Forces within an assembly are what make the parts move. The position and orientation of these forces are design choices that the designer makes, and so the reliable and carefree motions of a product are under the control of a designer. They are not the result of arbitrary effects; they are the result of how the designer applies physical principles in a product.
To properly design forces in a mechanical assembly, one should distinguish three forces that act on any moving part. The first is weight. The second type is a frictional force, which resists motion. The final type is an applied force (applied load), which a designer uses to create the motion. Whether a part moves freely or binds is completely determined by the interactions of these three forces.
As an example, consider a college design competition to deliver hockey pucks to a remote target. One design calls for a rack of pucks (mounted on a wheeled chassis) to be pushed off sequentially by driving a pusher with a worm drive, as shown in Figure 12.9. The left-hand design does not have the center of mass (weight) aligned with either the center of applied forces or with the center of friction. It will bind.
The second design has the center of applied forces aligned with the center of mass but not with the center of friction. It is less likely to bind and will avoid binding only through widening of the space between the linear bearings on the slide rails, creating a cantilever. This design is good for fast-moving applications, where the inertial force is higher than the frictional force.
The third design has the center of applied forces more aligned with the center of the friction forces. It does not have the center of mass aligned with either of the other two centers. This design is good for slower moving applications, where the inertial force is negligible. It is less likely to bind, and again, will only not bind through a wide linear bearing spacing on the slide rails.
Note that in the concept of Figure 12.9, it is impossible to align the frictional force centroid with the other two centroids unless one introduces an upper rail to frictionally impede the top surfaces of the
pucks. This results in a poor design-again, one should understand the heuristic and use wisdom in applying it.
V. SUMMARY AND "GOLDEN NUGGETS"
The focus of this chapter is concept embodiment, the stage of product development where concepts are transformed into physical realizations. A number of basic and advanced methods are available to aid in this transformation process. Yet, even with these methods, concept embodiment is a highly nonlinear, iterative, and complex process. It requires sound engineering skills, ranging from modeling and experimentation to manufacturing, assembly, and tooling design. Because of these characteristics, the remainder of this book is devoted to the topic of concept embodiment. Techniques and applications are developed in these chapters to help us understand and exercise the skills that are required.
Golden nuggets that can be derived from this chapter include the following:
Concept embodiment requires the input of a creative and feasible conceptual design. A great embodiment of a poor concept will result in a product that fails in the market.
To realize a product concept, the focus should be on the combined concepts of customer quality and engineering quality.
Engineering quality is equivalent to the implicit or "expected quality" that customers assume will exist in a product (at low relative cost).
Concept embodiment must consider the entire life cycle of a product, not merely the delivering of a product to the retail market.
Systems modeling is a skill that must be honed and applied during concept embodiment. Through systems modeling, products may be studied and refined using physical insights into a product's operation.
Design principles, based on past designs and physical principles, should be consciously applied during embodiment design. Through conscious application, a product's functions may be added to or modified, and its layout will change. The result will be a more robust offering.
Failure Modes and Effects Analysis (FMEA) is essential to create products that are robust and ethically responsible.
Design for manufacturing and assembly must be considered before and during concept embodiment. It is thus important to understand the manufacturing and assembly principles that govern any possible process choices for a product.
The nature of concept embodiment is complex and nonlinear. Thousands upon thousands of parameters and decisions must be systematically contemplated.