Mirage drive repair

To perhaps clairfy my point a bit here, the basic formula for stress due to bending is:

s=F*d*c/I

s = stress
F = Force applied
d = distance from the force to the area being analyzed
c = distance from centroid  of the cross-section to the point (usually at the extreme fiber) being analyzed
I = moment of inertia

So, all else being equal, you can reduce stress by either reducing F, d or C, or by increasing I.  Or any combination thereof.

The reason I say that making the bar solid wouldn't help as much is that the I that you add by doing that is very small.

Formula for I for a rectangular tube:
I = (1/12)*b*h^3
_______
|           |
|           |
|           |  h
|           |
________
      b

Forgive the crudeness of the drawing but that's supposed to be a rectangular cross section showing the base and the height dimensions.  If it's square, obviously the base and height will be the same.

For hollow tubes, you get the overall I by calculating the I as though it were solid, and then subtracting the I from the portion that is hollow.

E.g. For 3/4" square tube, 1/8" wall thickness
I=(0.75)(0.75^3)/12 - (1/2)(1/2^3)/12 = 0.02116 in^4

Make that tube solid 3/4" and the I goes up to 0.02637 in^4

If you increase the wall thickness by 0.076" you wind up with the same mass as the solid 3/4" bar, but your I goes up to 0.0498.

Now, you can't buy 0.901" square bar stock with 0.201" wall thickness at Home Depot.  But my point remains that you get a whole lot more bang for your stress reduding buck by adding the material to the outside of the bar than the inside.

What you can buy, though, is 1" square tube with 1/8" wall thickness.  That would yield an I of 0.03449.  Which would still be an improvement.

All of this is more an exercise in undestanding basic mechanical engineering principles.  You may, or may not, be able to fit 1" tube in this application, and that may, or may not actually solve the problem.  If there's something in the design that is causeing a particularly bad stress concentration you may still have the cracking problems, if perhaps after a bit more use, that you do now.

To perhaps clairfy my point a bit here, the basic formula for stress due to bending is:

s=F*d*c/I

s = stress
F = Force applied
d = distance from the force to the area being analyzed
c = distance from centroid  of the cross-section to the point (usually at the extreme fiber) being analyzed
I = moment of inertia

So, all else being equal, you can reduce stress by either reducing F, d or C, or by increasing I.  Or any combination thereof.

The reason I say that making the bar solid wouldn't help as much is that the I that you add by doing that is very small.

Formula for I for a rectangular tube:
I = (1/12)*b*h^3
_______
|           |
|           |
|           |  h
|           |
________
      b

Forgive the crudeness of the drawing but that's supposed to be a rectangular cross section showing the base and the height dimensions.  If it's square, obviously the base and height will be the same.

For hollow tubes, you get the overall I by calculating the I as though it were solid, and then subtracting the I from the portion that is hollow.

E.g. For 3/4" square tube, 1/8" wall thickness
I=(0.75)(0.75^3)/12 - (1/2)(1/2^3)/12 = 0.02116 in^4

Make that tube solid 3/4" and the I goes up to 0.02637 in^4

If you increase the wall thickness by 0.076" you wind up with the same mass as the solid 3/4" bar, but your I goes up to 0.0498.

Now, you can't buy 0.901" square bar stock with 0.201" wall thickness at Home Depot.  But my point remains that you get a whole lot more bang for your stress reduding buck by adding the material to the outside of the bar than the inside.

What you can buy, though, is 1" square tube with 1/8" wall thickness.  That would yield an I of 0.03449.  Which would still be an improvement.

I love it when you talk "nerd"!

-Allen

1" bar will not work for this application. Needs to be 3/4. Stock is 1/8 wall. 1/8 wall 3/4 stainless is nearly impossible to find but I think a found a source if the solid bar fails too.
Other option is solid stainless bar. Heavy as shit but shouldn't shear.

..... Back to the fabricating cave!

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Let me know the alloy you're considering and I'll see what kind of stress-life data is available for it.

304 would have a million cycle life in the neighborhood of 475MPa, so considerably better than even 7075-T6.  If you can find it in 3/4" tube with 1/8" wall that probably will get you what you need to be durable.

I think 304 and 316 seem to be the only stainless alloys I've encountered.
What else is out there?

How comparatively weak is stainless 3/4 1/16 wall? That is much easier to find!

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What is that hole for?  It is to allow adjustments at the attach point?  Can it just be welded permanently or permanently attached in some other fashion?

-Allen

I think 304 and 316 seem to be the only stainless alloys I've encountered.
What else is out there?

How comparatively weak is stainless 3/4 1/16 wall? That is much easier to find!

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I'm about to head out so I can collect my crab pots.  I'll work something up in the next day or so.  1/16 though, I think, will be problematic, but until I run the numbers I can't be sure.

That's just a really bad place to drill a hole that size in a crank arm. I bet if the hole was half the diameter or smaller there would be no problem. Better yet, weld whatever goes into that hole to the outside of the crank arm to keep the integrity of the beam.

As for fixing the current design, you might be able to weld, rivet or bond wall doublers to the outsides of the crank arm by the holes. That would effectively put a "splint" on the weak area.

Yes the hole could be replaced. We did the with Rory's emergency repair but you loose the ability to set the fin at the furthest back hole.

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Yes the hole could be replaced. We did the with Rory's emergency repair but you loose the ability to set the fin at the furthest back hole.

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Ahhh ... I see.  Drill another hole further back (position #8)?  Or fabricate some sort of "hook" that only needs to be secured on one side at hole #6?

-Allen

All feasible options. Personally I don't use hole 7 but that freakishly tall guy in that tiny little boat does.
We'll see how long the solid bar lasts before redesigning the entire system.

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All feasible options. Personally I don't use hole 7 but that freakishly tall guy in that tiny little boat does.
We'll see how long the solid bar lasts before redesigning the entire system.

Sent from my Motorola Flip phone.

Freakishly tall guy ... aren't you almost a freakishly tall guy?

I'm just thinking about an add on to the existing system ... for other users that may not want to replace the whole bar.

-Allen

What I ultimately did was go whine to the dealer til they gave me a free one

Ok, they just warranteed it. Doesn't fix the root problem tho!

All feasible options. Personally I don't use hole 7 but that freakishly tall guy in that tiny little boat does.
We'll see how long the solid bar lasts before redesigning the entire system.

Sent from my Motorola Flip phone.

Freakishly tall guy ... aren't you almost a freakishly tall guy?

I'm just thinking about an add on to the existing system ... for other users that may not want to replace the whole bar.

-Allen

That's why I like fishing with Bryce and Rory .... I'm the short one.

Sent from my Motorola Flip phone.