9.3 Form Generation
249
Figure 9.5 An example of integral architecture. Reprinted with permission
of Boeing.
A pull in the opposite direction from a modular architecture is to design an integral architecture. Integral architectures have fewer parts with all the functions blurred
together. An illustration is the Blended Wing Body BWB concept developed by Boeing, shown as a test vehicle in Fig. 9.5. In this design, the assignment of
functionality between wing, fuselage, and empennage are blended. A traditional aircraft uses wings for lift generation, a fuselage for storage of passengers and
cargo, the tail for pitch and yaw control. In the BWB, on the other hand, the integral “blended” body provides all three functions to some extent. This blending when
compared to a traditional plane leads to 19 lower weight and 32 less projected fuel burn per passenger per mile flown.
9.3.3 Develop Connections: Create and Refine
Interfaces for Functions This is a key step when embodying a concept because the connec-
Form Function
Material Production
Assembly Manufacture
Connections
Components Configuration
Constraints
tions or interfaces between components support their function and determine their relative positions and locations.
Here are guide- lines to help develop and refine the interfaces between components:
■
Interfaces must always reflect force equilibrium and consistent flow of energy, material, and information.
Thus, they are the means through which the product will be designed to meet the
functional requirements. Most design effort occurs at the connections between components, and attention to these interfaces and the flows through
them, is key to product development. During the redesign of an existing product, it is useful to disassemble it; note the flows of energy, information,
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Complexity occurs primarily at interfaces. and materials at each joint; and develop the functional model one component
at a time.
■
After developing interfaces with external objects, consider the interfaces that carry the most critical functions.
Unfortunately it is not always clear which functions are most critical. Generally, they are those functions that
seem hardest to achieve about which the knowledge is the weakest or those described as most important in the customers’ requirements.
■
Try to maintain functional independence in the design of an assembly or component.
This means that the variation in each critical dimension in the assembly or component should affect only one function. If changing a param-
eter changes multiple functions, then affecting one function without altering others may be impossible.
■
Exercise care when separating the product into separate components. Com-
plexity arises since one function often occurs across many components or assemblies and since one component may play a role in many functions.
For example, a bicycle handlebar discussed in Section 2.2 enables many functions but does none of them without other components.
■
Creating and refining interfaces may force decompositions that result in new functions or may encourage the refinement of the functional breakdown.
As the interfaces are refined, new components and assemblies come into existence. One step in the evaluation of each potential embodiment is to determine how each
new component changes the functionality of the design. In order to generate the interface, it may be necessary to treat it as a new
design problem and utilize the techniques developed in Chaps. 7 and 8. When developing a connection, classify it as one or more of these types:
■
Fixed, nonadjustable connection. Generally one of the objects supports the
other. Carefully note the force flow through the joint see Section 9.3.4. These connections are usually fastened with rivets, bolts, screws, adhesives,
welds, or by some other permanent method.
■
Adjustable connection. This type must allow for at least one degree of freedom
that can be locked. This connection may be field-adjustable or intended for factory adjustment only. If it is field-adjustable, the function of the adjustment
must be clear and accessibility must be given. Clearance for adjustability may add spatial constraints. Generally, adjustable connections are secured with
bolts or screws.
■
Separable connection. If the connection must be separated, the functions
associated with it need to be carefully explored.
■
Locator connection. In many connections, the interface determines the
location or orientation of one of the components relative to another. Care
9.3 Form Generation
251
Determine how constrained a component needs to be, and constrain it exactly that amount—no more, nor no less.
must be taken in these connections to account for errors that can accumulate in joints.
■
Hinged or pivoting connection. Many connections have one or more degrees
of freedom. The ability of these to transmit energy and information is usually key to the function of the device. As with the separable connections, the
functionality of the joint itself must be carefully considered.
Connections directly determine the degrees of freedom between components and every interface must be thought of as constraining some or all of those degrees
of freedom. Fundamentally, every connection between two components has six degrees of freedom—three translations and three rotations. It is the design of the
connections that determines how many degrees of freedom the final product will have. Not thinking of connections as constraining degrees of freedom will result
in unintended behavior. This discussion on two-dimensional constraints gives a good basis for thinking about connections.
If two components have a planar interface, the degrees of freedom are reduced from six to three, translation in the x and y directions in both the pos-
itive and negative directions and rotation in either direction about the z axis Fig. 9.6. Putting a single fastener—like a bolt or pin—through component A
into component B can only remove the translation degrees of freedom, but leaves rotation. Some novice designers think that tightening the bolt very tight will re-
move the rotational freedom, but even a slight torque around the z axis will cause
A to rotate. Using two fasteners close together may not be sufficient to restrain part A from rotating, especially if the torque is high relative to the strength of
the fasteners or the holes in A and B. Even more importantly, most joints need to
z
x
y A
B
Figure 9.6 Three-degree-of-freedom
situation.
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z
x
y A
B z
x
y A
B Figure 9.7
Block A restricted by a pin or short wall. z
x
y B
z
x
y B
A A
Figure 9.8 Efforts to fully constrain along the x axis.
position parts relative to each other and transmit forces. Thus, it is worthwhile to think in terms of positioning and then force transmission.
Fasteners like bolts and rivets are not good for locating components as the holes for them must be made with some clearance and fasteners are not made with
high tolerances. For positioning, first consider a single pin or short wall, as shown in Fig. 9.7. The effect of these will be to only limit the position of A relative to B
in the +x direction.
If there is a force always in the positive x direction, then this single constraint fully defines the position on the x axis. Putting a second support on the x axis
to limit motion in the negative x direction can have unintended consequences see Fig. 9.8. Due to manufacturing variations, block A will either be loose or
binding. In other words, even though block A looks well constrained in both the +
and −x directions, this will be hard to manufacture and to make work like it is drawn. Additionally, the second pin does nothing to constrain the motion in the y
direction or rotations about the z axis. If there are two pins or a long wall positioning the side of the block see
Fig. 9.9, then the x position and angle about the z axis are limited. If a sufficient force pushes in the +x direction, between the pins, then the
block is fully constrained in the x direction and about the z axis. However, if the force has any y component, block A can still move in the y direction.
9.3 Form Generation
253
z
x
y A
B z
x
y A
B Figure 9.9
Block A restriction in the x direction and z rotation. z
x
y A
B F
y x
Figure 9.10 Block A fully constrained.
Finally, if a pin is attached to component B so that component A is restrained from moving in the y direction and a force F is directed between the limits shown
in Fig. 9.10 the force has a positive x and y direction, then component A is fully constrained and has no degrees of freedom relative to component B. What is vitally
important here is that it takes exactly three points to constrain one component to another.
The three points to constrain component A relative to component B can take many forms. A few of these are shown in Fig. 9.11.
9.3.4 Develop Components
It has been estimated that fewer than 20 of the dimensions on
Form Function
Material Production
Assembly Manufacture
Connections Components
Configuration Constraints
most components in a device are critical to performance. This is because most of the material in a component is there to connect
the functional interfaces and therefore is not dimensionally criti- cal. Once the functional interfaces between components have been
determined, designing the body of the component is often a so- phisticated connect-the-dots problem.
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F A
B B
B A
F F
A
Figure 9.11 Other fully constrained blocks.
Fastening area Hinge line
Wing
5 cm 4.5 cm
9 cm 25 cm
Fastening area Hinge pin: 1 cm dia.
Loads: 100 N vertical 100 N horizontal
Figure 9.12 Requirements on an aircraft hinge plate.
Consider this example of an aircraft hinge. The spatial constraints for this and its interface points i.e., fastening area are shown in Fig. 9.12. The major functions
of this individual component are to transfer forces and clear not interfere with other components. The load on the component and the geometry of the interfaces
are detailed in the figure. The component is a simple structural member that must transfer the load from the hinge line to the fastening area. As shown in
Fig. 9.13, there are many solutions to this problem. The solutions in Figs. 9.13a
9.3 Form Generation
255
a
b c
d Solid block
with three holes Hinge pin
Fastener to wing
Machined block Welded structure
Forged part with holes drilled in
secondary operation
Figure 9.13 Potential solutions for the structure of the aircraft hinge plate.
Components grow primarily from interfaces. and 9.13b are machined out of a solid block of material. The solution in Fig. 9.13c
is made from welded sections of off-the-shelf extruded tubing and plate. These three solutions are good if only a few hinge plates are to be manufactured. If the
number to be produced is high, then the forged component in Fig. 9.13d may be a good solution. Note that all four of these components have the same interfaces
with adjacent components. One interface is fixed and may need to be removable, and the other has one degree of freedom. The only difference is in the body, the
material connecting the interfaces. All of these product designs are potentially acceptable, and it may be difficult to determine exactly which one is best. A
decision matrix may help in making this decision.
The material between interfaces generally serves three main purposes: 1 to carry forces or other forms of energy heat or an electrical current, for instance
between interfaces with sufficient strength and rigidity; 2 to act as an enclosure or guide for other components guiding airflow, for instance; or 3 to provide
appearance surfaces. We have said before that functionality occurs mainly at component interfaces; this is not always true. The exception occurs when the body
of a component provides the function—for example, needed mass, stiffness, or strength—in which case, shape can be as important as the interface.
It is best to connect interfaces with strong structural shapes. Strong shapes have material distributed to make the best use of it. Common strong structural
shapes are listed next.
■
The simplest strong shape is a rod in tension or compression. If a shape has two interfaces, as shown in Fig. 9.14a, and it needs to transmit a force from
one to the other, the strongest shape to use is a rod in tension, Fig. 9.14b. Once away from the ends interfaces, the forces are distributed as a constant stress
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Component Interfaces
Force Force
a
b Figure 9.14
A bar in tension. B
A
Figure 9.15 A triangulated component.
Triangulate Unless you have a very good reason not to. throughout the rod. Thus, this shape provides the most efficient in terms of
amount of material used to transmit the force shape possible.
■
A truss carries its entire load as tension or compression. A rule of thumb is always triangulate the design of shapes.
This is often accomplished by pro- viding shear webs in components to effectively act as triangulating members.
The back surface in Fig. 9.15 acts as a shear web to help transmit force A to the bottom surface. Take away the back surface and the structure collapses.
A rib provides the same function for force B.
■
A hollow cylinder, the most efficient carrier of torque, comes as close as possible to having constant stress throughout all the material. Any closed
prismatic shape exhibits the same characteristic. A common example of an approximately closed prismatic shape is an automobile or van body. As the
front right wheel of the van shown in Fig. 9.16 goes over a bump, a torque is put on the entire vehicle. Cutting holes in the sides for doors greatly weakens
the torque-carrying capability of a van, and it requires additional, heavy structure to make up the difference.
■
An I-beam is designed to carry bending loads in the most efficient way pos- sible, since most of the material is far away from the neutral bending axis.
The principle behind the I-beam is shown in the structural shape of Fig. 9.17.
9.3 Form Generation
257
Figure 9.16 Component that efficiently carries torque.
Fixed to wall x
x x
Figure 9.17 Example of an I-beam
structure.
Forces flow like water. Failures occur mainly in the rapids. Although not an I, it behaves much like one, as the majority of the material
labeled “x” is as far from the neutral bending axis as possible. Less stress is generally developed if direct force transmission paths are used.
A good method for visualizing how forces are transmitted through components and assemblies is to use a technique called force flow visualization. These rules
explain the method.
1. Treat forces like a fluid that flows in and out of the interfaces and through the
component. It makes no difference which way you assume the fluid flows. It is the path that is important.
2. The fluid takes the path of least resistance through the component.
3. Sketch multiple flow lines. The direction of each flow line will represent the
maximum principal stress at the location.
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Figure 9.18 Force flow in the tail stock of a clamp. Reprinted with permission of Irwin
Industrial Tools.
4. Label the flow lines for the major type of stress occurring at the location:
tension T, compression C, shear S, or bending B. Note that bending can be decomposed into tension and compression and that shear must occur
between tension and compression on a flow line.
5. Remember that force is transmitted at interfaces primarily by compression.
Shear only occurs in adhesive, welded, and friction interfaces. Two examples clearly illustrate many of the preceding rules. The first is from
the tail stock of the Irwin one-handed clamp Fig. 9.18a. Assume it is loaded at the worst possible condition with a force at the tip as shown. The free-body
diagram Fig. 9.18b shows the force balanced in the horizontal direction by a pin through the bar. The couple created by these forces is countered by a vertical
force couple on the two pins pressing against the bar as shown.
Following the rules just listed, the force flow in the tail stock looks as shown in Fig. 9.18c. The flow enters leaves at the tip of the tail stock and leaves
enters at the compression interface between the tail stock and the three pins. First, consider the bending created by the force on the tip of the tail stock. The
middle of the part is like an I-beam, the top is in compression, and the bottom is in tension. Thus, a compression flow line should go from the force on the tip of
the tail stock, down the top of the part to the pin. Since the I-beam cross section is in bending, the bottom of the tail stock must be in tension. At some point between
the compressive force at the tip and the tensile force in the body there is shear as shown. The tension then flows around the bottom pin to become compressive at
the interface with the pin. To visualize this shear take a piece of notebook paper, insert a pencil in one hole, and pull the pencil toward the nearest edge in the plane
of the paper. Note that the rip occurs in approximately 45
◦
, signifying a shear failure.
Besides the bending, the force at the tip applies a compressive horizontal load countered by the pin in the center. Depending on the geometry, the entire
9.3 Form Generation
259
part, including the bottom of the I-beam section may be in compression. Also, there will be some shear occurring in order to get the compressive force to the
pin. This force flow is shown by a dashed line in Fig. 9.18c. The tee joint in Fig. 9.19a represents a second example of the use of force
flow visualization. Figure 9.19b shows two ways of representing the force flow in the flange. The left side shows the bending stress in the flange labeled B; the right
side shows the bending stress decomposed into Tension T and Compression C, which forces consideration of the shear stress. The force flow through the nut and
bolt is shown in Figs. 9.19c and 9.19d. The force flow in the entire assembly is shown in Fig. 9.19e.
In summary, force flow helps us visualize the stresses in a component or assembly. It is best if the force paths are short and direct. The more indirect
the path, the more potential failure points and stress concentrations. Developing force flows comes with practice and comparison to detailed analyses from finite
element programs. With practice, you can learn where to look for failures.
In designing the bodies of components, be aware that stiffness determines the adequate size more frequently than stress. Although component design textbooks
emphasize strength, the dominant consideration for many components should be their stiffness. An engineer who used standard stress-based design formulas to
analyze a shaft carrying a small torque and virtually no transverse load found that
T T
T S
S S
C C
C
C C
C B
a
e d
c b
Figure 9.19 Force flow in a tee joint.
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it should be 1 mm in diameter. This seemed too small a gut-feeling evaluation, so the engineer increased the diameter to 2 mm and had the system built. The
first time power was put through the shaft, it flexed like a noodle and the whole machine vibrated violently. Redesign based on stiffness and vibration analysis
showed that the diameter should have been at least 10 mm to avoid problems.
Finally, in designing components, use standard shapes when possible. Many companies use group technology to aid in keeping the number of different compo-
nents in inventory to a minimum. In group technology, each component is coded with a number that gives basic information about its shape and size. This coding
scheme enables a designer to check whether components already exist for use in a new product.
9.3.5 Refine and Patch
Although not shown as a basic element of product design in Fig. 9.3, refining and patching are major parts of product evolution. Refining, as described in
Section 2.3, is the activity of making an object less abstract or more con- crete. Patching is the activity of changing a design without changing its level of
abstraction.
The importance and interrelationship of refining and patching the shape can be clearly seen in the following example. A designer was developing a small box
to hold three batteries in a series. This subsystem powered the clockcalendar of a personal computer. The designer’s notebook sketch of the final assembly is
shown in Fig. 9.20. The assembly is composed of a bottom case, a top case, and four contacts. Figure 9.21 shows the evolution of one of the contacts contact 1,
again through the sketches and drawings made by the designer. The number be- side each graphic image shows the percentage of the total design effort completed
when the representation was made. The designer was simultaneously at work on other components of the product. The circled letters in Fig. 9.21c were added for
this discussion and were not in the original drawings. The design of the battery contact is one of continued refinement. Each figure in the series moves the design
closer to the final form of the component. The initial sketch Fig. 9.21a shows circles representing contact to the battery and a curved line representing current
conduction. The final drawing of the contact Fig. 9.21e is a detailed design ready for prototyping. Figure 9.21c is of special interest, as it clearly shows the evo-
lutionary process. The designer began by redrawing the left contact, A, from the earlier sketch Fig. 9.21b. She also redrew line B, which represents an edge of the
structure connecting the contact to the wire. After beginning to draw this line, she realized that, since she last worked on this component, a plastic wall, C, had been
added to the product and the contact could no longer continue straight across.
At this point, she patched the design by tilting the connecting structure B up to position D. The sketch was then completed, with the wire connection still repre-
sented by an arc E. Moments later, the designer further patched the component by combining the wire and the connecting structure, making the structure between
contacts all one component F, and then immediately redrew it, as in Fig. 9.21d.
9.3 Form Generation
261
Figure 9.20 Complete layout of battery case.
The elimination of the wire simplified the component; there was no reason for a separate wire in the first place. The battery contact was patched by combining
two components. The component was then refined to a fully dimensioned form Fig. 9.21e.
From this example and others, we can identify many different types of patching:
■
Combining: Make one component serve multiple functions or replace mul-
tiple components. Combining will be strongly encouraged when the product is evaluated for its ease of assembly Section 11.5.
■
Decomposing : Break a component into multiple components or assemblies.
As new components or assemblies are developed through decomposition, it is always worthwhile to review constraints, configurations, and connections
for each one. Because the identification of a new component or assembly establishes a new need, it is even worthwhile to consider returning to the
Kategori