INTRODUCTION TO DESIGN

25

 

1

W

4 500 lb

− sec2

 

m =

=

f

=

lbf

=

a

139.9

139.9 slugs

( )

g

32.17 ft sec2

ft

For the ips system:

 

W

4 500 lb

− sec2

 

m =

=

f =

lbf

=

b

11.66

11.66 blobs

( )

g

386 in sec2

in

For the SI system:

4.448 N

W = 4 500 lb

= 20 016 N

lb

W

20 016 N

 

m =

=

=

N − sec2 =

c

2040

2 040 kg

( )

g

9.81 m sec2

m

For the cgs system:

4.448 E 5 dynes

W = 4 500 lb

= 2.002 E 9 dynes

lb

W

2.002 E 9 dynes

dynes − sec2

m =

=

=

E

=

E

d

2.04 6

2.04 6 g

( )

g

981 cm sec2

cm

2 For mass expressed in lbm, equation 1.4 b (p. 22) must be used.

g

m = W c =

=

386 in sec2

 

e

4 500 lbf

4 500 lb

g

386 in sec2

m

( )

Note that lbm is numerically equal to lbf and so must not be used as a mass unit unless you are using the form of Newton’s law expressed as equation 1.4 b.

 

 

 

 

1.10 SUMMARY

Design can be fun and frustrating at the same time. Because design problems are very unstructured, a large part of the task is creating sufficient structure to make it solvable.

This naturally leads to multiple solutions. To students used to seeking an answer that matches the one in the “back of the book” this exercise can be frustrating. There is no

“one right answer” to a design problem, only answers that are arguably better or worse than others. The marketplace has many examples of this phenomenon. How many different makes and models of new automobiles are available? Don’t they all do more or less the same task? But you probably have your own opinion about which ones do the task better than others. Moreover, the task definition is not exactly the same for all examples. A four-wheel-drive automobile is designed for a slightly different problem definition than is a two-seat sports car (though some examples incorporate both those features).

The message to the beginning designer then is to be open-minded about the design problems posed. Don’t approach design problems with the attitude of trying to find “the right answer,” as there is none. Rather, be daring! Try something radical. Then test it

Image 47

 

26

MACHINE DESIGN -

An Integrated Approach

 

 

1

with analysis. When you find it doesn’t work, don’t be disappointed; instead realize that you have learned something about the problem you didn’t know before. Negative results are still results! We learn from our mistakes and can then design a better solution the next time. This is why iteration is so crucial to successful design.

The computer is a necessary tool to the solution of contemporary engineering problems. Problems can be solved more quickly and more accurately with proper use of computer-aided engineering (CAE) software. However, the results are only as good as the quality of the engineering models and data used. The engineer should not rely on computer-generated solutions without also developing and applying a thorough understanding of the fundamentals on which the model and the CAE tools are based.

 

Important Equations Used in This Chapter

See the referenced sections for information on the proper use of these equations.

 

Mass (see Section 1.9):

 

 

 

W

m =

(1.3)

gc

Dynamic Force—for use with standard mass units (kg, slugs, blobs) (see Section 1.9): F = ma

(1.4 a)

 

Dynamic Force—for use with mass in lb = lb (see Section 1.9): m

f

ma

 

F =

(1.4 b)

gc

 

 

1.11 REFERENCES

1 Random House Dictionary of the English Language. 2nd ed. unabridged, S.B.

Flexner, ed., Random House: New York, 1987, p. 1151.

2 R. L. Norton, Design of Machinery: An Introduction to the Synthesis and Analysis of Mechanisms and Machines, 3ed. McGraw-Hill: New York, 2004, pp. 7-14.

3 Autocad, Autodesk Inc. , http://usa.autodesk.com

4 Unigraphics, EDS, Cyprus, CA, http://www.eds.com

5 ADAMS, Mechanical Dynamics, MSC Software, http://www.krev.com

6 Working Model, MSC Software, http://www.krev.com.

7 Pro/Engineer, Parametric Technology Corp., Waltham, MA, http://www.ptc.com

8 TK Solver, Universal Technical Systems, Rockford, IL, http://www.uts.com

9 Mathcad, Mathsoft Inc., Cambridge, MA, http://www.mathsoft.com

10 Excel, Microsoft Corp., Redmond, WA, http://www.microsoft.com

11 MATLAB, Mathworks Inc., Natick, MA, http://www.mathworks.com

12 Solidworks, Solidworks Corp., Concord, MA, http://www.solidworks.com

 

Image 48

 

Chapter 1