reverse engineering a bathroom scale reve

8/3/2019 Reverse Engineering a Bathroom Scale Reve

1/15

Reverse Engineering a Bathroom

Scale

W Dornfeld6Oct2011

Note: Some of the numbers in here were just made up

to illustrate the process. Others are valid.


2/15

How the Scale Works

Analog

Dial

Digital

Readout

Strain Gauge


3/15

Free Body Diagrams for 300 Lb User

150Lb150Lb

75Lb x 4

300Lb

150Lb x 2

During MeasurementGetting On & Off


4/15

Views of Analog Scale Mechanism


5/15

View of Digital Scale Mechanism


6/15

Cantilever Beam with Strain Gauge On Underside

Load

Strain Gauge


7/15

Minor Arm

Major Arm

Free Body Diagrams

for 300 Lb User

F1 F2

300/4 =

75 Lb

121mm

6mm

LbF

FMF

719.3121/450

)121()6)(75(

2

21

==

=

Fc

300/4 =

75 Lb

F2

F3

Lb

F

FFM

C

CF

048.6227/9.1372

227/]975)107)(719.3[(

)13)(75()107()227( 23

==

+=

+=

13mm227mm

107mm

Weight Fraction on Cantilever

8.24/10403.0150/048.6 ==


8/15

Worst Case Stress Analysis of Major Arm

7.44

150

144.9212.52

V

M

Max Moment = (145)

(0.51) = 74.17Lb.in

ksiI

Mc26.56

000207.0

)157)(.51.0)(145(===

Assuming this is 1040 steel with a yield strength of 51ksi, this

gives a static FOS = 0.906.

Cross Section Area has

an Area Moment of Inertia

= bh/12 = 0.000207in4

2.02 x 8 mm

7.44

150

12.52 Lb


9/15

Stress Analysis of Minor Arm

150

142.5620.46 Lb

V

M

Max Moment = (142.56)

(0.236) = 33.64Lb.in

ksiI

Mc40.33

000139.0

)138)(.236.0)(56.142(===

Assuming this is 1020 steel with a yield strength of 43ksi, this


Cross Section Area has

an Area Moment of Inertia

= bh/12 = 0.000139in4

2.02 x 7 mm


10/15

Load Sensing Arm

11.75mm

1.72mm

44.35mm

12.52 Lb

MPaI

Mc

mmbh

I

bend3.426

4.98

72/2)(44.35)(1.4.448N/Lb))(52.12(

98.412

)72.1)(75.11(

12

433

===

===

mmMPaEI

FlyDeflection 57.1)98.4)(10207)(3(

)35.44)(448.4)(52.12(3 3

33

=

==


11/15

Load Link

MPaLbNLb

A

F

mmtwA

axial285.6

2.63

)/448.4)(719.3(

63.2)94.0)(8.2(

2

2

===

===

2.8mm

0.94mm

Assuming this is 1020 steel with a yield strength of 295MPa, this



12/15

Fatigue Analysis of Major Arm

7.44

150

144.9212.52

alt

0

mean

1000

2000

3000

Stress

(psi)

0

20

40

60

0 20 40 60 80 100 120 140 160 180 200 220 240 260

Mean Stress (ksi)

AlternatingStress

(ksi)

Sut

Se

M

A

Assuming this is 1020 steel, this component will have infinite life.

Surface Factor = X

Size Factor = Y

Reliability Factor = Z


13/15

Summary

# Component Static FOS Life

1 Major Arm 0.906 > 10^6

2 Minor Arm 1.29

3 Cantilever 2.5

4 Load Link 46.9

5 Housing 100

6 Cover Vs 120


14/15

Recommendations

Do FEA on the Arms to get better stress analysis Make the arms out of thicker material

Test the scale on a tensile tester to see what breaks

first

Include effect of jumping on scale Test the material to determine its properties

Consider lateral buckling of the arms

Reference:

Diagram of Scale: http://home.howstuffworks.com/inside-scale1.htm


15/15

Minor Arm

Major Arm

121mm

6mm

13mm

227mm

107mm

11.75mm

1.72mm

44.35mm

Cantilever Beam

reverse engineering a bathroom scale reve

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