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# How much trial-and-error at your work location?7

## How much trial-and-error at your work location?

(OP)
I'm just curious as to the prevalence of trial and error in different industries. Below, I describe two situations with two different design methods. It may be sucessfully argued either way that one is better than the other.
Method 1:
Try a 1/4 inch shaft on the new conveyor belt design. Put the max load on it. It broke. Hmmmm, I wonder why. Let's try a 1/2 inch shaft. Install the 1/2 inch shaft. Put the max load on it. It worked for a few hours, then broke. Hmmmm, I wonder why. Let's try a 1 inch shaft. Install the 1 inch shaft on the new belt. Put the max load on it. It didn't break! Cool. We'll use a 1 inch shaft for production. Total design time: 8 hr.
Method 2:
An engineer models the shaft and coupling. It takes him about 12 hr to set up the CAD model. This model includes worse-case extremes of shaft misalignment, temperature, shock load, speed, etc. It takes him about 4 hours to go through the model with different shaft diameters. He concludes that a 7/8 inch shaft is the minimum acceptable to gaurantee the life of the product, so he adds a margin to increase the diameter to 1 inch and specifies a 1 inch shaft for production. Total design time: 16 hr.

How much of method-1 do you see at your work location?

### RE: How much trial-and-error at your work location?

2
Most here are engineers, so probably never use method 1 (except maybe at home in their garage.)

P.S. I would go with method 3, use an equation for shaft strength and assumptions for worst case load, then calculate; about 2 hours work maybe.

### RE: How much trial-and-error at your work location?

I agree with gibson's suggestion of method 3.

### RE: How much trial-and-error at your work location?

Q: How much trial-and-error at your work location?

A: More than I'd like given we're meant to be engineers. Sadly the way schedules are set and progress reported to management it often seems it's better to get to half aed hardware more quickly by minimizing analysis etc. rather than spending more time on design but taking longer.

In fairness some of our stuff is cutting edge, new applications... so at least some can be difficult to predict but still, I reckon we could do better sometimes.

(Oh and I don't mean just throwing every model into FEA etc. by default, I agree with 1gibson that sometimes the middle ground is most appropriate.)

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### RE: How much trial-and-error at your work location?

What does method 1 contribute to any knowledge of why the pin broke? How does one determine fatigue life in method 1? Who is going to explain method 1 to customer service when all of the pins breakl in the field?

### RE: How much trial-and-error at your work location?

If the words "let's try" are used during a discussion of a problem, you will be escorted to the gate

### RE: How much trial-and-error at your work location?

There are laboratory problems where you are so far on the bleeding edge that trial and error is the only way to develop algorithms to predict material behavior.

Outside of that, most of us will use Option 3 with varying safety factors. I'll usually calculate whatever limiting parameter is being discussed with a zero safety factor. Then do it again with a non-zero safety factor and compare the results and make an Engineering Decision on what to use. In my work if something breaks in the field it can cost millions of dollars to fix it and get back on line. We use large safety factors.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

### RE: How much trial-and-error at your work location?

I do a lot of product development work, on fluid dynamic devices, and so a lot of time is set aside for "cut and try". But not for easy problems, like burst pressure predictions, but more for tweaking flow rates to the last few percentage points, or adjusting a regulator to avoid chatter in certain piping runs/ flow conditions. And if you say, "why not use CFD", we can have a nice chuckle and go back to work.

### RE: How much trial-and-error at your work location?

Star for

#### Quote (btrueblood)

"why not use CFD", we can have a nice chuckle and go back to work

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

### RE: How much trial-and-error at your work location?

I've been that engineer. First solution takes many hours. Once the model is established and verified, following solutions take fifteen minutes each.

Option A is to keep breaking things without learning anything useful.

### RE: How much trial-and-error at your work location?

I suspect method 1 is too optomistic for time. Changing the shaft requires changing mating components. Other parts may also be destroyed when the shaft fails. We don't all have an inexhaustable supply of incremental parts and material.

Method 3 works well for me. Spend a several hours on hand calculations. The drawings/modelings are near final product. Build a test model to prove the design. Make adjustments. Test to prove. Done. This has served us well for real time to market with a product we won't see come back very soon because of design failure.

Ted

### RE: How much trial-and-error at your work location?

Method 4: Use the same diameter as the output shaft of the motor or gearhead.

-handleman, CSWP (The new, easy test)

### RE: How much trial-and-error at your work location?

#### Quote (1gibson)

Most here are engineers, so probably never use method 1 (except maybe at home in their garage.)

What he said. Engineering is thinking about things before you build them, not throwing different-sized rocks at a problem and hoping it goes away.

#### Quote (TheTick)

What does method 1 contribute to any knowledge of why the pin broke? How does one determine fatigue life in method 1? Who is going to explain method 1 to customer service when all of the pins break in the field?

Also what he said. If you build things without understanding, it's just a matter of time before you aren't allowed to build things anymore.

Testing is essential for good design - good final design. It's not a replacement for thought, and trying to use it as such on every nut and bolt along the way will bankrupt your company.

(As an aside: if it takes you 12 hours to model a shaft/coupling in CAD, you either need to get better at CAD or hire someone who is.)

"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." -Scott Adams

### RE: How much trial-and-error at your work location?

A fair amount.

We do a lot of looking for new applications for new materials. Thus we have to do some trials because the data is usually not available.

Secondly, the data we do get has often been through a marketing filter. The engineers supply figures but marketing takes whatever looks best and publishes that. We run our own tests because we want it to break in testing not at the customer’s.

Thomas J. Walz
Carbide Processors, Inc.
www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

### RE: How much trial-and-error at your work location?

Our guys in the field do it all the time -

Famous words of their wisdom...

I have been doing this for 20 years and it always worked.

It worked last time...

My responses vary from ...

And where did you get your engineering degree from..

After 20 years - you should know better OR you have been doing it wrong for 20 years!!

### RE: How much trial-and-error at your work location?

I also agree testing is the key. Nothing demonstrates this better than F1 racing for me.

They have top quality engineers who specialise in a specific discipline, they have cutting edge technology, workshops, materials, simulation software, analysists and testing facilities at their disposal and huge budgets, but still things do not perform as expected and it all comes down to time on the test track.

### RE: How much trial-and-error at your work location?

Your method #1 isn’t engineering. It’s classic DIY’er. and shows a lack of being able to apply any engineering knowledge or experience. And, your method #2 doesn’t show much engineering knowledge or judgement either, since most experienced engineers on that type of problem wouldn’t bother with FEA on that problem, at least at first, and would probably only spend a few hours to develop his/her initial design. As mentioned above your time comparisons are way out of line also for the reasons mentioned. Unfortunately there are too many people and companies using method #1 and selling that as engineering to the public, and they are hiring people to do their engineering who don’t have enough real tech. or engineering ability or knowledge to do anything other than your method #1 in arriving at a solution. Engineering has become a bastardized term, the way many people and companies us the term today.

Most good design is done with a considerable amount of technical, materials and engineering knowledge, experience and judgement in the mix. There will likely be knowledge of a previous product or project to refer to for a starting point. There is also just a good general engineering education and background which gives you some direction as you start a project. The process generally isn’t a hit-n-miss endeavor. That doesn’t mean there is no place for testing, but that generally isn’t the starting point of most general engineering design and product development work.

### RE: How much trial-and-error at your work location?

"Testing" is not "Blind trial and error". The formula 1 example above is illustrative. The people working on a multi-million dollar car are not going to "try a 1/2 inch shaft and see if it breaks" on a race car. They are going to apply engineering principles and balance material properties vs. weight and pick an answer within the matrix of acceptable results. Then they'll test a sound engineering design to see how the design fits with the system and performs with the thousands of other cogs in the machine. That is about as far from the OP's "option 1" as it is possible to get.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

### RE: How much trial-and-error at your work location?

I have been fortunate to work in a couple places that value engineering (not just DIY hacksmanship). Engineering isn't just calculating, it's acquiring knowledge for future use. This means a combination of real data and calculations. An engineering model needs to be refined for the appropriate amount of precision and for valid assumptions and simplifications.

It can take some effort to refine one's "math model" to match reality, but once they sync up, great things are possible. A well-refined model saves many precious hours not trying things that are destined to fail. A well refined model also can be used to predict things beyond experience.

### RE: How much trial-and-error at your work location?

At least *try* to do a T/J = t/r = G@/L before destroying the first few shafts...

### RE: How much trial-and-error at your work location?

The allowable Trail-and-Error in a company is inversely proportional to the amount of "Error-then-Trial", if you get what I mean.

That is to say..., if failure is not an option and things are expected to work correctly right out of the gate then there may be a "trial" after any "error".

But if failures are of little consequence -- in terms of safety and cost -- then trial-and-error can be acceptable. Why design a \$1000 solution for a 10-cent problem?

But most companies with engineering departments have them for a reason: they do want/need the design to work correctly right out of the gate. Overkill on the up-front design effort can avoid lots of costly errors/failures downstream.

16 hours of one designer's time is a lot cheaper than 4 hours of 5 techs and 2 managers staring at the failed design (i.e. 4 x 7 = 28 man-hours). Not to mention the additional cost due to "There is never time to do it right, but always times time to do it over" syndrome.

The good news here is that seasoned designers don't reinvent the wheel. Sizing a shaft, for example, will typically be calculated once and then future designs can be quickly WAG'ed based on the experience of the previous design calculations.

Sort like if a 300-lb man can walk across a 5-ft long 2x4 board calculated to withstand that load, then a year later a designer will confidently recommend using a 5-ft 2x4 to support a 200-lb man.

Unless there is an overpowering reason to optimize the design, the previous calculated design becomes the rule-of-thumb. And, sure, although testing will almost always be conducted to validate a design, the calculations minimize the number of test failures -- which would lead to another round of costly tests.

So pure trail-and-error should be considered a no-no except for rare situations of precious little significance and consequence.

### RE: How much trial-and-error at your work location?

"if failure is not an option "

"So pure trail-and-error should be considered a no-no except for rare situations of precious little significance and consequence. "

Then no new devices would ever get created. Real R+D is a lot of educated guessing, followed by testing, followed by analysis and then repeated as required until the error (what you've got vs. what you wanted) is reduced to an acceptable level. Sometimes, not very often I would agree, but often enough, you find yourself blazing truly new territory. It's pretty fun when it happens, and you have to be willing to stub your toes and break stuff in trying to get stuff working the way you think/hope it ought to.

I find it funny that a discrete control PID loop is essentially a trial and error process used to control all kinds of widgets in the real world.

### RE: How much trial-and-error at your work location?

Btrueblood makes a good point. Many people have a complete misconception as to what "trail and error" means. In this usage, error does not have the connotation of failure or catastrophe. It simply means a deviation from the desired result, as in error signal in a control loop. We drive to work using trial and error. When the car's path deviates from the desired path by more than a comfortable amount we take action to correct the error. Too many people take the term error personally and react by denying that there is an "error" because that would mean that they are "wrong". Trial and error is probably the most common and most useful method of problem solving, even for engineers. All that the analysis and calculations do, is allow for the size of the errors to be kept smaller. I guess the flip side of "trial and error" is "paralysis by analysis". There has to be balance and perspective.

### RE: How much trial-and-error at your work location?

Where I work, we have no room for trying things. Except for the pure research in the R&D lab, it's expected that any/all mechanical designs should be done "correctly" on the first time through without failures. We are constantly doing variations of existing products ranging from minor alterations to completely custom, so especially with the custom cases a prototype with testing is required to ensure flawless operation but we're never given the opportunity. In the cases where a prototype is planned, sales sells the prototype before it's even been built.

David

### RE: How much trial-and-error at your work location?

With method 1, it may take 8 hrs of design time but think of the cost in labor including supervision, injuries , parts, tool replacement,utility useage, unless your plant is in China, India etc...

### RE: How much trial-and-error at your work location?

As important as trial-and-error may have been in past centuries, personally I don't support the assertion that this method is of value in engineering. I do recognize that engineers are typically recipe types who reference closed form solution sets from technical references, this per-supposes there is an understanding in the model limits or the assumptions inherent to that paradigm anyways. So on that note, I guess we are all guilty of some measure in trial-and-error, philosophically speaking.

Regards,
Cockroach

### RE: How much trial-and-error at your work location?

There's a lot of trial and error when you are fine tuning something. For example, crankshaft bending dampers are fitted to improve sound quality, and despite the attempts of many, there is no conclusive way to compare the sound of two engines graphically or by calculation . Similarly, tuning shock absorbers for ride and handling largely comes down to suck it and see. Indeed, tuning the boost curve for the power assisted steering also comes down to a mixture of measurement, calculation and observation.

To some extent all of these are open loop investigations, there is often no specific target and the result is the best you can get as a judgement, in the time available, with the hardware to hand.

Cheers

Greg Locock

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### RE: How much trial-and-error at your work location?

#### Quote (geesamand)

In the cases where a prototype is planned, sales sells the prototype before it's even been built.

Welcome to every company ever :)

NX 7.5.5.4 with Teamcenter 8 on win7 64
Intel Xeon @3.2GHz
8GB RAM

### RE: How much trial-and-error at your work location?

FWIW, my definition of trial and error, and I think Greg's, does not align with the OP's example. Nobody I know uses trial and error as a first design method; instead it is a method to refine a model beyond the limits of analysis available. The following comments continue using my definition:

Every custom design outfit, even geesamand' shop, is using trial and error, but they refuse to admit it. Their trials, in the form of one-off designs, get tested in the field, by their customers.

Cockroach, would you say that testing is also of no value then? I.e. why bother to test a product, since you "know" that you have designed it correctly? We test products because we know there are deviations that occur between real world objects and our simplified analytical models, and that these are beyond our limited ability and/or finances to completely control, and we hope that those deviations don't reduce the performance of the product beyond acceptable limits. But if you don't test it, your customer will. Just hope that the feedback loop gets closed by the customer directly, and not via his lawyers or those of his survivors.

### RE: How much trial-and-error at your work location?

I'm mechanical design - oilfield equipment, BTrueBlood. On occasion we might test for shear screw strength only because of the uncertainty in the brass chemistry, but for trial-and-error in prototype development, never. So I agree with you on your first design methodology comment.

But don't get trial-and-error mixed up with "testing". We test all the time, usually destructively for on the first-off. This is consistent with ISO 9001 Design Control were the interest is in "verification", not "validation". By the time the prototype is build, we have a real good handle on validation. If something doesn't test within our strict set of limits, then the model is in error and we search for the understanding as a design error. Validation has failed. This essentially means we go back to the drawing board and after a full review, design again to get it right. Hopefully it is a mathematical error, maybe a poor model or something, but more times than not it is a fundamental oversight in the input design statement(s).

Perhaps it is my mathematical physics background, but I just don't buy into taking a good guess and homing in on reality following a few good tests. That's how I regard trial-and-error, aside from Greg's comment on "fine tuning".

Regards,
Cockroach

### RE: How much trial-and-error at your work location?

I'd argue that trial and error is done all of the time in engineering (though not in the way that is described in the initial post). Generally there are tons of unknowns that go into a design. Often loading is not well-understood when designing a machine, or it is simplified to make the calculations possible. Heat transfer and temperature distributions are especially difficult to predict, since we often rely on empirical models and environments are never any more than an approximation of what the device will actually see. Flow network models assume flow coefficients (which have been part of engineering since the Romans) for idealized geometries. Tons of approximations.

So, we (engineers) make educated assumptions about such things and refine our assumptions as a design progresses. Critical areas of a machine which push the mechanical or thermal limits of the machine's components are often validated with component tests, which further help us to understand their behavior. Finally, once we have done the best we can at producing a working design, we build the machine and test it as well.

At the end of the day, designing is all about trial and error. We use the best tools we have to make the smartest decisions about our designs that we're able to... and as we learn more about the thing we're designing, we iterate. Finally, after a lot of iteration, crumpled up papers, and frustration, we arrive at a design that we think is adequate and meets its design requirements... and we build it. From there, we see how we did, and how the device performs. We monitor it in service and listen to what customers like or dislike about it. We monitor performance, follow maintenance issues, wear issues, and other serviceability issues. We learn what worked well and what could be improved, and we use that input when designing our next machine.

Engineering is all about trial and error, actually. However, it's also about using your knowledge of physical sciences to make smart design decisions and minimize design iterations.

Could you imagine building an automobile by making parts through trial and error as was described in the initial post? There are thousands of parts which need to be sized. How much would it cost to have so many failed tries? How would you ever arrive at a competitive design?

### RE: How much trial-and-error at your work location?

flash3780, arriving at a competative design of an IC now primarily by trial and error most likely would be impractical. However, 100+ years ago one suspects quite a bit of what we might now consider trial and error went into such things.

Hence the various comments of there still being trial and error on some 'cutting edge' applications - as well as on those which might not be cutting edge as such but we still have trouble fully defining/analyzing.

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### RE: How much trial-and-error at your work location?

KENAT, I agree... though 100 years ago machines were often simpler (though not all were; steam locomotives are quite complex). There were innovative methods that engineers used to determine design adequacy, though. Gustave Eiffel, for example, used Tresca's maximum shear failure criteria when designing the Eiffel tower. Clearly there was no room for trial and error in such a venture.

Test components were often constructed for machines, however. One method for evaluating them involved painting parts with shellac; areas where the shellac cracked were assumed to have excessive strain and the design was modified to correct it.

Design has always been a bit scientific... and always has involved some level of trial and error.

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