Looking for ideas for a Mechanical Engineering problem 21

[Lisa_247]

We are looking to take on an Aerospace Mechanical Engineering apprentice and I would like to give the candidates a problem to solve during the interview. I dont want anything too technical, just something that will give us an insight into how they approach problem solving, we are looking for someone who thinks logically but also thinks outside the box. Does anyone have anything I can use, or does anyone have any ideas please? Many thanks

 

[Compositepro]

This shows how useful the question is. Will a simple question be turned into an impossible to answer complexity.

 

[3DDave]

Seems like that makes it the best Gotcha! question. The chances for coming up with either a truly novel answer or one that perfectly matches the interviewer's previously determined "right" answer means that the majority of perfectly fine applicants will be rejected.

 

[Compositepro]

There is nothing gotcha about it. It simply tests for knowledge of some of the most basic principles of physics and problem solving skill. Getting bogged down with irrelevant details is one of the most common failings of engineers. Even the points about NPSHA, while perfectly correct, are unnecessarily complicated. Using the term NPSHA shows you have had courses on pumps. But the question can be answered correctly after having taken Physics 1A. I have found that many graduate engineers get through school by memorizing formulas and plugging in numbers but do not have a good understanding of the basic principles which allows for deriving the equations and getting to the solution to a problem.

 

[CWB1]

The question is needlessly vague about a simple system and without a definitive answer, suggesting that the person asking is either poorly prepared, not technically competent, or simply playing games as mentioned previously. In any case, the only thing it shows is that the candidate should consider the likelihood of a poor company culture when considering an offer.

Given the problem statement, we cannot definitively say where the pump will nor where it will not work. IME engineering depts tend to seriously frown on engineers making statements to outsiders that cannot be proven. Paint yourself into a corner by incorrectly telling the customer or leadership what can/cant be done and you'll likely be walked out. Not sure why anyone would expect an interviewing candidate to be any less conservative.

 

[Compositepro]

I'm sorry, CWB1, but your logic is the same one that is used by those who promote perpetual motion machines. If you can make an argument for any location of the pump but on the ground, please do so. No time limit, no limit to researching your answer. The original question had no constraints, like the ground gets flooded. The question also asked for the reasoning for the answer. This gives a great deal of opportunity to see how a candidate thinks and solves problems.

 

[verymadmac]

how does one get the water drum to the roof, well catapult of course.

I have always thought a good assessment of graduate engineers would be to ask them to assemble a flat pack desk (with or without instructions depending on what skill sets you want).

Show a broken bit of structure and asking why did it likely break or how you would improve the design could sort the wheat from the chaff.

 

[handleman]

CWB1, you're the one who's painted yourself into a corner, and now you have to support an argument that defies physics or changes the original question. The origninal question is ABSOLUTELY crystal clear, and not vague in the slightest until you start throwing in garbage that wasn't in the original statement. There absolutely are right and wrong answers. Your argument at this point is reminiscent of the parent who goes in and yells at their kid's teacher for flunking them when they say 2+2=5.

However, the original question also left an opening to "explain your reasoning". Gives some opportunity to redeem yourself. Relative to hiring, I'm pretty sure it was never stated that the one question is not a right/wrong, hire/fire thing.

 

[bcd]

An old engineer gave me a test during my first interview. Once I was hired, I learned his nickname was "Putz". His question: If I know the air supply pressure and the piston diameter, can I figure out how much force it will produce?

I was asked by another Engineer if I had a copy of Roark.

 

[JohnRBaker]

NOT having a copy of Roark's would certainly cause me to have second thoughts.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[btrueblood]

I have two. One for each shed.

 

[JStephen]

Actual problem from the Dynamic's textbook from college:

The question is: Given mass m and spring constant k, what initial velocity must the mass "m" be given in order to stop at the horizontal position at point "A"?
As I said, actual problem, which the good professor didn't see any problem with, either.

On Roark's- depends entirely on what your work involves. I remember a line from a surplus "stuff" catalog long ago: "Handy if you need it, useless if you don't", and that applies to Roark's and a whole lot of other stuff, too.

 

[FACS]

I'd be curious to hear a method for making the pump work from the roof.

A hand pump will work from the roof. Hand pumps supply water from hundreds of feet in depth.

Archimedes screw pump will work from the roof.

 

[handleman]

A hand pump will work from the roof. Hand pumps supply water from hundreds of feet in depth.

Archimedes screw pump will work from the roof.

By "hand pump" I assume you refer to the old water well style handle pumps.... However, with these mechanisms, the actual pump is at the bottom of the well where the water is. The main body that you see above ground that most people call the "pump" is nothing but handle and a long rod running down the pipe with which to transfer the mechanical energy from the top of the well to the bottom where the actual pump is.

It's pretty tenuous to classify an archimedian screw as a pump... However, if we were to do so, you would have to consider the entire length of the screw and housing as the pump. If you had to answer "where is the pump", you would have to say that it's at the top, and at the bottom, and everywhere in between. You could put the motor at the top, but that would hardly be considered putting the pump at the top.

 

[CWB1]

CWB1, you're the one who's painted yourself into a corner, and now you have to support an argument that defies physics or changes the original question. The origninal question is ABSOLUTELY crystal clear, and not vague in the slightest until you start throwing in garbage that wasn't in the original statement. There absolutely are right and wrong answers.

Crystal clear with absolutely right and wrong answers eh? Ok...

Scenario 1 - Pressure out of the lower drum raises water 49' to a minuscule pump which easily raises it the last foot. Move the pump to ground level and it cannot raise water 50' being undersized for that effort. Fits the problem statement perfectly, doesnt defy physics, incorrect per CP as the pump isnt on the ground.

Scenario 2 - Pressure out of the lower drum overpressures the pump's PRV and fills the upper drum without any pumping work being done. Pump location therefore is irrelevant to the task. Once again it fits the problem statement perfectly, doesnt defy physics, and is also incorrect per CP and yourself.

Scenario 3 - With zero outlet pressure at the lower drum the pump works fine halfway up the line under all conditions, cannot provide adequate suction if placed on the roof, but lower it to ground level and it cannot overcome head from the upper drum. Again this fits the problem statement, doesnt defy physics, and is also incorrect per some folks.

I wouldnt call anything with wildly varying interpretations "crystal clear," especially when the folks being asked to interpret the problem statement have seen many variations of it, albeit decently defined, in the majority of their undergrad coursework in physics, thermo, fluid dynamics, hydraulics, machine design, heat transfer, HVAC, and CFD, plus others I'm surely forgetting or unfamiliar with. I never took HVAC but am fairly certain I had similar problems in each of the others, hence three scenarios given with less than a minute's thought.

Ultimately it is a "gotcha" bc rather than proving someone's technical competence and sound engineering judgement it proves the exact opposite.

 

[handleman]

The first two scenarios you have made up add information that was not present in the problem statement... They do not defy physics, but they certainly do change the original question. There is nothing in the statement to suggest that pressure is available in the lower drum. Adding some pressure-assist for no other reason than to put the pump on the roof needlessly complicates the simple problem statement, just so you can try not to be wrong. Adding pressure-assist in the lower drum is no different from adding a janitor with a bucket to carry the water halfway up the building to a third, intermediate drum. You're still wrong. Your third scenario HAS ALREADY BEEN DEBUNKED IN THIS THREAD! Delta P across the pump is the same, regardless of whether it's at the bottom or middle of the building. If it can't push water to the top, it can't pull water halfway and then push it to the top.

There are a lot of "creative" answers in this thread for getting water to the top. Some of them don't even use a pump! But it's funny that you mention "sound engineering judgment". Engineers sometimes have to make decisions based on limited information. Based on ONLY the information provided, and not adding any weird scenarios or other energy sources etc, in your "sound engineering judgement", which solution is reasonable? I would argue that absolutely zero of the suggestions other than putting the pump at the bottom show "sound engineering judgement".

 

[SandCounter]

How many quarters need to be stacked to reach the height of the Empire State building?
The right answer is not important. The important thing is to see if they ask clarification questions and if they think through it logically.

 

[3DDave]

20 quarters. At least I think that's what a ticket to the observation deck costs. Or less, if there's an appointment to an upper floor of the Freedom Tower. Or let go of a helium filled balloon. Depends on what it means to reach the height.

See Empire State building barometer answers.

http://www.mrao.cam.ac.uk/~steve/astrophysics/webpages/barometer_story.htm

 

[SnTMan]

Now that I like

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[moon161]

If you're only going to pump it once, this is a problem for 5 people and 10 buckets instead.

 

[FACS]

By "hand pump" I assume you refer to the old water well style handle pumps.... However, with these mechanisms, the actual pump is at the bottom of the well where the water is. The main body that you see above ground that most people call the "pump" is nothing but handle and a long rod running down the pipe with which to transfer the mechanical energy from the top of the well to the bottom where the actual pump is.

It's pretty tenuous to classify an archimedian screw as a pump... However, if we were to do so, you would have to consider the entire length of the screw and housing as the pump. If you had to answer "where is the pump", you would have to say that it's at the top, and at the bottom, and everywhere in between. You could put the motor at the top, but that would hardly be considered putting the pump at the top.

Do you even read the words you write?

"Making the pump work from the roof"

I went on the roof, I pumped the handle, and the pump worked.

It DOES NOT MATTER where the pump is. You asked how to make it work from the roof. You did not say the pump was on the roof. I know that's what you meant, but since you are being "one of those guys" you should change your statement to "I would like to know how to put a pump on the roof, and have it pump water from the ground to the roof."

I made it work:
Work: Moving a pound of water, one foot, in one second.

Your comments on Archimedes screw is just argumentatively stupid.