Sheesh, I can't believe I've never had to deal with this before! Nor do I know where to *begin*!
How does one evaluate "wire" (cable?) for durability? E.g., how do you evaluate the quality/longevity of something like "ear buds" (in terms of wire failure rates)?
Are there tricks to improve reliability/durability? E.g., most of the molded "strain reliefs" seem to be an afterthought. While I suspect they do *some* good, I doubt there is much "engineering" that goes into them.
Is the solution to just use litz wire for everything?! :-/ (hmmm... I think I have misremembered that name :< )
It has been a very long time since I had to do work on that topic. Can't quite recall if the Mil-HDBK has anything on the topic. Might be worth checking out though.
I have often put cabling around flexible bits of machinery. The main trick is never to exceed the bend radius specifications, never exceed the tension load specification and observe the flexibility coefficients of the insulation materials (usually the first to go) under repeated motion. You may also have to consider UV exposure, chemical environment around the cables, radiation and thermal effects etc.
Occasionally you may find it easier to make up a special cable with a tension wire included amongst the cores and provision of a solid anchor point for the tension wire.
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********************************************************************
Paul E. Bennett...............
Forth based HIDECS Consultancy
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-510979
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************
While mechanisms pose a problem due to *inexhaustible* abuse, you, at least, have some control over how the cable is routed, supported, etc.
I'm more concerned with cases where the user has free reign over how the cable is "abused" -- hence the "ear bud" reference.
Of course, protecting against the bozo who wants to use the cable as a *rope* can be challenging... :> (though I suspect you can play games with the strength of the jacket, etc. to help absorb these strains). But, the fatigue that inevitably manifests "at the joints" (i.e., where connectors are attached) is the harder one to guard against.
E.g., the molded strain reliefs seem laughable when you think about them. I have many examples of cables where the wire fatigued *within* the strain relief portion of the cable. Or, immediately *past* the strain relief (so, all the strain relief did was move the failure point).
You (I) want the cable to be flexible yet this flexibility seems to be the root of the problem (i.e., cable run in EMT rarely suffers from "wire fatigue"! :> ).
I think that would help tensile strength but probably not do much by way of torsional or bending strains.
I think I can include some hardware provisions to at least allow me to determine cable faults "during use". But, that won't help the user anymore than a "check engine" light helps a driver whose car won't start!
I guess I'll have to talk to some wire and cable houses and see how they even *quantify* this sort of thing. I.e., it may turn out to be some bogus metric that is hard to relate to
Maybe take a look at what's used in robotics and automated machinery. Our pick-and-place machine does a LOT of cable flexing, though is inside a flex trackway.
In the 'old days' we would sometimes build-up our own strain reliefs with different length and diameter - but overlapping, pieces of shrink tubing. Crude but helpful. And putting a 360deg turn, or a 'helical' turn, in when space allowed as well, helped to distribute the wear. It also served as a 'service loop' for at least one or 2 future repairs.
My experience is that after all the careful selection and design, the off-shore manufacturer substitutes locally available (and usually far inferior) wire, and omits anything that is not visible without opening the device.
Sniff around the automotive industry for hints. Requirements for engine electronics are only slightly less stringent than those for spaceships and military, and the specs are not classified.
One thing I guess to mention is that, there could be significant differences between designing for staitic 'strain relief' ,occasional handling and actual 'movement'.
If Don is using this cable on a portable device, then perhaps he might find that a coiled lead might stand up better than a straight lead. Even with the moulded on connectors these tend to resist the ravages of normal "IQ0" usage. However, as evidenced on a nice set of stereo headphones, after about
20 years failures will creep in. I suppose the failures might have happened earlier had I handled the headphones very roughly.
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********************************************************************
Paul E. Bennett...............
Forth based HIDECS Consultancy
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-510979
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************
There are two different sets of problems I have to address.
One is figuring out some "standard" to reference for the cable manufacturer to ensure that the cables I get fit my needs/expectations.
But, the bigger -- *prerequisite* -- problem is figuring out what my *needs* are!
E.g., it's easy to figure out what type of resistor to put in a circuit iv you have a voltage V impressed across it and want to derate it by P percent.
It's easy to specify a response time of a system if you know the characteristics of the process interacting with it.
It's even easy to figure out how long a *harnessed* cable will survive in a repetitive motion application.
But, how does one go about quantifying the sorts of abuses to which a cable will be put by a user? E.g., we've all seen folks unplug power cords by "tugging on the cord" (instead of grasping the 'plug' itself). Or, people wrap telephone cords around their hands absent-mindedly while talking (back when phones *had* cords :> ).
One approach is to just say, "Hey, you don't respect this device and treat it 'properly' then you get what you deserve!" That sort of attitude never flies -- people hate it when you're *right* about their misbehavior.
Another approach is just to have N different cords made up (by M different vendors) and *contrive* some sort of test by which you can (subjectively) rate them. Then, hope your test is an accurate representation of the sorts of "typical abuse" you are likely to encounter.
E.g., safety (eye)glasses mys withstand the impact of a 1" steel ball dropped from a height of 50". Great! And how does that reflect the types of hazards a person *wearing* safety glasses is likely to encounter in an industrial environment??
Grab a copy of the Belden Master Catalog (AFAIK, the latest available for download is the 2006 volume). Belden is (are?) the "Cable Guys" and their catalog is quite informative. It's not internally hyperlinked but it is bookmarked, so navigation is reasonable for an 800 page catalog.
Start with the "Cable Finder Guide" section and drill down from there. If you can identify a suitable cable, then you can spec your vendors to supply, say, Belden 1694SB or equivalent. It's a de-facto standard and many other manufacturers will show a "Belden equivalent" chart.
Read through the Technical Information chapter as well.
Also, if you're getting into the wire/cable spec'ing arena, pick up a copy of the NEC Handbook. There's a new one out for 2011 (a three-year release cycle). Do get the "handbook" version. It contains the complete text of the base NEC with a lot of explanatory and supplemental material that can be quite useful.
You can only look at the physical needs of your product and the environment you expect it to operate in, then come up with notional figures for what punishment it might have to put up with.
If it is a portable device, then I would expect it to survive, unharmed, from being dropped from 1.5m height onto a concrete slab. Any cable into the device thus dropped should either auto-disconnect or be able to take the strain of the device being dropped and held by the cable. Not too difficult to get some force figures out of that.
Down to how you envisage the device being used, then looking for permissible worst case usage that you will expect it to contend with.
Once you have figures from considerations I outlined above, you may then look at the figures and decide on your comfort zone. Most drop scenarios look for at least 2.5 times as a margin.
So you are already familiar with scenarios and only need to make some measurements based on those scenarios to determine what to specify.
I have, at times, put very special, positively keyed, colour coded screws into a box to close it and give indication of un-authorised opening. Un- authorised access invalidated warranties and rendered the user to the repair charges. That was, though, on equipment that was expected to work within a rigorously specified harsh environment.
Considering that Safety eye wear was first introduced in the machine shops where projectile portions of work could fly off the lathe or milling machine, then the ballistic standard seemed quite sensible.
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********************************************************************
Paul E. Bennett...............
Forth based HIDECS Consultancy
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-510979
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************
Yeah. The trick is ensuring that *your* imagination is at least as bizarre as your users'! :-/
I've already compiled a list of some of the "static"/one-time abuses that I expect to encounter (frequently). Dropped from a table (weekly?), submersed (bi-annually?), dropped-then-kicked (e.g., imagine someone walking, dropping it in front of them and, before having time to react, *kicking* it with their extended leg -- *into* something... *hard*, of course).
What I can't envision is "pathological" behaviors that could arise from continued use/familiarity. E.g., the corded telephone is a perfect example! Do you think the designers ever envisioned people *wrapping* the cords around themselves? Or, winding portions of it around their twirling fingers/hands? Or, dropping the handset while holding onto the cord (so the cord supports the handset)? Or, dropping the *base* (so the cord supports the base)?
Aside from the obligatory "twisted cord syndrome" that seemed to befall all such cords within seconds of their installation, the telephone cord seemed to be an amazingly robust (in terms of maintaining continuity) implementation.
I've been doing post mortems on various bits of consumer kit to try to identify the ways cords die. And, guess at the abuses they may have seen (e.g., *knots* in cords surely aren't A Good Thing). So far, the only consoling idea I've had is to make the cord disposable and easily replaceable. But, that adds more connectors (and more failure points). And, if it is *too* "disposable", users will complain about "always having to replace the *^&()*^ cord!"
But, only *some* scenarios. I'm sure I will never be able to anticipate all the clever ways people will see to misuse it. E.g., damn near everyone has cradled a telephone handset between shoulder and chin to free up both hands for "whatever". Why wasn't the handset designed with this *common* usage practice in mind? Why haven't *cordless* phones (or cell phones) picked up on this, either?
User will not be able to open the device without seriously damaging the case. I.e., cases are considered disposable depot items. I don't want to get into any "he said-she said" arguments regarding whether or not the case was tampered with. No stories about "little johnny" taking it apart with his toy screwdriver before mommy and daddy realized what he was doing" (i.e., requiring you to call the user a *liar* if you disbelieve their excuse)
Of course, if he *cuts* the cord with a pair of shears and claims "little johnny did it", then I'm in the same boat. But, then there's just a cord involved.
"Yes, I know how mischievous young children can be! You should have your repaired unit back in a couple of weeks. (if that inconveniences you, you might want to consider being more careful around -- ahem -- 'little johnny')"
Yes. But why 1" from 50"? If you've ever seen a diamond grit grinding wheel shatter, you *know* that it would easily go straight through a pair of safety glasses and the cranial cavity behind them! And then keep right on going! (next time you're in a machine shop, look up at the rafters and I suspect you'll see evidence of one or more such incidents)
Or, how ineffective "safety shoes" are if your foot is run over by a forklift -- or, a pallet set atop it!
Of course, the standard is a tradeoff. No one wants to wear eye protection (esp if you don't "need glasses") that weighs more than the hardhat on your head! Or, "Herman Munster" safety shoes... ;-)
*Somewhere*, there is an analysis of the types of things they are hoping to protect against. And, the types of things they just threw up their hands and said, "Pity the poor sucker who does *that*!". I wonder how imaginative they were in their hypotheses? Or, did they just pick a couple of simple cases and extrapolate from there?
... which we all perfectly well know is impossible. Trying to make things fool-proof is an exercise in futility --- evolution has the advantage of infinite experience and patience in making better fools; you'll never beat that.
Not initially, I guess. But they learned, eventually. Thus the wide-spread use of spiral cord for the headset, then the invention of the anti-twirl gadget to take care of the residual disruption.
At least here in Germany, a good part of the reason for that was that the phones in private homes were actually owned by the phone company, not the users. You rented them. As such, there was substantial interest on the company's part for those things to be robust enough to survive the usual abuse, so they wouldn't have to replace them unduly often.
And those thing weren't just robust --- they were effectively indestructable. I.e. if you managed to break one, the company could usually make a convincing case about gross negligence, if not outright sabotage.
Yet they're virtually unavoidable on some types of cord, with earbud cords again being the prime example. It's a known fact they'll develop knots just by regular usage (stuff into a pocket, sit or walk a while, take them out --- et voila, a knot).
[...safety equipment...]
Not at all. Because the people who decide about those things aren't engineers, they're insurance analysts. Those guys routinely don't have any imagination, nor a need for it. They have statistics of thousands upon thousands of past accidents, and they'll use that to find the sweet spot where the cost for the safety equipment balances the savings in insurance pay-outs.
No. They use so many actual cases that they can _interpolate_.
Correct. But, you can make *some* attempt to think of the
*likely* (ab)uses that you will see.
E.g., a hand tool manufacturer complained that they were being sued by a bozo who used a WOOD CHISEL to cut ALUMINUM GUTTERS (and an aluminum shard flew into his eye).
*My* response, when hearing this, was: "Wow! What an excellent way to trim aluminum gutters!"
:-/
Despite the prevalence of the "twisted cord syndrome", I've only ever seen the "anti-twirl" gadget used in businesses. Every home I ever visited still suffered from this malady -- despite the fact that you could purchase those devices for a few bucks.
The same was true here (until ~30 years ago). You also rented your
*phone number*.
This should make my motivation pretty obvious, eh? :>
TPC invested a *lot* in its infrastructure. Western Union did a lot of engineering and research to figure out how *not* to spend money (time/personnel) after the fact maintaining things "in the field" (customer's premises). When there is no competition, you want to keep *your* costs as controlled as possible over the long term -- instead of short-term as the bottom-feeding market-driven approach of today exhibits.
Mine would have been closer to "Too bad; another hopeful Darwin award nominee failed to pull it off."
Some lawsuits really don't deserve any reaction from the courts except an immediate dismissal with prejudice, and a bill for the costs of the procedure including a fine of one month's salary for contempt of the court.
In summary at least two parties to that story would have got what they deserved.
Well, some home makers do consider their own labour cheap enough they can afford to un-twirl it themselves.
The Law is perverse. If you look at things in that microcosm, everything "makes sense". I.e., "if we have agreed on *this*, then *that* naturally follows. And, from *that*, then this *other* thing also follows..." It's only when you step back and look at the *entire* route you've "logically" followed that you realise just how totally insane it is!
E.g., growing up with "winters", one learned NOT to shovel the sidewalk (on your premises... not the one along the street) to the front door/mailbox when it snowed. If it was NOT shoveled, the postman was not obligated to deliver mail to you. OTOH, if you *did* shovel it and the postman slipped and *fell* (on a patch of ice that might have formed after you shoveled it), then you were liable for his injuries. You *assumed* responsibility by shoveling it and inviting the postman onto your property.
This *seems* to make sense if you look at each individual line of reasoning. But, when you step back and think about it on the whole, its ludicrous!
E.g., if a person has collapsed and not breathing, you are not legally required to render aid (unless you are a doctor/nurse in which case I think you are). *But*, if you elect to "get involved", now you *are* liable. (this has led many localities to pass "Good Samaritan" laws to indemnify you if you act "in good faith". of course, you;ll still have to hire a lawyer to defend yourself!)
I've learned to do *all* of my own work around the house, cars, etc. I've been burned by too many bad experiences with "professionals" -- whose goal is to get *paid*, not to "do a good job".
Whether it's the landscape architect (who apparently has no idea as to what plants actually *will* survive in this climate), roof-coater (who thinks diluting roof paint with 50% water results in an equivalent quality product), to house painter (who doesn't understand how to patch stucco and concrete so that, once painted, it *remains* where it was intended), to auto-mechanic (who gets grease all over the car interior), or the plumber that installed the antisiphon valve for the irrigation system (and made *no* attempt to ensure the highly visible, upright pipe was "vertical"), etc.
People you hire have little concern for the quality of the product they leave behind -- especially as most folks are clueless as to what that quality could (ideally) *be*!
IMO, medical procedures are terrifying because you have so
*little* control over their quality. E.g., the Bill Cosby "Ooops" routine...
Huh??? I don't understand how that will help. :< My codebook makes no mention of these sorts of issues -- though I now know how to protect UF cable runs from accidental shovel strikes! :>
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