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Discussion Starter #1
I am fed up with metal fatigue; I had a reinforced cooling shroud crack on my 56 LI. need some ideas on additional bracing and dampening of vibration on the shroud. I was thinking that I could try modifying the gastank bands.
 

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Discussion Starter #2 (Edited)
I was thinking that I could try modifying the gastank bands, so that there is a second location for the bands to mount to. also what is the rating of steel that the tank bands are made of
 

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Gravely bug bit.
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It looks like your shroud is suffering a bad case of metal fatigue. Might pay to find another to replace it with.
 

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Discussion Starter #4
the problem is that all shrouds have the same inherent design flaw. I would like to find a fix for this problem.
 

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the problem is that all shrouds have the same inherent design flaw. I would like to find a fix for this problem.
I have never had this problem with any of my shrouds. I have only had some of the old style gas tank bands that are bent break on me. But other then that, no shroud problems.
 

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That type fatigue is ALWAYS from vibration.

If its a shakin', its a crackin'!!

The extra brace is the best resolution.

Stiffness raises the natural frequency of an assembly, the extra brace makes it stiffer.

If the natural frequency is high enough, the fatigue will stop.

When that gas tank is cracking quickly, the resonant frequency of the item cracking is VERY close to the resonant frequency of what is shaking it, in this case, engine speed, of 50Hz.

Get the bracket stiff enough so its resonant frequency is 60Hz or so, and it will never crack.

Is that technical enough?? :dunno: :ROF

Sorry, it just "came out" :hide:
 

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Discussion Starter #7 (Edited)
CADplans no harm done. :thanku: that alleviates some uncertainty about reinforcement of the shroud. I had an idea, would a piece of a failed tank band welded to the bottom of the existing tank bands with the other end bolted to the bottom bolts of the shroud work? any other options other than replacing it?
 

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I agree with CADplans.

We cannot see from the photos if it is an electric start tractor, probably not; but with the tractor running, take a close look at the timing pinion shaft, that is the one with the fan pulley on it.

Can you see any noticeable 'wobble' with the engine running? If so, then the flywheel assembly is out of alignment, and that would give you, "a whole lotta shaking going on".

Hardware for the handle bars-to-shroud has to be in good condition and tight.

Roger,
 

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And there is an explanation why some users see the cracking and others do not, of an identical part.

If the engine is run 500 RPM higher, or lower, the cracking will be minimized.

That is not much of a change.
 

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And there is an explanation why some users see the cracking and others do not, of an identical part.
I would think amount of usage would be the dominate factor, if in fact all else is equal.
 

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And there is an explanation why some users see the cracking and others do not, of an identical part.

If the engine is run 500 RPM higher, or lower, the cracking will be minimized.

That is not much of a change.
Could it be those that keep the tank of fuel closer to full vs the crowd that runs closer to empty? Mass difference would change the reaction to harmonics...or it could just be old!

Sent from the MTF Free App
 

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Discussion Starter #14
Jim, that is a idea since I do fill up the tank to near 1.75 gallons and plus my yard is not exactly the smoothest. I also can never seem to get the shroud to stay tight enough. On a second note, what about having a fatigue simulation done on the shroud in Inventor or SolidWorks?
 

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Jim, that is a idea since I do fill up the tank to near 1.75 gallons and plus my yard is not exactly the smoothest. I also can never seem to get the shroud to stay tight enough. On a second note, what about having a fatigue simulation done on the shroud in Inventor or SolidWorks?
The heck with simulation, put it on an actual shaker table!!

Who wants to donate one!!?? :dunno:

Careful with those "old" jokes!! :crybaby:
 

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Discussion Starter #16
I personally don't want to do that type of destructive testing, however it does have its merits. One of the things that I would be concerned about is the load on the crankcase bolts should I bolt a reinforcement there.
 

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Discussion Starter #17
I think I have determined what I am going to do with the cooling shrouds. After reinforcing both cooling shrouds for both the model L and LI I am going to have them annealed and then redo the heat treatment on them. I hope this does not cost to much.
 

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I think I have determined what I am going to do with the cooling shrouds. After reinforcing both cooling shrouds for both the model L and LI I am going to have them annealed and then redo the heat treatment on them. I hope this does not cost to much.
I do not know what a cooling shroud is (pic!?) but, if it is sheetmetal, you can anneal.

If you anneal sheetmetal, there is no re-heat-treatment. Strength is ONLY added to typical sheet steel by work hardening (rolling or bending, etc...)

You could carbo-nitride, but, bring your checkbook!! :dunno:
 

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Discussion Starter #19
Update on the fan shrouds: I have determined that the rivets on the top of the shroud are pivot points for the shroud to expand, contract, and flex. Therefore, in order to keep the integrity of the fan shroud intact, I have revised the reinforcement plans. One thing observed is the need to prevent the gas tank brackets, that are bolted to either side of the shroud, from working loose, and or wallowing out depending on the style of bracket. Other than the fan shroud bowing out where the handlebars bolt to the shroud, the gas tank vibrating is the other. Another thing noticed, from past experience is the worst place for someone to weld together is the two sections that are riveted on the top of the shroud. This is due to the fan shroud not being able to flex. What do you all think?
 

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As I'm sitting here waiting for my computer to do an FEA analysis, I thought I'd type in a few points on structural design. My only walk-behind is a 5265, so I don't have one of these fan shrouds to study, but the comments I'll add here are generic enough to apply to anything.
- Remember that stress flows through a section and doesn't like turning corners. Sharp edges and abrupt section changes are stress risers and kill your fatigue life.
- Make sure your edges are smooth. Rough edges, when viewed microscopically, are just fields of notches, each one of which are crack initiation points.
- When you form a part, you are usually cold working the material. While cold working makes the material's yield strength increase, it doesn't change the strength at which it would fail (ultimate strength). This increase in yield strength does not help you in fatigue.
- Depending on the adjacent parts, some areas need to flex. If you make a part stiffer, you can make it fail faster. This is especially true of thinner, weaker parts mounted to larger, heavier parts. If you have an area that is going to flex no matter what, try lengthening the flexible area. This spreads the deflection over a larger area, reducing the unit strain and thus stress. I've gotten a lot of mileage out of this type of solution in the structural analysis work I've done.

One other thing to consider is a thought experiment I remember from my physics classes. If you have a plate with a hole in it, and you heat it up, what happens to the diameter of the hole? Answer - it increases. Remember that the hole would expand just as if the disk of material removed from it were still there. For reference, the coefficient of thermal expansion for steel is roughly 0.0000065 in/in-deg F. Thermal effects are worse when you have different materials (different expansion coefficients), large temperature swings, or large differences from one end of a part to another.
 
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