Mechanical Elements

1

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, …)

2

Mechanical properties of materials:

Behavior of materials in response to mechanical loads. Useful in material selection.

Most basic property: stress-strain relation

3

Stress:

Strain :

)(

)(

Aarea

Fload

)(

)(

0lngthoriginalle

lgelengthchan

Strength of a material

4

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, …)

Stress-Strain relation

5

The stress caused by load must not exceed the failure stress.

Always consider a Factor of Safety.

FS= σall /σdesign

σall = Allowable Stressσdesign = Design Stress

6

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

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

7

Mild steel 0.1% - .25% Carbon e.g., AISI 1018 Medium carbon steel .25% - .45% e.g., AISI 1040

Machine elements

8

•Motor•Gear•Belt, pulley•Chain, sprocket•Universal joint•Friction drive•Cam-follower •Mechanisms (linkages)

•Bearing•Joining methods (welding, brazing, rivets, bolts, screws, etc)

Motor types

• DC motors

• Stepper motors

• Gearhead motors

• AC motors

9

a) Permanent magnet (brushless)

b) DC Shunt motorc) DC series motor

Characteristics

10

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)

Characteristics

11

Stepper motor

• Can rotate in both directions

• Moves in precise angular increments (steps)

• Sustain a holding torque at zero speed

• Easy to control

Comparison of stepper and DC permanent-magnet motors

12

Selection factors

When selecting a motor consider these issues:

• Speed range• Torque-speed variations• Reversibility• Required power• Load inertia

13

•Starting torque•Size and weight restrictions•Price •Maintenance

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?

14

Useful information sources

Experts Manufacturer specification

sheets Product catalog Design handbooks Motor nameplate Web

15

Gearhead motors

16

Force ratio for gear trains

17

21

21

RRR

rWrrF

W=Load force F= balancing force

rFFR 1

R

r

Gear train speed ratio

18

driver

driven

ddd

DDDspeedratio

321

321

driven

drive

NN

NN

n

n

42

31

1

4

n= speedN= number of teeth

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.

19

Motion and torque transmission examples ([5])

20

•Right angle crossing shafts•Self locking•High friction and wear•High speed reduction

•Right angle I/O torque•Smooth tooth interaction•Low noise

Rotary-Linear power transmission

Bearing types

21

Ball bearing

Tapered roller bearing

Needle rollerbearing

Thrust bearing

Important factors in bearing selection• Loads (radial, axial)• Operating speed• Size and weight

22

A quick way of bearing selection

Information sources:• Experts • Manufacturer’s catalog (SKF, TIMKEN, FAG,…)• Design handbook

SKF online interactive catalog (www.skf.com)

SKF interactive bearing selection example page

23

Other ways of motion transmission: Linkage mechanisms

24

Intermittent rotary motion[6]

Rotary to back and forth motion[7]

Alternative mechanism design

25

Flexible transmission

• Chain-sprocket• Belt-pulley

26

Ref[8, 9]

Ref[10]

Joining methods• Welding• Brazing• soldering

27

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)

28

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

29

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

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)

30

31

Arc Welding: Setup configuration [11]

1- power supply2- electrode holder3- workpiece4- work clamp

Welding position

32

1- workpiece 2- work clamp3- slag4- electrode5- electrode holding position

33

Electrode position in groove welding

Electrode position in fillet welding

34

1- stringer bead (steady movement along seam)

2- weave bead (side to side movement along seam)

3- Weave pattern

Single stringer bead for narrow grooves

Weave bead for wide groove or multiple stringer bead

35

36

Properties of a good welding

37

Excessive spatter

Cause: 1-Amperage too high2-Arc length too long

Correction:1- Decrease amperage or select larger electrode2- Reduce Arc length

Porosity

Cause: 1-Dirty workpiece2-Arc length too long3-Damp electrode

Correction:1-Remove all grease, damp, oil, dust,..., from work surface 2-Reduce length3-Use dry electrode

Lack of penetration

Causes: 1-Improper weld technique 2-insufficient heat input

Correction:1-Reduce welding speed 2-Increase amperage, use larger electrode

Troubleshooting

38

Burn through

Cause:Excessive heat

Correction:Decrease amperageuse smaller electrodeIncrease travel speed

Wavy bead

Cause:Unsteady hand

Correction:Use two hands practice

Distortion

Cause:Excessive heat

Correction:Use clamp to hold base materialUse lower amperageMake tack weld along the edgeWeld in small segments, allow cooling between welds.

Troubleshooting

39

Example of good and bad welds