Is smaller really better?

In the September 2002 Motion System Design article, “How Efficient Is Your Servomotor?” the measure of a servomotor’s efficiency is Km, torque per square root watt, and was not mentioned in the article. Servomotor Kms range from 0.5 to 200 ftlb/ square root W. Increasing slot fill increases Km. All things equal, larger motors have a higher Km and therefore are more efficient at converting watts to torque. A lower Km will require more power from the amplifier to deliver the same torque.

There are reasons for choosing a smaller motor and inertia in many incremental positioning applications, but in general it is difficult to defend using a smaller motor on the basis of efficiency.

Bob Anticole
anticole@ieee.org

Let’s first look at the definition of efficiency and of Km.

The definition of efficiency is “the ratio of power output to power input of a machine, expressed as a percentage.” Efficiency is calculated by the formula: eff = (output power/input power) x 100%.

Kmis “motor constant.” This is a figure of merit used for servos to define the ratio of torque produced to the electrical power losses of the motor. Motor constant is calculated by the formula: Km = (continuous torque) / (sqr((continuous amps)2 (resistance))).

The article documents testing that shows the motor’s efficiency is, relatively, quite flat over the range of 40 to 90% of the continuous stall torque.

The guideline suggests, if you are operating at the 40 to 50% stall range, to consider a smaller motor, thus having the motor operate in the 80 to 90% stall range. There are, of course, other application considerations, such as inertia matching, which should be considered (see previous Motion System Design articles), and these are handled by proper selection of gears, pulleys, and other components. This smaller motor may cost 15% less (for 2 to 3-in. diameter motors) to 25% less (for 5 to 6-in. diameter motors). That’s something worth considering.
— John Mazurkiewicz,
Baldor Electric Co.

Lose the lubes

The section entitled “A lot of Gall,” on page 50 of the October 2002 issue of Motion System Design, speaks about a coating system to reduce/eliminate gall. Will this coating allow users to go to a nolubrication system?
John Burhoe
Fairlawn, Ohio
john_burhoe@goodyear.com

Yes. The coating system, which matches tungsten disulfide molecules to the scale of part surface variations, has been used successfully in a number of “dry” and “near dry” applications. Two examples are in aerospace (where the use of lubricants is not possible) and in drive mechanisms (where lubricants are prohibited.)
— Ed.

Under new ownership

In regards to “Please Don’t Squeeze Our Engineers,” in the October 2002 issue of Motion System Design, I have to strongly disagree with your interpretation of who the “bad guys” are in this situation. It is true that this engineer, or any engineer, while involved in the design and development of a manufacturing process or a new product, also develops and refines marketable skills. Some possible skills learned in the cited example would be the proper planning, execution, and analysis of a designed experiment, effective project management, or the artful application of new technology to an existing process resulting in an edge on the competition. It is these individual skills that will be the springboard when moving on to bigger and better things. The difference in the situation you cite is that this individual passed along company trade secrets to which he had no personal rights or ownership.

I could agree with your entire argument if the engineer involved had used his own money to purchase or lease a building, buy or rent all of the associated machinery, and then, all on his or her own, developed the new process. The engineer would then have the personal right to sell his or her collective knowledge about the process and its machinery. What I just described, however, is what we call self-employment and entrepreneurship. I speak from experience, as I was once selfemployed.

What allows engineers like the one in this example to practice their profession and execute leading-edge projects, is that a company exists in the first place, and it made the financial investment years ago to buy buildings and equipment to make a product at a profit. The drying process is just a small part of the total infrastructure of converting wood pulp to bath tissue and paper towels. Apart from Procter & Gamble Co., the opportunity for this engineer to develop a special drying process would never have existed.

Again, I speak from experience. I have made major improvements to a manufacturing process for my current employer. This is my second term of employment with my current employer. While working at another job after my first term of employment, I was contacted by a competitor of my current employer to act as a consultant on their similar process. The patents for this process expired during my first term of employment. Enough time had passed since my first term of employment that all secrecy agreements I signed had expired. The competition was looking for the “easy way out,” and I had the knowledge. The only reason I had the knowledge, however, was because my company had made the initial investment to build machinery, develop the process, and service a market. I was being asked to give away something that was the result of years of continuous investment, and to which I had no personal rights or ownership.

You tried to argue that other professions are immune from the legalities of using their experience and skills to land a more lucrative position. An accountant can move from Procter & Gamble Co. to the competition and do better because they have the proper individual skills necessary for success as an accountant. If that accountant, however, were to use past knowledge of the inner working of Procter & Gamble Co. to steal money from them, he would be subject to applicable legalities.

The heart of this controversy is not the technical aspects. It is an issue of morality and ethics. I refused to act as a consultant. I was not willing to steal from a previous employer for personal financial gain. The engineer cited in this situation stole from Procter & Gamble Co. and was only concerned with their personal gain. He “bit the hand that fed him,” and now it is time to pay the consequences.

Robert Chipriano
Senior Project Engineer
Foamex International Inc.
bchipriano@foamex.com

Thanks for your comments. In the bigger picture, I have to agree with you, but what about other professions? Like salespeople. There is no way they could develop their valuable contacts if some employer didn’t foot the bill for them to travel around, wine and dine customers, and otherwise build up a lot of personal goodwill. Yet, they can move between employers, taking a lot of the buying influence with them, influence they developed on the previous employer’s nickel. — Larry Berardinis

Kudos and brickbats

Kudos, Mr. Berardinis, on your excellent editorial regarding the manipulation of the intellectual property created by so many engineers. Your points regarding this exploitation are, unfortunately, all too true. It has happened to me, and I’m sure that it has happened to most engineers over the course of their careers.

Your mention of keeping engineers content as a way of keeping them from wandering off to greener pastures is good as long as the package includes the official recognition of their contribution to the financial success of their employer — not just money, but true recognition of their intellectual skills. That sort of thing tends to feed on itself; creativity is easier to foster and achieve in a pleasant working environment.

And now the brickbats. This is the second time I have written asking that you refrain from imposing your particular religious beliefs on readers. Motion Systems Design is not your private forum for advancing your brand of religion on your readers. As one letter writer correctly pointed out, Christians are a very small minority in the world and while they may, at the moment, be a majority here in the U.S., that does not give them the right to belittle those who do not believe as they do. That, as I understand it, is exactly the opposite of why the folks who founded this country left their homes and came here — religious freedom and the right to think as one chooses. The writer who likened negative comments with evil by using the pejorative phrase “communists and atheists” is using exactly the kind of thinking that should frighten all reasonable people and sounds an awful lot like the phrases uttered by the radical Muslims who demand that all people should think only their way.

Keep up your good technical writing, but please keep your sermons to yourself.

Robert B. Price, C.Mfg.E.
Automation-Gears-Machinery
gearknox@nycap.rr.com

Thank you, Robert. Yes, it is unfortunate that corporations in general do not recognize what’s truly behind their success. Perhaps America as a whole suffers from the same problem. Which leads me into your second point. I appreciate your opinion, but I’m a little disappointed. Never in print (that I can recall) have I espoused any religion, Christian or otherwise. The extent of my “sermons” is my basic recognition of the Bible as a source of wisdom and truth. If that offends you, I apologize.— Larry Berardinis