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?)

 

[CoryPad]

I can see the joints pulling together when the bolt is running down, This says joint is soft right?

Maybe. As I said on 12 Apr 07 12:38, "soft" is a misnomer. If your parts are deflecting elastically, that can be considered a soft joint.

If I see the joint compressing, does that mean its damaging the joint?

Again, maybe. If the compression is all elastic, then you aren't damaging them. If you are indenting under the screw or nut, or deforming the the entire volume of the joint members between the fasteners, then yes you would be damaging the parts.



Regards,

Cory

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

 

[desertfox]

hi preload,

If your joint is embedding, then it would mean that the material under the head is yielding. This will affect the preload, ie as the head slowly sinks into the joint material you will lose tensile load.
This can also occur in the threaded portion of the joint.
As I said earlier, this might explain the excessive angle you are having to turn some of the bolts through.
It seems to me the bottom line is that the preload for the joint is too high. Without actually seeing the joint it's hard for me to say whether you're damaging the material on the surface or not. I would have thought that there would have been a slight witness mark on the surface of the material, which should be visible to the naked eye or possibly with a magnifying glass.
I think the bolts are failing because you are exceeding the tensile stress in the bolt whilst trying to tighten it.
This needs to be reviewed by the design team. The only way forward now is to reduce the preload preferably by reducing the torque setting or by increasing the friction in the joint.
Either way your design office need to give you a preload for that joint which is suitable for its service conditions.
The best way, once they have given you a preload, is to monitor bolt elongation, torque and possibly angular movement of the wrench.
Once you have done a few and built up some confidence you can drop the bolt elongation measurement and then use either torque measurement or angular measurement depending on what tolerance the design office give you for the design preload.

Regards,
desertfox

 

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"""The best way, once they have given you a preload, is to monitor bolt elongation, torque and possibly angular movement of the wrench.
Once you have done a few and built up some confidence you can drop the bolt elongation measurement and then use either torque measurement or angular measurement depending on what tolerance the design office give you for the design preload.""""

Desertfox, This sounds like a great idea. Will let you guys know the specified load as soon as I get the info.

Cory, Thanks for the help.

 

[dimjim]

The article is for high strength bolts used in structural
components where it is typical to take the bolts to the
yield levels into the elastic range. These are basically static load applications like bridges etc.

Most mechanical bolt applications will see other external loads being applied to the joint and require that only 75 percent of the proof load be the general guide for the joint assuming you might have a plus or minus 25 percent
error in your tightening method. If you have only a plus or minus 10 percent error in your tightening method, you can use 90 percent of the proof load as the tightening spec. Because you are not using steel on steel conditions the angles must be determined by tests to understand the deflection differentials that are taking place in the aluminum parts vs steel parts.

The only reason that I referenced the article was to show you that the upper proof load was reached and the clamping force remained relatively constant for greater degrees of rotation until the bolts finally broke.

I will have to review the c'bored holes to see if you have
enough support material around the holes at the interfaces to ensure proper contanct between the castings.
The dscf1648zi9.jpg drawing looks really suspect as to whether you have enough material around the interfaces where the castings meet? Is there non supported material
as pictured? If so, I can see you would have a very hard time analysing the 4 bolt application.

 

[preload]

""""
I will have to review the c'bored holes to see if you have
enough support material around the holes at the interfaces to ensure proper contanct between the castings.
The dscf1648zi9.jpg drawing looks really suspect as to whether you have enough material around the interfaces where the castings meet? Is there non supported material
as pictured? If so, I can see you would have a very hard time analysing the 4 bolt application.""""

Dimjim,

I am sorry, I didn't quite understand the above lines. Could you please explain that again.The picture u quoted is the crankcase and the bolt head would sit on those 6 holes u seeing in the picture. I can get you more pics in different angles if you want.

Thanks

 

[dimjim]

It may have been the angle of the picture of the
4 bolt casting but the c'bored holes looked
strange like they were not flush or parallel
with the top surface of the casting.

 

[dimjim]

Preload,
It finally sunk in that those holes are not threaded
and simply clearance holes for the bolts. Sorry about
that. I assumed the heads were on the opposite side
and the bolts were coming thru those openings and
imposing a moment load on the bolts.
dscf1648zi9.jpg

 

[sreid]

My appologies beforehand if I missed something but are there washers under the bolt heads?

 

[preload]

No washers. But the bolt head is a flange head.

 

[desertfox]

Hi preload

Any more info yet?

regards

desertfox

 

[dimjim]

Preload,
The photo that I am talking about does not have
6 holes in it. It has 4 recessed holes in it.

 

[preload]

http://img248.imageshack.us/img248/4562/dscf1648zi9.jpg

dimjim, please see the picture, you can see 2 more holes on the left most corner of the picture.

Desertfox,

I am still awaiting load data from engineering.

 

[preload]

General question.

Bolt 1 is a hex flange head and also indented head
Bolt 2 is a hex washer head and no indentation

everything else is same (plating,threads,size of bolt.....)

My question is,

if we use both these bolts in same application, will there be a significant change? if yes, why is that?

I feel indedntation shouldn't make a difference, but flange or washer head should make a difference, but is that difference real significant? and which is better choice? and why?

 

[CoryPad]

The effects of an indented head are reduced fastener mass and increased fastener compliance (very small effect). These have no real influence on joint behavior.

Your description of flange head and washer head are not sufficient to provide conclusive answers. The head height and outer diameter influence fastener compliance, fastener mass (and cost), bearing friction torque, and allowable surface pressure. If the washer head has a larger outer diameter than the flange head, then it will also generate a large bearing friction torque (less preload) and a lower surface pressure (less indentation of clamped parts). The design engineer must balance these characteristics with the joint requirements.

Regards,

Cory

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

 

[preload]

Cory,

Flat washer head has same outer diameter as flange head bolt and the head height of the flange head bolt is lil bit more than the flat head bolt. I just measured them.

So now that the outer diameters are same, what is the flange head bolt effect vs flat washer head bolt effect on a joint.

 

[CoryPad]

Is it a washer head or a bolt plus washer?

If it is a washer head, then there is no difference.

If it is a bolt plus washer, then the advantage is that you are not rotating the fastener against the clamped parts, which reduces coating damage (if applicable) and applied shear stress.

Regards,

Cory

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

 

[desertfox]

Hi preload

According to the site I found for flat washer hex head screws the flat dia is 0.72- 0.78" compared with the flanged
hex head screw which is 0.81".
As CoryPad stated the larger flat face would reduce pressure
on the joint surface which leads to less pre-load lost to embedding but increases the frictional torque required to tighten the fastener. So if you take the proof load of 9300lbs and divide them by the area under the head it will
give you an idea of the pressure variation under the head for a given tensile load:-

9300/(3.142*(0.72^2- .375^2)/4)= 31344.4 lb/in^2

9300/(3.142*(0.81^2-.375^2)/4) = 22971.3 lb/in^2

http://www.smithfast.com/hexheadms.html

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

Don't know what the tolerance is on the flanged head but the above illustrates possible pressures under screw head which according to the above is 27%.

typical wrought aluminium alloy for cylinder heads is
2018-T6 which has a yield stress of 43ksi which is higher
than the above stress values.

regards

desertfox

 

[preload]

Cory,

Yes it is a washer head not a washer plus bolt.Thanks for the explanation

Desertfox,

Thanks for the calcluations. I measured the washer dia of both bolts and they are almost same. (0.74 and .75 in)

 

[desertfox]

hi preload

Then there should be no difference.

regards

desertfox

 

[dimjim]

Desertfox,
The minor diameter should not be .375 but should be
the hole size thru the casting. The 9300 pound clamping load is probably exceeded when they are turning the bolts greater than 90 degrees. The bearing stresses under the
heads must be exceeding the yield limit of the castings.