understanding. Graphs or charts should have descriptive axis labels including appropriate units. If more than one set of data is plotted, then the graph needs to include a legend. Engineers need to carefully consider what type of graph or chart to use; the choice depends on the nature of the data and the type of insights that need to be understood by the reader.
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each showing no damage index. Presenting the data in this form clearly shows the potential O-ring damage at the forecasted temperature range. Even with a simple trend line, the damage is predicted to be signifi cantly worse than at any test case point. It is possible that an effective
presentation of the technical data could have prevented the Challenger disaster. Understanding and applying the principles of effective technical communication are an essential skill of mechanical engineers, especially when our decisions directly impact the lives of others.
3.7 Communication Skills in Engineering
Figure 3.9
O-ring damage chart.
Presidential Commission on the Space Shuttle Challenger Accident, vol. 5 (Washington, DC:
US Government Printing Offi ce, 1986.) pp. 895–896.
78°
12 12
63°
11 11
57°
10 10
70°
9 9
73°
8 8
72°
7 7
67°
6 A
6
68°
5 A
5
B B A B A B A B A B A B A B
80°
4 A
4 B
69°
3 A
3 B
70°
2 A
2 B
66°
1 A
1 B
* No Erosion History of O-Ring Damage in Field Joints
O-Ring Temp (°F)
SRM No.
E
E
H
58°
24 24
76°
23 23
75°
22 22
79°
21 21
76°
20 20
81°
19 19
70°
18 A
18
67°
17 A
17
B B A B A B A B A B A B A B
75°
16 A
16 B
53°
15 A
15 B
67°
14 A
14 B
70°
13 A
13 B O-Ring
Temp (°F)
SRM No.
S
B B
E *
*
E B
Figure 3.10
Revised O-ring damage graph.
Courtesy of Kemper Lewis.
025
35 45 55 65 75 85
12
Temperature (°F) Temperature forecast
for January 28
O-ring damage index
10
8
6
4
2
A mechanical engineer was running some tests to validate the spring constant of a new spring (part #C134). A mass was placed on a spring, and the result- ing compression displacement was measured. Hooke’s Law (discussed more in Chapter 5) states that the force exerted on a spring is proportional to the displacement of the spring. This can be expressed by
F 5 kx
Where F is the applied force, x is the displacement, and k is the spring con- stant. The data was recorded in the following table in SI units.
mass displacement
0.01 0.0245
0.02 0.046
0.03 0.067
0.04 0.091
0.05 0.114
0.06 0.135
0.07 0.156
0.08 0.1805
0.09 0.207
0.1 0.231
The engineer is tasked with developing a professional table and graph that com- municates the data and explains the Hooke’s Law relationship for the spring.
First, the engineer needs to calculate the resulting force from the applied mass using w mg and construct a table that illustrates the force and displace- ment data.
Example 3.10 Written Communication
Table 3.7
Results of Spring Test Data
Mass (kg) Force (N) Displacement (m)
0.01 0.098 0.0245
0.02 0.196 0.0460
0.03 0.294 0.0670
0.04 0.392 0.0910
0.05 0.490 0.1140
0.06 0.588 0.1350
0.07 0.686 0.1560
0.08 0.784 0.1805
0.09 0.882 0.2070
0.10 0.980 0.2310
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Note the following best practices regarding Table 3.7.
• The engineer has added the calculated force values • Units for each column have been added
• Appropriate borders to separate the data have been added
• The number of signifi cant digits in each column is now consistent • The headings are capitalized and bolded
• The data is aligned to make each column easy to read
Second, the engineer must communicate the spring rate relationship in the data table. A scatter plot is chosen and created in Figure 3.11. This graph effectively illustrates the relationship between force and displacement and demonstrates how well the data aligns with the linear relationship predicted by Hooke’s Law.
Example 3.10 continued
Note the best practices regarding Figure 3.11:
• The axes are clearly labeled, including appropriate units • A descriptive title accompanies the graph
• A trend line clearly demonstrates the linear relationship between the variables
• The number of gridlines is minimal and used only for visual aids
• The data spans the axes, eliminating large areas of empty space in the graph
3.7 Communication Skills in Engineering
0.000
0.2 0.4 0.6 0.8 1
0.12
Displacement (m)
Validation of Hooke’s Law for Spring #C134
Force (N)
0.10
0.08
0.06
0.04
0.02
Figure 3.11
Example of a professional engineering graph.
Courtesy of Kemper Lewis.
S UMMARY
Engineers are often described as being proactive people with excellent problem-solving skills. In this chapter, we have outlined some of the fundamental tools and professional skills that mechanical engineers use when they solve technical problems. Numerical values, the USCS and SI systems, unit conversions, dimensional consistency, signifi cant digits, order-of-magnitude approximations, and the ability to communicate technical results effectively are, simply put, everyday issues for engineers. Because each quantity in mechanical engineering has two components—a numerical value and a unit—reporting one without the other is meaningless. Engineers need to be clear about those numerical values and dimensions when they perform calculations and relate their fi ndings to others through written reports and verbal presentations. By following the consistent problem-solving guidelines developed in this chapter, you will be prepared to approach engineering problems in a systematic manner and to be confi dent of the accuracy of your work.
Self-Study and Review
3.1. Summarize the three major steps that should be followed when solving technical problems in order to present your work clearly.
3.2. What are the base units in the USCS and SI?
3.3. What are examples of derived units in the USCS and SI?
3.4. How are mass and force treated in the USCS and SI?
3.5. What is the major difference in the defi nitions of the slug and pound- mass in the USCS?
3.6. What is the difference between the pound and pound-mass in the USCS?
3.7. One pound is equivalent to approximately how many newtons?
3.8. One meter is equivalent to approximately how many feet?
3.9. One inch is equivalent to approximately how many millimeters?
3.10. One gallon is equivalent to approximately how many liters?
3.11. How should you decide the number of signifi cant digits to retain in a calculation and to report in your fi nal answer?
3.12. Give an example of when the technical problem-solving process can be used to make order-of-magnitude approximations.
3.13. Give several examples of situations where engineers prepare written documents and deliver verbal presentations.
Using the table and graph, the engineer can quickly estimate and communi- cate the spring constant for the spring as 4 N/m and validate that against the design requirements.
Example 3.10 continued
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