Journal Bearings Lecture0

8/13/2019 Journal Bearings Lecture0

1/84

Chapter 12

Lubrication &Journal bearings

(7(7 -- 8 Lectures)8 Lectures)


2/84

TOPICS

1. Definitions and Objectives 2. Types of Lubrication3. Dynamic Viscosity

4. Bearing Characteristic Number5. Stable & Unstable Lubrication6. Hydrodynamic Lubrication

7. Design Considerations QUIZ48. Heat Balance-Self-Contained Bearings9. Clearance

10. Pressure-Fed Bearings11. Loads and Materials12 Boundary lubrication

13. Types of Journal Bearings


3/84

Definitions and objectives

The role of a bearing is to provide relative positioning and rotational freedom while transmittinga load between a shaft and a housing.

There are two general types of bearings:1. Rolling-contact bearings (anti-friction bearings, rolling

bearings). In the rolling-contact bearings the load istransmitted by rolling rather than by sliding.

2. Journal Bearings (plain bearings, bushings, sleeve bearings). In journal bearings, the load is transmitted bysliding and the problem of this class of bearings isessentially a lubrication problem .


4/84

Definitions and objectives

Journal Bearings : cylindrical or semi-cylindrical bushing made of a suitable material .

The Journal is the part of shaft or pin in bearing

Among applications :1. High speed, high temperature, high varying loads:

Automotive engines: connecting rod, crankshaft, Metal alloys Turbo machinery: Metal alloys

2. Light loads, low speeds with little or no lubrication: Nylon, Teflon, rubber


5/84

Types of Lubrication

1. Hydrodynamic Lubrication(HDL)(a) Full, thick Fluid film lubrication - surfaces

separated by bulk lubricant film; Filmconditions required for lubrication.

2. Boundary (Thin Film)Lubrication(b) partial lubrication ( mixed) - both bulklubricant and boundary film play a role; (c)

boundary lubrication - performance dependsessentially on boundary film


6/84

3. HydrostaticLow speed, light load

4. Elastohydrodynamic

For rolling contact(gears, rolling bearings)

5. Solid FilmExtreme Temperatures

(Graphite or Molybdenum disulfide))

Types of Lubrication


7/84

Viscosity(HDL)Shear Stress

dydu

AF

==

is absolute or dynamic viscosity(lbf.s/in 2 or reyn . In ips system and

Pa.s in SI system)

du/dy is the rate of shear or velocitygradient

If rate of shear is constant: du/dy = U/hWith h= c (clearance)

c=

Fig. 12.1

cU

hU

AF

===


8/84

Viscosity vs. temperature

In general, Viscosity decreases with temperature increase.The increase in temperature comes from friction


9/84

Petroffs Law

=

===

==

==

==

= cr

P N

f s f

f

s

s

T T

rlW

Pr lPr f fWr T

clN r

lr T

rl Ar AT

crN

cU

22

2;)2(

4)(2

2;)(

2

322

Bearing Characteristic Number (Sommerfeld Number )

S cr f

c

r

P

N S

22

2

=

=

Petroff used a concentric shaft to define a group of dimensionless parametersThat allow the prediction of an acceptable coefficient of friction .

r/c = clearance ratio

Shear torque in lubricant

Friction torque

S bl d bl l b i i


10/84

0.08-0.14For steel onBronze

a

bc

12

1

2 f = 0.001-0.005Similar to precision BB

Stable and unstable lubricationThe McKee Brothers Plot

(a) Full, thick Fluid filmlubrication - surfacesseparated by bulk lubricantfilm; Film conditions requiredfor lubrication.

(b) partial lubrication ( mixed) - both bulk lubricant and boundary film play a role;

(c) boundary lubrication -

performance dependsessentially on boundary film

Boundary lubrication should be expected for slow speeds:

U


11/84

Hydrodynamic Lubrication (HDL)

For lubricated bearingthe minimum film

thickness h 0 occurs tothe left of load line because the shaft is

pushed by the pressure build up on the right.The shaft is playing therole of a pump.


12/84

Hydrodynamic Lubrication (HDL)Nomenclature

Fig. 12.6

1. e: eccentricity

2. h 0 minimum film thickness3. = e/c = eccentricity

ratio

4. bearing angular length

H d d i L b i ti (HDL) Th


13/84

Tower investigated bath-type lubrication in 157

partial bearing. He was ableto determine the pressuredistribution in oil film inaxial and radial directions.

Reynolds used Towersfindings to propose arelationship betweenfriction, pressure andvelocity. His work is given

under mathematical form inthe following.

Hydrodynamic Lubrication (HDL)-TheoryB. Tower


14/84

Hydrodynamic Lubrication (HDL): theoryO. Reynolds

Assuming pressure varies in x-direction only (no leakage)

Assuming Velocity varies in x & y directions

)1(

0

ydxdp

dxdzdx y

dxdz pdydzdydzdxdxdp pFx

=

=+++=


15/84

Hydrodynamic Lubrication (HDL): theory

Assuming Newtonian viscous fluid + u = u(x,y)

Assuming Constant viscosity and substituting Eq. (1) into (2):

Integrating (3) twice (holding x constant):

Assuming no slip at boundaries :

)2( yu

=

)3(12

2

2

2

dxdp

yuor

yu

dxdp

=

=

)4(1 2122

C yC ydxdpu ++=

)5(@

00@0

1

2

=== 2

===dphU

C h yU u

C yu

dxh


16/84

The velocity distribution in film is:

Flow rate/unit width:

Incompressible flow:

The above is the Reynolds Eq . For one-dimensional flow.

Considering Leakage (2-D):

Hydrodynamic Lubrication (HDL): theory

( ) )6(21 2 yhU hy yu dxdp +=

)8(0 adxQd

=

)1112(633

=+

dx

dhU z

ph

z x

ph

x

)7(122

3

0 dxdphUh

udyh

Q ==

)1012(63

=

dx

dhU

dx

dph

dx

d


17/84

Hydrodynamic Lubrication (HDL):Theory

There are no general analytical solutions tothe 2-D Reynolds Equation.

The Summerfeld Solution to Eq. 12-11

)1212(2

= P N cr f cr

S


18/84

Design ConsiderationsTwo groups of variables in the design of sliding bearings (eq:12.12)

A- The independent variables :1. The viscosity ,2. The load per unit of projected bearing area, P (unit Load)

3. The speed N= N J (see equation 12-13)4. The bearing dimensions r, c, and l ( important l/d )B- The dependent Variables or performance factors :

1. The coefficient of friction f 2. The temperature rise T3. The volume flow rate of oil Q

4. The minimum film thickness ho

The first (A) are somewhat under designer control and the second (B) are not. Certainlimitations should be set for variables B and A varied to satisfy them

)1212(2

=P N

cr f

cr

General Design Criteria for Journal Bearings


19/84

General Design Criteria for Journal Bearings( See Lab Manual for details )

1. The value of the important parameter l/d is taken between 0.25 and 1.5 . Values up to 2 and 3 were usedin earlier designs. Nowadays, the value of l/d is

confined between 0.25 and 0.75 . Short bearings are preferred when shaft deflections and misalignments areexpected.

2. The nominal value of clearance ratio r/c can be takenapproximately as:

1000 for precision bearings when 25< d


20/84

General Design Criteria for Journal Bearings

( See Lab Manual for details )

3. The minimum film thickness h0 can be estimated fromTrumplers design criteria :

or

4. The outlet temperature of the oil should be kept below250 F (121 C). A value around 70 C (160 F) is usuallyspecified as the average operating temperature.

5. Starting unit load P st=W st /ld is kept below 300 psi6. Design factor on starting load should be at least 2.

)(0004.0005.0)(00004.00002.0

0

0

mmd h

ind h

+ +)(00025.00 ind h =

Relationship bet een ariables


21/84

Relationship between variables

Viscosity ChartsIn IPS units



22/84


Viscosity ChartsIn SI Units



23/84


Viscosity Charts



24/84


Minimum Film Thickness &Eccentricity ratio Chart

Opt ima l de s ignZ one



25/84


Minimum Film Thickness Angular position vs. S



26/84

Relationship between variablesCoefficient of friction variable vs. S



27/84

Relationship between variablesFlow variable vs. S



28/84

p

Maximum pressure ratio vs. S



29/84

pTerminating Position of film pressure & maximum film pressure vs. S



30/84

pLubricant Temperature rise T

)(211

2a

QsQT QC T sQQC T QC p ps ploss H

=+=

Taking T 1 as reference temperature:

)(Pr 42 bc fr

J lNc

J TN

loss H ==The heat loss due to friction

( ) ( )[ ] )(

//5.01/

4c

rcNlQQsQc fr

P

C J T p

=

Equating (a) to (b)

( ) ( )[ ] )1512(//5.01/70.9

=

rcNlQQsQc fr

P psiF

T With = 0.0311 lbm/in 3 &

C p = 0.42 Btu/lbm. F for petroleum lubricantsand J=9336 lbf.in/Btu



31/84

pLubricant Temperature rise vs. S


32/84

Sample problems on HDLThe analysis problems are of two general categories:1) When the viscosity is specified as in example 12-

1 through 12-4 of 7 th ed. The solution is straight

forward.

2) The problem becomes more complex when only the

lubricant inlet temperature is specified. To solvethis type of problem an iterative procedure has to

be followed. An example of the procedure is

given in the following.

Sample problems on HDL


33/84

p pProblem 12:3: Given T av=T operating =150 F

Solution : to find any of the performance factors we need to havethe bearing characteristic number : S .

1.65

l bl


34/84

Sample problems on HDL

Problem 12:3: Continued

Problem # 12-12 (Modified)


35/84

( )A 2-1/2 x2-1/2-in sleeve bearing uses grade 20 lubricant. The axial-

groove sump has an inlet temperature of 110 F . The shaft journalhas a diameter of 2.500 in and the radial clearance is 0.002 in. lf

journal speed is 1120 rev/min and the radial load is 1200 Ib f .Estimate

(a) The magnitude and location of the minimum oil-film thickness.(b) The eccentricity.(c) The coefficient of friction.(d) The power loss rate.(e) Both the total and side oil-flow rates.(f) The maximum oil-film pressure and its angular location.(g) The terminating position of the oil film.(h) The average temperature of the side flow.(i) The oil temperature at the terminating position of the oil film.

Given : d = 2 5 in b = 2 504 in c = 0 002 in W = 1200 lbf SAE = 20Problem # 12-12


36/84

Given : d = 2.5 in, b = 2.504 in, c min = 0.002 in, W = 1200 lbf, SAE = 20,T 1 = 110F,N = 1120 rev/min, and l = 2.5 in.

Required (see list)

Solution : to find any of these performance factorswe need to have the bearing characteristicnumber : S .

To find average viscosity ( From Fig. 12-11; 12) we need to have

the average operating film temperature T f (Eq. 12-14) :Procedure: (good for IPS and SI system)1. For a first trial assume T = (General) 20 80 F ( 10-50C )

For our case take T = 40 F2. T f =130 F

( ) avavavavP N

cr

S

42 108.3192

67.18625

5.25.21200

67.182

002.25.12

==

==

21T

f T T +=

Problem # 12-12


37/84

3. Find av = 3.8 reyn (From Fig. 12-11; 12) using T f = 130F

4. Calculate S = 3.8x104x3.8x10

-6= 0.144

5. Calculate TF or TC using 12-18 or Fig 12-23; 24 withS=0.144 and l/d =1

6. Recalculate T fcal = 110+25.7/2 122.85 F7. Compare T fcal to T fassum if |difference| less than 6 F or 3 C

Recalculate, For our case Tfassum -Tfcal = 130-122.85= 7.15 >6 Fneed to re-iterate:

1 assume T =30 F2 T f = 125F 5 TF 27 F3 av 4.3 reyn 6 T fcal = 110+27/2 123.5F4 S 0.163 7 T fassum -T fcal = 125-

123.5=1.5


38/84

av = 4.3 reyn (From Fig. 12-11 for oF; 12 for oC) using T f= 125F yielding S=0.163

a) Using Fig 12- 16 with S=0.163 and l/d =1

h0/c = 0.49 h

0= 0.0098 in

Using Fig 12- 17 = 56 b) e= c- h0 =.002-.00098 = 0.001in. or using Fig. 12-

16 =e/c = 0.5 e = 0.001 in .c) f :Fig 12- 18 (r/c)f= 4f = 4/625=0.0064

d) Power loss: H=(2 TN)/(778x12)= (2 fWrN)/778x12=H = 0.121 Btu/s =436 Btu/hr H = 126 j/s=453 KJ/hr

Problem # 12-12


39/84

e) Using Fig 12- 19 with S=0.163 and l/d =1 Q/rcNl = 4.15 Q = 4.15x1.25x0.002x18.67x2.5=0.48

in3/sUsing Fig 12- 20 Q s /Q=0.61 Q s = 0.29 in 3/s

f) Using Fig 12- 21 P/P max = 0.44 P max = 192/0.44=436psig

g) Using Fig 12- 22 Pmax

= 18 & p0

= 82 h) See part (a) Tav = 125F

i) T2= 110+30=140 F

NOTE: In cases where l/d curve is not available the interpolationequation (12- 16)

may be used when necessary.

Sample problem on Design of HDL Journal


40/84

Sample problem on Design of HDL JournalBearings ( to be solved during help session )

Design a journal bearing to carry a radial load of1500 lb while the shaft rotates at 850 rpm. Theshaft stress analysis determines that the minimumacceptable diameter at the journal is 2.10 in.

The shaft is part of a machine requiring good precision.

Power loss in the bearing should not exceed 1% ofthe 15 hp driving power.


41/84

Procedures for design of oil lubricated journal


42/84

bearings

Step2 : The value of the important parameter l/d istaken between 0.25 and 1.5 . Values up to 3 wereused in earlier designs. Nowadays the value of l/d isconfined between 0.25 and 0.75 . Short bearings are

preferred when shaft deflections and misalignmentsare expected.

Step3 : The minimum film thickness h0 can beestimated from one of these equations:

)(0004.0005.0

)(00004.00002.0

0

0

mmd h

ind h

+

+)(00025.00 ind h =



43/84

bearings

Step4 : The nominal value of clearance ratio r/c (r = bearing radius and c = clearance) can be takenapproximately as:1000 for precision bearings when 25< d


44/84

bearings

Step5 : Now the bearing characteristic number ( S =Sommerfeld number) can be determined from thechart of Fig. 12.164 .

Step6 : Next, the viscosity of the oil is determinedusing:

Where:

P (unit load) = W/ld, with W being the applied load. N = speed in revolutions per second.

N P

r cS

2

=



45/84

bearings Step7 : The outlet temperature of the oil should be kept

between 200 F (93 C ) and 250 F (121 C). A value of 70 C(160 F) is usually specified as the average operatingtemperature [2, 9, 18-20]. The chart of Fig 12-11 or 12-12[3,4]) can be entered to select an oil grade. If the selectedlubricant has a viscosity higher than the value computed instep 6, recalculate S and find the new h0.

Step8: Now, find the friction coefficient from Fig. 12-17 . Thefriction coefficient should be kept as low as possible consistentwith h0 (i.e. in the optimum zone between the minimumfriction line and the maximum load line in Fig. 12.14 [3,4]. Asa general rule friction coefficients below 0.01 are acceptable(see Table 28-1 of the Standard Handbook of Machine Design[5]).

Procedures for design of oil lubricated journalb


46/84

bearings

Step9: Power loss due to friction can be calculatedfrom:

Its value can be compared to the input power to take adecision concerning f and h0.

Step10: Select a suitable bearing material from Table 12-5[3,4] or from Tables 28-2 to 28-4 of the Handbook [5]. Unit

load, maximum operating temperature and conditions should be used as criteria for material selection.

Step11: Write a summary of your design results .

)(1050

hp fWrN

H =

Self-Contained Bearings


47/84

Pillow-blocks or pedestal bearings are used for:

Fans,BlowersPumps and small motors

Examples of Pillow-blocks withPolymer Bearings

Ring oiled bearing

Self-Contained BearingsTwo general types of lubrication: 1) Oil-Ring and 2) Oil Bath


48/84

Since the warm lubricant stays within the bearing housing ; it shouldbe designed such that the heat generated by friction is dissipated.

As seen above the heat generated (in Btu/s ) by friction can be estimated:

)(Pr 42 bc

fr

J

lNc

J

TN gen

H ==

Where J= 9336 in.lbf/Btu

The heat to be dissipated & surface temperature of housing (=T bearing ) are respectively:

)19;1712(1

)19;1712()(

1bb

T T T

aaT T A

f b

f CR

loss H

++=

+=

h

)(1050

hpin fWrN gen

H =

See Eq. 12-18 for CR =

Combined overall coef. Of rad-conv.=2, 2.7,5.9 Btu/(hft 2 F)

and Table 12-2 for

Or in ( hp )

T f is the average film temperature which is unknown and found bytrial and error to satisfy Hgen =H loss as in the following example.See also (Eq. 12-20) for T f


49/84

Example on self-contained bearings


50/84

Ring oiled bearing

Example on self contained bearings


51/84

Example on self-contained bearings

ClearanceAmong the independent variables under designers control, clearance is the


52/84

Among the independent variables under designer s control, clearance is themost difficult to hold accurate during manufacture and It may also increase

during service because of wear .When selecting a clearance for a JB a number of performance variablesand expected in service wear should be taken into account.

Bearing Noisy+h0 decreases


53/84

Clearance


54/84

Temperature limits for mineral oils

Oils with antioxidants + O 2 supply unlimited

O 2 insignificant


55/84

Pressure-Fed BearingsUnit load


56/84

Unit load

w

( )

=

==

2'

2312'4'2

2/

wll

rlW

rlW P

Velocity Profile

( ) )2112(4'822

= ycl p

u s

Pressure-Fed Bearings


57/84

Example of pressure-fedGrooved bearings

Centrally located full annular groove

Circumferential groove axial pressuredistribution

Pressure-Fed BearingsPressure-Fed lubricantNatural circulation of oil


58/84

Use charts with l/d

)6(21 2

yhU

hy yu dxdp

=

'8

)2112(4'82

max

22

lc pu

ycl p

u

s

s

=

=

( ) )2212(5.11'3

23

+=

l

rc psQ

s

( )2312'4'2

2/ ==rl

W rl

W PrlW

P 2=

Q s from Fig. 12-20

Velocity

Side-Flow

Unit load

Use charts with l/d

( )

( )( )( )

)2512(5.11

/)10(978

)2412(5.11

/0123.0

42

26

42

2

+

=

+

=

r pW S c fr T

r p

W S c fr T

s

C

s

F

Temperature rise

T from Fig. 12- 24

Example on Pressure-Fed Bearings


59/84

Problem 12-16 (modified)

An eight-cylinder diesel engine has a frontmain bearing with diameter 3.5 in. and length2 in. The bearing has a central annular oilgroove 0.250 in. wide. It is pressure-lubricated

with SAE 30 oil at an inlet temperature of180F and at a supply pressure of 50 psi.Corresponding to a radial clearance of 0.0025

in, a speed of 2800 rev/min, and a radial loadof 4600 lb, find the temperature rise and theminimum oil-film thickness.

Given : d = 3 .5 in, l = 2 .0 in, P s = 50 psi, w = 0.25 in; c min= 0 0025 in W = 4600 lbf SAE 30 T = 180F; N = 2800

Problem # 12-16 (modified)


60/84

= 0 .0025 in, W = 4600 lbf, SAE 30, T 1 = 180 F; N = 2800

rpm Required: TF, h 0 , P max , Pmax & p0

Solution: Use Eq. 12-24 to compute TF

psirl

W P

cr

inwl

l

751875.075.14

4600'4

7000025.0

75.1

875.02

'

=

==

==

==

)( )

)2412(5.11

/0123.042

2

+

=r pW S c fr T

s

F

( ) avavP

N

cr avS

410045.375160

28002700

2 ===

Problem # 12-16


61/84

1. For a first trial assume T = 30 F2. T f = 180+30/2 = 195 F3. Find av = 1.4 reyn (From Fig. 12-12) using T f = 195F4. Calculate S = 0.04265. Use S = 0.0426 and l/d = to find = 0.93 from Fig. 12-16 & (r/c)f

= 2.2 from Fig. 12-18

6. Calculate TF

6. T fcal = 180+22.64/2 191.3 F7. Compare T fcal to T fassum if |difference| >6 F Recalculate,

For our case Tfassum -Tfcal = 195-191.3= 3.7


62/84

Using Fig 12- 16 with S=0.0426 and l/d =1/4h0/c = 0.07 h0 = 0.000175 in

Trumpler Trumpler ss Criteria satisfied?Criteria satisfied?1)1) hh

0 0 0.0002+0.00004(3.5)=0.00034 in0.0002+0.00004(3.5)=0.00034 in not satisfied?not satisfied?

2)2) T T max max = T = T ss ++ T= 180+22.64=202.64T= 180+22.64=202.64 FF


63/84

A- Loads : Typical values of unit load P

JOURNAL BEARING LOADS & MATERIALS

B- Materials : To minimize wear of journal bearings, Metallic


64/84

j g ,

Materials (Table 12-6 for Hydrodynamic Lubrication and 12-7for Boundary Lubrication) are selected for:1. Mechanical Properties

Conformability : to compensate for small shaft misalignmentsand deflections (i.e. Low E and yield: Lead base Babbit=90%Pb + 10% Cu)

Embeddability : to allow foreign particles to becomeembedded into the bearing which prevents scratching of shaftand sleeve (Tin base and Lead base Babbit )

High Fatigue Strength : to support the compressive cyclicloading (Trimetal, Silver, Steel base, Solid Brass)

JOURNAL BEARING MATERIALS2. Thermal Properties


65/84

High Thermal Conductivity : to remove heat rapidly from thebearing (Ag, Cu, Pb ). Thermal Coefficient of Expansion not too different from that

of casing and shaft.3. Metallurgical Properties

Compatibility : to avoid fusing under heat and contactdissimilar materials (Mainly not same melting point ) for shaftand bearing are more compatible than similar materials.

4. Chemical Properties Corrosion Resistant : to resist corrosion by lubricant

improvement additives (Sn, Al, Ag...).

Non-Metallic Materials (Table 12-6) such as Wood, Rubber, CarbonGraphite, Derlin, Teflon, Nylon Most have low thermalconductivity.


66/84

Boundary Lubricationthin-film, Olite, Oiles, bushed pin

In certain applications boundary lubrication should be


67/84

In certain applications boundary lubrication should bedesigned for ( see your lab manual and earlier notes andexample for the procedure of boundary lubrication design ).

Boundary lubrication should be expected for slow speeds ( start

ups and shut downs) : U


68/84

Mix animal or vegetable oils with lubricantUse porous metallic materials (Table 12-7)Use non-metallic materials (Table 12-8)Use indented bearings

K in Table 12-8 and f 1 and f 2 in tables 12-10 & 12-11

( )2712)(21 == PV K f f t wratewear

JOURNAL BEARING MATERIALS-Boundarylube


69/84

JOURNAL BEARING MATERIALS-Boundary lubrication


70/84

Sample problem on Design of Boundary-LubricatedJournal Bearings


71/84

g

Design a boundary lubricated plain-surface bearing to carry a radial load of 2.5 kN from ashaft rotating at 1150 rpm. The nominal minimumdiameter of journal is 75 mm.

Given: Boundary lubricated JB. W=2.5 kN; n= 1150 rpm ;d = 75 mm


72/84

Solution : 0.25< l/d


73/84

(Plain Bearings, sleeves)

Radial

Thrust Journal Bearing


74/84

Thrust

Journal Bearings


75/84

Types of bearings


76/84

Plain Bearings

Journal Bearings

Self-lubricated Journal Bearings

Bushes Polymer Bearings

Types of Bearings


77/84

Radial Journal Bearings for PinionShaft in Gear Box for GE Turbine

ESCOSA-DAMMAM

Types of Bearings


78/84

Housing for Gear Boxshowing Radial JournalBearing Supports

Types of Bearings


79/84

Radial Journal Bearings for PinionShaft in Gear Box for GE Turbine

Types of Radial Journal Bearings


80/84



81/84



82/84

Typical Groove Patterns



83/84



84/84

Thrust Bearing for GETurbine Shaft

Journal Bearings Lecture0

Documents

Transcript of Journal Bearings Lecture0