Perfect Reliability? The product lasts too long?
In the poem by Oliver Wendall Holmes, The One Hoss Shay, a deacon is confounded by the various parts of his carriage the fail.
And, he decides to do something about it.
But the Deacon swore (as Deacons do,
With an “I dew vum,” or an “I tell yeou,”)
He would build one shay to beat the taown
‘n’ the keounty ‘n’ all the kentry raoun’;
It should be so built that it couldn’ break daown:
“Fer,” said the Deacon, “t’s mighty plain
Thut the weakes’ place mus’ stan’ the strain;
‘n’ the way t’ fix it, uz I maintain, Is only jest T’ make that place uz strong uz the rest.”
Translating from old English, it basically means he wanted to craft a carriage using the best materials and techniques. Later, he built a very sound carriage where every part is just as strong as all the other parts.
And it is a fine craft that works well. It remains as new as the day it was built and outlives the builder.
There is more to the story written in the 1800’s, which you can enjoy.
Stress-strain in the design
The deacon’s idea was to craft each part such that it can “stan’ the strain”.
We do this today with component derating, stress/strain analysis, stress testing, and good engineering practices. Yet, we still have failures.
Since the “shay” outlasted the builder, did it outlast its useful life?
No. The chaise continued to operate for subsequent owners and performed well.
By crafting every part to be as good as any other part, there wasn’t a weakest element of the design.
A carriage, while a complex vehicle in its own right, is not as complex as an automobile today. Just the number of parts and range of functions make the design a much more complex process, yet we use the same approach.
Make each part just good enough to “stan’ the strain”.
Too much longevity
The deacon only made one carriage, and it lasted for 100 years and a day without repair or maintenance.
A thing of beauty. You could say it was over designed and lasted well beyond the expected or useful life.
It may have been bad for the horse carriage business as the owners did not need to purchase another carriage or care about maintenance.
As long as horse-drawn carriages were the preferred mode of transportation, it wasn’t obsolete.
If all carriages were built to such standards and just worked a very long time, would new designs and features have been invented?
Maybe new features would slowly evolve the design, yet just making the basic functions better, there would be little need.
There would be no shopping for a new carriage as the old one was breaking down too often (wear-out, costly to maintain, etc.)
From a carriage-making business point of view, this is a bad thing, to make a product that would guarantee to need to buy another one. That limits the market and profits.
Yet, how long is good enough?
Each product will be different and our ability to define what is long enough is fraught with uncertainty. Wear out prematurely and your customers will go elsewhere.
If it lasts too long, they do not need to come back.
Not to worry
It is rare that we design a product that lasts too long.
Sure it happens and in many cases one or two elements of a product tend to fail first, thus making all the other parts over designed.
The hard part is understanding which part is going to fail when under all the variations of use and environmental conditions.
Ideally, we make every part as good as any other part. Yet it’s hard enough to know if the weakest part is good enough, much less the other 1,000 or so components.
Create designs that are solid and have no life-limiting faults within the expected duration of useful life.
Start there. That is often a much harder challenge than many believe.
If we design and build products as the deacon did, we can then consider how to give up robustness to shorten the operating life.
Until we have that problem, let’s not worry about it too much.
10 Ways to Find Reliability Value (article)
Purpose of a Reliability Program (article)