mechanics of materials lecture slides

8/12/2019 mechanics of materials lecture slides

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Intro to Bearings

Lecture Nov 4, 2013

Cheryl Quenneville, Ph.D., P.Eng.

[email protected]


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Bearings

Every free body has 6 degrees of freedom

A bearing is a device to: Constrain some of the DOFs while leaving others free to move

Carry a load (i.e. provide stiffness) in constrained directions

Reduce friction in unconstrained directions

video


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Bearings

Bearings are designed to carry loads in certain directions,

e.g. radial and/or axial for rotary bearings:


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Applications

Anything with a rotating component!

Found in: Dental drills

Mars Rover

Cars Bicycle wheels

Alignment systems

Machinery

Robots Skateboards, Rollerblades


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Journal Bearings

Journal bearings, also called Plain bearings, have sliding,

rather than rolling, contact Note that hydrodynamics topics are outside this course (you

cover this in fluids)

Consider an experiment by B. Tower in 1883:


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Bushings

Independent plain bearing inserted into a housing

Flanges on bushings used to locate or to provide thrustbearing surface

Sizing: XXYY-ZZ where XX is ID in 16th of an inch, YY is

OD in 16th of an inch, and ZZ is length in 8th of an inch Often secured using retaining rings instead of pressed in

Made from durable, low friction, resistant materials


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Plain Bearing Materials

A wide variety of bearing materials are available from

different manufacturers, with different properties:

Note that some of these (e.g. bronze, wood) may require

lubricant while others dont (e.g. Teflon, Carbon-Graphite)to sustain these values


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Plain Bearing Materials

Load (P) is the pressure on the bearing. If a force of W is

placed on the bearing, with a projected area (A) then P =W/A

Speed (V) is the surface velocity at the bearing

PV is the product of load and speed. Because of frictionalheating, max PV is always less than product of max Pand max V


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Lubrication

Class 1: bearings requiring application of alubricant (e.g. oil, grease)

Class 2: bearings containing a lubricant within the

walls of the bearing (e.g. bronze, graphite) Class 3: bearings made of materials that are the

lubricant (e.g. Teflon)


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Lubricants

The object of lubrication is to reduce friction, wear and

heating of bearings Most common are mineral oils Rated according to kinematic viscosity (strongly affected by

temperature)

Seals are required to keep oil in machine / dirt out Grease is oil that has been thickened with additives.

It is used when: Temp < 100C

Speed is low Unusual protection is required from foreign matter

Simple bearing enclosures desired

Operation for long periods with little attention

Solid lubricants (e.g. graphite) are useful at high temps,and when dirt is a problem


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Lubrication Types

Different lubrication regions can be defined:

Boundary lubrication where asperities of the surfacecome into contact

Hydrodynamic lubrication where there is a stable fluidfilm between surfaces and no contact

Mixed lubrication is a region of unstable fluid film inbetween

Hydrodynamic bearings do not wear except whenstarting and stopping E.g. auto engine bearings may last at least 109

revolutions before failure

Friction coefficient may be very low (e.g. = 0.007)


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Hydrostatic Bearings

Have the lubricant

pumped into the bearingto create the fluid film E.g. air hockey tables use

pressurized air

500 ton Mt. Palomartelescope rotates with1/12th HP

Hydrodynamic and

hydrostatic bearingdesign involvessignificant fluidmechanics calculation


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Boundary Lubricated Bearings

In many applications speeds, loads and/or life

requirements are low, so boundary lubricated (orunlubricated) bearings suffice

The bearing can be as simple as a drilled hole,

or inserted bushings

This bushing has a

flange, allowing it tocarry both radial andthrust loads


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Antifriction Bearings

Use rolling contact (e.g. points in ball bearings &

lines in roller bearings) Friction usually much reduced (e.g. = 0.015 for

ball bearings vs. 0.15 for bronze bushing on steel

shaft


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Rolling Element Bearings

Low friction, moderate to high speed applications,

moderate to high life, handle higher moments


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Cylindrical roller bearings Higher load capacity than ball bearings,

lower capacity and higher friction underloads perpendicular to primary direction

Dont perform well if misaligned

Needle roller bearings Long thin cylinders, often tapered

Thin, so compact bearings, fatigue quickly


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Tapered roller bearings

Conical rollers & races, support both radialand axial loads

Carry higher loads due to greater contactarea, used for wheeled vehicles

Expensive, acts as wedge, tries to ejectroller, therefore greater friction

Spherical roller bearings Outer race is spherically shaped inside

Take both static & dynamic misalignment

Expensive, higher friction (sliding)


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Configurations

Thrust loads

Axial loads, such as vertical shafts Commonly spherical, conical or cylindrical rollers

Radial loads Used for axles

Light loads ball bearings, heavy loads tapered rollers

Linear motion For shafts or flat surfaces

Rollers mounted in cage, e.g. drawer slides

Recirculating balls (in a groove)

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mechanics of materials lecture slides