Reliability of prototype 2

[buoy]

We are developing a new product. It uses off the shelf components in ways that differ from typical usage, so it is hard to get accurate information about component life. Because the design is still being refined, we will not be building and testing multiples for a while. Is this too early in the game to estimate life and reliability, which we know we need to improve due to failures of earlier prototypes? Is there a recommended approach to reliability and life estimates when a design is immature?

 

[MikeHalloran]

Testing for reliability is very slow, and requires many exemplars for statistical validity.

Repeated detailed analysis of each design iteration should be faster and cheaper; that's pretty much why engineers have jobs.

At least where components like bearings are involved, the hard work has already been done, math models exist and have been validated, so all you have to do is figure out how your design maps to what is known.

If you're going completely 'off label' for everything, you're on your own, and will have to work from the basics. ... and you should be constantly re-evaluating the wisdom of using standard components in odd ways.

On the other hand, test failures do give you signposts that can guide your investigation, by reverse-engineering each failure to its root.

On the other, other hand, robust designs don't usually fail in test unless you try really, really hard, and sometimes not even then, because you're relying on safety factors built in by other people, and you don't know how much margin you really have, or how it will change when a component vendor changes their process in some unanticipated way, which provides yet another argument for not getting too creative.




Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

"It uses off the shelf components in ways that differ from typical usage"

What exactly does this mean? If you are still using them within their rated performance, then the reliability estimates are still valid. Only if you are running too high a supply voltage, or are running them excessively hot, would you have to recalculate the reliability numbers. MIL-HDBK-217, while old, still describes the basic process for estimating how reliability changes as a function of environment, even if the fundamental failure rates listed in the handbook are not relevant.

TTFN
I can do absolutely anything. I'm an expert!
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[buoy]

"It uses off the shelf components in ways that differ from typical usage": for example, we are using parts not designed for a marine environment, one aspect of which is that waves produce variable rather than cyclic loading (i.e. amplitude and period are not constant). We just have an atypical application compared to what's available on the market.

 

[IRstuff]

So what? where does any reliability prediction say that it's only valid for cyclic vibrations? Do you really think that Army trucks have only cyclic vibrations?

I think you should contract hire a reliability engineer, because you seem to be making things up.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[buoy]

Not making things up, just not communicating the problem well (taking a component designed for controlled conditions and using it in a marine environment where you can get 'Perfect Storm' conditions).

 

[IRstuff]

But, that's just normal stuff; that's why we do environmental/ESS testing, and at the integrated circuit level, you can buy industrial grade or military grade components. I'm puzzled as to why your company hasn't already purchased a a few exemplars of the components you're going to use and tested them, or at least, asked the suppliers for more data, or for the possibility of them building full-environment parts.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[buoy]

Since we are basically a R&D startup trying to develop a new technology, we have limited $$ and test time (and expertise). So we have identified some key components based on past failures and tested those. But our device is complex and there are many more components/assemblies that we can't test individually. Suppliers are helpful but often don't have data on our application's conditions. And we are seeking to avoid having our suppliers develop custom parts if they don't have experience with our application's conditions. These issues are why I want to know the approach taken by people who have made a prototype that works and is reliable. It must differ from the approach you would take to improve reliability if there are comparable devices already out on the market and components that have worked for them. I think Mike is right, basically don't get overambitious.

 

[IRstuff]

There is no magic, secret, formula. Barring the ability to verify by testing, you select the components with the widest operating ranges and move on.

TTFN
I can do absolutely anything. I'm an expert!
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