Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Engine Main bearing bolt yielding help 9

Status
Not open for further replies.

preload

Marine/Ocean
Apr 12, 2007
176
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?)
 
Replies continue below

Recommended for you

Hi preload

yes to your last question and yes grip length=engagement length
 
Godddd, I realised just now that I dint bring my data tranfer cord for my digicam. I cant upload the pics now. I will do them as soon as I go home today.

I will get you all other details on monday as we are going to do some test rundowns.

I can paste the load and torque for previous rundowns for one engine

T(ft-lbs) F(lbs)
32.50 12180
32.10 12880
32.10 7550
32.20 8860
32.10 11970
32.50 7550
32.30 12180
32.10 8860

above is one v6 engine (8 bolts) worth sample data. tension numbers are calculated by measuring elongation and torque from the tool.u can see the scatter of preload and torque pretty contant. I dont know why
 
I disagree with desertfox (that doesn't happen often).

Preload does not drop during assembly if the bolt yields. Bolts are made with strain hardening steels, so the preload increases when the bolt yields. The preload can decrease if a service load yields the bolt and then is removed, resulting in less preload.

Engagement length in this context is the length of bolt threads in contact with tapped hole threads. The grip length is the length from under the bolt head to the start of the tapped hole.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
 
I can't tell you guys the engagement length because I don’t see any part of the bolt except for the the head as everything will be in the housing
 
\\\so the preload increases when the bolt yields.\\\\

Cory,

so u saying that the >12,000 lbs (my prev post numbers) are the ones that are yielding?

this bolt 3/8 - grade 8 bolt has a min proof load of 9300 lbs
 
You have to calculate the engagement length from the design documents or by measuring the disassembled components.



Mike Halloran
Pembroke Pines, FL, USA
 
Mike I will do that on Monday for sure
 
My first thought is some bolts have proper hardness and others missed heat treat, wrong material, or some other strength problem. You have two groups of data; the 12,000 group and the 8,000 group. The question "Are the parts to print" often leads to the root cause here where I work.

32.50 12180
32.10 12880
32.10 7550
32.20 8860
32.10 11970
32.50 7550
32.30 12180
32.10 8860
 
This is set of data from another engine rundown

32.40 10666
32.00 8855
32.20 7245
32.20 11571.875
32.10 9358.125
32.00 7245
32.50 6238.75
32.20 5131.875


 
The torque-tension data won't help find low strength parts. It will have huge variation due to friction variation, and relies only on length change from the ultrasonic measurement.

Regards,

Cory

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

Your right on both counts I was leaving work in a hurry and wasn't thinking straight.Yes the bolt tension would increase due to work hardening then drop off before final bolt failure.
Anyway I was going to suggest that from the 9300lb proof load quoted in the first thread and using the formula "preload" as provided we can theoretically work out
the friction factor to achieve 9300lb.

K = T/FD

which based on an average 33lb-ft would be 0.1135 friction factor.

We can see from the figures now posted by "preload" that
most of the bolts have seen more than 9300lb.
If we assume the bolt engagement is sufficient for the joint design which it appears to be if only the bolts are failing, then the problem lies with the friction factor in practice and possibly in the original design of the joint, or screw thread geometry etc not conforming to standards.
Preload I assume somebody did a joint analysis and established a torque figure for the working conditions of
this joint?
It seems to me that you need to do a lot of experimental work and get your fastening sequence consistent in terms of bolt lubrication, fastening rotation and method of measuring bolt tension force and not least of all certified
materials if the joint is so critical.
There are quite a few methods of measuring and controlling
bolt pre-load all of which have varying degrees of accuracy
I will provide a few links which give some information on this topic:-
Finally you may find that you either need to increase your number of 3/8" bolts or increase the size of the bolts you have already, or reduce the existing pre-load if the design can stand it.

regards

desertfox
 
Cory, Thanks for your analysis. I am new to this job (plant quality eng) and my manager says we get specs (only torque) from our engineering group and for some reason they almost always go to proof load on all joints be it critical or non critical. So we in the plant, when problems arises, find the root cause of the issue and if the spec is too high we need to prove it some how and tell the engineering group to reduce the spec.
Now on this problem, we need to find the rooot cause and analyse why the bolts are yielding and if torque is the culprit we got to let them know. We need to prove things before we complaint to engineering dept.
This problem arised even before I joined the company and I dint see the assembly of this joint. Will see it on Monday. I can give u guys a very good information and calculations the present technician used.
My concern is,
Bolts is steel material and joints are aluminum-copper casting(so the tapped hole thread are aluminum threads). If the bolt yields, then the aluminum threads in the joint should yield even before that right? I "think" it is not happening, only the bolt is yielding at the threads. How come a high strength bolt yield first even before aluminum threads?
 
I am sorry, my prev post is addressed to Desertfox.

 
If the thread engagement is large enough, then the bolt (even a high strength steel one) will fracture before stripping of the bolt or nut threads (even if they are weaker Al alloy ones). This is due to the differences in areas (bolt stress area vs. bolt thread shear area vs. tapped hole thread shear area).

Designing the fastener to be at proof load is common for highly stressed powertrain joints like bearing caps, etc.

You need to measure dimensions, material properties, and torque-tension performance. If you don't have the skills and facilities to do this yourself, use an outside lab to do it for you. One such company is Archetype Joint:


Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
 
Have you verified that the bolts are really grade 8? The reason is that there are still a lot of counterfeit and improperly heat treated fasteners out there.

It is unusual, though not unheard of to see double ended studs made to the grade 8 standard.

If you can find a Skidmore-Wilhelm see if you can get some testing done. You will probably have to make a few fixtures but it will save a lot of testing on real components.

If you find that you need a little stronger stud you might want to contact ARP.

 
Hi preload

Well to find the root cause start at the beginning first look at the original design ie calcs etc.
Then look at the joint materials and get the material properties for the crankcase and head.
If the thread engagement is designed correctly it is possible that he bolt thread will fail first this is because
the female thread fails in shear and it has a bigger shear area than the shear area of the bolts, in addition the bolt can fail in tension as it is subject to tensile loading whereas the female thread which is acting like a nut is subject to compression. The criteria now is to establish what pre-load the joint actually needs with a saftey margin built in to enable the joint to function in service.
Having done this you need to then to get experimental evidence by building a few units and testing them to ensure
theory and practice mate up.

regards

desertfox
 
Desert,

I agree with you 100%. cylinderhead is acting like a nut and the grip length here is just the length of the crank case.

But,

I dont find any kind of pattern, for example, 6 bolts in the joint exhibit totally different clamploads varying from 5000lbs in one bolt to 12000 lbs in another. If torque is very high then why am I getting a very less (5000lbs) clamp load in another bolt in the same joint?
 
Hi preload

Thats because all pre-load methods are not highly accurate and you get a spread even when you torque to a specific figure.
the roymech site gives the following accuracy for various
pre-load methods as:-

Preload Setting Error
Operator "Feel" +/- 35%
Torque Wrench +/- 25%
Angle Torquing (Turn of nut) +/- 15%
Load Indicating Washer +/- 10%
Measuring Bolt elongation +/- 5%
Hydraulic Bolt pretension +/- (1% to 10%)
Strain Gauges / Ultrasonics +/- 1%

so your angle torquing is +/- 15% at least this can also be affected by bolt class (fit like 6H etc)and geometry of thread, pre-loading bolts is not an exact science more of a black art.
Maybe you need to use one of the last 2 methods to achieve better results.
Your biggest problem I think is the friction factor, I had a similar problem once a few years back, I had specified some torque figures for joints which had no lubrication and should have been fine, however I got a call to go and look as the fitters were stripping threads.When I arrived the fitters said the first one they did was fine but after that they all started failing. I asked them what there system was and found that after the made the first joint okay they were using electrical contact grease on the assembly as it was a (conductor bolted joint) consquently this grease was on their hands tools etc and yes on the threads too.
I told them to make all the joints dry before they went near any grease and that cured the problem and running a rough calc on the bolts I estimated the grease reduced the friction factor by half and doubled the pre-load and hence
thread stripping followed.

regards

desertfox

 
Desert thanks a lot for ur suggestion.

Here I am attaching two snaps of cylinder head and crank case joints.in the first snap u will see only 4 holes of cylinder head and second snap u can see all 6 holes of crank case where the bolt is tightened. I know these photos are notreally valued added but uploading them for reference.



I approximately measure the engagement thread length and it is more than an Inch for sure (almost to 1.5 inch). I can get you guys the exact length on monday.
 
Ah. That helps clarify your situation.

Those threaded holes are truly blind, right? They don't go 'thru', and they don't intersect other holes. So as you run bolts into them, you are trapping air, and some oil. Probably more oil as you work faster to squirt the oil and insert them, and variable depending on who is doing the squirting.

At some point, you may have enough oil in there to hydraulically stop the bolts from entering the holes any farther, at which point the heads may not even be seated, and the shank may yield in pure torsion.

Did you save any of the bolts that visually yielded? Examine them under magnification.

;---

Try applying the outboard lube oil to the bolts, and then setting them down to drain for a little while before inserting them, or find another way to make sure the amount of oil on the bolts is uniform, and not more than you absolutely need for lubrication.



Mike Halloran
Pembroke Pines, FL, USA
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor