Metal Coil Spring Failure for the General Public

How many times do you hear that a coil spring or set of coil springs have gotten old and soft?  Sounds reasonable, but is it?

The real truth is that the stiffness of coil springs is governed by their geometry, the type of metal that are made from, and the state of that material as it originally was when it first came out of the box.  It is not affected by use/age.  I’ll tell you why in just a minute.

There are many ways a coil spring can fail.  Here are some:

It can corrode (rust) and thus lose material and/or become cracked,

It can be overloaded beyond what it was designed to take,

It can wear out from constant rubbing on something,

It can lose its characteristics or even fail by being exposed to  temperatures higher than it was designed to meet,

It may take a “set” early in its use (essentially change its geometry) if that isn’t taken care of before it is installed for use, and there are ways to prevent that,

It can buckle (not remain straight) if it is not designed correctly for its application.  We had that happen on a large spring used in the Apollo Back Pack and had to redesign the spring.  Fortunately, we found the problem long before anyone had to use the Back Pack.

It can develop a “fatigue” crack from being flexed a significantly number of times beyond its planned life.  Oddly enough, this will not necessarily change its operation or stiffness.  We had a spring once that was cracked half way through, and it did not affect the stiffness of the spring at all.  I could explain why, but it’s complicated for a non-engineer (no insult meant).

If the ends are not ground flat properly, or not ground flat at all, the life of the spring before actual failure will be reduced.

The primary characteristic of interest that determines a spring’s stiffness once its geometry has been settled is a thing inherent in the actual material, its “modulus”.  The modulus is sort of a spring stiffness that is a natural characteristic of the material itself.  It can be affected by temperature, but not by very much in metals.  And, except for temperature, it stays the same regardless of the list I gave above.

So, unless a spring actually falls into one or more pieces, it does not get soft in use.

So, the next time someone tells you that the coils springs on your car have gotten soft, just laugh.

Now, if you want to make a spring out of a non-metal, you have my sympathy.

San Andreas

I watched this yesterday on disk with my wife, son, and his oldest son.  As disaster movies go, it was up there with the best.  Pretty well cast.  Pretty well written.  Pretty well directed.  No complaints about the acting, good job.  Plenty of action from start to finish.  Excellent special affects.

I’m not a geologist, but even as an engineer, I could see some faults in it here and there (a little play on words can’t hurt).  However, for the type of movie it is, those things don’t matter much.

If you like disaster movies, watch it if you missed it in theaters.

However, given the now often up close news coverage of real disasters, it takes a lot to match reality.  I happened to be having trouble sleeping at the time of the Japanese psunami.  I found it live on TV by accident.  It wasn’t the visual spectacle of fiction, but the realization of the actual reality dwarfs even the most spectacular disaster movie!


A Few Good Men (On ROKU)

One of my favorites.  We only had a VHS copy, so ROKU came in handy so that we could see it in HD.  Watched it again a week ago.  Been awhile.

I’m not a lawyer, so the realistic level of what happens is not mine to judge.  However the story is strong right up to the climax.  It is military.  So, if you aren’t into military films, then you may not like it.  However, it is not about war.  It is a legal battle.

Writing, acting, directing, and casting are all exellent!  I highly recommend it!

The Martian

It was Good enough to keep my interest, but it was definitely science fiction.  The list of things that wouldn’t happen in a real Mars encounter is long.  The hype about the movie far outweighs its performance.  The acting is fine, but the writing and directing needs work in my opinion.  Hey, I worked on the design of the Apollo Backpack and the Shuttle Environmental Control System, I can afford to be critical.

When you think about it, the movie makes NASA management look like political hacks.  The one lowly worker who thought up the right way to perform the rescue got no thanks or recognition.  That was by far the closest thing to what happens in real life in the whole movie.

Oh well, most people will like The Martian.  It’s OK, just not as good as it could have been.

By the way Hollywood, when you burn hydrogen and oxygen, the flame has no color!  That’s one of the reasons why hydrogen is so dangerous.

Writing Fiction

I’ve had to write nonfiction as a part of my work as an engineer…a book chapter, and many memos, all technical.  That, I’m used to.  I feel safe doing that.

So, how dangerous is writing fiction?  Certainly nothing will catch fire or explode, which could be the outcome of poor nonfiction writing, but what about the insecurity that comes from writing a novel when you are not already a well known author…or even if you are a well known author?

So, let me ask one first question.  Why do you want to write fiction?  Is it for money?  Is it for fame?  Do you have a cause that you want to put before your readers?  Or do you just want/like to write?  It might even be a little of each.

For me, it’s mainly the last one.  I like to write.  Fame, in the sense that people like and buy my books, shows up somewhere in my list, but not first on the list, and certainly not fame like a movie star.  I just want to share my joy in the writing with readers.

A cause?  Let’s see, do I have a cause?  Hmmm.  I don’t think so.  I try to stay away from politics and environmental controversies, wars, and things like that.  I’m really more interested in the smaller things in people’s lives.  Mainly, the mysteries that befuddle us, and the emotions that hound us, the fears, our limitations, our defeats, and our victories.  And I write using characters that I know from my fifty years as a working engineer.  Some of it is right out of past reality.  Some of it is simply made up.

I’ve read a number of books on writing and selling fiction.  With few exceptions, they are the worst books I have ever read.  One notable exception is Stephen King’s,  On Writing, at least his section of suggested rules to follow.  I found that I was pretty much already following them, particularly the one about not restricting yourself to a plot.  I have an overall idea of the story, but the storyline of each chapter comes from the content and event logic of the previous chapter.  Beyond that, I pretty much don’t know the whole story.  In the book I’m writing now, the characters come from what was actually my very first complete novel, Fall to Earth.  When I started this new novel, all I had was an idea for the first chapter.   I only found out who the murderers were recently, after having written nearly fifty thousand words.  What that does however, is make the story as much a mystery to me as it eventually will be for the reader.  It makes it more serendipitous, more alive, harder to predict.

In fact, although my books are mysteries, I think any good book, even nonfiction needs a sense of mystery and discovery about it.  The reader should always want to read the next chapter.  And the next chapter, although connected, should not be predictable.  However, it does have to be credible.

I try to link the incredible to the credible, an event that is crucial to the story that has a real connection with everyday life that the reader probably never thought much about.  Here’s an example that I plan to use as a preface for a future book.


Bungee jumping, your life hanging by a single thread…certainly that’s something for the foolishly courageous. OK, you may see things differently, and I respect your right to jump from a bridge only to be stopped before your certain death by a single rubber band. We all have our favorite risks that we think are safe or at least worth it, safe or not.

The list of activities that might kill us is long. Which one is the worst can be judged by its history, but we all know that life is never that simple. Much of safety depends on what others do and what we do ourselves. Controlling those sources of risk is not always easy, thus the need for training, but what about the mechanism itself. How do you “train” a mechanism?

It’s simple. You don’t train a mechanism. You design it.

So, let’s talk about that. The first and most primary rule of fail safe design is that no single failure of the equipment used in a design, shall cause a catastrophic event. Such an event is usually interpreted as one of two things:

1 – loss of mission (in other words, the loss of the capability to accomplish the purpose of the design), or

2 – loss of life!

And now you can see clearly why I picked on bungee jumping. Even when parachuting you have a backup parachute strapped to you, but in bungee jumping, it is obvious that there is only one bungee cord. And a single failure of that single bungee cord is the difference between life and death…your life and death!

In my world, designs, plans, and schemes are said to be “single threaded” if just one failure of just one part results in a catastrophe. There are no bungee cords in the story I am about to tell you, but single threaded scenarios are all around you everyday.

Can you see them coming…the next oncoming car that slides across that single white line, that sharp knife whose one slip cuts into your flesh, or that natural gas appliance whose design has that one fatal flaw?


OK, does that catch your interest?  If now I wrote a chapter about a woman  bungee jumping to her death, would you have a better feeling for how realistically dangerous it would be, even though the story was simply fiction?

The Preface, which is essentially the previous chapter, caused the chapter that followed to happen.  It left a mystery waiting to unfold.  And now the next chapter in line needs to resolve the mystery of why the cord failed.  Why was she the one who died, and not someone else.  Was it an accident?   I don’t have to know the answers before I start to write.  All I have to do is create a credible situation that has danger, or failure, or death, or injury, or confusion, and let it open up questions in the reader’s mind.  From there I pick a credible question or group of questions that lead to the next chapter.  And once you start asking questions, they invariably lead to other questions.  Essentially, the story builds itself from within, and answering the matrix of the exploding tree of questions becomes the full time, increasingly desperate task of the writer, although to the reader, it looks like the protagonist of the story is doing it.

I would have to say that if the writer isn’t as stumped as the protagonist in the story, it isn’t much of a mystery!