates frictional losses, which are converted into heat energy leading to the temperature rise of the bearing.

  The most important objectives of a bearing design is to reduce

frictional losses, insure stable running of the rotor system in the extreme and normal condition ensuring long life of the bearing, and particularly no wear to the rotating journal etc.

  There are different types of bearings used for different speeds

and applications. A quick review of various type bearings and their field of application is made. But the main focus of this project work is on journal bearings.

  As it is possible to reduce the weight of the machines, by

increasing the speeds, it is not a common practice to go in for high-speed machines. These high-speed machines naturally need suitable bearings to withstand high dynamic faces and seven operating condition like misalignment, axial thrust etc. Lubrication also plays an important role in the bearing design and application. Hence lubrication aspects have also presented in this project work.

 

TYPES OF BEARINGSBased on the principle of operation, the

bearings are divided into two main categories. These are:Anti-friction bearings(rolling element bearings)Sliding contact bearings Anti-friction bearings are further divided

into:i) Ball bearing ii) Cylindrical roller bearingsiii) Taper roller bearingsiv) Spherical roller bearingsv) Needle roller bearings

TYPES OF BEARINGSSliding contacts bearings are further divided into: i) Journal bearing ii) Thrust bearing  Journal bearings are further sub divided into (i) Cylindrical journal bearings(ii) Two-lobe journal bearings (Elliptical bearings)(iii) Three-lobe journal bearings (iv) Pivoted (Tilting) pad journal bearings Thrust bearings are sub divided into(i) Flat-land thrust bearings(ii) Taper-land thrust bearings(iii) Pivoted shoe thrust bearings(iv) Spring mounted flexible plate thrust bearings

Types of bearings .

DESIGN PROCEDURE INTRODUCTION TO DESIGN PROCEDURE:  The primary requirements of good bearing design

are: (i) The specific bearing pressure shall be with in

limits.(ii) The operating temperature of the bearing shall

be well below the safe limits of the material used(Babbitt plasticity temperature limit). This means that sufficient oil flow must be there to take away the heat developed by the frictional losses and keep the bearing cool.

(iii) The stiffness of the bearing oil film (together with the support and foundation stiffness) shall be such that, the natural frequency of the rotor bearing system is away from the operating speed.

TWO LOBE BEARINGS AND THEIR DESIGN

ROTOR SHAFT

TWO LOBE BEARING FOR ROTOR SHAFT TO COUPLE WITH TURBINE.

1 ) SPECIFIC BEARING PRESSURE . This is the static load per unit area of the projected surface of the

bearing   P = (Wb) / (L x D) Kg/cm2 …………………………………………...........2.1 Where, Wb= is the load on the bearing in kg L = is the length of the bearing in cm D = is the diameter of the bearing in cm If specific bearing pressure is too high, the oil film will be very thin

and there may be a chance of occasional metal to metal contact. If the specific pressure is too low, the oil film will be very thick and the rotor will be just floating on the oil film. The rotor will not be stable and for any small disturbance, the rotor becomes unstable. More over lightly loaded condition leads to a phenomenon called HALF FREQUENCY WHRIL.

  For journal bearings, the guide line for the limits of specific bearing pressure

is about 8-25 kg/cm2

If the specific bearing pressure is less than 5 kg/cm2, the bearing will be unstable.

 

2) L/D RATIOGenerally in journal bearings, the L/D ratio of

bearing is maintained in the range of 0.6-1.0. For bearings in the speed range about 3000 rpm, generally L/D ratio is taken as 0.8.

Peripheral velocity: The peripheral velocity of the journal

in the bearing is given by the equation u = (3.14 x D’ x n)/60

meters/sec2where,D’ is the diameter of the journal in meters.N is the speed in rpm.

3 BEARING CLEARANCES AND CLEARANCE RATIO:

(i)Cylindrical bearing  Diametrical clearance = Bearing inner diameter - journal diameter Cd = Db - Dj

  In cylindrical bearings, the diametrical clearance is generally

maintained 0.15-0.2% of the journal diameter. (ii)Elliptical bearings (two-lobe bearings) In this case, the horizontal diameter of the bearing Dh is more than the

vertical diameter of the bearing Dv. Generally the following values are maintained.

  Vertical clearance = Cv = Dv – Dj

Horizontal clearance = Ch = Dh – Dj

  From practical point of view (measurement point of view), half the value

of horizontal clearance is called SIDE CLEARANCE. During assembly of the bearing and during inspections, when the journal is stationary, the journal will be resting in the bearing The bearing clearances are measured as TOP and SIDE CLEARANCES and compared with the design values

4 ECCENTRICITY AND ECCENTRICITY RATIO:Eccentricity is the distance between the bearing center and the

journal center. The eccentricity depends on the load on the bearing, viscosity and peripheral velocity of the journal. This is denoted by e.

  ECCENTRICITY RATIO: It is a dimensionless quantity, which is the ratio of eccentricity to

the radial clearance.   N = ((e)/(c/2)) = ((2e)/c)  ELLIPTICITY RATIO:   (Horizontal diameter -Vertical diameter)/ Horizontal diameter

CLEARANCE RATIOThis is the ratio of diametrical clearance to the bearing diameter. This is a non dimensional quantity. m =Cd/D.This acts as a guide line for selecting the clearance for different journal diameters of any particular type of bearing

5) FRICTIONAL LOSSES IN A BEARING:The frictional losses in a bearing are given by P = {frictional force on the rotor} x {distance

traveled by the rotor journal in one sec} Frictional force on the rotor = u x Wb

 WhereØ u =coefficient of friction of hydrodynamic

filmØ Wb =load on the bearing in kgØ D’ = diameter of the journal in meters.Ø N’ = Speed of the journal in rps

6) Quantity of oil flow requiredThe quantity of oil flow required shall be such that,

the oil should be adequate to carry away the frictional heat generated in the bearing and keep the bearing temperature with in the limits.Maximum allowable temperature for Babbitt is 120ºC. In the process of cooling the bearing, the temperature of oil increases. The rate of flow is dependent on

  (i) Heat generated due to frictional losses. (ii) The specific heat of oil (iii) The temp rise allowed for the oil

Flow rate requirement of lubricating oil =P/(r x n x )…Where,

P -Frictional losses in KWr -Density of oil on gm/cm2n -Specific heat of oil -Temperature rise allowed for oil.

7)BEARING  STIFFNESS AND DAMPING COEFFICIENTS:In actual practice the journal is supported on an oil

film of the bearing, and the bearing itself is supported bearing pedestal (or simply called the support) which in turn rest on the foundation. So in addition to the oil film stiffness, there are stiffness of support and stiffness of foundation in series.

The rotor natural frequency (critical speed) is

influenced by the combined stiffness of the oil film + support + foundation.

  The representation of the rotor bearing system

as mass, spring and damper elements .

BABBITTING OF BEARING SHELLS:Babbitt metal is most commonly used as a thin surface layer in a

complex, multi-metal structure. Babbitt metal is soft and easily damaged, which suggests that it might be unsuitable for a bearing surface.

However, its structure is made up of small hard crystals dispersed in a softer metal, which makes it a metal matrix composite. As the bearing wears, the softer metal erodes somewhat, which creates paths for lubricant between the hard high spots that provide the actual bearing surface.

When tin is used as the softer metal, friction causes the tin to melt and function as a lubricant, which protects the bearing from wear when other lubricants are absent.

There are many Babbitt alloys in addition to Babbitt's original. Some common compositions are:

90% tin, 10% copper89% tin, 7% antimony, 4% copper80% lead, 15% antimony, 5% tin76% copper, 24% lead75% lead, 10% tin67% copper, 28% tin, 5% lead

PROBLEMIn comparison with most other types of engineering design

work, the design procedure for bearings is unusual in that the theory cannot be used to produce a design. The reason for this that bearing theory and practice are, in a sense, diametrically opposed to each other.

In bearings the rotor rotates continuously thus generating a lot of heat. We circulate cooling oil in a way to dissipate heat. It is one of the vulnerable practical problems in oil ingress. Primary requirement is to avoid leakage of oil and thus reduce the loss of oil and improve the aesthetics of the machine.

 Oil leakage occurs due to (i). Pressure variation:Due to rotation of the shaft, the heat is developed and

vaporizes the lubricating oil. Thus vapor pressure is developed and this vapor pressure is greater, that outside atmospheric pressure causing oil ingress.

 (ii). Surface tension: The liquid surface exerts tension upon the adjacent portions of

the surface with which they are in contact. Oil due to the surface tension is carried away along with the rotor.

REMEDY To reduce the vapor pressure developed, two holes are

provided on either side of the pedestal. For any substance, generally the flow takes place from the high-pressure side to the low-pressure side. So the vapor forces the oil outside.

To avoid this leakage, we provide a sealing ring in addition to the knife edge scrapper fins that are available with a number of pins. If there is a contact between the rotor and the sealing rings, vibrations are produced. In order to avoid this small clearances of the order of few microns are provided between the and the rings. But some oil is found to leak through the clearances provided.

` Initially a plastic sealing is provided on the pedestal. This results in vibrations. Then sealing with felt is provided. Felt has very less durability i.e. Worn out and influences the insulation. A suction pipe is provided on both sides, into the holes and vapor pressure is sucked out. The vapor pressure is then completely removed.

.Another improvement in the above

arrangement is to provide a annular groove in the sealing ring. A tapping of high-pressured air from open fan zone have been extracted and connected to the annular ring thus providing circular air sealing.

This sealing proved to be effective and around 95% of oil entering was slotted.

Thank you!

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