mechanical elements lecture
TRANSCRIPT
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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)
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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].
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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)
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Arc Welding: Setup configuration [11]
1- power supply
2- electrode holder
3- workpiece
4- work clamp
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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