Posts Tagged ‘PE’

Gear Reduction Worked Backwards

Sunday, March 9th, 2014

      Last time we saw how a gear reduction does as its name implies, reduces the speed of the driven gear with respect to the driving gear within a gear train.   Today we’ll see how to work the problem in reverse, so to speak, by determining how many teeth a driven gear must have within a given gear train to operate at a particular desired revolutions per minute (RPM).

      For our example we’ll use a gear train whose driving gear has 18 teeth.  It’s mounted on an alternating current (AC) motor turning at 3600 (RPM).   The equipment it’s attached to requires a speed of 1800 RPM to operate correctly.   What number of teeth must the driven gear have in order to pull this off?   If you’ve identified this to be a word problem, you’re correct.

Machine Design Expert Witness

      Let’s first review the gear ratio formulas introduced in my previous two articles:

R = nDriving ÷ nDriven             (1)

R = NDriven ÷ NDriving             (2)

      Our word problem provides us with enough information so that we’re able to use Formula (1) to calculate the gear ratio required:

R = nDriving ÷ nDriven = 3600 RPM ÷ 1800 RPM = 2

      This equation tells us that to reduce the speed of the 3600 RPM motor to the required 1800 RPM, we need a gear train with a gear ratio of 2:1.   Stated another way, for every two revolutions of the driving gear, we must have one revolution of the driven gear.

      Now that we know the required gear ratio, R, we can use Formula (2) to determine how many teeth the driven gear must have to turn at the required 1800 RPM:

R = 2 = NDriven ÷ NDriving

2 = NDriven ÷ 18 Teeth

NDriven = 2 × 18 Teeth = 36 Teeth

      The driven gear requires 36 teeth to allow the gear train to operate equipment properly, that is to say, enable the gear train it’s attached to provide a speed reduction of 1800 RPM, down from the 3600 RPM that is being put out from the driving gear.

      But gear ratio isn’t just about changing speeds of the driven gear relative to the driving gear.   Next time we’ll see how it works together with the concept of torque, thus enabling small motors to do big jobs.


Do The Best Schools Turn Out The Best Engineers?

Sunday, August 30th, 2009

     Your initial response to this question is undoubtedly, “Of course they do!”  And your answer would have been formulated through a series of brain functions responsible for logical reasoning skills.

     The bad news is that logical reasoning skills have no place in the response to this question, at least not according to a recent article and report dated August 19, 2009, by U.S. News & World Report entitled, “America’s Best Colleges 2010.”

     The report is replete with names of institutions of higher learning which we equate with true excellence in education, names like Harvard,   Princeton, the Massachusetts Institute of Technology, and the California Institute of Technology taking the top positions.

     But was it through a process of logical evaluation and reasoning that these leaders of education once again placed in the Top Ten?  Hardly.

     How they got to the top of the list will surely surprise you. And here it is in a nutshell as taken from the article itself:

“The U.S. News & World Report rankings measure up to 15 indicators of academic performance for each college and university.  Quantitative data that assess a college’s performance in areas such as graduation and retention rates, faculty resources, financial resources, student selectivity, and alumni giving account for 75 percent of a college’s score.  The other 25 percent is based on a peer assessment survey the magazine sends to top officials at each school asking them to rate the other colleges in their category.”

      So what are the factors used for assessment of America’s top colleges?  The first stated criteria is the number graduating within six years.  Now call me old fashioned, but I thought a Bachelor’s Degree was traditionally to be earned within four years?  And how these graduates perform in the real world is also anyone’s guess because no assessment is offered.

     This is followed by “retention rates,” meaning the number of students who remain with the institution after one year.  If marriages were measured by this yardstick we’d be in real trouble.

     Then the criteria gets really interesting.  Interesting in that its sole focus seems to be on the institution’s financial resources:  “…faculty resources, financial resources, student selectivity, and alumni giving account for 75 percent of a college’s score.”  “Student selectivity” meaning that institution’s method for selecting its student body. 

     And in case you missed it the first time around, here’s the cherry on top of this massive pile of whipped cream:  “The other 25 percent is based on a peer assessment survey…”  Hmm, educators assessing other educators.  Isn’t that akin to letting the other teenagers in the subdivision decide whether your teen is towing the line within your household or not?

     Rankings are wonderful tools, if they are based on an objective, unbiased assessment.  When it comes to objectively assessing engineers one organization stands head and shoulders above the rest, the National Council of Examiners for Engineering and Surveying (NCEES).  It is the same entity responsible for the licensing of Professional Engineers.

     Here’s what they have to say about their assessment process and objectivity as found through this link:

“NCEES is a standards development organization (SDO) of the American National Standards Institute (ANSI). As the U.S. representative to the International Organization for Standardization, ANSI oversees the development of standards for various products, services, and processes throughout the United States. Its membership includes more than 100,000 government agencies, corporations, and academic and international bodies.”

      Which assessment process would you trust to hold up your cherry?