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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Desertfox,

we had this problem from 2004 or 2005.I and my managaement dosent know why this problem started from 2005.

or it should be there since many years but may be we found the problem in 2005.

but nothing has changed in lube,process or materials

 

[desertfox]

Hi preload

Well I am surprised your companies had it this length of time.
The problem I am convinced is friction factor to low as previously stated and the calcs I have done seem to confirm this in my mind at least.
Try what I said earlier make some joints without lube I don't think any bolts will fail

regards

desertfox

 

[MikeHalloran]

And you've always dipped the bolts in the exact same brand and type of 'outboard lube oil', and its formulation hasn't changed ... right?



Mike Halloran
Pembroke Pines, FL, USA

 

[CoryPad]

desertfox,

Were you calculating shear stress on the fastener shank using the entire applied torque of 33 lbf-ft? It appears that you were. This is not realistic (since the head friction torque opposes the applied torque) and not typical for bolted joint calculation.

When I do calculations, I find that 33 lbf-ft should not yield the screw.

Regards,

Cory

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

 

[desertfox]

Hi Cory

Yes cory I was using the torque of 33lb-ft on the shank
but combining it with the tensile stress to get the principle stress of the 134,000 I quoted previously.
The shear stress alone I agree would not yield the material
but combine it with the tensile stress and it will.
I just used a plane stress model with torsion and tension.
What value of friction do you use under the head so I can reduce the torque on the shank?
If theres oil under the head the friction would be quite low I thought, so I used the friction factor of .152 something to get the tensile force and just went from there.
I think its a fairly good representation of whats happening when you consider that there using the maximum recommended torque for a dry bolt and putting lubricant on it.
The reason some bolts aren't failing is the spread of friction and the fact not all the bolts will have minimum yield of 130000.
What calculation are you doing? are you just considering the shear due to torque?

regards

desertfox

 

[CoryPad]

desertfox,

I used the last equation in faq725-536. It accounts for tension loading and torsion loading, but the torsion loading is only due to the pitch torque and thread friction torque. The sum of these two is equal to the difference of applied torque and head friction torque. It is widely documented that the rotating fastener's contact surface creates a torque equal to approximately 50% of the applied torque. Here is a link to one source:

http://www.boltscience.com/pages/casestdy.htm

Your analysis is incorrect in that it uses the entire applied torque on the shank. This is why I have stated multiple times that, with the data provided, friction variation has not been proven as the cause of this problem.

Regards,

Cory

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

 

[desertfox]

Hi Cory

I will post later just starting work at the minute.
I'll look at that faq link.
I still think that its a friction problem.
One of the sites I posted states a friction factor of 0.15,
recommended torque of 33lbs-ft and a preload of 6975lb

using p= T/(.15*d)

now if the torque is 33lb-ft and I reduce friction by 50%
get 2*6975lb tension in my fastener which is greater than the proof load of 9200lbs it was this that started me thinking that it might be friction.
Bear in mind the 33lbs-ft is for an unlubricated joint.
I will post properly later.

regards

desertfox

 

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?M = Rp0.2 / [1 + 3(3/2 · d2/d0 · {P/(? · d2) + 1.115 ?G})2]0.5

MA = FM (0.16 · P + 0.58 · d2 · ?G + ?K · DKm/2)

Cory,
In the above equations from the thread you referred to calcluate assembly torque..

Is d0 - shank cross section dia same as Nominal dia?
how can I get the ?G,?K,DKm values?
Is there any chart which can give us these values for any type of fastener?

Thanks

 

[CoryPad]

d[sub]0[/sub] is the diameter of smallest shank cross section. This is thread minor diameter for your parts.

[μ][sub]G[/sub] & [μ][sub]K[/sub] are measured with laboratory equipment (like the Skidmore-Wilhelm machine mentioned by unclesyd above). If you can't obtain measurements, then assume 0.1 for the low end and 0.2 for the high end. There are many published sources of friction coefficients (Internet sites like RoyMech, books like Handbook of Bolts and Bolted Joints, and standards like VDI 2230).

D[sub]Km[/sub] is the effective contact diameter for the rotating fastener. You will need to measure your parts or look at your part drawings.

Regards,

Cory

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

 

[desertfox]

Hi Cory, preload

Firstly I was incorrect in using all the torque in my analysis and only a proportion should have been used as rightly stated by Cory.

Now going back to one of my earlier posts which had a link
to this site:-

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

It gave figures for the 3/8" bolt with a recommended torque
of 33lbf-ft with a clamp load of 6975lbf and using a friction factor K of 0.15,associated with the formula P= T/(K*D) which we have seen on other posts here.
Now with reference to The Machinery's Handbook 25th Edition.
I was able to calculate the torque req to overcome thread friction, bearing friction under the head and the torque req to produce the axial load or pre-load in the above bolt.
I started with the pre-load 6975lbf and the dry friction factor .15 given which according to the Machinery's handbook split into 0.12 for thread friction and 0.1 for the bearing friction.I calculated the torque from the formula and found it to be in good agreement with the 33lbs-ft torque quoted on the website I mentioned earlier (my calculated figure was 32.871196 lbs-ft).
I then began to reduce the friction factors for both the threads and bolt head bearing keeping the torque constant and working out the increased pre-load that this created.
From the pre-load figures and the torque required to achieve it, I worked out the maximum principle stress.
I found that if the friction factors for thread and bearing
friction reached 0.07 then the principle stress would exceed the 130000lbs/in^2 minimum yield quoted for that bolt grade during assembly.
On this last calculation the axial load would be about 9894lbf which would generate a stress of 127664.5 lb/in^2 just below the minimun yield but when combined with the torsional stress it just fails.
I have uploaded a file which shows the calculations and formula I used it can be found at:-

http://www.uploaddownload.eu/download.php?file=353014b5cbd05ec1c20b31370f3276ce

One thing I didn't show on the above file was the K factor
that would be used in the formula P= T/KD for this last calculation where the bolt reached yield, you can work it out from the formula that are there but for ease it will be
0.106.
I still believe that this problem is caused by reduced friction given that the bolts seemingly fail during tightening which is when they're most stressed.

regards

desertfox

 

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Desertfox,

Thanks a ton for all the effort and time you put in those calculations. I really appreciate you help.

 

[desertfox]

Hi preload

Your welcome, hows it going with this problem any more info yet?

regards

desertfox

 

[preload]

Desertfox,

meeting on this problem tomm with the design guys. Will let you gusy know the minutes and what we have decided.

Most probably,everyone is gonna agree upon angle strategy, and eventually I am the one to develop the strategy.

 

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Desertfox,

I got couple of general questions (not on this specific problem though)

1) How reliable is clutch tool compared to dc electric? Is it ok using a cluth tool on critical joints even if we have very good confidence on the tool?

2) Which one gives less scatter of preload? Slow Hand tightening or clutch/pulse tool? For a given torque, which tightening method gives less clamp load compared to other (taking friction into consideration?)

3) I am gonna ask you guys a real stupid question.

All the tables give torque and tension numbers for different fasteners.

For a given torque, if the grip length changes, tension in the joint changes right? Then how come the tables give a tension number with out specifying the grip length?

 

[TVP]

1) Pulse tools are not reliable and should not be used on critical joints.
2) DC electric gives less scatter than the others. I would assume hand tightening has less scatter than pulse tools. The problem with pulse tools is variation, not that it is always less clamp load for a given torque. Hand tightening would probably result in the lowest consistent clamp load.

 

[desertfox]

Hi preload

I posted earlier regarding setting accuracy ie:-

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%

However when you get the info ie:- preload etc you need to ask them what tolerance is on that preload then we can look at methods etc.

regards
desertfox

 

[preload]

Desertfox,
thanks for the info. What I am asking for is if tightening speed is more, then clampload decreases or increases?

And also
All the tables give torque and tension numbers for different fasteners.

For a given torque, if the grip length changes, tension in the joint changes right? Then how come the tables give a tension number with out specifying the grip length?

 

[MikeHalloran]

Increasing the grip length increases the strain energy stored in the joint for a given torque. If you think of the bolt as a spring, longer bolts and longer grip lengths have a lower spring rate.

Changing the spring rate in turn changes the 'feel' of the joint. A short grip length will come up to torque very rapidly, whereas it will take more rotation to achieve a given torque for the longer grip length. One side effect is that the extra tension per unit of extra rotation is dependent on grip length, so if you were using shorter bolts, you would have a smaller rotation limit, and you would have to measure it more precisely.

In theory, the tension produced for a given torque should be independent of grip length. Because of the nonideal way in which the various wrenching machines work, there may be some accidental correlation.

To give a contrived hypothetical example, suppose a given wrenching machine actually stopped applying torque at some fixed time or some fixed angle after detecting the correct torque level. In the former case, the actual torque and tension would be speed- sensitive, and in the latter, it would be grip- length sensitive.



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi preload

As Mike as already said grip length and tension should be independant and the strain in the bolt will change dependant on length.
Not sure about effect of speed although it may effect how the joint relaxs after initial tightening I will try to find some info.
Can you use longer bolts in your current joint without them bottoming?


Regards

desertfox

 

[unclesyd]

Here is an outstanding paper on bolts, metric. But the discussions are relevant to the ongoing discussions in both of preload's threads.
Some very good information on the spread expected from the various tightening regimes.

http://www.kamax.de/pdf/Bolt_and_Screw_Compendium.pdf