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]

Problem statement: You have to pump water from a 50 gallon drum at ground level to a second drum on the roof of a 50 foot tall building. Where would you place the pump; on the ground, roof, or in between? Why?

Over half of the graduate engineers I've asked get the answer wrong.

 

[MintJulep]

I wonder if Compositepro would hire me?

Spoiler

On the ground.
So that there is NPSH at the pump for startup so that I don't have to mess around priming the pump.

 

[handleman]

Spoiler

A pump cannot "suck" water up 50ft

 

[SuperSalad]

Compositepro, I can't help myself

Spoiler

At the top won't work since the pressure, even at full vacuum, couldn't get the water up 50 ft to your pump using an unpressurized container as your feed.

Though there should be enough pressure (assuming the pump can provide full vacuum) to place it halfway. That would not be my choice, though you could spec a less powerful pump and save on costs.

I would place it at the bottom because there is no question about supply pressure being adequate in any foreseeable situation. You can spec the pump to provide however much pressure you need to overcome the elevation. But for me, probably the most important aspect is, serviceability. It is a lot easier to service/replace/whatever at ground level than halfway up or on the roof. I think that aspect outweighs the potential operational savings from the halfway option.

Did I get the job?

Andrew H.

http://www.mototribology.com

 

[Compositepro]

This could be fun. Post a suggested question for Lisa and give the answer under a spoiler (the gift box icon in the posting options). Then everyone can test themselves. It might be more fun to see people post answers, as Mint Julep has done, but I do not think that most engineers are brave enough to publicly post potentially wrong answers.

Here is the answer to my question, above:

Spoiler

Mint Julep gets hired! The pump should be on the ground. You cannot suck water up a pipe more than about 30 feet because it is only atmospheric pressure that is pushing the water up. Water vapor pressure reduces this height somewhat from the theoretical value. In most cases, installation and maintenance are also easier at ground level.

A common wrong answer is to place the pump on the roof because it is easier to pull than to push.

 

[handleman]

Years ago someone posted an actual question on here. It was regarding conveyor belt speeds, nothing particularly complex.

Anyway, it was a great question because

1. There were LOTS of numbers given in the question. Many of them were seemingly related but superfluous to the solution. This is much more realistic than textbook problems. Textbooks typically give you exactly what you need to solve the problem. No more, no less. To the point where if you get an answer without using all of the givens, you've probably done it wrong! In the real world, you almost always have available much more information than the bare minimum to solve the problem. You must be able to quickly understand the situation and realize which things are actually necessary.

2. There was ONE piece of info missing to solve the problem using the full gear train analysis, which would have been the straightforward sort of "textbook" type solution. However, it would have easily been determinable by a quick physical look at the system. This is also very real-world, becuase you often have to recognize what info you currently lack to solve your problem.

3. The solution was actually determinable without doing a full gear train analysis by recognizing a couple of ratios.

I had copied it myself into a word document for a similar purpose, but unfortunately I don't have access to it anymore.
Edit: Well daggum, I found the post... When I made the Word doc I think I dressed it up a little more with a diagram or something...

https://www.eng-tips.com/viewthread.cfm?qid=232164

 

[handleman]

You could spec a less powerful pump

Can you please elaborate?

 

[SuperSalad]

I think my fingers went too fast for my brain! .

The total required minimum pressure would not decrease due to pump location.

I won't edit it out of my original post though for posterity's sake.

Andrew H.

http://www.mototribology.com

 

[SnTMan]

Anybody who'd say "put it on the roof" has not worked on roofs

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

 

[rb1957]

It would be interesting to see their response to redundant systems.

A simple mechanical system would be flap drives, quite straight forward until you bring skew sensors into play. This would be a good example of responding to failure cases.

But these are not fair questions for an apprentice. I have very simple interview questions for structures graduates, to see how much they'll learnt. At the apprentice level, what have they learnt ? Maybe focusing on the personality is more valid ? why do you want to do this ? attention to detail. personal responsibility

another day in paradise, or is paradise one day closer ?

 

[hendersdc]

A couple questions I like to use for mechanical engineers to see their thought process:

- Hand them a complicated sheet metal part and ask them to draw the flat pattern

- Give them the attached drawing and ask them to draw the missing internal line and isometric view of the part. Pg 1 is given to the applicant, pg 2 is the answer. No one ever does a good job on the isometric view but you just need enough from them to get the point across.

Another one I have heard of but never used is to ask how much your fuel economy drops if you turn the headlights on in your car. Don't provide any numbers to them, just get them to make a whole lot of assumptions and see where it takes them. I heard of one guy that knew the energy density of a gallon of gas off the top of his head and made pretty quick work of it.

 

[3DDave]

I would winch it up. Less chance of spills and avoids buying the hose/tubing and the second barrel. Since other things are subject to delivery to the roof, there should already be a means to lift the barrel. It also works regardless of temperature.

 

[moon161]

I'd order a different drum of water from Mcmaster-carr, and make the delivery address the rooftop.

 

[moon161]

I worked at a place, an intern there had gotten a job at the national mint. He said part of his interview was the question, 'how do you weigh a car with something in your pocket' or it may have been more direct 'how do you weigh a car with a dollar bill'

Spoiler

You 'weigh' a car with a dollar bill by inking the tire and imprinting the contact patch of one of the tires on the bill. Assuming nominal tire pressure, 25%/corner weight distribution and measuring the area of the contact patch from the imprint, you get W = 4PA

 

[SuperSalad]

Y'all are killing me with these answers.

Spoiler

But as for weighing a car with a dollar, I think it would be more cost effective and accurate to pay a scale operator a dollar for the info

Andrew H.

http://www.mototribology.com

 

[danschwind]

Spoiler

Wouldn't that imply that you had ink and a measurement device in your pocket too? Hahahaha

Daniel
Rio de Janeiro - Brazil

 

[dvd]

As opposed to catching some poor soul off guard, I would give them a heads up that I want to chat about a particular subject or two in qualitative terms. The kid who is ruled out with a pop quiz may have been the best candidate. Offer up something in the interview about how you would approach an unknown problem and let them know what you are looking for first, as an ice breaker.

Otherwise -

I think that knowing that similarly loaded and configured steel (metal) parts have similar deflections due to similar Young's modulus in spite of vast differences in steel yield and ultimate strengths shows insight.

Knowing how materials expand with temperature seems like a good conversation point. If a part doesn't have degrees of freedom to expand what happens? i.e. strains or buckling

Basics of combustion air requirements and expansion of gases after combustion would be appropriate in certain industries.

 

[JohnRBaker]

You could always ask them to finish the simple drawing below, by adding the missing Side View:

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

 

[3DDave]

Easy

Spoiler

It's a thin gasket at 45 degrees with a ~1.414:1 aspect ratio. So the side view is a 45 degree line, maybe leaning left, maybe leaning right.