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Confessions 24

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D Scullion

D Scullion

Mechanical
Sep 1, 2016
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Many interesting links shared detailing other peoples screw ups. Anybody on here willing to admit to their own?

Not trying to entrap anybody, don't admit to anything you could still get prosecuted for. Just want to hear a few funny stories.

I'll start: My first job in a small engineering firm with very little senior experience, almost every day was a near miss:

- Prototype 24 ton dump trailer, my first folly with hydraulics, on placement from uni. I designed the small hydraulic rams that open and closed a gull wing style back door to operate on the same circuit as the very large rams that lift the 24 ton laden body, without any flow controls. On first operation the back door closed so hard and fast it damaged the solid steel door.

- Ordered a laser cut sheet steel kit for a first production batch of a new product. Didn't realise AutoCAD was set so that all files were scaled 10:1. Received a delivery of miniatures.

- Made a tolerance error on the pivot of a large arm that moved a prototype piece of machinery into operating position. It seized up so tight only the hydraulic ram on the arm could turn it, couldn't even get grease into it. Prototype was on a tight deadline for a demonstration in front of a large group of customers which we had to go ahead with. Started to move the arm and the screeching from the pivot was mortifying. needless to say we waited until everyone had left after the demo before folding it back in again.

 
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Watching units is always important especially when using software. Pipe weights may be off by a factor of 144 if you don't watch density units...
 
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2-months ago, the hydrotest pressure on isometric dwg appeared incorrect. i did the calc & i got a higher value. oscar sierra . . . i got distracted and revisited 2-weeks ago. hydro test pressure was incorrect - too low. ouch.
meanwhile, about 60 pipe spools were prefabricated, hydrotested, and in the process of being shipped to site. double ouch.
engineer, not me, used 2014 code to determine test pressure and stress values were lowered in 2016 code, resulting in higher test pressures at higher design temps.
contract was signed before the 2016 edition became effective, so 2014 edition is code of construction. a goodie!
fortunately, 95% of the prefabricated spool pieces just barely met the test pressure by 2014 edition, but not 2016 edition. Whew!
the remaining 5% did not meet 2014 edition test pressure by 2-psig. final and bigger ouch, but a lesser eventual rework.
when suspecting or encountering an error, immediately followup and do not wait. trust your instincts. darn gremlins . . .
 
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Everyone who works in a fab / assembly shop has stories of things gone wrong. It's human nature. It happens. You try not to let it happen ... but it does.

Back in my mechanical design days ...

I was design lead on a special-purpose CNC machine. In design review, the boss wanted additional workpiece clamping, so that assembly was designed after the main part of the machine was already drawn up. I completely forgot that the place where the boss wanted extra clamps already had another critical part of the machine occupying that space. This wasn't found out until the parts landed in the shop (this was LONG before the days of 3D CAD models). Question arose, "What do we do about it", my reply, "Don't install the clamp assembly, and see what happens." Fortunately, it worked fine without, and the only remaining sign of the oopsie was a pattern of holes in the base of the machine which was no longer used for anything, and a couple of blanking plates on the hydraulic manifold where the valves were supposed to be.

Sometimes the designers screwed up, sometimes the shop screwed up. Best one we had involved a hydraulic lift table with 4 cylinders tipping a plate from horizontal to vertical. Of course the project ran late, and the first time it was ever tested was when the customer was there for run-off. (Lesson: Don't do that. Don't let it happen. Delay the run-off if you have to.) That's when we found out that someone didn't crimp a fitting properly on one of the hydraulic hoses. It made it about halfway up before letting go ... and spraying the customer's representative with hydraulic oil.

I deal with robots nowadays. Anything you can touch ... you can smash into.
 
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When I was first out of college, I was tasked with doing a quantity take-off of crane rail for a power house on the east coast that my employer was bidding on. I tallied up the total length in feet and used the value in the steel manual to determine the total weight. Of course everyone knows that rail is called out as so many pounds per yard. (Only item in the steel manual listed as such)



 
 am
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Way back, a few years after graduating, I designed a large mall... the fabricator wanted to change all the 'purlin' to girder connections to single angle connections... I spent half a day and decided this was a good revision. I should, however, have issued it as a Notice of Change... There was a huge savings that was never accounted for.

I'm of the opinion that contrition is not good for the soul.

Dik
 
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One of my first jobs in process engineering a long time ago was to to set up a pump on a pressure controller. The system included a pressure tank. I was told that the system had to operate at a certain pressure, i don't remember the exact numbers now but lets say it was "400" . The gauge was in psi but what did i know and it was in a difficult spot to read and it was half dark.

Of course the 400 was kpa which is about 60psi but i was trying to get 400 on the gauge which it went to. The pump i was using had enough stages it may well have gone to 400psi. On the first attempt, a pipe blew. Second attempt the pressure tank started to emit pinging and cracking sounds passing about 200.

It finally clicked that the tank probably should not be making such sounds and i had better shut down and investigate.

Lesson learnt.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
 
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We had an operator who failed to check whether all the gasses our diffusion furnace were turned off, and when she opened the end of the tube, she got a nasty little explosion that could have killed her. High purity oxygen and hydrogen running through a 900 C furnace and all that; luckily the flow rates were usually pretty low. For some whacked reason, she didn't get fired, but got essentially promoted to a QC position elsewhere in the facility.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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As a graduate, I once analysed a 12 storey RC frame building intended to be reinforced concrete construction, but I inadvertently used the Young's modulus & self weight of steel in place of using concrete material properties in my analysis model.

I picked up on it eventually and we all had a good laugh about how the simplest of things can be missed.
 
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As an EIT I designed a wastewater lagoon for a single family house (allowed where the soils didn't "perc" on lots > 20 acres.

I had done the comps to size it then set about to drafting it in CAD, had the comps/plans reviewed, and out the door they went.

Fast forward a couple of months, I was designing another lagoon and pulled up my CAD file to reuse some of the elements. It was at that moment that I realized, "damn that thing looks kinda big..."

Turns out, while my comps were right, my drafting was not. I had computed the diameter needed for a truncated cone of a particular volume and depth. But AutoCAD queried me for a radius when I drew that first proposed contour. So I wound up with a lagoon twice as wide and 4X the target volume.

It has passed our QC review and was permitted. But fortunately had not yet been constructed when I brought it to everyone's attention. So aside from some wounded pride and quick rework, there was no real harm done.
 
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Another engineer caught an error in my calcs, but after the fact on a very fast paced job. Turns out the steel connections for the hydrotest load were "overstressed" by a by something like 50%. And, the hydro test was scheduled for the same day we found the error.

I was calling up the contractor to warn them not to do the hydro test on this particular piece of equipment. It had already been done, would be the only time during the short life of this structure that it would be done. Nothing damaged, nothing failed..... Thank the lord for safety factors.
 
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Telling my boss that I thought I was close to finishing something and being off by months. I hadn't done the job before but I honestly though it wouldn't be that much work. Now, if someone ask I always think about how much time I think it could take and then double it.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.
 
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Timely thread. I was just thinking about something from my past work.

We were studying the load capacity of members existing within an old steel truss bridge which shall remain nameless. Bridge was built during the time of rivets and the only rolled steel sections you could get back then were angles, so in order to get larger box sections you had to build them up with angles in the corners, and either plates or steel x-lacing on the four faces (ie. built up sections). I developed shop drawings for a 3/4 scale model of one of the box beams, with plates on several faces and x-lacing on the others, and sent them to a professor, who shall also remain nameless, from the local university that was overseeing the testing program. The one thing I overlooked was to also scale down the spacing "between" the bolts stitching all of the pieces together.

Test took place in front of TV cameras, because this test was a big deal in the community. Immediately after failure the professor jumps in front of the cameras and goes on and on about classic plate buckling behavior. Problem was the main failure spot likely initiated BETWEEN two bolts along the corner of the section. I immediately saw it and knew my oversight. The professor didn't have a clue at the time, because I don't think he reviewed anything I ever sent him.
 
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D Scullion said:
- Ordered a laser cut sheet steel kit for a first production batch of a new product. Didn't realise AutoCAD was set so that all files were scaled 10:1. Received a delivery of miniatures.
Click to expand...

I got a part made in inches when I designed it in mm. Got a part that was 25.4 times bigger than expected. Fortunately they were tiny parts so the duplicates weren't too ridiculous to deal with.
 
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Metric VS Imperial;
In laws were visiting from Boston. We had a three hour drive to the airport when they flew home.
Highway driving and for mile after mile I had the speedometer pegged on 100.
My sister in law sat quietly in panic for quite awhile but finally said something about my speed.
She didn't realize that Canada is metric and that the speedometer was in KPH not MPH. The speed limit was 100 KPH, or 62.5 MPH.
She had a reputation as a fast driver and had she been driving with an MPH speedometer she would have been driving even faster.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Since beginning engineers seem to make more mistakes than old guys, I did a drafting job the summer after first year as a CE student. There we had beginning surveying courses and did some map preparation. A furniture factory was upgrading their shop and placing new machines,etc. They put me to work mapping the shop. About half way through field measuring and drawing the plans, I found that the steel tape had more than 10 tenths per foot. Having learned to map in feet and tenths, what gives? Turns out the tape was feet and inches. Luckily they had a neat powered eraser. Guess we should have been taught that engineers and architects are different as to scales.
 
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When I was younger, there was an engineer that often had his Z28 up at 150... and not kilos... He borrowed survey equipment from the University and chained out a measured quarter mile on the perimeter highway one morning... and used proper traffic paint, borrowed scrounged from the highways department, to mark the distance. There was an area of the perimeter used for racing on Saturday mornings.

Dik
 
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