Engine Main bearing bolt yielding help 9

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Hi

I am new to this forum, but used to be in ROM (Read only mode). I am new to the fastening field but have quite a bit knowledge. My problem is we are yielding some bearing bolts right after installation on the cylinder block and crank case joint. I will provide you guys the info on bolt and joint parts and environment.

Bolt – carbon and low alloy steel flanged head 3/8-16 grade 8 bolt.
Joint – both cyclinder head and crankcase are made of Aluminum – copper alloy casting (soft joint compare to the bolt)

We also use a bead of gel seal in between the mating parts (cylinder head and crank case), when in contact, the gel seal uniformaly gets distributed on the mating surfaces. (making the joint even more soft?)

We make two engines V4 and V6. for v4 we have 6 bearing bolts and for v6 we have 8 bearing bolts. The tool used is two spindle dc electric torque angle monitor. For v6 We do bolts 1-2 then 3-4 then 5-6 then 7-8 and again do 1-2 (re torque due to elastic interaction) (I will try to upload a picture for better understanding)

Torque used is 31-15 lbs-ft on each bolt.Min proof load of the bolt is 9300 lbs.

Do you guys know why we are yielding bolts on a soft joint (if it is?)

 

[desertfox]

Hi preload

When you say after installation do you mean during engine running? is there any temperature involved at failure point? how are the bolts failing? information regarding yield stress of castings? and finally when you make the bolted connection is there any lubricant involved on the threads?
I would of thought that the softer materials may have yielded and taken a set before the bolt fails, perphaps when we see a picture and more info we can help further.

regards

desertfox

 

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What I mean installation is during assembly (engine not running). So no temperature involved at failure point, except for the temperature produced while torquing. Right now I don’t have any info on yield stress of casting, but will try to get that.

Regarding lubrication, yes we apply outboard lube oil to the bolt and also some gel seal which is applied to the mating surfaces will be in contact with the bolt (please see the pic, the pic despcription: casting shown is cylinder head and u can see the blue gel seal layer on the surface which is also in contact with the bolt and hole. You can see the fasteners in the pic and the crank case (not in pic) will be installed on that and assembled. The yielding occurs right in the thread area u can see.)

http://img410.imageshack.us/img410/6853/dscf0016ef7.jpg

Fyi – the pic is the brand new cyclinder head with bolts ready for assembly not the failed one.

 

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\\\\ I would of thought that the softer materials may have yielded and taken a set before the bolt fails,\\\\\

But with out any external loads acting on the joint , why would the bolt yield after the joint yielded?

 

[CoryPad]

Low friction (from intentional or unintentional lubricants on the parts) can cause high preload from the tightening torque, leading to bolt plastic deformation.

High friction can cause torsional yielding of the bolt.

Are you sure the applied torque is correct? Measured by an accurate means?

Your characterization of the joint as "soft" is not appropriate. "Soft" is a misnomer - a soft joint is elastically compliant, which is not related to indentation hardness, but rather to elastic modulus and joint clamped length and joint cross-sectional area. For your engine, the bearing joints' cross sectional area is large enough that the joint compliance is much less than the bolt.

The missing information here is friction coefficient (we can assume ~ 0.1), as well as clearance hole size. Also, is that supposed to be 15 lbf-ft or 51 lbs-ft for the torque?



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

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We are using torque control and angle monitoring with dc electric which has +/- 5% variation. Torque is 31-35 lbs-ft (I am sorry about the typo 15 lbs-ft).

when u say friction coeff , do I need to get the thread friction and bearing friction from tables? The lube oil we use decreases the friction coffecient. The gel seal which comes in contact with bolt (u see in pic), does that make any effect as it is not applied to the threads of the bolt?

 

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while doin angle monitoring, we see some fasteners achieve thier clamp in less than 20 deg of turn , but some take 260 deg of turn.we dont know if those 260 deg turnd fasteners are the one which are yielding but we think those are the ones.

we dont know why they get that much high rotations on some?

 

[CoryPad]

The fasteners in your picture appear to be studs with no heads. Do you use a nut to tighten these?

If you are using liquid engine oil as your lubricant, you shouldn't need to worry about the gel as it likely won't have a significant effect.

If you have a graph of torque as the ordinate and angle as the abscissa, are you saying that some of the fasteners have a much smaller slope? Are you accounting for the variation in angle at the start (the free running portion of the assembly)? You can use a non-zero torque (something like 5 lbf-ft) as the snug torque, and then count angle from there. Do you still see a difference?

Based on your information, I would look strongly at thread dimensions of the bolt and tapped hole, and bolt material strength.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

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cory

we are doin torque only but monitoring the angle.

we re-hit/ retorque two fasteners (please see new picture attached).

http://img64.imageshack.us/img64/8936/seqpt2.png

first we torque 1-2 then those two spindle move and tighten 3-4 and then 5-6 and then 7-8 and at last spindles re torque 1-2 again (just to make sure we dint lose any torque on elastic interaction)

sequence 5 is re-hit of 1.

in angle monitoring we see some fasteners get to 260 deg of turn to achieve 32 ft-lb torque and some only less than 20 deg to get the same torque. I dont know why?

why r we acheiving very high deg of rotation some factors in the same joint?

 

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If we are yielding the bolt , then why there is no damage to parts? are the parts stronger than the bolt?

 

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One thing I noticed from my data is if we have high degree of rotation then the torque achieved is low always.

 

[MikeHalloran]

As Cory said, the objects in the photo are studs, not bolts. We get confused enough; don't make it worse by using inaccurate terminology. You are probably torquing flanged nuts over them to retain the crankcase halves.

And that may be what's confusing your nutrunner.

By which I mean, and this is a wild-ass guess, that it's twisting some of the studs farther into the block while it thinks it's actually tensioning the studs.

I think I remember that studs are actually stronger if they're not fully bottomed in a blind hole ... but that leaves them free to rotate, which could confuse a torque angle monitor.

Mike Halloran
Pembroke Pines, FL, USA

 

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Mike,

I am sorry for the confusion. The pic I uploaded is a different engine in which we use studs and nut.

But the problem is with the other engine in which we use just the flanged bolt torqued down into crank case and into the cylinder head tapped hole. No nut used here

 

[CoryPad]

I agree with Mike regarding studs, confusion, flange nuts, wild ass guessing, studs not fully bottomed, free rotation and confusing a power tool monitor.

Any time I see high angle with low torque, I think deformation of parts (bolt threads, tapped hole threads, embedment of fasteners into parts).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

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\\\Any time I see high angle with low torque, I think deformation of parts (bolt threads, tapped hole threads, embedment of fasteners into parts). \\
Agreed. So what do u think would be the reason for this high angles and yielding?

 

[CoryPad]

Non-conforming bolt thread geometry

Non-conforming tapped hole thread geometry

Non-conforming bolt material strength

Lower than expected tapped hole material strength

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

Are you sure you've got enough engagement so that you're nut just stripping the threads out of the block?



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi preload

How do you know what tensile force you have in the bolts
how are you calculating that ie what formula are you using?
What is the engagement length of the bolt?
How have you determined that the bolts have yielded are you measuring bolt lengths ?

Regards
desertfox

 

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Desertforx,
Visually we can see the bolt yielded.
After the installation we are measuring the length of the bolt using ultrasonics. Then calculating the clampload using T=KDF.
Is engagement length same as grip length?
In our tool, we set 260 deg as max turn and when ever the tool shows 260 then the tool shows a red light saying something wrong. Then we take out the bolt see visually for any deformation and also measure the length using ultrasonics. We have a threshold/snug torque too.
Today I brought digicam, will take the pics of exact assembly I am talking about and upload them in an hour

 

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I have a question, when the bolt just starts yielding does the clamp load drops immediately?