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Ozark Sage
User Rank
Silver
Re: Experienced False assumptions
Ozark Sage   1/25/2012 6:26:50 PM
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My Dad, an ME PE and experienced hyralic machine designer and company owner contracted to buy for his customer several zink injection casting machines at auction.  All the machines were inspected working before he offered the customers  winning bid.  All were delivered during my Christmas break from college, so I asked Dad if he needed some help finishing the job he had begun earlier in the week.  Since it was a union shop and all their employees were given the Saturday before Christmas off he said yes.  

Arriving early Saturday at the install site we both went to work on seperate machines.  Lunch time came as I compleated my last assigned machine so I asked Dad if he wanted some help on the machine he was working on for the last hour.  Saying yes, I asked the problem he was having.  He remarked that "the machine was working yesterday when he left the plant and he could not understand why it didn't work now!" 

As he was just about finishing assembly of the hydraulic pump for the third time I waited til he finished and asked him to turn it on.  He did and I said turn it off an back on after the motor stops.  He did and I told him go have a soda and I'l fix it.  I VERY cautiously chuckled while he had his Coke and I rewired the 3 phase feed to the machine.

After a call to the plant manager we were told the assigned union electrition was told not to report on Monday since the owner's in house people would do tne clean up.  And, by the way,  those motor casting rotation arrows are there for a purpose.

 

jlinstrom
User Rank
Gold
False assumptions
jlinstrom   1/24/2012 3:08:38 PM
I think every engineering student should hear a version of the following fable -

A grouchy king imprisoned 3 subjects who displeased him: a mathematician, an engineer and  a commoner. He told them of his anger and said if they could not free themselves by daybreak tomorrow, they would all be executed.

The mathematician looked at the cell door lock and began to compute the number of possible variations, trial time for trying all variations in order to work out  a survival strategy with his cellmates.

The engineer looked around the cell and began to inventory the possible materials he could use for a MacGuyver door key.

The commoner walked over to the door and tried the handle. It was unlocked!

Sort of like checking that the AC cord is plugged in. Verify your problem before you try to find a solution.

Rob Spiegel
User Rank
Blogger
Re: Assumptions can bite!
Rob Spiegel   1/24/2012 1:04:37 PM
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This is a good story, OldGeek. I'd like to use it as a Sherlock Ohms posting. It would need to be a tad longer, but I'm sure you could flesh that out with a bit more detail.

If you're interested in the fame that comes with Sherlock Ohms let me know at rob.spiegel@ubm.com

Also, we would like to include a brief bio of two to four sentences.

OldGeek
User Rank
Iron
Assumptions can bite!
OldGeek   1/24/2012 10:39:37 AM
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I'm definitely with you on the assumptions causing problems!

I once installed a sound system in a local church. A few weeks later I got a call that they were having trouble with noise. I fairly quickly narrowed the problem to a small pre-amp I'd had to add to get the microphones up to the desired volume. Since it was a "zero budget" job and the preamp was simple I decided to repair rather than replace. As I remember, the whole amp consisted of two transistors and a few passive components, powered by a 9 volt battery.I had no test equipment with me, but did have a pretty good assortment of parts, so my troubleshooting approach was to just replace the first transistor. No change! Nor did replacing the second transistor!

Puzzled, I started trying to isolate the problem by shorting signal to ground with a spare electrolytic cap. Shorting the input had no effect. Shorting the output of the first stage reduced the noise, but did not kill it completely. Jumping around, I found that putting the cap across the power supply (the 9 Volt battery) killed the noise! The cure was to replace the battery, and for good measure add a large electrolytic across the battery.

Never before or since have I found a noisy battery, but that one taught me not to assume that a battery is always a clean power source!

William K.
User Rank
Platinum
Power supplies cause confusion
William K.   1/24/2012 10:00:52 AM
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This is an example of what happens when we assume that an item that we pull from "stock" is OK, meaning that it meets specifications. It also points out the need to check calibration stickers, at least when accuracy is required. I wonder if the supply in the story had a calibration sticker, and if so, how far out of the calibration time it was. 

But it is an interesting tale, and I have come across other instances where the assumption that some piece of equipment was functioning correctly was wrong. The result is that items not "brand new" now always get checked prior to use. This often helps preserve sanity and avoid a lot of frustration.

Charles Murray
User Rank
Blogger
Amazing
Charles Murray   1/23/2012 7:17:00 PM
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This is another of those stories where it appears more obvious in retrospect. As I've said in response to previous Sherlock Ohms stories, I'm amazed by the tenacity of our readers. I never would have thought to check the "working" supply.

Ann R. Thryft
User Rank
Blogger
Re: Watch your assumptions
Ann R. Thryft   1/23/2012 3:24:20 PM
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That reminds me a bit of the first three or four steps in troubleshooting electrical appliances, like my stereo system: 1) Is the power cord plugged in? 2) Is the main unit switched on? 3) If so, is the cord frayed? If the first two are true and the third is not, then you start suspecting incorrect connections among components, and if those are OK, you may have a faulty stereo component. But this Sherlock almost sounds like one of these first questions could not even be asked, or at least not correctly answered.


Rob Spiegel
User Rank
Blogger
Watch your assumptions
Rob Spiegel   1/23/2012 2:36:36 PM
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Another fine Sherlock Ohms story where the answer to the dilemma can only be solved when the on-the-scene Sherlock drops all assumptions. Perhaps that's the most important quality of a good Sherlock -- that nothing is assumed.



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