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 dimjim

I used the site given by unclesyd to get the equivalent lengths for the joint. ie:-

http://www.riverhawk.com/boltloading.php

However when you posted your question I just did a rough check like this:-

[σ]*L/E = x (deflection)

where [σ]=stress= F/A approx (4500*4)/(3.142*0.375^2)

L=bolt length 3.625"

E= modulus elasticity = 30*10^6 psi

punching this through my calc gives:-

x= 0.0049225"

Maybe I have missed something but I can't see what, besides working in inches is alien to me, were metric in the uk.

regards

desertfox

 

[preload]

Desertfox,

1) Engg specified 33ft-lb with lube oil.But no where in the specification says about clamp load.
2) Yes, we always go upto 95% of the proof load and sometimes go to 95% of the yield load. Everyone knows it’s a mistake but its been working like this for yrs. So they don’t want to change anything. I agree if 4500lb is the specified load then we are 50% of the proof load on a joint which is really critical.
I am learning things from you guys. Being from quality dept and moved to work on a new field of fastening, exciting yet challenging. May be in 3-4 month time I will be on my own. I started reading some introduction books by Bickford.

 

[desertfox]

Hi preload

Hope you enjoy your new venture.
Anyway my approach in your situation would be to get the design dept to give you the pre-load figure then abandon for the moment your torque figure and use the ultrasonic kit
to measure bolt elongation till you get the pre-load you're looking for, at the same time you can monitor torque, angle rotation etc after you get confidence in either the torque setting, angle rotation or whatever whilst using the ultrasonic measurement, after which you can dispense with the ultrasonic measuring once you achieve fairly consistant results, I suppose it will depend on the tolerance the designers give you to work with.
Alternatively you could use the same method to achieve the 90% proof load and send the info back to the design dept for approval and put the ball in their court.

regards

desertfox

 

[dimjim]

Desertfox,
Thanks. I cannot see anything wrong with the method
you used or the results. I am at a loss now to understand
the recommended turn of the nut angles. Thanks for your patience.

 

[desertfox]

Hi dimjim

Thankyou, is there anywhere on here I can see your turn of nut reference?
Just a thought does the angle of rotation apply to a diameter of a thread as they will have different pitches.

regards

desertfox

 

[dimjim]

Desertfox,
Structural Bolting Handbook
SSTC
Steel Structures Technology Center, Inc.
Publication No. SBH-2, 2st Edition
2nd Printing: June, 2001
ISBN 0-9707400-1-8

They reference RCSC Table 8.2.
Research Council on Structural Connections
I think I have seen the later on the internet
in the past.

 

[desertfox]

Hi dimjim

Yes I have seen a reference to it now and the figure quoted for extension doesn't tie up with the extension calculated the way I did it. Interesting to note though that a 1/2 turn
on a 7/8" UNC would be 0.055r" according to its pitch.

regards

desertfox

 

[desertfox]

Hi dimjim

Reading more closely though it seems the 70% of tensile strength required as a minimum preload seems to be well exceeded and it also appears they are taking the bolts into the inelastic range have a look at this site and go to page 14.

http://www.cisc.ca/files/publications/techpubs/design/bolting/HighStrengthBolting.pdf

regards

desertfox

 

[dimjim]

Desertfox,
I think that answers my question. It also assumes all
parts are steel. I guess that explains why the bolts
are failing if:

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?

Thanks for the article.

 

[desertfox]

Hi dimjim

Thanks for your response, its the only thing a could see that might explain the difference, interesting though.

regards

desertfox

 

[desertfox]

Hi preload

Some easy questions if you can answer please:-

1. what is the depth of the tapped hole and its tolerance.

2. what is the tolerance on length of the bolt from under the head

3. of the 2.766 grip length what is the tolerance on that too

regards

desertfox

 

[dimjim]

Dessertfox,

I too think the drilling and tapping
tolerance is suspect to have that much
difference in angle or pitch tolerance
on the bolts. Go NoGo guages can quickly
verify this. Chips in the threaded holes
if bottom tapped may also be a cause.

 

[preload]

Desertfox,

Grip length tolerance is +/- 0.02
Tap size tolerance is 1.12 inch +/- 0.02 and drill size tolerance is 0.16 in dia +/- 0.02
Bolt length tolerance is 3.5 inch + 0.04 and – 0.06

After seeing the numbers and if u feel there is some problem, could you please explain how these things effect the angle,clampload.....?

 

[dimjim]

Preload,
You have more than sufficient tap depth if .814 is the
length of depth that the threads are engaged in the
casting. The 1.10 depth exceeds .814 by .286 inches.
You could screw in the bolts 15 full revolutions without
binding as a check for the proper depth and ensure
clearance.

The tapped hole size should be .318 to .325 diameter
before tapping for a 3/8-16UNC-2B thread which should be the minor diameter of the threads.

 

[desertfox]

Hi preload

I was trying to see whether the bolts were bottoming in the tapped hole this doesn't appear to happen,however I couldn't get to a thread engagement of 0.814 from your figures around 0.79 was the best and as low as 0.6?? from memory.
What is interesting though is that if your bolts in the joints were just snug ie no free play and you turn them by around 20 degrees that should generate a pre-load of about 4500lb and roughly 40degrees would the proof load.
Therefore at a rotation of 260 degrees the tensile force would be very large in comparison.
One other thing to check is that were the bolts that failed of the longer or shorter threaded lengths:- ie are the failures limited to one length of threaded bolt either longer or shorter if some can you tensile test some.


regards

desertfox

 

[preload]

Desertfox,

I think I know why some of them take more angle (Correct me if I am wrong).

I have to make some things clear first. The 260 deg angle is just the monitor limit.
Maximum number of bolts fall in the range of 70 deg to 150 deg. But some of them touch the 260 deg mark. When the operator sees that angle, they will back off the screw and observe for any yielding. Even if they don’t observe visible signs of yielding at 260 deg we will scrap the screw. So bolts which sees 260 deg should be scrapped irrespective of yielding signs.

Our concern is if we see yielding at 260 deg, then yielding might have occured well below 260 deg, which we are saying OK because it passed the 260 deg angle monitor.

Basic point is we really don’t know when the bolt yields.

Now coming to your point, you said we should not see more than 40 deg of turn after a nice tight snug to reach proof. When u say nice snug, what would be the approx torque value?
Our process is, we snug to 3.2 lb-ft and then start monitoring torque from there. So at 3.2 lb-ft the joint is not really snug right? I think this is the reason we are seeing high angles to reach the final clamp. And also the joint is made of aluminum and bolt is carbon steel. When the operator runs the bolt down, I can visually see the Aluminum parts compressing like a soft joint.

So when we see more than 70 deg angle is because of the low snug torque. And when we see high angles, bolts are yielding. But one thing I don’t understand is why some of the bolts seeing 260 deg are not visually yielding as others?

 

[desertfox]

Hi preload

When I say snug I mean like finger tight but not putting any tension in the bolt which is very difficult to do and then tightening the 20 degress or so.
Now you have moved the goal posts here the parts were originally a copper-aluminium alloy and now aluminium?
Also what your seeing when the aluminium compresses is embedding of the screw which you haven't mentioned before.
Whats happening in this last case is the aluminium is yielding under the bolt head because the tensile load your putting into the bolt is to high for the alumium to withstand, it could be that that causes the high angle rotation,ie as you rotate the bolt instead of increasing tension the alumium gives way and therefore you need to keep turning the bolt to try and reach your torque figure.
Early on you said there was no damage to the parts see your quote:-

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

You need to get the pre-load from that designer and really now find the mechanical properties of the component parts.

I'll try looking into the embedding a bit further and post again.

regards

desertfox

 

[dimjim]

http://www.cisc.ca/files/publications/techpubs/design/bolting/HighStrengthBolting.pdf

Desertfox was kind enough to post this. If you read page 14 to 20, it may explain the answer to many of your questions.

I would simply use the turn of the nut method and turn
the bolts to 60 to 90 degrees and forget the torque specs.
Though the clamp force remains the same beyond this, the
bolt is knecking at the threaded portion and putting greater strain on the bolts. Almost all of the elongation is taking place thru the threaded portion which is obvious too when looking at the failed bolts that you have posted.
I also notice what appeared to be a small amount of c'bore
or c'sink in the castings before the threaded portion. Is
this always specified and called out?

 

[preload]

Desertfox,

Thanks for explanation. Yes they are copper-Aluminum joint . When I say there is no damage to the joint, I mean there is no physical damage to the bearing surface of the joint. But I see the joint compressing (what I mean compressing is , I can see the joints pulling together when the bolt is running down, This says joint is soft right?).

If I see the joint compressing, does that mean its damaging the joint? I dint know that. I am sorry.
Regarding the load specification, I am bugging our design guy like anything..

Dimjim,

I will definetly go through the link.
Regarding the Counterbore, you are exactly rightttttt sir. Today in our meeting, we found out that our technician (who measures the grip length and do minimax studies) did not take into account the counter bore length. So there will be a change in grip length now. By how much??? I would know that soon and will post here.

Thanks a lot guys for helping.

 

[preload]

Dimjim,

The article says

" However, for longer bolts, 1/2 turn may not be
sufficient to bring the pretension up to the desired level,
whereas for shorter bolts 1/2 turn might twist off the bolt.
Laboratory studies show that for bolts whose length is
over eight diameters but not exceeding 12 diameters, 2/3
turn of the nut is required for a satisfactory installation."

Length of my bolt is over 9 diameters, so I would fall under the 2/3 turn of the nut category. i.e is 240 deg of turn for satisfactory installation. And also he says this rule is acceptable only for steel joints. My joints are little soft Al-Cu alloy parts. So I need more turn.

If I go by the article, then I would end up again turning the nut over 240 deg and yielding the bolt. Am I missing something?

And also how to determine the snug torque? Is there any procedure or just an estimate?

I just got the counterbore difference on clamp load from my tech.The clamp load difference between the old block dimension and the new counter bore block id about 7% less.