01 rolling contact bearing 2012 2

7/30/2019 01 Rolling Contact Bearing 2012 2

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Rolling-Contact

Bearings

Design II

Mechanical Engineering FacultyEngineering Division

Universidad Santo Toms


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initial test

Adapted from Budynas-Nisbett, 8ed.

1. Describe two differences between ball

and roller bearings

2. In a bearing mount with two bearings

of same reference (L10) what is reliabilityin this mount?


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Ball Bearing Nomenclature

Rotary ball bearings are

defined through geometry and

performance.Geometry:

Outside diameter

Inside diameter

Width



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Ball Bearing Nomenclature

Performance:

Load capacity (radial, axial,

moment)

Stiffness (radial, axial,

moment)

Runout

Bearing life

Allowable speed

Lubrication, etc.Adapted from Budynas-Nisbett, 8ed.


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Ball Bearing Types



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Roller Bearing Types



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Bearing Seals

Adopted from Juvinall RC, Marshek KM, Fundamentals Of Machine Component Design


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Ball and Roller Bearing

Types

Adopted from Collins JA, Mechanical Design of Machine Elements and Machines


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Ball and Roller Bearing

Types



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Ball vs. Roller Bearings

Roller bearings are stiffer and have a higher load

capacity than comparably sized ball bearings

This is due to the type of contact, line contact for

rollers vs. point contact for balls

Ball bearings have a lower friction

This also is a function of contact type.

Ball bearings can often be operated at higher speeds

Most ball bearings can take modest axial loads for free

Only tapered rollers can take axial loads

Ball bearings are less expensive than roller bearings


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Ball vs. Roller Bearings



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Bearing Failure Modes

Surface Fatigue Surface fatigue is the dominant

failure mode

The cyclic subsurface Hertzian

shear stresses produced by thecurved surfaces in rolling

contact may initiate and

propagate cracks that ultimately

dislodge particles and generate

surface pits

Typically, the raceways pit first,

resulting in noise, vibration, and

heat



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Bearing Failure Modes

Brinelling Static loads on stationary

bearings may cause brinelling

of the races The resulting local

discontinuities cause vibration,

noise, and heat

Adopted from

http://www.ejsong.com/mdme/memmods/MEM30009A/Bearings/Bearings.html


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Bearing Selection Criteria

When selecting bearings, both failure modes need

to be considered. As such, we need to examine:

Resistance to surface fatigue, expressed as basicdynamic load rating Cd

Resistance to brinelling, expressed as basic static

load rating C0



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Basic Load Ratings

Basic load ratings are a standardized measure provided

by the bearing industry to quantify the rolling element

bearings ability to resist surface fatigue and brinelling

The basic static load rating C0 is a measure of theresistance to failure by brinelling

The basic dynamic load rating (C)Cdis a measure of

resistance to failure due to surface fatigue

Adopted from SKF.com


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Bearing Life

Two identical bearings tested under different loads F1 and

F2will have respective lives L1 and L2according to:

Where: a = 3 for ball bearings; a = 10/3 for roller bearings


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Example 1: Timken Bearing

SKF ball bearing is rated as follows:

At a rated load of 2140 lb, the bearing has a life of 3000 hr at

500 rpm


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Basic Dynamic Load Rating

The basic dynamic load rating Cd is defined to be

the largest stationaryradial load that 90 percent of

a group of apparently identical bearings will survivefor 1 million revolutions (inner race rotating, outer

race fixed) with no evidence of failure by surface

fatigue..



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Basic Dynamic Load Rating

The bearing life L for a given load P can be determined as

follows:

Where: L = bearing life (revolutions to failure); Cd

= basic

dynamic load rating (90% reliability); P =applied bearing

loada =exponent (a = 3 for ball bearings and a = 10/3 for

roller bearings)


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Reliability Adjustments

Reliability adjustment factors, based on actual failure rate

data, allows a designer to select bearings for reliabilities

higher than 90%

LR= reliability-adjusted

bearing lifeKR= reliability life-

adjustment factor

L = bearing life



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Adjustments for Impact Loads

Rated load ratings are basic on static, continuous loading.

Time-varying loads reduce the bearing life and need to be

considered when selecting bearings.

Adopted from Collins JA, Mechanical Design of Machine Elementsand Machines

The applied equivalent

dynamic loadFeis

modified by the

estimated impact

factorIF

:

F = IF

Fe


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Adjustments for Impact Loads

You can also use.

Adopted from Urugal, A.C. Mechanical Design. 2004.


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Bearing Selection Procedure

1. From free-body diagram, determine the radial

loadFr

and axial thrust loadFa

2. Determine the design life requirementLdfor the

bearing(calculate or use table of recommended

design life for bearings)


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3. Determine the reliabilityRappropriate to the

application and select the corresponding life-

adjustment factorK

R

4. Assess the severity of any shock or impact

associated with the application and select an

appropriate Impact FactorIF

5. Select the type of bearing


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6. Calculate the dynamic equivalent radial loadPe

Where: V = 1 when the inner ring rotates, and V = 1.2 when the outer

ring rotates. In Self-aligning bearings V = 1 for rotation of either ring.

In the first iteration use following table


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Others bearings: Checksuppliers engineering catalog


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7. Calculate the basic dynamic load rating requirement:

Cd

= required dynamic load rating to give a bearing reliability of R percent

L = life (revolutions)

KR

= reliability adjustment factor

IF

= application impact factor

Fe = equivalent radial load

a = exponent equal to 3 for ball bearings or 10/3 for roller bearings

8. With Cd

enter a basic load rating table and find the

smallest bearing with a load rating of at leastCd

Fe


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9. Note C0 , f0and other design parameter.

10.To obtain a new value ofY2

11.Find Cd

12. If the same bearing is obtained, stop.

13.If not, take next bearing and go to step 7.


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Example 2: Rolling Element

Bearing SelectionA support shaft for a new product has been designed with a

diameter of 1.6 inches. The force analysis shows that:

Radial bearing load Fr = 370 lb Axial bearing load Fa = 130 lb

Shaft speed n = 350 rpm

Design life specification is 10 years of operation, 50

days/year, 20hr/day Design reliability specification is R = 95%

The shaft is V-belt driven


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Bearing SelectionThe shaft shown in Figure is to be supported by two

bearings, one at location A and the other at location B. The

shaft is loaded by a commercial-quality driven helical gearmounted as shown. The gear imposes a radial load of 7000

lb and a thrust load of 2500 lb applied at a pitch radius of 3

inches. The thrust load is to be fully supported by bearing

A(bearing B takes no thrust load). It is being proposed to use

a single-row tapered roller bearing at location A, and anotherone at location B.


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Bearing Selection


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Bearing SelectionThe device is to operate at 350 rpm, 8 hours per day, 5 days

per week, for 3 years before bearing replacement is

necessary. Standard strength-based analysis has shown thatthe minimum shaft diameter must be 1.375 inches at both

bearing sites. Select suitable bearings for both location A and

location B


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Bearing Mount for Thrust

WARNING!!!!!

Thermal expansion of the shaft creates thrust force,

possibly overloading the bearings..


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Bearing Mount for Thrust

RECOMMENDED!!!!

The thrust force is provided by a single bearing. The second bearing only

takes radial loads because it is allowed to slide axially.


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Bearing Mounting

Shaft/bearing bore has a light interference fit.

Housing/outer race has a slight clearance fit.

Check manufacturers catalog

Match maximum permissible fillet radius.

Shaft or housing shoulders not to exceed 20%

of diameter.


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Permissible misalignment

Cylindrical and tapered roller bearings is limited to

0.001 rad.

For spherical ball bearings, the misalignment

should not exceed 0.0087 rad.

But for deep-groove ball bearings, the allowable

range of misalignment is 0.0035 to 0.0047 rad.



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References

Norton, R.L. Machine Design: an integrated

approach. 3rd ed, 2006.

Budynas and Nisbett. Shigleys MechanicalEngineering Design. 8th ed, 2006.

Mott, R.L. Machine Elements in Mechanical

Design. 4th

ed, 2004. Urugal, A.C. Mechanical Design. 2004.

Collins JA, Mechanical Design of Machine

Elements and Machine, 2009.


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References

http:// www.skf.com

http://www.fag.com

http://www.timken.com/es-

es/Knowledge/students/Pages/default.aspx
http://www.fag.com/http://www.skf.com/http://www.fag.com/http://www.timken.com/es-es/Knowledge/students/Pages/default.aspxhttp://www.timken.com/es-es/Knowledge/students/Pages/default.aspxhttp://www.timken.com/es-es/Knowledge/students/Pages/default.aspxhttp://www.timken.com/es-es/Knowledge/students/Pages/default.aspxhttp://www.timken.com/es-es/Knowledge/students/Pages/default.aspxhttp://www.fag.com/http://www.skf.com/http://www.fag.com/


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01 rolling contact bearing 2012 2

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