Author Topic: Broken ARC billet crank  (Read 56583 times)

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Offline tsmith1499

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Re: Broken ARC billet crank
« Reply #90 on: June 23, 2011, 10:44:20 am »
I have raced cars for years, (NHRA, IHRA, NASCAR)  and we use 4340 in LOTS of parts. I think the difference between the crank and the rod is that the rod doesn't really get any lateral pressure like the crank does. It see's stretch more than lateral force.  

There's gotta be a ton of lateral pressure being exerted on the pto end of the crank.

I was also wondering if the metalurgy results have been sent to the manufacterer and what they or their foundry have to say about this subject. Now that we're discussing it.  Not looking for ANY blame, just educational information that I think we all would like to know.  t
Thanks for keeping us informed,  Tom
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Offline Tom Cole

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Re: Broken ARC billet crank
« Reply #91 on: June 23, 2011, 10:52:19 am »
By the way, about 90% of all single cylinder crank failures in engines like these are on the flywheel side of the rod journal.  The reason is because it is the weakest part of the crank and in the zone where the most flex occurs.  Just saying so we don't go down a path thinking that common fracture points are a revealation.  What it does indicate is that there was not a flaw in the material at the point of fracture since it would be very unlikely that two cranks would have a material flaw in exactly the same place.  Especially when chemical analysis indicates consistency in the material composition.
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Offline DanW

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Re: Broken ARC billet crank
« Reply #92 on: June 23, 2011, 01:23:39 pm »
How much oiling does the flywheel side crank journal get?  It has an oil hole coming from the pump I would hope..

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Re: Broken ARC billet crank
« Reply #93 on: June 23, 2011, 03:35:48 pm »
Most of these Briggs OHV engines do not have a true pump.  Most use a slinger that flings the oil to places that it needs to go.  The Briggs OHV engines that have oil pumps often do not pump oil to the flywheel side of the crank.  The oil is often pulled from the sump through the filter, then "pumped" to the PTO side only, it often does not oil the connecting rod journal directly.
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Offline berthyd

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Re: Broken ARC billet crank
« Reply #94 on: June 23, 2011, 07:34:23 pm »
The SAE Chemical Composition specs of SAE 4340 "Chromoly" Steel (hardened)
are as follows:

Carbon  0.38 - 0.43 
Chromium  0.7 - 0.9 
Iron  Balance 
Manganese  0.6 - 0.8 
Molybdenum  0.2 - 0.3 
Nickel  1.65 - 2 
Phosphorus  0.035 max 
Silicon  0.15 - 0.3 
Sulphur  0.04 max 

The way I read the spectrometer test results posted above, ALL chemicals are present and within spec.
So, the 6562-C crank is made of the material we advertise.  The test above not only proves that, but it also shows that the material is consistently within spec, as multiple tests seem to have been done.

Is there stronger material?  Sure!  BUT...

We make cranks from a material that is CONSIDERABLY tougher than hardened chromoly...It is also CONSIDERABLY more expensive than hardened chromoly.  The blank itself would be more than $200 and a length of that material would cost us about $4000.  Figure that along with the increased machine time and tooling cost and that crank would cost the racer somewhere around $600.  A $600 crank won't sell when it is cheaper to just start with a better engine!

Why this one failed is a mystery to me because I have not seen enough pieces of the puzzle to come anywhere close to being able to speculate.

I just looked the spec up today for this material (just to show Snowman that he was wrong) and that sulpher number (.040) is absolutely correct.

I stand by my opinion that this is a design flaw that a billet crank may not adequately address.

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Offline George Herrin

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Re: Broken ARC billet crank
« Reply #95 on: June 23, 2011, 11:16:23 pm »
I agree Bert the problem lies in the BLOCK TOO MUCH FLEXING going on and seems like Paul K> is the only one that has figured out how to eliminate it. He turns his motors hard!!! And he doesn't break cranks.
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Offline Smowkin Joe

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Re: Broken ARC billet crank
« Reply #96 on: June 23, 2011, 11:35:30 pm »
Is it not possible, that eliminating some flex with a external support, would possibly focus and intensify the flex in a smaller area? I imagine it's much the same as a large speaker, poorly placed, in a small room that will encounter frequency cancellation due to 1st, 2nd, and 3rd order reflections. In the same line of thinking, without proper placement, absorbtion and diffusers, the same poorly placed speaker will generate signifigant amplification in other frequencies.  Just my thoughts on a different angle to the subject.
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Offline THawley08

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Re: Re: Broken ARC billet crank
« Reply #97 on: June 23, 2011, 11:56:22 pm »
Needs a support on the flywheel side....or you could just replace it with an opposed.

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Offline redline

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Re: Broken ARC billet crank
« Reply #98 on: June 24, 2011, 08:40:34 am »
or you could just replace it with an opposed.



Nah, I will just break a few engines, spot you the 18+ cubes and finish on the lead lap.
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Offline Old Goat

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Re: Broken ARC billet crank
« Reply #99 on: June 24, 2011, 11:03:38 pm »
By the way, about 90% of all single cylinder crank failures in engines like these are on the flywheel side of the rod journal.  The reason is because it is the weakest part of the crank and in the zone where the most flex occurs.  Just saying so we don't go down a path thinking that common fracture points are a revealation.  What it does indicate is that there was not a flaw in the material at the point of fracture since it would be very unlikely that two cranks would have a material flaw in exactly the same place.  Especially when chemical analysis indicates consistency in the material composition.

Over the last couple of years, I've broken a LOT of Briggs cranks. I have no idea why, but they all broke in the exact same place... right at the PTO side of the rod journal. I've never broken any on the flywheel side.
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Offline GRUBDIGGER

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Re: Broken ARC billet crank
« Reply #100 on: June 24, 2011, 11:13:04 pm »
SORRY GOAT, MELISSAS STOCK CRANK BROKE AT THE FLYWHEEL END.SO IS IT FLEX ON BOTH ENDS? WE TURN THIS ENGINE 6400 RPMS. WHAT DO I NEED TO LOOK FOR SO I DONT BREAK IT?(THE ARC CRANK)
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Re: Broken ARC billet crank
« Reply #101 on: June 25, 2011, 07:16:07 am »
The 4 cranks that I have broken were all on the flywheel side.
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Offline sandsnipe

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Re: Broken ARC billet crank
« Reply #102 on: June 25, 2011, 07:45:53 am »
Is it a truth that a case that has had a crank problem will more than likely have another?
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Offline George Herrin

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Re: Broken ARC billet crank
« Reply #103 on: June 25, 2011, 08:17:46 am »
Is it a truth that a case that has had a crank problem will more than likely have another?

yes

And all i have broke (stock wise) were flywheel side
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Offline Huffy044

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Re: Broken ARC billet crank
« Reply #104 on: June 25, 2011, 11:03:23 am »
Some thoughts I'll share, everything vibrates, me, you, your house. We aLLhave a "Natural Frequency" that can be exiced to the point it causes damage. I once got close to a large industral blower that had a damaged silencer on the intake side, the closer I got, the worse I began to feel, back away and the bad feeling would go away. The frequency of the sound it was eminting was messing with the natural fequency of my body.

I work in a large industral enviroment and we have problems with big machinery vibrating, mainly when we try to increase production by running the machines faster than they were designed for. Speeding them up almost certainly guarentees us of a problem. One thing we do is to add mass, increased mass asorbs some of the energy created by the vibration. We do this by filling the support beams with concrete. We also add rigidity, we do this by increasing the amount of metal in the structure, this raises the vibration point and allows the machine to run nice and smooth.

When Briggs built the 28 ci OHV, it was designed to run well below what we do with them. it was also designed for 28ci not what we have them out to now. Personally I beleive (along with the Vibe Techs at work) that case flex is a real factor in these cranks breaking. We have a program that can be atached to a machine and it will read the amount of deflection occuring in a machine. This is transfered to a moving picture and the results are amazing! I'd be willing to bet bothe the top and bottom of the cases that support the crank are moving all over the place.

How do we fix it? Add mass or rigity. The support bearings on either end will help, but we feel not cure the problem. There is very little room inside the block to add "Gussets" (triangle peices of aluminum) Rules are specific that the outside must "apear stock". So I don't really know, a billett block like "Blockzilla" might do it?

Just rambling thoughts.
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