Why did all the high-grade flywheel bolts shear on our race car?

[Ranger Mike]

High grade ARP bolts used to attach flywheel to crank shaft..all broke. race car came intoi the pits with engine running and no power to the drive wheels. we pulled off the body panels and could see the rear of the crank shaft. it turned when we turned the front pulley on the crank snout..but the fly wheel did not. after disassembly sure enough..all 6 3/8 bolts were sheared. We have been told this was common. The titanium coated drills were a joke. we used an eazy out to remove the sheared potion. we took third in next days race because a S car almost spun us..last race of the year..the good news is that the car is in one piece and everything is pointing the right way on it.
just thought i would tell this on mech eng forum for your knowledge
RM

 

[pantaz]

... We have been told this was common...

Um, "common" is not a word that should be associated with catastrophic failures.

 

[Ranger Mike]

it happens..more often then one would like...first time for us..from now on i am replacing these every rebuild!
One - this engine is a 4 cylinder vibration machine..it shook the electric water pump apart..vibrated a dry cell battery until an internal short developed...the jam nuts on the roller rockers vibrate loose and the chassis develops stress cracks...6 bolts holding on the flywheel will certainly fracture at some point and spinning the tires going over a hill ( we run road courses) will shock t he drive train...plus we don't remember the age of these bolts??
getting old sucks!

 

[brewnog]

Doesn't bear thinking about! Any clues from the fracture surface of the one which went first? Are you torquing correctly? I've never trusted ARP studs since a few of them snapped in my first 911 engine.

 

[Q_Goest]

Do you think the bolts are breaking in shear or fatigue? Even if you don't think there's cyclic stress, there probably is. Consider bolts are very stiff springs, and they're holding together parts (the joint) that are also modeled as very stiff springs. The joint has to be much stiffer than the bolt to reduce cyclic stresses on the bolt. Cyclic loads impose cyclic stresses on the bolts regardless of the stiffness of the joint, so the key is to maximize joint stiffness and minimize bolt stiffness to minimize cyclic stresses. If the bolts break due to fatigue, you should be able to see beach marks under a strong magnifying glass. On the other hand, if the bolts are breaking due to shear, you can try increasing the frictional coefficient between the parts being held together. Locktite for example, can help reduce shear load on the bolt by taking up the shear load between the parts of the joint. Another option for shear loaded joints is to have one part dig into another by having a knife edge cut into one of the two joint parts which digs into the opposite flat side. I've used this tactic successfully in one application where the shear loads were too high.

 

[Ranger Mike]

we use Loctite RED religiously on every bolt not safety wired. I think it was a combination of what you said as these bolts were so old Rex and I could not remember when we replaced them..Thanks to all for the insight. we have to pull the engine apart to repair the crank as material transferred in the process. the flywheel holes got hogged out too.

 

[danev2]

Why is the engine vibrating so much?
Have you done a study on the primary vibration levels?
Can engine and trans mounts be redone to include isolators?

D

 

[mender]

Was the flywheel doweled? 4 bangers making over 200 hp definitely have to be.

Had a similar failure but on the ring gear of a RWD Ford Escort recently, suspect that the ring gear bolts had insufficient shoulder to properly locate the ring gear and worked loose, "porting" the aluminum case and puking out the oil.

 

[Ranger Mike]

no dowel. I have to check to confirm but i think the bolt has material that goes into counter sink on crankshaft flange then has screw threads. If i remember correctly.
A 4 banger will vibrate mainly due to firing every 90 degrees. lots of vibes

 

[mender]

I'd recommend installing at least three and preferably six evenly spaced dowels when you have the crank out, same size as or bigger than the bolts. Press fit into crank, very snug fit into flywheel. Make sure your machinist knows how to dowel a crank (http://www.geneberg.com/article.php?ArticleID=206). Your crank won't be as critical as the ones in the article but the procedure applies.

There's a set of eight at the middle of this page:
http://www.dansperformanceparts.com/buggy/epc/buggyep%203%20crank.htm

I'd go for the 11/32 ones. There are valve guide reamers that come in .001" steps to allow you to get the precise size needed on the crank and on the flywheel. Same thing applies if you want to go for 3/8".

Once you have the dowels in, send the complete crank assembly (balancer, flywheel, pressure plate) out and have it balanced together, that should help the vibes.

 

[Ranger Mike]

Mender, Thanks for that post...we will be disecting the flywheel/crank to do a post mortum and will be adding the dowels..

question to the Mech Eng community,,what is minimum number of dowels and bolts required to handel 160 hp engine..am in Cleveland and don't have my dyno sheet on torque..present 6 bolts are 3/8 diameter and go into counter sink .100"

 

[mender]

What are your engine specs: engine size, compression ratio, cam specs, number of valves per cylinder, rpm at peak hp, etc. I can make a guess and say that it would likely be around 130-140 ft.lbs but as you said, it's the shock loading during shifting and lofting that's more the issue.

 

[Danger]

I'm not familiar with the concept of doweling. Can you please explain it to those of us not in the current scene?
Meanwhile, my thought was toward whether or not some sort of polymer (such as Torlon) bushings between the bolts and the flywheel would be helpful. (To clarify, by "bushing" I meant something like a plastic Heli-coil.)

 

[mender]

Danger, the idea behind doweling is to provide a solid stop (or six) to prevent the flywheel from turning on the crank flange and introducing a side load on the flywheel bolts.

If the torque applied to the flywheel exceeds the "traction" of the flywheel/flange interface, the flywheel will move and apply a shearing force on the already stressed flywheel bolts. Properly torqued bolts are close to their yield point and shear much more easily than dowels in single shear (assuming the same material and size) because of that. I'm sure you know the difference between cutting a piece of cloth that is held on one side only and one that is being stretched tight.

As has been mentioned by others, increasing the friction at the interface can delay or even prevent the shearing action. Dowels however take over the job of keeping the flywheel from rotating and will do their job even when the flywheel is almost falling off due to loose bolts.

 

[Danger]

Thanks, Mender. It turns out that I'm familiar with the technique from the old days, but we didn't call it that. If memory serves, we referred to it as "pinning" or "pegging" or something like that.

 

[Ranger Mike]

wil get into the trailer and look up the dyno sheet
2000 cc engine running 12 to 1 comp ration
cam lift is .412" duration 268 degrees on both intake and exhaust, one int and 1 ex valve per cyl and single overhead cam
6800 RPM peak hp

 

[mender]

What engine brand? With those specs I'd put your peak torque around 145 ft.lbs depending on intake/carb(s) and header config.

I'm sure you can figure out the loading needed to shift the flywheel on the crank flange assuming normal steel to steel contact. Have you seen fretting at that surface before or is this new?

 

[Ranger Mike]

i don't know..will have to ask my engine builder as he saw it last.
thanks for the tip on fretting..always nice to pick up some knowledge here

 

[Ranger Mike]

we did it again..sheared the bolts...found out what happened...when we used loctite on the threads we did not follow the proper assembly technique. it involves using a speed handle to tighten down the flywheel bolts in proper sequence and then hitting the top of each bolt with a hammer ( like driving a nail) to seat the bolts andthen bringing up to proper torque...one more critiacl thing we did not do..the crank shaft register ( large O.D. boss that sticks through the large flywheel hole, must fit SNUG to the point of needign a hammer to properly seat it. This and having dowel pins in addition to the bolts should take care of this..