Engine Main bearing bolt yielding help 9

[preload]

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?)

 

[unclesyd]

Two things bother me.
Were did you get the 10% reduction for the torque value.
The values I have say that for oil you reduce torque values 35%-40%. .2K to .12K is a 40% reduction.

The values when using 70% UTS are 9250 min strength using 39 ft lb dry torque. The very max.

Using 75% proof load there is a clamp load of 4940 lb using 30 ft lb dry, 23 ft lb(K=.15) lubricated. This will give around 60,000 bolt stress.

85000 psi proof load for grade 5.
85000*.0775= 6587 psi proof load for 3/8-16 grade 5
Tensile area = .0775 in^2

Where do you get the 9300 psi proof load for your bolt?

 

[preload]

Unclesyd,

The bolt I am using is a grade 8 bolt.The 10% reduction of torque value for lube oil is from the link

http://www.morbark.com/Service/belttorque.pdf

 

[desertfox]

Hi preload

Your exceeding the proof load & stress because yes the friction factor is to low or as you say your lubricating the bolts too much.
However the bolts are failing during tightening when both
shear stress and tensile stress are applied to the bolt at the same time as I have mentioned in my earlier posts. When you cease tightening the shear stress due to the applied torque drops off and your left with tensile stress in the bolt, but at that point its too late the bolt as already yielded.
I think I see were your coming from with 7500lb I don't quite get that figure but fairly close but that assumes a friction factor of 0.15 which is much higher then what your getting in practice.

Do you understand the Mohr circle now or not?

Finally if you get the pre-load from the designer we can go from there.
If you post any pics of bolt fractures I won't get to see them before next friday so maybe you can describe the surfaces in your post.

Hi unclesyd

The figures come from the links to sites I posted earlier
ie:-

http://www.morbark.com/Service/belttorque.pdf

http://www.almabolt.com/pages/catalog/bolts/proofloadtensile.htm

http://www.almabolt.com/pages/catalog/bolts/tighteningtorque.htm

The bolt used is a grade 8 with 16 threads per inch and the 9300lbf is a force not a stress in psi.

I mentioned earlier also that the figures are not for flange head bolts as that may have an effect but I think the
priciple cause of this failure is friction factor being to
low for the pre-load torque there trying to achieve.

regards

desertfox

 

[unclesyd]

Another senior moment with my last post.
Your 9300 psi proof load is correct for the grade 8.

My information uses .2K for dry steel and .11K-.12K for various lubricants. It uses .15K for Cadmium plating and Aluminum Plating. Both would reduce torque by 25%. I'm trying to find a paper on various combinations of plated and lubricated surfaces.
My tables show a clamp load of 7000 psi with a dry torque of 45 ft lb(k=.2) and a lubricated torque of 35 ft lb (K=.15) for a 3/8-16 grade 8.

As mentioned in my earlier post it would be a great help to get on Skidmore-Willhiem tester.

http://www.skidmore-wilhelm.com/sw_industrial_torque.asp

 

[desertfox]

Hi unclesyd

The site I had earlier said .15k for just plated threads unlike your last figure in your post just shows how arbitrary these friction factors are.

This table is based on IFI 5th Edition Technical Data N-12/N-16, using Equation (1) and a torque coefficient, K=0.20 for non plated steel fasteners and K=0.15 for plated fasteners.

 

[unclesyd]

desertfox, preload,
You are correct the fine print got me again. I concur that something is amiss in what value of K to use if the fasteners are good.

The numbers I have come from Fastening Reference, Machine Design, Nov 1977.

I'm kinda like POGO, "I have found the enemy they is me"

 

[desertfox]

Hi preload

Just correcting my figures if you take the proof load of 9300lb and combine it with a torque of 33lb-ft the principle stress would be 148459lb/in^2 not 136500 as previously stated acting on a plane at 23.645 degrees above the horizontal and not 14.24 degrees below.
Similarly for the 6975 clamp force at 33lb-ft I obtain a principle stress of 124056lb/in^2 at an angle of 27.65 degrees above the horizontal note also that this is just slightly above the proof stress of 120000 lb/in^2 which is to be expected.
Sorry for any confusion this may have caused but it still clearly shows why those bolts are yielding.

regards

desertfox

 

[preload]

Desertfox, Thnks for those links. I "kind" off understood mohrs circle.

 

[desertfox]

Hi preload

your welcome

 

[sreid]

A statement and and two question;

1) A torque of 33 lb-ft should be quite reasonable for a grade 8 bolt.

2)A grade 8 bolt should have a minimum yield strength of 160,000 PSI?

3) A proof load is a load which should show no evidence of yielding?

 

[desertfox]

Hi sreid

I believe your right with your statement about proof load.
Looks like I misunderstood and took proof load to be related to proof stress.
definition of proof stress:-0.2% proof stress. Stress at which the material undergoes a 0.2% non-proportional (permanent) extension during a tensile test.

33lb-ft is the recommended torque for a grade 8 bolt when unlubricated in a joint, if you lubricate it then friction
reduces significantly and your pre-load can go above that which is required for yield.I believe this is the cause of this problem. Also during tightening the bolt see's maximum stress which is a combined stress due to tension and shear loads caused by the torque.

This site shows a grade 8 bolt having a minimum 130000lb/in^2

http://www.americanfastener.com/technical/grade_markings_steel.asp

regards

desertfox

 

[sreid]

Desertfox,

Thanks. I guess I get spoiled using Unbrako cap screws. I'll bet that most of the time, bolt yield is well above the 130 ksi spec for grade 8 bolts. Now with a batch (say) at the lower limit, some will strech oiled at 33 lb-ft.

 

[desertfox]

Hi preload

Anymore information?

You could try this for an experiment:- make a joint without
any lubrication, torque all bolts to 33lbs-ft and I bet you don't have any bolts fail.

regards

desertfox

 

[CoryPad]

desertfox,

By fail, do you mean exhibit visually detectable yielding?

I don't think there is enough information to conclude there won't be high angle results with your proposed experiment.

If this indeed is a friction related problem, I believe it likely would be due to variation at the bearing surface (some parts have no oil, some parts have oil) rather than in the threads (since all parts likely will have oil in the threads).

Regards,

Cory

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

 

[desertfox]

Hi cory

I think the main problem is friction being to low because of the lubricant and then torquing the bolt to 33lbs-ft
which is the recommended torque figure without lubricant.
If you do a combined stress problem ie:- torque loading plus tension in the fastener, I calculate that the shear stress at 33lbs-ft in the bolt is about 65170lb/In^2.
Using a friction factor of 0.1427 and calculating the tensile load from this info you get 7595lbf.
That equates to about 98000lb/in^2 tensile stress in the bolt.
At the point of tightening if you combine 98000lb/in^2
with 65170lb/In^2 shear stress you get a principle stress of 130536.6lb/in^2. this latter figure is just over minimum yield stress for the bolt.
However if you consider lubricant the friction factor would be a lot lower than 0.1427 maybe less than 0.1 in which case if you do the calc again your well into the yield stress for the bolt.
My idea of doing a dry joint was just to show that friction was the problem and if failures occured in the dry joint then theres something else wrong.

regards
desertfox

 

[dimjim]

Why not just use angle control and be done
with this. It is no surprise to anyone that
torque is not a good method to tighten bolts
for all of the reasons posted so far. I would
expect any bolt to fail or yield after turning
it 260 degrees.

 

[preload]

Dimjim,

I am trying to develop a angle strategy on this issue (that is my mnagments goal to improve the capability), but before that I am just trying to prove that it is a friction problem. And with desertfox's caluculaions we can see that.

Please see the failed bolts pics,

http://img442.imageshack.us/img442/4011/dscf1701kp1.jpg

http://img477.imageshack.us/img477/7199/dscf1703ho8.jpg

Please let me know if the pics are ok. The other part of the bolts are in the tapped hole,as they are difficult to remove.

I am starting a new thread on another problem , It would be great if you guys help me on that issue and continue helping me in this thread too

 

[desertfox]

Hi preload

Can't see the pics till Friday pm but thanks for posting them.
Whats happening regarding my questions posted last week ie:- joint design pre-load, change of lubrication etc?

No point in trying to develop an angle stratergy till you get the designed pre-load for the joint.
The ony way currently to prevent this bolt failure is reduce bolt pre-load by reducing torque or increasing friction and you can't do that till you know what the joint design needs.

regards

desertfox

 

[preload]

Desertfox,

lubrication is not changed,proces is also not changed,but we are seeing the problem since 2-3 yrs.no one exactly dont know what changed,but to thier knowledge nothing has been changed.

regarding joint design preload,my meeting with design guy is on 2nd may,he is gonna let me know the design preload.

 

[desertfox]

Hi preload

Thanks for the response, when you say you are seeing the problem since 2 or 3yrs, do you mean that you have not had this problem for 2 or 3 years or you have had this problem for 2 or 3 years?

regards

desertfox