General Process of Product Embodiment
http://www.youtube.com/watch?v=tmXtt_JcfeE
Embodiment design is where we deal with specific parameters and layout, that is, parametric and layout design of the product. At the end of concept development, we have a concept that has been logically chosen from a number of developed solution alternatives.
In general, how do we deal with complex characteristics of embodiment design? The general idea is to iteratively refine the geometry and layout of a product from an abstract form to a concrete one. Figure 12.6 illustrates a general process for implementing this idea (Pahl and Beitz).
The process begins by considering the product specifications. Using the customer needs and these specifications (Chapters 4 and 7), the critical needs are identified that will drive the embodiment of the product. Examples include size-determining specifications, arrangement determining specifications, and material-determining specifications. After choosing the driving specifications, an overall layout of the product is drawn to scale based on the concept drawings. At this stage, the drawings (sketches) should not be fully detailed and care should be taken not to over constrain the layout. Through these drawings, the following items are illustrated: maximum dimensions of the product, clearances between relative subsystems, installation paths, and the general arrangement of components relative to one another.
Embodiment Checklist
To supplement the general embodiment process (Figure 12.6), a second basic method is the application of an embodiment checklist (after Pahl and Beitz). Such a checklist, as illustrated in Table 12.1, provides a systematic approach to apply proven design principles during product development. This checklist is created from generic (and historically proven) design principles of ensuring robustness, clarity, simplicity, and safety in a product.
Robustness is the design principle that seeks to minimize the variability in performance of a product under all expected environmental and user conditions. This principle provides a basis for understanding the impact of noise on a product's performance.
Clarity is the basic principle that all functions should be unambiguously specified, in form, parameters, manufacturing, and assembly. Unintended functions should not be present in a product. It also assumes that product functions (or function chains) will be implemented as independent as possible. In doing so, the performance of each product function (or function chains) can be controlled and modified without deteriorating or compromising the performance of other product functions.
The design principle of simplicity, on the other hand, is the minimization of information content within a product design.
Example: Drive Train Subsystem of a Radio-Controlled (RC) Car
http://www.youtube.com/watch?v=r_zZGXWFM5w
Consider a product found in hobby stores: a RC
car. Electric RC cars are entertainment products that emulate
full-scale on-road or off-road vehicles. They are controlled by a
transmitter (speed and steering) and a
re
powered by 6-7 D-cell batteries. An embodied version of a popular RC
car is pictured in Figure 12.7.
Applying the checklist to the RC car, one of the questions (Layout,
Geometry, and Materials category) is stated as, "Does the chosen layout ensure freedom from resonance?" Many components are subject to resonance, including all of the rotating components (bearings, shafts, etc.) and the helical suspension springs. Of these components, the drive shafts of the car are the most susceptible due to their nominal long length and small cross section.
Figure 12.8 illustrates a possible drive shaft layout. One issue with drive shafts is whether the shaft will excessively vibrate and thereby cause premature failure or annoying dynamics. One mode of the vibration is when the shaft is not centered as it rotates-when the middle of the shaft is radially offset outward as the shaft rotates. Based on a downward deflection of the shaft due to its weight, the critical speed of shaft may be calculated by
w
here
Wcr
is the critical speed for resonance (in
rpm), g is
the acceleration constant due to gravity, and 8 is the lateral
deflection of the shaft (Juvinall and Marshe). Substituting variables
from Figure 12.8, the critical speed for the current shaft layout is
approximately 3600 rpm. On the other hand, based on the choices of
motor and gear train variables, the maximum driven speed of the shaft
will be 2500 rpm. Based on this analysis, the shaft will not
resonate. If the shaft speed were close to or greater than the
critical speed, the shaft dimensions would have to be modified
(radius increased slightly) to satisfy the resonance checklist item.
As shown by this example, embodiment checklists such as the one illustrated in Table 12.1 motivate the careful consideration of both performance issues and possible failure modes of a product. Checklists of this type are constructed from historical data and past product developments. They should be used, in conjunction with the general embodiment process, to carefully create detailed subassembly layouts and properly select parameters that will ensure a robust design.