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Leveraging Manufacturing, Part I

I recently finished a long series of articles on the power of the digital age in the form of software to connect the designer to the process, with an eye to achieving all the normal requirements of a product but, in addition, incorporating measures to drive sustainable product design and green manufacturing. There is certainly more that can be said about that.

But, not now.

I’d like to get back to a subject that was mentioned first about one and a half years ago in an earlier article – leveraging.

This was a follow on to a discussion centering on the “buy to fly” ratio concept used in the aerospace industry and discussed in another article in November 2010 covering the impact of manufacturing on product performance.

That article cited some results from VW on the role of manufacturing in the life cycle impacts of a particular VW automobile. It turned out that for a VW Golf example the data showed that there was a 20 percent manufacturing phase versus 80 percent use phase contribution to the life cycle impact of the vehicle. I then did some simple calculations about the effect of some significant savings in one phase of manufacturing due to some “greening” efforts (like using lower energy machine tools, for example) and it turned out that, when this ripples through the production and use phases of the vehicle, we get, at most, single digit improvements in the life cycle impact.

So, the question was, why bother?

I rationalized that if you are paying the electricity bill for the factory and this small technology wedge improvement is added to a lot of others in machine operation it can add up to real savings. But, maybe still not impressive compared to the full life cycle of the auto.

But, I reasoned, if we follow that logic we are leaving a lot of potential impact reduction from manufacturing on the table.

Then I gave the example of something discussed in another prior posting on precision manufacturing about a major German auto manufacturer (but not VW in this case) who has been working to improve the “power density” of some of its diesel engines over the past years and has seen an improvement of almost a factor of 3 in power per unit of displacement. That means, for the same engine size (displacement) they have managed to squeeze three times as much power out. Coupled with advanced fuel injector systems operating at very high pressures (once thought absurd) they see enhanced performance in a small engine – increased fuel economy, improved acceleration (due to reduced mass), and reduced emissions. And this was due to advanced manufacturing.

When we ripple that effect through the life cycle of the vehicle, the impact is enormous. Since most of the life cycle impact (think CO2, for example) is due to operation of the vehicle and the production of the fuel to consume in it, savings due to engine efficiency are highly leveraged.

And, the savings are attributable in large part to manufacturing.

There had been a similar example with respect to improved machining tolerances for airframe structural components in aircraft. Tighter tolerances due to improved machine tool control lead to less weight for the structural components (since we can still meet size/strength/performance requirements without “overbuilding” the component) and that means either more cargo per flight or lower fuel consumption due to decreased aircraft weight. Either one improves the performance of the aircraft. Another example of leveraging.

So, we need to look at this in more detail.

The example I’d like to use to illustrate the fundamentals of leveraging is from a recent paper from our research group at Berkeley. The full citation is “Evaluating the relationship between use phase environmental impacts and manufacturing process precision,” CIRP Annals, 60, 1, 2011, pp. 49-52 and I encourage you to look this up (or contact me and I’ll send a copy) for all the details. It is co-authored by two of my research students Moneer Helu and Athulan Vijayaraghavan (now with System Insights).

The example focuses on another aspect of vehicles and transportation – the gear train.

We saw the example for the German auto maker how manufacturing precision can have a strong effect on the operational efficiency of an automotive engine. The operational efficiency of an automobile can be generally measured based on its fuel economy. The fuel economy is strongly influenced by the construction of the powertrain, where tight tolerances and high quality surfaces in the camshaft and crankshaft bearings are required to ensure relatively low losses. Tight tolerances are also required between the piston, piston ring, and cylinder surfaces to enable the use of lower viscosity oils that reduce frictional losses in the engine. In addition to the powertrain, the drivetrain is another component of automobiles that is vital to fuel economy.

Recent work has shown that the efficiency of gear systems is due to a variety of factors including the surface roughness of the mating surfaces, assembly errors (e.g. shaft misalignments), and other manufacturing errors (e.g. form errors). Because the vast majority of environmental impacts of an automobile occur during the use phase as we saw illustrated in the VW example, the impact of increased manufacturing precision through better surface finish on the final drive reduction of an automotive manual transmission drivetrain presents the ideal case study for this investigation.

For a good review of the terms powertrain and drivetrain I suggest any book in automotive engineering or our friend Wikipedia! The term powertrain usually includes the engine, transmission, drive shafts, differentials, and the final drive (drive wheels) but it sometimes refers only to the engine and transmission. Wikipedia sums up the case well:

“Competitiveness drives companies to engineer and produce powertrain systems that over time are more economical to manufacture, higher in product quality and reliability, higher in performance, more fuel efficient, less polluting, and longer in life expectancy. In turn these requirements have led to designs involving higher internal pressures, greater instantaneous forces, and increased complexity of design and mechanical operation. The resulting designs in turn impose significantly more severe requirements on parts shape and dimension; and material surface flatness, waviness, roughness, and porosity.”

It’s this last bit – about imposing stricter requirements on, among other things, surface features including waviness and roughness – that we are going to focus on here.

I’ll start on that topic in the next article.

David Dornfeld
David Dornfeld Director, Laboratory for Manufacturing and Sustainability University of California, Berkeley55
 
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