Engineering Calculation Presentation 3

[teaaddict]

I am a Design Engineering Manager of a company that designs bespoke machinery

Recently I have been given sets of calculations to check that were difficult to understand.

Mathematically they were correct however their presentation was not in a methodical sequence for example there was no distinction between the type of stress bending or shear.

These are things that I do automatically without thinking. I have searched the internet for guides for best practice and have been unable to find any. The kind of think I envisage is a "Style Guide" for calculations. Various books have part of a chapter devoted to the topic for example Wiley Engineer's Desk Reference - Wiley (1998).

Does anyone know of either a book or network resource that would be of help?

 

[GregLocock]

I don't, but if this calculation was by one of your minions (yes that was a joke) then perhaps you could prepare a sample calculation sheet and get it issued as a company standard? You could even get it included in future contracts for your suppliers to adhere to.

I must admit I wish I had had that pummelled into me at some point, even using MathCAD I often find it quicker to start over than revisit some old worksheet.

Bear in mind there is an element of job preservation in NOT making your calculations too transparent, this has been discussed elsewhere on this site.

I'm also a bit confused - are you checking the arithmetic, checking the equations and assumptions... or checking the answer? I'd have thought the latter was the most robust, in which case you don't need to see the other guy's working at all (that's my excuse).





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[chicopee]

The way I would present my calculations would be to have assembly and component sketches detailed as free body diagrams on the right half side of the paper and show the calculations on opposite on the left hand side with heading for each formula. Obviously, all the work that you are doing belongs to the company so they may hand you a company notebook for all the drawings and calculations that you are doing for the company to keep when you leave.
Before starting such work, it would be preceeded by an objective statement.

 

[hydtools]

I suspect you are methodical without thinking because that was the way you were trained. Me, too. My college training, many years ago, taught me to be methodical and clear. The text books also taught methodology and clarity.

Before computers and spreadsheets there were chart and table organization of calculations. That just dated me.

Ted

 

[teaaddict]

My company do not have a history of formalising their calculations this is something that I wish to instigate so we don’t have any historic examples. I have done calc’s for example nuclear lifting equipment which are very formal but probably excessive for our purposes.

I thought checking them would be a half hour job but once I had started I realised I could not understand them. Just little details made them unreadable for example bending stress I would workout the “I” and “z” of the section, find the maximum bending moment and then work out the stress. This way you can check the “I” and “z” for realistic values then check the bending moment. In this case the stress was calculated directly with the I Z and BM substituted in to make a complicated formula (using MathCad). In the narrative no indication that it was bending and not another type of stress so the first thing I did was try and work out what the formula was supposed to calculate then expand it to see if it made sense. It was correct but took time.
Probably we need to develop our own best practice I may even have to write my own “Engineering calculations for Dummies book”.

 

[rb1957]

i'd suggest talking it over with the author. i've reviewed many calcs and 99% of things i couldn't understand at first pass could've been explained by the author, in a semi-rational way. other than that it's back checking (is the moment the max, re-calc'ing I and Z) ... pretty much as greg suggested, start again and do the analysis yourself !

i doubt there is anything out there other than motherhood ... explain your assumptions, rationalise why they're conservative, explain your equations (how/why they apply, what geometry values you're inputing), what loadcases you're considering, ... probably something you'd see in a uni. tech. writing course.

 

[Twoballcane]

Hmmmm...I think your jumping the gun. Is this the first iteration of checking? If it is, just redline it and add your comments. It will be up to the author to correct it and make it clearer, some times the more you try to make it clear the more complicated it gets. I've been to a meeting where they used flow charts to show how they did the analysis and it ended up looking like the New York City Subway map. I think it also depends on the audience, if it is full of Mechanicals, I’m sure they can follow along. If not, maybe just showing the formula and the end results is good enough.

Tobalcane
"If you avoid failure, you also avoid success."

 

[Twoballcane]

Oh sorry, just to add, the way I present my calcs is the same way text books present their examples. I show a pic and present what is known, state assumptions, and then state what we have to find. Show calcs and the final answer with the three dots.

Tobalcane
"If you avoid failure, you also avoid success."

 

[chicopee]

If you do use a spreasheet or a computer program with your calculation sheets, it may not hurt to include a logic diagam and obviously a hard copy of your computer program and output.
and Hydtools, you cannot be methodical without thinking--got it!!!

 

[RossABQ]

Here is an excellent, short and sweet, description of what a calculation should include. Sorry if it is not really specific, but it's a starting point...

 

[unclesyd]

Though both of these products are aligned with structural steel you might be able to glean some information from them. The best bet would probably be the Mote Method as they have 2 downloads.

http://www.motagg.com/Books

http://www.matrix.co.nz/Engineering_Calculations_156.aspx

 

[KENAT]

This is one place where having Dyslexia actually helps me.

I struggle with mental arithmetic or jumping lines in simplifying equations etc.

So, I have to fully right down every step in the process.

This tends to make it easy (if a bit long winded) for others to follow.

We had it drummed into us all through school to ‘show your workings’, most people never did it well, I had to do it just to help myself.

I agree with others about starting out with known information, a diagram, assumptions etc. For all but the most common equations state a reference/where you got it from, explain any variables etc. At school we were taught to substitute actual numeric values only at the very end, I still tend to do this.

Sometimes this is over the top, but for anything needing a record it’s an absolute minimum.


KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[GeorgeDon]

Proper listing of assumptions, references, mathematical simplification, etc., is a developed practice.

Here is one reference for the basics of Engineering Calculations, used in the Nuclear Regulatory environment:

http://www.efcog.org/bp/p/doc/BP65.pdf

 

[tygerdawg]

I concur with RossABQ & GeorgeDon. Without realizing it, I had taught freshmen engineering students how to do this. My students were Nintendo-generation kids with powerful graphic calculators who hadn't been taught to think yet. After I saw how they were struggling with the mechanics of doing homework problems and exams (and everybody had their own format) I laid down the law to them:

1. clean sheets of engineering paper, and use plenty of it. I told them it was their Patriotic Duty to support the US Paper Industry.
2. clear statement of problem and clear statement of desired results (reinforced their writing / communicating skills). Use sketches and diagrams for clarity.
3. statement of known quantities
4. statement of unknown quantities
5. statement of assumptions and justifications of assumptions with quoted references if possible
6. formulaic development, with units. Use sketches and diagrams for clarity.
7. after final formulaic reduction, then final insertion of numeric quantities (with proper significant figures) and calculation of results
8. Double Check results: "does it make sense?" results, proper units, proper significant figures.

I told my students that if they did Steps 1-6 in their exams, then that was worth 80% credit. After they adopted this strategy, then their grades all improved and the panic level during exams was significantly reduced.


TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[MotorVib]

Being the Engineering Manager I would set down some rule on what you would like to see in all cal's. tygerdawg & RossABQ have some vary good ideas to start with if not copy in total. My is always units for all final number.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[IRstuff]

I would modify "always units for all final number" to always units for ALL numbers. While there are few programs, Like Mathcad, that can do transparent unit conversion, dimensional analysis is a critical tool for verifying syntactic correctness of calculations.

Hopefully, someone in the opensource community will come up with a simple tool for doing this type of calculation.

TTFN

FAQ731-376

 

[MotorVib]

A quick modify I like to see what units were used at the start and end of an equastion. If you don't know what you started with how do you know what the end should be. Also units are a good check.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson