The Steel Guitar Forum

I am using a 5/16" diameter steel axle to support the fingers. If I calculate the load on it at, say, 25lbs per string, the bending stress works out at 39,100 psi. Pretty high eh? I arrived at that number as follows....

Moment = Wl/8 = 25 x 10 strings x 3.75"/8 = 117.2 lb-in

and s = pi x d cubed/32 = 3.142 x 0.3125 x 0.3125 x 0.3125/32 = 0.003 in cubed

and M/s = f = 117.2/.003= 39,066 psi where Fy yield stress is 36,000psi !!

Is this correct?

I'd spend more time practicing.

Its a distributed loading not a point loading on the axle. . Not sure is that is properly handled in the formula.

Kudos on having the math skills to evaluate this on paper!

I'm afraid I haven't the math chops to check your work (I should've gone to college afterall). I have however made my living designing and making all manner of mechanical stuff for the last 40 years. I can say with a fair bit of confidence and no math that a 5/16" diameter metal changer pivot axle held only at the ends will not be stiff enough to handle the loads it would see on a pedal steel without unwanted deflection. However a simple support (ala Sho-Bud and others) between fingers should eliminate most if not all deflection.

If you're ever in the Petaluma area, stop by and we can have some fun nerding out on this kind of thing.

I suppose that one of the key words is "unwanted"- is any deflection wanted? I recall a physics professor saying that when a fly lands on a steel bar it bends- just because you might be unable to see it, the deflection can be measured.
It has been my experience that, in general, 12 string guitars have a tendency to suffer from more cabinet drop than their 10 string counterparts and it has been my opinion that the one of the reasons for this is the increased length of the changer axle being unsupported. I also know that extremely small changes can be responsible for audible changes in frequencies of a vibrating string. A supporting anecdotal incident comes to mind when I was perplexed about the severe cabinet drop in a keyless 12 string guitar that I built a number of years ago. After trying everything I knew to minimize it, I called the revered builder of the Anapeg in Australia Noel Anstead and the first question that he asked me was "How thick is the top plate in your keyless changer?" When I replied that it was .350" thick and chromed(which is supposed to increase rigidity dramatically) he said- "Ah- there is your problem- it needs to be at least 1/2" thick!" While I disagreed initially because I was unable to see any movement in the plate in question, I happened to have a dial indicator right there and measured it while still on the phone with Noel by mounting it on the cabinet of the guitar and put the other end on the center of the keyless tuner plate and stepped on the B pedal and saw a dramatic movement in the needle. My jaw dropped and was relieved to have found the source of my problem.

I smile sometimes when I hear cabinet drop! There are a couple steel guitars that have so called cabinet drop. By taking a look at the other components (not the cabinet) you can see just that,some steels have ruined material in key places. I found many years ago while running an engineering department that many times THE PROBLEM is a result of other symptoms. This in deed one of those cases.

Ten or more years ago I pointed this out to a great steel player and was told that it had to be the cabinet flexing. I simple changed the subject. Later I found that same guy had little of no mechanical abilities or "common sense". But I still wish I could play like he plays.

I just build seats.

Glad this subject came up on the forum.

Joe Naylor
www.steelseat.com.

Many years ago I was an inspector in a factory that used large electric motors (up to 1100 H.P.). One of my jobs was to check the run out (wobble) at the end of the shaft. The shaft was approx. 3" in dia. and about 12" long. With a dial indicator set-up I could move the needle by putting my hand on the shaft. If I pressed down hard on the shaft the needle would move a lot.

@ Tom, no it's a formula for a udl (uniformly distributed load).

Well I used this size axle in my previous home-build and it seemed to work fine. There was cabinet drop but not a whole lot to my ears. It was prob shaft bending. I like the idea of a centre support so i'll either go with 3/8" dia. shaft or a centre support on build #2.

@ Ross, when I'm up your way i'll def give you a shout. thanks for the thought.

@ Erv, practicing psi? or psg? ha ha

@ Georg, You tested with smoke and mirrors you say? Not sure if that will come off as very scientific even though it is. ha ha. You mentioned modification.....was this the photo you posted? Of extra supprt for axle?

super interesting Georg. Would you say the vibrations from the strings/changer only enters the body at the 4 screw points then? Changer stops are independent of wood body?

A true study in paralysis by analysis.

I think it is a legitimate endeavour for a pedal steel manufacturer.


The numbers seem to be coming out around the yield stress for steel.

Titanium or bust!

Was just thinking you need to use Mohrs stress circle to first establish maximum sheer stress...


eg.. the stress caused by moment is a different plane than yield stress.

I used to be able to do this stuff in my sleep. .. 30 years ago

Georg,
If we're nearing the stress point for steel, there's no question in my mind that what's needed is titanium for changer axles. You should know that!

Oh goodee! I Love puzzles!

I wonder, though, if you might be making some unrealistic assumptions that'd throw off your calculations by a lot.

The nature of the load. distributed and discrete, and not a point load, and not a uniformly distributed load. You might want to consider summing up Wl calculations for each string.

But consider again what W and l are. W=25 pounds tension per string is way too convenient. Press a pedal and the tension might go to 32 pounds. (http://steelguitarforum.com/Forum5/HTML/012534.html) I'd think you need to use the max load. And there's more. What happens to the tension when you put a lateral load, as in a steel bar, on a string? Half a pound downward pressure will translate into surprisingly large linear forces.

You said the rod is 3.75", but that's not the l the formula requires.

And the string isn't in line with the axle, but is offset by the distance from where the string rests to the center of the axle. It may not seem like much, but consider the geometry. The string exerts a rotational torque on the finger that's countered by the finger stop. If that string rest on top is the same distance from the center of the axle as the finger stop is, the linear force on the axle would only be half of what it'd be if the force was instead aligned with the axle.

There are on-line tools for just about everything. Here's one for calculating bending moment. It'll give you a perfectly precise, but perfectly wrong, answer if your assumptions aren't correct: http://steelguitarforum.com/Forum5/HTML/012534.html

Enjoy!

My dear late friend James Morehead and I had some lengthy discussions about changer axle size years back. I had sold him my '59 Perm, and I had a Fingertip that I gotten from George King. James and I noticed that the axle sizes were larger than usual, and we wondered if this had any effect on how wonderful the guitars sounded. They were over 1/2 inch in diameter, 9/16s maybe? And the thicker axle also changed the mass of the changer fingers due to the larger hole. Makes me wonder?
RIP James!