Transcript Basic Mechanical Parts - Simon Fraser University

Simon Fraser University
Mechanical Elements
ENSC 305/440
Esmaeil Tafazzoli
Spring 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:
load( F )
 
area( A)
lengthchange(l )
Strain :  
originallength(l 0 )
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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 and
design method:
Brittle (concrete, cast iron, glass,…)
Ductile (gold, Aluminum, copper, steel, …)
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Stress-Strain relation
The stress caused by
load must not exceed the
failure stress.
Always consider a Factor
of Safety.
FS= σall /σdesign
σall = Allowable Stress
σdesign = Design Stress
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Stress-strain curve for structural steel[1]
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Stress-strain curve, Aluminum[1]
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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
Medium carbon steel
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0.1% - .25% Carbon
.25% - .45%
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e.g., AISI 1018
e.g., AISI 1040
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Machine elements
•Motor
•Gear
•Belt, pulley
•Chain, sprocket
•Universal joint
•Friction drive
•Cam-follower
•Mechanisms (linkages)
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•Bearing
•Joining methods
(welding, brazing, rivets,
bolts, screws, etc)
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Motor types
• DC motors
• Stepper motors
a) Permanent magnet
(brushless)
b) DC Shunt motor
c) DC series motor
• Gearhead motors
• AC motors
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Characteristics
DC motor
a) Speed proportionate to voltage applied
b) Suitable current/torque and speed/torque curve
properties
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
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•Starting torque
•Size and weight
restrictions
•Price
•Maintenance
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Selection factors (cont’d)
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
Wrr1 r2
F
RR1 R2
W=Load force
F= balancing force
R
FR  F1r
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r
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Gear train speed ratio
D1 D2 D3 driven
speedratio

d1 d 2 d 3
driver
n 4 N1 N 3
drive


n1 N 2 N 4 driven
n= speed
N= number of teeth
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Different kinds of power transmission or
motion 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 friction
and wear
•High speed
reduction
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•Right angle I/O
torque
•Smooth tooth
interaction
•Low noise
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Rotary-Linear power
transmission
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Bearing types
Ball bearing
Thrust bearing
Tapered roller
bearing
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Needle roller
bearing
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Important factors in bearing selection
• Loads (radial, axial)
• Operating speed
• Size and weight
Information sources:
• Experts
• Manufacturer’s catalog (SKF, TIMKEN, FAG,…)
• Design handbook
A quick way of bearing selection
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 400°C
None
Good
Brazing
Good
800-1000 °C
Minimal
Excellent
Welding
Excellent
above1500°C Likely
Fair
e.g., Brazing with Bronze alloy as filler with 870-980°C for joining mild
steel with melting temperature of 1600°C[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 resistance
caused 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- electrode
5- electrode holding position
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Electrode position
in groove welding
Electrode
position in
fillet
welding
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Single stringer
bead for narrow
grooves
Weave bead
for wide groove
or multiple
stringer bead
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1- stringer bead
(steady
movement along
seam)
2- weave bead
(side to side
movement
along seam)
3- Weave pattern
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Properties of a good welding
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Troubleshooting
Excessive spatter
Porosity
Lack of penetration
Cause:
1-Amperage too high
2-Arc length too long
Cause:
1-Dirty workpiece
2-Arc length too long
3-Damp electrode
Causes:
1-Improper weld
technique
2-insufficient heat
input
Correction:
1- Decrease
amperage or select
larger electrode
2- Reduce Arc length
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Correction:
1-Remove all grease,
damp, oil, dust,...,
from work surface
2-Reduce length
3-Use dry electrode
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Correction:
1-Reduce welding
speed
2-Increase amperage,
use larger electrode
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Troubleshooting
Burn through
Wavy bead
Distortion
Cause:
Excessive heat
Cause:
Unsteady hand
Cause:
Excessive heat
Correction:
Decrease
amperage
use smaller
electrode
Increase travel
speed
Correction:
Use two hands
practice
Correction:
Use clamp to hold
base material
Use lower amperage
Make tack weld along
the edge
Weld in small
segments, allow cooling
between welds.
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Example of good and
bad welds
2006 Miller Electric Mfg Co
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1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
References
http://en.wikipedia.org
Robot mechanisms and mechanical devices illustration,McGraw Hill 2003
www.igusa.com
http://www.bodine-electric.com
http://www.engr.utexas.edu/dteach/Experience/mechanisms/brief_mecha
nisms.htm
http://www.cabaret.co.uk/education/geneva.htm
Mechanism Design: Enumeration of Kinematic Structures According to
Function, Lung-Wen Tsai, Copyright © 2006 Taylor and Francis group,
LLC.
http://www.thesaabsite.com/95/95pulleys
http://microhydropower.net/index.php
http://www.goldstarind.com/prdndx1.htm
Slides 32-39 were adapted from: ‘Guideline for shielded metal arc
welding, http://www.millerwelds.com/pdf/guidelines_smaw.pdf
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