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?)

 

[desertfox]

Hi preload

Thanks for the images, If your thread engagement depth is at least an inch or more then that would probably be about right and explains why only the bolts appear to be failing.
Usually if your tapping a hole for a bolt and it as a lower strength than the bolt material you make the tapped hole depth about 3or4 x "bolt dia" and make sure that the bolt screwed portion engages the majority of this tapped length.
The machinery's handbook as formula for checking strength of threads for both similar and disimilar materials, however you need the mechanical properties of all the materials as mentioned previously.
I'll try and respond on monday but its difficult because I work away from home all week and can only access the net after working hours. Friday evenings and weekends its easier
as I have unlimited access.

regards

desertfox

 

[desertfox]

Hi again

One further point try to establish the forces the joint as to withstand in service that way we can advise you better
regarding the bolt pre-load.

regards

desertfox

 

[preload]

Cory,

Yes those threaded holes are blind, It dosent go "thru".
Yes I have the yileded bolt with me and the failure is in the threaded portion of the bolt.I will get the image of yielded bolt too on monday. What do u want me to check them on magnification?

Regarding the oil lubrication, I think its a very good Idea.I will check how the operators doing that on monday.

Desertfox,

I will get the material properties of joint and bolt.Bolts are not failing on the serivce loads but they are failing during assembly.But its still a good idea to find out the service loads on that joint as it wil be usefull if the joint fails after the durability test.But for now the problem is they are yielding during assembly itself.

 

[desertfox]

Hi preload

I understand that the bolts are failing during assembly but unless you know what the forces the joint see's in service
you cannot revise the torque setting or more importantly
the pre-load for the assembly of the crank case and head.


regards

desertfox

 

[preload]

Desertfox,

u r absolutely right, I wasn't thinking in that point of view.I will do that. Thanks

 

[MikeHalloran]

What you're looking for under magnification is an indication of relative rotation across the yielded area, so you can guess if there was any tension in it when it yielded.



Mike Halloran
Pembroke Pines, FL, USA

 

[preload]

Mike, your point sounds intersting. could u please explain it more, like how to do that? what do u mean my relative rotation here? and how to check that?

Does yielding occur when bolt is not in tension too?????

 

[MikeHalloran]

There are normally natural axial striations in a bolt. In the case of cut threads, there will be witness marks where the die stopped, but there may be other more subtle markings that were in the original wire.

If you can track one of those across the yielded area, it may provide clues about what was going on when the bolt yielded.



Mike Halloran
Pembroke Pines, FL, USA

 

[VF10]

1. Assemble all the joints
2. Thread on until finger tight in the prescribed sequence
3. Mark the head of the bolt extending to the crankcase
3. Turn 45 degrees with a wrench
Without even knowing the length of the bolt it's just an educated guess
Cheers

 

[desertfox]

Hi preload

Just some further points to add in case you had not considered them already:-

When you get the case and head material info also get the coeff of linear expansion for them as well and of the bolt
material also.
When you look at the design analysis of the joint it should
include stresses due to thermal expansion of the materials as well as external loading.
Depending how hot the joint gets you may find the point at which the material reaches its yield stress drops due to the
running temp again this should have already been looked at.
Although now I am wondering why your assembling with such a high intial pre-load ie:- bolt material reaching proof load
before service conditions are encountered.

regards

desertfox

 

[preload]

Checked the process.

They got a small container of lube oil and the operator is dipping the threaded portion of the bolts in the oil (absolutely no pattern/consistency, just dippin them and placing) and placing them in to the crank case holes into the cylinder head threaded holes. I can see the oil drippin off the bolt end while placing in to the hole.

After placing the bolts, they don’t hand start the bolts (I don’t know if this makes much difference in my issue)

 

[Screwman]

I am wondering how the load numbers were obtained? Was it ultrasonics or a load cell?
High angle and low torque is indicative of something in the joint yielding, the question is what is the component that is yielding. Measure both the male and female threads to see if they are in print. The tap drill size might be over on some of the holes, resulting in thread yeilding.
One thing to look for is any kind of a pattern for the location of the bolts that are exhibiting this result. Often times it turns out that a problem is located only in certain hole locations and if that is the case it becomes much easier to trouble shoot.
One question that may seem dumb: are you sure it's not an equipment problem with the programing of some of the spindles on the gun? I've seen that before several times.

 

[preload]

Screwman,

Load is obtained from measuring the elongation (ultrasonics) and then the load.

Bolt is yielding in the threaded area. Bolt threaded area is almost fully engaged in to the cylinder head thread hole.

What do you mean by print?

Could you please explain this more " The tap drill size might be over on some of the holes, resulting in thread yeilding."

Regarding the pattern, I will check into that. I think the equipment is calibrated recently but I will have them check it again to make sure.

 

[desertfox]

Hi preload

Any news on design and material information?

regards

desertfox

 

[strokersix]

Preload, Have you checked the hardness of the studs yet? Specifically a good one versus a bad one?

 

[preload]

Desertfox,

Bad news is that I cant get the material properties of the joint.I am here in the plant quality group (I just get torque spec from engineering group) and my manager said that our engineering design dept cant help me get the material properties as they don’t know. This design is a carryover since 20 yrs.

So from the plant end I can only work with the bolt point of view. What all calculations I need to do, I can do that only using bolt specs. So now the problem boiled down to friction.

As you guys said, we all believe the problem is with the friction (lube oil) and I got two options now to address the issue.

1) Don’t use lube oil and run some tests and see if I can reduce the scatter of clamp load.
2) Use angle control strategy and see if I can reduce the cloam load varaiation.

The variation now is

5000 lbs – 9000lbs – 12000 lbs, 9000 being the proof load

Our goal is to reduce the variation to 6500 – 9500 lbs

 

[MikeHalloran]

Uh, no. Option 1 should be to use a controlled amount of lube oil, not none.



Mike Halloran
Pembroke Pines, FL, USA

 

[CoryPad]

preload,

You have not proven that this is a friction problem. Screwman and I have both mentioned the possibility of deformed parts. If you were simply overstressing the bolt due to low friction, you wouldn't have high angle and low torque.

You need to do fundamental quality control checks on your parts. Measure part dimensions and material properties. You must do these steps before trying to re-engineer the joint.

Regards,

Cory

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

 

[desertfox]

Hi preload

Using no lube will probably increase your scatter and not to mention reducing the clamping load of the joint.
In addition without the material info you run the risk of failure in service if you reduce the preload without knowing the service conditions.
Someone somewhere will have access to the info how about the foundry were there cast for instance.
Advice from Cory & Mike should also be followed.

regards

desertfox

 

[MintJulep]

Please post a picture of a yielded fastener.