me 343: mechanical design-3 - school of engineeringaly/courses/me343/slides3_bearings1.pdf · me...

ME 343: Mechanical Design-3

BearingsBearings

Dr Aly Mousaad AlyDr. Aly Mousaad AlyDepartment of Mechanical Engineering

Faculty of Engineering, Alexandria University

What are “bearings”?What are bearings ?

• Machines with moving parts must have some way to guide their motions in turning, sliding, y g g gor rolling. Structures that enable these motions are often called bearingsmotions are often called bearings.

• Just think of anything that rotates and you are f b f hsure to find a bearing in one form or other in

it.

Slides 3: Bearings 2

Why bearings?Why bearings?

Th bj i i d f i i d• The mean objective is to reduce friction and wear.• Example adding a sleeve to protect a rotating shaft.• They assist in easy rotary action thereby reducing the

effort or power required. Just think about pulling your luggage at the airport without any wheels or withoutluggage at the airport without any wheels or without any bearings to the wheels.

• They assist in carrying loads or assist in transferring the• They assist in carrying loads or assist in transferring the loads from something that tends to rotate to something which is stationary and has the capability to g y p ycarry the loads.


HistoryHistory

• The oldest instance of the bearing principle dates to the Egyptians when they used tree gyp ytrunks under sleds. There are also Egyptian drawings of bearings used with hand drillsdrawings of bearings used with hand drills.


Types of bearingsTypes of bearings

l i b i l k b h ifi• Plain bearing, also known by the specific styles: bushings, journal bearings, sleeve bearings, rifle bearings.

• Rolling-element bearings such as ball bearings g g gand roller bearings.

• Fluid bearings in which the load is carried byFluid bearings, in which the load is carried by a gas or liquid.M ti b i i hi h th l d i i d• Magnetic bearings, in which the load is carried by a magnetic field.


Journal and roller bearingsg


WindmillWindmill

Diameter range:0.5 m - 6m, Weight range::75.8kg-4000kg


Dental drillDental drill

• Pneumatic bearings can allow for very highPneumatic bearings can allow for very high speeds (300,000 to 400,000 rpm ).


Roller bearingsRoller bearings

if i i b i ! d d d h• Antifriction bearings! Load, speed, and the operating viscosity of the lubricant do affect the frictional characteristics of a rolling bearing.

• An issue with ball and roller bearings is that the balls or rollers rub against each other gcausing additional friction which can be prevented by enclosing the balls or rollers in aprevented by enclosing the balls or rollers in a cage (separator or retainer).


Roller bearings versus sleeve bearingsRoller bearings versus sleeve bearings



Roller bearings Sleeve bearingsRoller bearings Sleeve bearings

The main load is transferred through elements in rolling

The main load is transferred through sliding contactthrough elements in rolling

contact.through sliding contact.

The starting friction is about Very high starting frictionThe starting friction is about twice the running friction.

Very high starting friction (metal to metal contact).

Require less axial space but Require more axial space butRequire less axial space but more diametrical space

Require more axial space but less diametrical space

High initial cost Low initial costHigh initial cost Low initial cost

Low maintenance cost High maintenance cost

Lower reliability High reliabilityLower reliability High reliability



Characteristics Roller bearings Sleeve bearingsCharacteristics Roller bearings Sleeve bearings

Shock Oil film, excellent Direct contact between rolling element and fixedrolling element and fixed and rotating parts.

Vibrations Oil is excellent for No damping effect,Vibrations Oil is excellent for damping

No damping effect, vibration due to changing rigidity during rotation.

Dust < ho and embeding, no contact with

Cause small pits, i.e. failure

surfaces

Maitenance Permenant care b l b

Lubrication, grease can babout lubrication. be once or twice a year



Characteristics Roller bearings Sleeve bearingsCharacteristics Roller bearings Sleeve bearings

Repair Can be repaired No repairRepair Can be repaired No repair

Service life Perminant oil Fatigue lifeService life Perminant oil infinite life (take care of running up)

Fatigue life

Accuracy of rotation

Easier to reach Depends on case excentercity, roundness and accurecey of the rolling.


Comparison of bearing frictionsComparison of bearing frictions

• It is advantageous to use ball bearing and roller bearing at low speeds. Journal bearings are mostly suited for high speeds and high loads.


Advantages of rolling bearingsAdvantages of rolling bearings

C ffi i t f f i ti i l ( 0 001 0 003)• Coefficient of friction is low (μ = 0.001 – 0.003) compared with plain bearings especially at low speeds This results in lower power lossspeeds. This results in lower power loss.

• Wear is negligible if lubrication is correct. • They are much shorter than plain bearings and• They are much shorter than plain bearings and

take up less axial space.• Because of extremely small clearance they permit• Because of extremely small clearance they permit

more accurate location; important for gears for example.p

• Self-aligning types permit angular deflection of the shaft and misalignment. g


Disadvantages of rolling bearingsDisadvantages of rolling bearings

Th id di i l• The outside diameter is large. • The noise is greater than for plain bearings, especially

t hi h dat high speeds. • There is greater need of cleanliness when fitted to

achieve correct lifeachieve correct life. • They cannot always be fitted, e.g. on crankshafts

because of there lower shock resistancebecause of there lower shock resistance. • They are more expensive for small quantities but

relatively cheap when produced in large quantitiesrelatively cheap when produced in large quantities. • Failure may be catastrophic.


Main characteristics of sliding bearingsMain characteristics of sliding bearings

i h i f i i d l l• High starting friction due to metal to metal contact. This requires the use of low-friction metals like bronze alloys and babbitt metals.

• Under certain combination of force, speed, , p ,fluid viscosity, and bearing geometry, a fluid film (hydrodynamic film) forms and separates ( y y ) pthe containing surface.

• A fluid film can also be formed with a• A fluid film can also be formed with a pumping unit (hydrostatic film).


Classification of roller bearingsClassification of roller bearings

• Bearings are manufactured to take pure radial loads, pure thrust loads, or a combination of pthe two kinds.

• Essential parts of a roller bearing are the outer• Essential parts of a roller bearing are the outer ring, the inner ring, the balls or rolling

l helements, and the separator.


Bearing nomenclatureBearing nomenclature


Types of rolling bearingsTypes of rolling bearings



Cylinder roller bearings provide greater force because of the y g p g f fgreater contact area.

Combines the advantages of ball and roller bearings.


Service LifeService Life

F l i b i i l i h l lif• For plain bearings some materials give much longer life than others. Some of the John Harrison (1693-1776) clocks still operate after hundreds of years because ofclocks still operate after hundreds of years because of the lignum vitae wood employed in their construction, whereas his metal clocks are seldom run due to potential wear.

• Although long bearing life is often desirable, it is g g gsometimes not necessary. For example, a bearing for a rocket motor oxygen pump is not required to have

l h lif f i f th l t fseveral hours life, far in excess of the several tens of minutes life needed.


Service LifeService Life

• Fl id d ti b i h ti ll i d fi it i• Fluid and magnetic bearings can have practically indefinite service lives. In practice, there are fluid bearings supporting high loads in hydroelectric plants that have been in nearly continuous service since about 1900 and which show no signs of wearsince about 1900 and which show no signs of wear.

• Rolling element bearing life is determined by load, temperature, maintenance, lubrication, material defects, contamination, handling installation and other factorshandling, installation and other factors.

• The service life of bearings in one application was extended dramatically by changing how the bearings were stored before installation and use as vibrations during storage caused lubricantinstallation and use, as vibrations during storage caused lubricant failure even when the only load on the bearing was its own weight.

• Bearing life varies because microscopic structure and contamination vary greatly even where macroscopically they seem identicalvary greatly even where macroscopically they seem identical.


Choice of lubricantChoice of lubricant

Grease is used for slower rotational speeds, lower temperatures, and low toGrease is used for slower rotational speeds, lower temperatures, and low to medium loads. Grease is used in situations where maintenance is more difficult or irregularly scheduled. It can be used in dirty environments if seals are provided


provided.

Comparison between grease and oil lubrication

Use grease when Use oil whenUse grease when Use oil when

•Temperature is not over 90 C (special types of grease may allow

•High temperatuers

for higher temp.)

•Low speed •High speeds

•Unusual protection is required from the enterence of forign matter.

•Piltight seals are readily employed.

•Bearing enclusures are desired •Bearing type is not suitable for graese

•Opperation for long period without •The bearing is lubricated from aOpperation for long period without attenion to lubrication.

The bearing is lubricated from a supply whish is also used for other machine parts (gearbox).


Roller bearings: maintenanceRoller bearings: maintenance

• M b i i i di i t t t t• Many bearings require periodic maintenance to prevent premature failure, although some such as fluid or magnetic bearings may require little maintenance.M t b i i hi h l ti d i di l b i ti• Most bearings in high cycle operations need periodic lubrication and cleaning, and may require adjustment to minimize the effects of wear.B i lif i ft h b tt h th b i i k t l d• Bearing life is often much better when the bearing is kept clean and well-lubricated. However, many applications make good maintenance difficult. For example bearings in the conveyor of a rock crusher are exposed continually to hard abrasive particlesrock crusher are exposed continually to hard abrasive particles. Cleaning is of little use because cleaning is expensive, yet the bearing is contaminated again as soon as the conveyor resumes operation Thus a good maintenance program might lubricate theoperation. Thus, a good maintenance program might lubricate the bearings frequently but clean them never.


Defective bearing installationDefective bearing installation

(a) Misaligment (out-of-line), (b) shaft deflection, (c) crooked or tilted outer race, (d) crooked or


tilted inner race.

Defective rolling bearingsDefective rolling bearings

R dl f th f il h i d f ti• Regardless of the failure mechanism, defective rolling element bearings generate mechanical vibrations at the rotational speeds of eachvibrations at the rotational speeds of each component. These characteristic frequencies, which are related to the raceways and the balls or yrollers, can be calculated from the bearing dimensions and the rotational speed of the

hi M h i l ib ti l imachine. Mechanical vibration analysis techniques are commonly used to monitor these frequencies in order to determine the conditionfrequencies in order to determine the condition of the bearing.


ReadingReading

• Mechanical Engineers Reference Book, Ch. 9, “Tribology”.gy

• Shigley’s Mechanical Engineering Design, Eighth Edition The McGraw Hill CompaniesEighth Edition, The McGraw−Hill Companies, Inc., 2006. (11. Rolling−Contact Bearings)


HWHW

S l bl 2 f h t 1 (Sh t1 Sh ft d i• Solve problem 2 of sheet 1 (Sheet1 Shaft design (continue).pdf).

• Select two deep groove ball bearings to carry the• Select two deep groove ball bearings to carry the loads at A and E.

• Solve problem 3 of sheet 1 (Sheet1 Shaft design• Solve problem 3 of sheet 1 (Sheet1 Shaft design (continue).pdf).

• Please submit the assignment to your TA during• Please submit the assignment to your TA during tutorials in the week starting Saturday 23, April 2011.

• Late assignments will not be considered.


me 343: mechanical design-3 - school of engineeringaly/courses/me343/slides3_bearings1.pdf · me...

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