mechanical elements lecture

7/30/2019 Mechanical Elements Lecture

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Simon Fraser University

Spring 2006 ENSC305/440 1

Mechanical Elements

ENSC 305/440

Esmaeil TafazzoliSpring 2006


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Outline:

Mechanical properties of materials

Machine elements:

1) Motors (types, selection considerations)

2) Power transmission devices (gears, belt, pulley, chain,

friction drive)3) Miscellaneous: Bearings(types, selection issues,

catalogs), springs, pins, retaining rings

4) Other motion generation devices (linkage mechanisms,

cams)

5) Joining methods (welding, brazing, soldering, bolts,

screws, rivet, )


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Mechanical properties of materials:

Behavior of materials in response to mechanical loads.

Useful in material selection.

Most basic property: stress-strain relation

Stress:

Strain :

)(

)(

Aarea

Fload

)(

)(

0lngthoriginalle

lgelengthchan


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Strength of a material

Maximum stress that can be applied to material before failure.

Types:- Tensile Strength- Compressive strength

- Shear strength

The definition of failure varies by the type of material anddesign method:

Brittle (concrete, cast iron, glass,)Ductile (gold, Aluminum, copper, steel, )


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Stress-Strain relation

The stress caused byload must not exceed the

failure stress.

Always consider a Factorof Safety.

FS= all/design

Stress-strain curve for structural steel[1]

all=Allowable Stress

design = Design Stress


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Stress-strain curve, Aluminum[1] Stress-strain curve for a brittle material [1]


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Standards

SAE (Society of Automotive Engineers)

AISI (American Iron and Steel Institute)

ASTM (American Society for testing and Materials)

ANSI (American National Standard Institute)

AISI-SAE designation for carbon and alloy steel

Mild steel 0.1% - .25% Carbon e.g., AISI 1018

Medium carbon steel .25% - .45% e.g., AISI 1040


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Machine elements

Motor

Gear

Belt, pulleyChain, sprocket

Universal joint

Friction drive

Cam-follower

Mechanisms (linkages)

Bearing

Joining methods

(welding, brazing, rivets,bolts, screws, etc)


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Motor types

DC motors

Stepper motors

Gearhead motors

AC motors

a) Permanent magnet

(brushless)

b) DC Shunt motor

c) DC series motor


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Characteristics

DC motor

a) Speed proportionate to voltage applied

b) Suitable current/torque and speed/torque curveproperties

c) Easy to control

d) Position/Speed Servo

e) No brush noise, durable, clean (brushless)


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Characteristics

Stepper motor

Can rotate in both directions

Moves in precise angular increments (steps)

Sustain a holding torque at zero speed

Easy to control


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Comparison of stepper and DC permanent-magnet

motors

Robot mechanisms and mechanical devices illustration, McGraw Hill 2003 [2]


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Typical DC motor characteristic curves [3]


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Selection factors

When selecting a motor consider these issues:

Speed range

Torque-speed variations

Reversibility Required power

Load inertia

Starting torque

Size and weight

restrictionsPrice

Maintenance


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Selection factors (contd)

Consider these questions when selecting a motor:

1. Will the motor start under load?

2. What is the maximum speed the motor can produce?

3. How much power does the load require?

4. Is the load to be driven at constant speed?

5. Is transmission gearbox required?


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Typical specification sheet[4]


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Useful information sources

Experts

Manufacturer

specification sheets

Product catalog

Design handbooks

Motor nameplate

Web


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Gearhead motors


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Force ratio for gear trains

21

21

RRR

rWrrF

W=Load force

F= balancing force

rFFR 1

R

r


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Gear train speed ratio

driver

driven

ddd

DDDspeedratio

321

321

driven

drive

NN

NN

n

n

42

31

1

4

n= speed

N= number of teeth


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Different kinds of power transmission ormotion control capability of gears:

a) Changing rotational speed.

b) Changing rotational direction.

c) Multiplying or dividing torque.

d) Converting rotational to linear motion.

e) Offsetting or changing the location of rotating motion.


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Motion and torque transmission examples ([5])

Right angle

crossing shafts

Self locking

High frictionand wear

High speed

reduction

Right angle I/O

torque

Smooth tooth

interactionLow noise

Rotary-Linear power

transmission


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Bearing types

Ball bearing

Tapered roller

bearing

Needle roller

bearing

Thrust bearing


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Important factors in bearing selection

Loads (radial, axial) Operating speed

Size and weight

A quick way of bearing selection

Information sources: Experts

Manufacturers catalog (SKF, TIMKEN, FAG,)

Design handbook

SKF online interactive catalog (www.skf.com)


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SKF interactive bearing selection example page


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Other ways of motion transmission:

Linkage mechanisms

Intermittent rotary motion[6]

Rotary to back and forth motion[7]


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Alternative mechanism design

Robot mechanisms and mechanical devices illustration, McGraw Hill, 2003 [2]


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Flexible transmission

Chain-sprocket

Belt-pulley

Ref[8, 9]

Ref[10]


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Joining methods

Welding

Brazing

soldering


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Joining methods

Welding: (melting both material and filler, generally

used for welding ferrous materials)

Brazing: (melting nonferrous metal, brass or bronze,as filler to join base materials by capillary action)

Soldering: ( same as brazing but at lower

temperatures)

Si F U i i


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Joining

method

Joint

strength

temperature Distortion Aesthetics

Soldering Poor up to 400C None Good

Brazing Good 800-1000 C Minimal Excellent

Welding Excellent above1500C Likely Fair

e.g., Brazing with Bronze alloy as filler with 870-980C for joining mildsteel with melting temperature of 1600C[1].

Si F U i it


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Welding types

Arc welding:An electric arc between material and filler melts

them at the joining point.

Gas welding (oxyacetylene):

Widely used for welding pipes and tubes and repair work

Resistance welding:

Generating heat by passing current through resistancecaused by joining metals. (widely used in automotive

industry)

Si F U i it


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Arc Welding: Setup configuration [11]

1- power supply

2- electrode holder

3- workpiece

4- work clamp

Si F U i it


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Welding position

1- workpiece

2- work clamp

3- slag

4- electrode5- electrode holding position



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Electrode positionin groove welding

Electrode

position in

fillet

welding



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1- stringer bead

(steady

movement alongseam)

2- weave bead

(side to side

movement

along seam)

3- Weave pattern

Single stringer

bead for narrow

grooves

Weave bead

for wide groove

or multiplestringer bead



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Properties of a good welding



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Excessive spatter

Cause:

1-Amperage too high

2-Arc length too long

Correction:

1- Decrease

amperage or select

larger electrode2- Reduce Arc length

Porosity

Cause:

1-Dirty workpiece2-Arc length too long

3-Damp electrode

Correction:

1-Remove all grease,

damp, oil, dust,...,

from work surface

2-Reduce length

3-Use dry electrode

Lack of penetration

Causes:

1-Improper weldtechnique

2-insufficient heat

input

Correction:

1-Reduce welding

speed

2-Increase amperage,

use larger electrode

Troubleshooting



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Burn through

Cause:

Excessive heat

Correction:

Decrease

amperage

use smaller

electrode

Increase travel

speed

Wavy bead

Cause:

Unsteady hand

Correction:

Use two hands

practice

Distortion

Cause:

Excessive heat

Correction:

Use clamp to hold

base material

Use lower amperage

Make tack weld along

the edge

Weld in small

segments, allow cooling

between welds.

Troubleshooting



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Example of good and

bad welds

2006 Miller Electric Mfg Co



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References1) http://en.wikipedia.org

2) Robot mechanisms and mechanical devices illustration,McGraw Hill 2003

3) www.igusa.com4) http://www.bodine-electric.com

5) http://www.engr.utexas.edu/dteach/Experience/mechanisms/brief_mechanisms.htm

6) http://www.cabaret.co.uk/education/geneva.htm

7) Mechanism Design: Enumeration of Kinematic Structures According to

Function, Lung-Wen Tsai, Copyright 2006 Taylor and Francis group,LLC.

8) http://www.thesaabsite.com/95/95pulleys

9) http://microhydropower.net/index.php

10) http://www.goldstarind.com/prdndx1.htm

11) Slides 32-39 were adapted from: Guideline for shielded metal arc

welding, http://www.millerwelds.com/pdf/guidelines_smaw.pdf

mechanical elements lecture

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