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?)

 

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Thnaks Unclesyd for the paper.Thanks Mike and DesertFox for the answers.

I got a small update on this issue.

I have two 4 inch length bolts and one has a shank length of 2.75 in and thread length of 1.25 inch.

The other bolt has a shank length of 2.5 in and thread length of 1.5 inch.

We get a mix of these bolts for this assembly; can we attribute the yielding problem to this change?

 

[MikeHalloran]

If you can correlate the yielding with the threaded length, you've got a problem other than threaded length. It shouldn't make a significant difference for bolts that meet the spec.

Are you by chance buying the bolts offshore?



Mike Halloran
Pembroke Pines, FL, USA

 

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Mike,

No I am not corelating the issue, but one of my technician is doing that. I am positive that this shouldnot make a significant difference if the parts meet the spec. I just want to make sure with u guys and go a ahead with my thoghts.

 

[desertfox]

Hi preload

My terminology was incorrect the strain will remain the same for longer and shorter bolts but the elongation of the bolts will be different.
By the way the bolt you are using what class is it? I assumed class 3A but is it 2A?.
Have you seen evidence of the the bolts being to spec yet ,or just taking managements word.
You could take one of the bolts and have your lab do a tensile test on them perhaps.

regards

desertfox

 

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Desertfox,

Its fit class 2A not 3A.

How does it make a difference in your calculations?

Reg the tensile data I am still awaiting the info from my supplier.

 

[desertfox]

HI preload

Yes it will slightly effect the stress for a given torque
but I don't think its very much.
Have you had your meeting yet with the designers?


regards
desertfox

 

[preload]

Desertfox,

If the fit is 3A then the stress is going to be more than 2A right? because of the tight fit for 3a.

Yes, I met the designer and he dosent have info, he said he has to Dig in to it and will give me the range. He is also gonna give me the tensile data.I am also awaiting the data from my supplier too.

Did u see my recent update?

Though the major problem is friction, we also found something yesterday. A year back we used bolts which has less thread length (so for the same griplength, the joint sees less threads). Now we are using the same bolt but more thread length (so for the same grip length joint has more threads)

I also got the history load data

For the bolts we are using present (more thread length) avg clamp load for a 24 pc sample is 9300 lbs for the same torque

For the bolts we used 1 yr back(less thread length) avg clamp load for a 24 pc sample is 7800 lbs for the same torque

What do u think? Because the threaded length is more now, are we seeing more stretch and more clamp load?

 

[Screwman]

Proload,
Your clamp load variation is more likely due to frictional variation between the two lots of bolts. For critical engine bolts the auto industry requires each lot to pass a torque tension test into a referee nut and bearing surface to assure that the friction is consistent from lot to lot.
The change in thread length that you have will cause a variation but it will be very small: less than 10%.

I second the comments about pulse guns and critical joints. They are not prefered. Best is DC using angle control and torque monitoring. That method will catch parts or joints that are yielding during assembly or that are cross threaded or stripped out. Hand torquing will have more variation than a well maintained power tool and clutch because of the slip stick effect going from static to dynanic friction as the final torque is applied with the torque wrench. (more good joints have been messed up by 'inspectors' and their torque wrenches, than you can imagine).

 

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Screwman,

I have clamp load and elongation data for both bolts (ultrasonic). I have different load factor values for both as the goemetry change.

one bolt lot has 7800lbs and other has 9300 lbs. Its 15% difference.

With the data I have , how can I say that all this 15% difference is not due to thread length change? By taking the load factor percentage?

 

[unclesyd]

If the thread engagement stays the same you will have your percentage difference in the effective length of the bolt.

To calculate the effective length.

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

 

[preload]

the load factors are as follows: For the current (short shank) fastener the Load Factor is 1.00625 and the (long shank) is 1.07224.

 

[preload]

Will explain it clearly.
The thread engagement length remains same, but we have two different bolt cross sections in the effective length (grip length). In one case we have 1/4th of the effective length as threaded length and in other case we have 1/3rd of the effective length as threaded length. This is because our supplier can supplier can supply different bolts for the same application until he stays with in the spec.

Will I get more clampload for the same torque on a bolt which has more threaded part in the effective length? If yes why is that?

 

[unclesyd]

Using only torque as the measurement and keeping the same thread engagement length you should get the same clamp load or the same bolt stress. They parameters that affect the torque will stay the same, that is the under head friction and the thread friction.

 

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Unclesyd,

so are u saying that it shouldn't matter if I use a fully threaded bolt or half threaded bolt as long as I use the same size of bolt.

so,

if the engagement length is same,torque is same,effective grip length is same, then I should get the same clamp load for a fully threaded bolt and non-fully threaded bolt.

But my question is,

for fully threaded bolt, the area is different and for half threaded bolt the area is different (shank area + threaded area).If area is different clamp load changes right?If the answer is no, then where does that calcluated area difference go?

I am thinking, if the bolt is fully threaed, then for the same torque, the stretch will be more,so the clamp load will be more compared to the half threaded bolt.I am really confused.

 

[unclesyd]

If you use the information available in the Riverhawk site posted above and use your elongation measurements and measurments to calculate the bolt stress based on elongation you can see the difference between the thread lengths if the thread engagement stays the same. This is why you want to use the minimum thread length to get the job done.

Again using torque you really don't know what the clamp force is until you measure it experimentally or by elongation of the fastener and calculate it. If you know the bolt stress you can calculate the expected elongation.

A given torque is just a value that will get you in the ballpark for the required bolt stress to preload the joint to do it's intended function. The torque values given in the tables only use the fastener pitch not the number of threads on the fastener.

In critical joints the length of the threaded section is always kept the same, at a minimum. That is why there are standard dimensions for threaded fasteners to eliminate one of the variables in any joint.

 

[desertfox]

Hi preload

I will run some numbers when I get home later today.
Regarding thread length none of the formula I have for working out the tensile load contain anything to do with thread length, the only time thread engagement length comes in is when you are considering thread stripping.
As Mike said earlier the longer bolt as a lower stiffness than a shorter bolt.

I'll post later

regards

desertfox

 

[desertfox]

Hi preload

What exactly is the thread engagement of the 2 different length bolts?

regards

desertfox

 

[preload]

Unclesyd,

Exactly.

What I am doing now is the same. I have two different threaded length bolts. After tightening them to the same torque, I measure the elongation using ultrasonics and that instrument will calculate the load for me. Inorder to caluculate the load for me I need to do some inputs to the ultrasonic instrument and then it calculates a "load factor".

So,

Ultrasonics instrument asks me to enter a area of cross section for the bolt. So when I enter those numbers I get a different load factor for these two different bolts because the area of bolt cross section changes for these two bolts as the threaded length is different.

So when I measure elongation, for the same elongation I get different load values for these two different bolts because load factor is different.

But as Desertfox says,

When we do T=kdf, the d we enter here is same for different threaded length bolts as long as the size of the bolts remains same. And so we don’t see any difference in the final load.

So, finally,

Is T=kdf an approximate and what I am doing now is accurate? Does the different threaded lengths have different clamploads for the same torque if the thread engagement is same. This subject is so complicated. lol

 

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My thread engagement length is 0.814 In
Effective grip length is 2.77 in

For bolt 1: 2 inch is the shank length and 0.77 in is the threaded length in the whole grip length
For bolt 2: 2.4 inch is the shank length and 0.37 in is the threaded length in the whole grip length

Why we have these different threaded lengths is because, our engineering drawing says a min thread length, but it dosent specifies a max thread length, so our supplier has authority to supply different threaded lengths as it will be per our chart spec.

So when I develop a new strategy I have to develop the torque keeping in mind this variability in threaded length.

 

[dimjim]

Whoa! If the thread engagement length is .814,
how does the .37 and .77 thread length engage
the .814 depth?

Why isn't the wrench angle cutoff point
set to 180 degrees?