Letters 01/08/2009

Jan. 8, 2009
Regarding Don Vorwerk’s letter “MPG Squabbles” (Nov. 6), I have heard many people say auto mileage would improve significantly if only we would go back to manual transmissions


A reader suggests modern automatic transmissions have erased any mileage gains once available with manual transmissions. And another reader chimes in with praise for Toyota’s approach to product development and tolerancing. GM, are you listening?

Pinning down MPG
Regarding Don Vorwerk’s letter “MPG Squabbles” (Nov. 6), I have heard many people say auto mileage would improve significantly if only we would go back to manual transmissions. These people always wonder why automakers don’t offer more vehicles with manual shifters in this country. It is my understanding that modern, computerized automatic transmissions, which also feature locking torque converters, have been optimized by the manufacturer for best fuel economy and for the best possible performance given that economy. The transmission controller has split-second information and look-up performance tables. Human drivers have only their skill and intuition, coupled with seat-of-the-pants “feel” and a tachometer on the dash. I am sure it is possible for skilled and attentive drivers to get the best possible MPG, but certainly the vast majority of drivers will get better results from a modern automatic. It is my impression those who are asserting otherwise are either thinking of older and less-efficient automatics, or are deluding themselves regarding their driving prowess.

Paul Schmidt

I am sure you are right about transmission controllers and their splitsecond optimization curves. But there is also a loss associated with the fluid coupling between the transmission and the driving and driven elements, right? I have no idea what that loss is. I suspect it can’t be ignored when you figure out mileage efficiency. And I also suspect automatic trannies are heavier than their manual cousins, so there would be a bit of a mileage penalty there as well, no? — Leland Teschler

It is my understanding there is no loss in the fluid coupling anymore during normal driving because transmissions have torque-converter locking. This bypasses the fluid coupling except when needed during shifting. I am also sure automatics weigh more than manuals, but when compared to the entire weight of the car, the extra weight is insignificant.

This brings to mind another consideration. When official MPG figures are calculated, are they done with one driver, or a driver plus one passenger, or a whole load of people? And are those people some idealized weight, or more realistic American-sized people? I would expect that more mileage is lost due to our American bulk than the extra weight of an automatic.

Paul Schmidt

Name that gadget

Be the first to identify this device from a past issue of Machine Design and win a fabulous prize, along with the honor of seeing your name in an upcoming issue. E-mail entries to [email protected] and put “Gadget” in the subject line.

No one even came close to identifying this device from 1940. It’s a fur-cleaning machine powered by a 1/3-hp motor through a ball-bearing transmission. A smaller motor drives an exhaust blower and the lever on the right controls a shutter to remove cleaning compound from air-tight compartments inside.

Toyota and tolerances
Excellent article on Toyota’s development process (“How to develop products like Toyota,” Oct. 9) A point you made but didn’t spend much time on was “Toyota’s practice of leaving manufacturing tolerances to be set by die makers rather than by design engineers creating the prints. Die makers make die dimensions as close as practical to those in the CAD database, but have the flexibility to modify them so body parts fit together well. Manufacturing engineers then set tolerances around manufacturing capabilities.”

Our company injection molds plastic products with heavy walls and tight tolerances, and it’s a shame how much time is spent trying to teach this concept to customers. When I designed for a company that built molds for plastics, we did exactly what was described in the article. Everything was dimensioned to the middle of the tolerance on the customer drawing, shrinkage was added, and toolmakers did everything within the state of their art to make the molds exactly correct.

Once the tool is built, the practical way to arrive at “good” parts is to use statistical methods to inspect resulting parts and reveal what the tolerances need to be. The designer decides which features must change and which do not, and the tool is changed to bring parts into conformance where possible. Again, statistical methods tells us what the molding tolerances must be.

If an as-molded dimension/tolerance is close but not critical, it is changed. If it is critical and statistics say it can be held in molding, the tool is changed. If the dimension is critical but cannot be held in molding, the part must be machined. And if machining is too costly, there is no point in molding the part at all.

Once this thought process is taught to customers, things go smoothly both for us and for them. The key is to find out what can be done. Designing around what we know can be made is much less costly than trying to make things work that defy the laws of physics. W. Edwards Deming taught this decades ago, and it sounds like Toyota applies it well.

Jon Roesler

German faux pas
As a German engineer working in the U.S., I found the article on German engineering (“Engineering in Germany,” Nov. 6) interesting. But there were a few things in it that make it hard to believe a German coauthor was working on it. For example, in the listing of cities in Germany with Technical Universities, the two cities hosting the two best Technical Universities in Germany are missing, so is Germany’s capital. It would have been better to list the best universities rather than cities, since almost every bigger city has a Technical University or Fachhochschule.

And Germany has 16 states, not 18.

I also think the education situation in Germany, especially the apprenticeships, is not well explained.

If you ever plan doing something similar again, I would be happy to assist you with it.

Carina Buck

Wrong URL
I was reading the recent issue, and in the article on the future of mechanical engineers (“Careers: Mechanical Engineers in the Year 2028,” Nov. 6). The article references and gives a link to a report that describes what a day in the life of a mechanical engineer will look like in the year 2028. However, every time I try to access this link, I get a URL error. Can you please either provide the full URL address?

Alex Mlsna

Alex, try this one:

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