Letters 02/18/2010

Feb. 17, 2010
A few readers questioned whether some of the news we recently reported were signs of better times ahead or indicators that things are headed the wrong way

Good news or bad?
A few readers questioned whether some of the news we recently reported were signs of better times ahead or indicators that things are headed the wrong way. And one steely-eyed reader ferreted out several missteps an author made in a recent article.

This is good news?
The first sentence in the Dec. 10 “Careers” column went like this: “The good news is more women are entering the ranks of engineers.”

Why is this good news? Engineering is a profession which relies primarily on math, physics, chemistry, and other “hard” sciences, as well as logic and common sense. If I were to hire a group of engineers to design a airplane, engine, or wind turbine, I can think of no reason whatsoever why it would be advantageous to hire a woman rather than a man. But to me, your statement implies that women make better engineers than men do. If this is true, I haven’t seen any data to support it.

Don’t get me wrong, I don’t have any problems with women being engineers, I just don’t see any reason to push them into it or get worked up by the fact that engineering is a male-dominated profession. I remember seeing other articles in past issues boasting about various organizations giving women special incentives to get involved in engineering, and I thought to myself that this was blatant sexual discrimination. To me, the gender distribution in engineering is neither good nor bad; it just is what it is. The only thing that should be important is the quality of the engineer, not the race, gender, or any other personal characteristic.

Mike Wubben

How smart is this grid?
The article on the smart grid (“Embedded design for the smart grid,” Nov. 19) brought up some old memories. Our local electric utility tried a variant of the smart grid years ago. It was called the “Summer Cycler” program and consisted of a module attached to the side of your air conditioner’s outdoor condenser. At peak times, the utility would send a signal through the power lines to this module to take your condenser off-line. They would give you a credit on your electric bill if they shut down your air conditioner. But after coming home to a 100°F home several times in one week, I told them to come get their module.

And now the folks behind the “smart grid” expect to install smart meters on every house to cycle a clothes dryer? Are we really going to spend untold billions of dollars to cycle a small percentage of clothes dryers?

This smart-grid stuff for homes is totally irrational and illogical. A better way to encourage people to use less energy is to have a multiple-tier rate system. The first few hundred kilowatt-hours is at one rate. The next few are at a higher rate, and so on.

This smart grid is a fleecing of the American taxpayer who will pay for it through subsidies.

Now excuse me while I get on-line and buy some “smart-grid” stocks.

Jock Stucki

Most smart-grid proposals I am aware of give consumers the ability to overrule dialed-down periods of energy use. And the smart grid is supposed to reduce power used during peak periods when power is expensive to generate. The alternative is to build more peak-generating capacity which consumers would ultimately pay for.

Leland Teschler

Getting straight on steel
The recent article on steel metallurgy (“Should you stick with stainless?” Dec. 10) touched on nearly all of the questions a good engineer would raise when considering approval of a mechanical design, particularly if it was of an aerospace or hi-tech nature. Unfortunately, I found it contained several errors:

The AISI 440C stainless steel in the materials comparisons was a poor choice. Its use is almost exclusively limited to rolling-element bearings and some cutlery applications. Because it’s brittle when fully heat treated, it is not generally considered a structural material. AISI 410 stainless steel would have been a much better choice for these comparisons.

In the discussion about yield strength of materials, it talks about 2% strain. It should be 0.2%.

In the “Stainless selection” table, the high end for the ultimate tensile strength (UTS) for austenitics, 325 ksi, is only true for fine music wire or, perhaps, ultrathin foils. It is not representative of the metals generally used to make hardware.

In the “Alloy-to-Alloy” table, a shear strength of 11.6 ksi is given for AISI 4130. Assuming a UTS of 236 ksi and using the well-proven rule of thumb that shear strength is 50 to 60% of UTS, then shear strength should be 140 ksi. And for the given UTS of 440C stainless, 285 ksi, shear strength should be about 170 ksi.

In the corrosion and passivation section, it states” “Stainless steel fasteners on an aluminum panel quickly corrode.” That statement is not true. Over the years, I have signed off on many aluminum sheet structures that had austenitic-stainless-steel fasteners and they have proven to be durable when exposed to the environment. However, using aluminum fasteners in a stainless-steel structure is inviting disaster.

Irving W. Glater

My main point in writing the article was to point out that stainless steel is like any other material and that low-end stainless steels contain carbon and are susceptible to corrosion. But let me address your points one by one:

I listed 440C because there is more data available for comparative analysis with other materials. Compared to AISI 410, AISI 440C has more carbon for greater hardness and more chromium for better corrosion resistance. However, the minimum mechanical properties of the two are close. I personally use AISI 440C for tooling material.

Thank you for pointing out the error. The correct value is indeed 0.2%.

The table lists the maximum achievable UTS for austenitic steels in general, regardless of application. The value listed for AISI 304 austenitic stainless steel in the Alloy-to-Alloy table is more indicative of the forms of steel used for hardware.

In reviewing your comments, I came across several errors in the Alloy-to-Alloy table. A corrected version appears at www.machinedesign.com.

A lecturer at an FAA-sponsored event brought to my attention that using stainless-steel fasteners on aluminum structures invites a greater risk of galvanic corrosion and requires a barrier or insulator. Another reason I avoid stainless-steel fasteners is that, unless I know where the fasteners were made and type of material used, I have doubts as to their purity. — Norm Ellis, Ellis & Assoc.

Standardizing confusion
The recent article on the revised dimensioning and tolerancing standards (“New standards for GD&T,” Oct. 22) asserts that users of the new revised standard will find it “much easier to read and understand” than the previous version. The authors’ enthusiasm to embrace and use the new standard is tempered by the candid admission that certain aspects related to the Datum Reference Frame are “...a little confusing at first.” This statement could not be more true.

Section 4 of the revised standard describes Datum Reference Frames by simulating a simulator which requires discernment between theoretical and physical datum simulators. How can that do anything but bewilder those trying to use the standard?

Those using the new standard are also presented with new terms replacing True Geometric Counterpart (imitated by associated processing equipment). But it conveys the same concept in the last revised standard which was easier to understand.

True Geometric Counterpart remains in the text of the 2009 standard which further adds to the confusion and may lead users to wonder why the substitution was made at all.

Kevin C. Parson, a GD&T instructor

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