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?)

 

[preload]

Dimjim,

You got me wrong. When I say .37 and .77 thread length, its not the whole thread length.
It is the length of the thread left over after the thread engagement.

So the thread length for

Bolt 1 is .37+.814
Bolt 2 is .77+.814

If we set 180 as angle monitor, many of the non-yielded bolts might fail for angle. And we also not sure how much angle monitor we need to keep. So to avoid all this problems, we want to go to angle control.

 

[MikeHalloran]

>>>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.<<<

Huh?

Preload,
How are you computing the cross section area that you enter into the ultrasonic instrument?



Mike Halloran
Pembroke Pines, FL, USA

 

[preload]

Mike

Mike,

F=E*A*DL/L

A/L changes for different cross sections of the bolt.

For 1 bolt : As/Ls + At/Lt
For 2 nd bolt : Ls and Lt are different

As is area of shank
At is area of threaded portion

 

[unclesyd]

Your effective thread length should be:

Your shank length (l[sub]1[/sub])
+
Your exposed thread length + 1/2D (l[sub]2[/sub]) (you use only 1/2D of your thread engagement.

The stress area of the above two have to be considered independently.

y = stretch

F= yE/L/A L/A = l[sub]1[/sub]/A[sub]1[/sub] + l[sub]2[/sub]/A[sub]2[/sub]

Area[sub]shank[/sub]= .7854(D)[sup]2[/sup]
Area[sub]threaded poertion[/sub]= .7854[D-(D-.9743/n)][sup]2[/sup]

 

[desertfox]

Hi preload

Firstly I am confused you stated in one post:-


"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."

Then in another the shank lengths changed to:-

"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"

Which are the correct shank lengths?

Next question you state in a very earlier post that the tapped holes are blind and therefore for a given joint configuration you surely cannot have the same thread engagement for each bolt but you quote 0.814, depending
which bolt you use one must have more thread engagement than the other, can you please sketch the joint and upload it so we can see exactly what your trying to say.

Regarding Class 2A threads, in the threads calcs I uploaded
I assumed that it was 3A but if you substitute the pitch dia
of the 2A thread in the formula it actually reduces the torque req to overcome friction on the threads and would therefore generate more tensile load and stress in the screw
although I don't think its very significant.

The stress calculations in a given bolt or screw are based on bolt or screw stress area which is normally equivalent to the mean of the minor and pitch diameters and is therefore an assumed area for calculations.

However with a bolted joint, the components bolted together
usually have far more stiffness than the bolt itself, so when you tighten a bolt it stretches more than the components compress, if you know the relative properties of the components you can draw a force deflection diagram of the joint, examples of which can be seen at:-

http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf

Sadly you haven't been able to get the material info of the component parts so we are stuck with the bolt spec.
The different length bolts you have however can be viewed like two different springs lets say one as a stiffness of 2lb/in and the other 4lb/in and you wish to compress them individually to give a force of 6lb in order to do this your given a screwed stud mounted on a fixed plate, in addition you have a loose plate and a nut.
If you place the 2lb/in spring over the stud and rest it on the fixed plate, then place the loose plate on top and screw the nut down on the thread till you achieve the 6lb force, at this point you will have compressed the spring by 3" :-
3" * 2lb/in =6lb

Consquently if you do the same with the 4lb/in spring the deflection to achieve the same force is only 1.5".
What I am trying to convey is that which Mike Halloran and uncle syd stated that even if your bolts vary in stiffness and stretch differently the outcome for a given joint configuration and a target force should be the same.
In simple terms if your aim is a given preload and your bolts vary in stiffness then you have to stretch each bolt
by a different amount, so it is no surprise if you stretch
the bolts by the same amount as you have stated, you will get different loads but what is the difference that you have measured? Looking at the load factors you quote the difference between them is just under 7% does that reflect the load readings?
For us to help you further we need correct and consistant information. I find it quite amazing that a designer with at least a weeks notice of a meeting to discuss a problem joint turns up and says I'll have to go and find the design pre-load.

regards

desertfox
turns up

 

[preload]

Sorry for any confusion.

Firstly, the bolt lengths are not different, length is same but the threaded length is different. Yes I agree because of the load factor difference the difference in clamp load is just under 7%.But we made sure with our supplier last friday, from now onwards we will be getting a consistent threaded length.

this is the exact dimensions
2.77 is the grip length for both bolts
"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"

0.81 is the thread engaement length (as the bolt length is same, the thread engagement length will be same).

thanks for the explanation and regarding the joint material properties, the only thing I know is the joint materials are Aluminum and bolt is low alloy carbon steel. Yes Its reallyy amazing to me that he dint turn up with exact number and said "I can get you the info some time next week, but if I remeber that correctly load spec should be around 4500 lbs"

 

[unclesyd]

When you are saying grip length are you taking into account the 1/2 D that should be added to the threaded section to account for the thread engagement?

 

[preload]

I dont think we are adding that. I will make sure to add that.

is it 1/2D or D/2? D being the nominal diameter of the bolt.

 

[desertfox]

Hi preload

So if the bolts are both 4" long in total and they both have
0.81" of engagement length then the height to the underside
of the bolt head should be:-

4"-0.81= 3.19"

according to this site I found the effective length for the
bolt in a tapped hole should be:-

3.19"+0.4*.375= 3.34"

http://www.co-design.co.uk/dpg/bol/bol3.html

The effective joint length according to the above site would
be:-

3.19"+.375= 3.565"

preload can you please confirm that this is the joint
configuration?

If you can confirm above and based on the 4500lb pre-load I can do some more calcs?

regards

desertfox

 

[preload]

Desertfox,

will explain again clearly,

Bolt length (from head to bottom) - 3.78 in(ultrasonic)
thread engagement length - 0.814 in
effective grip length - 2.77 in (we din't consider the 0.4*D of the bolt in the grip length, I know its a mistake, but I will make sure we will change the grip length value)

the above values are common for both the bolts we are using.but what different is

"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"

But, In future we made sure to get only bolt 1 from our supplier.

 

[desertfox]

Hi preload

The original 4" length came from your earlier post
now your saying that between the bottom of bolt to underside of head is 3.78" inches long so therefore :-

2"+0.77"+0.814"= 3.58" I thought it would add to 3.78"

and 2.4"+0.77"+0.37"= 3.58" again not 3.78"

Have you made a typo error?

Lets just try to get this right firstly:-

1/ are the tapped holes blind or not?

2/ are you including the 0.814" in your effective grip
length or not? I assume not

3/ what is the thickness of the flange the bolt passes
through before entering the tapped hole as it stands at
the moment I am subtracting 0.814 off 3.58 which leaves
me a flange thickness of 2.766"

I know that the .77 and .37 are the tapped portions not in the region and therefore if the holes are blind I know they are in the grip length as you call it.
Might it be easier to upload a sketch of the joint so we can all see it.

regards

desertfox

 

[preload]

Desertfox,

when I say 3.78 in, its not from the underside of the bolt to the bottom of the bolt.It is from the very top of the bolt to the bottom.

yes 3.58 in is from the underside of the bolt head to the bottom. which you have calcluated right i.e 2"+0.77"+0.814"= 3.58" and 2.4"+0.77"+0.37"= 3.58".

answrs for ur questions
1) yes blind hole.
2)no I am not including 0.814 in. We are just taking the length of the top joint plate as the grip/effective length, which is wrong.I have to add 0.4*D to the grip length.
3)yes u r right the flange thicknes is 2.766 which is our grip length.

 

[unclesyd]

I have to agree with Desertfox's that a posting a a drawing or sketch would be very nice.

I'm still at a loss as to the exact form of this fastener if it's a hex head you will have two entities, one threaded section and one shank section, in your calculations and if this is a set-in stud you will have three unties to consider, two threaded sections and one shank section.

 

[desertfox]

Hi preload

Thanks thats cleared that up except to say that normally when you talk about a length of a bolt or screw in engineering its from the underside of the head.

unclesyd from posts earlier if my memory serves the bolts are hex flange head.

preload if we are now talking (4500lb pre-load) with the infomation I now have,I calculate that to achieve this pre-load you would need to stretch a bolt with the largest threaded length by 0.00453385" and 0.00429564" for the one with the shortist threaded length. The difference in these extensions is just over 5%.
Going back to my earlier calculations which I uploaded, I now calculate that to achieve a 4500lb pre-load you will need about 21lbs-ft of torque with an assumed K of 0.1497
which is unlubricated.

regards

desertfox

 

[dimjim]

Desert Fox,
Are your stretch values off by a multiple
of 10? .00453386 would only require a
26 degree angle to achieve that amount of
stretch? Maybe I am off by 10 factor.

 

[desertfox]

Hi dimjim

I will check, but I am only considering a 455lb preload not the previous figure of 9300lb

regards

desertfox

 

[desertfox]

Hi dimjim

I checked my figures they seem okay I used 30*10^6 for E
modulus of elasticity.
My last post should have said 4500lb not 455lb


preload if your design department only wanted the loading in the bolts to 4500lb on assembly which means your well inside the yield stress how did these figures you quoted earlier and have records for, ever came into being:-

"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."

regards
desertfox

 

[preload]

Desertfox,

I think plant is using high torque. The only thng plant gets from engineering is torque spec. They gave a spec of 33 ft-lb target torque for this joint.

When I talked to the design guy , he said "if he remember that correctly, it is 4500lb" if he confirms the number, then the spec specified by engineering is wrong.

I am awaiting his conformation regarding the load.

 

[dimjim]

Desertfox,
Normally for a bolt with a length of 8 times
the diameter, the turn of the nut method
recommends 180 degree of rotation to reach
75 percent of the proof load which would be
equivalent to .03125 stretch equal to 7000
pounds clamping force. From this I would
expect .020 stretch for 4500 pounds.
Maybe the turn of the nut method guideline
that I have from SSTC is wrong.

 

[desertfox]

Hi preload

Yes I am aware you're waiting confirmation and it is certainly not your fault that the company have had this problem.
Couple of points though:-

1/ Did the engineering specification specify using a
lubricant with the 33lbs-ft. If they didn't specify
lubricant then the joints should have been made dry.

2/ Whilst delving into this problem, I have to say, I have
learned a few things too regarding bolted joints which
brings me to my next point. From what I understand with
joints from within your industry torquing bolts upto 90%
of the yield stress is quite common so I am a bit
surprised that your department was given only a torque
figure and not a pre-load, given the equiment you have
to measure bolt elongation. Finally if the 4500lb figure
is correct then your bolt is only seeing about 48% of
the proof load given in the sites I posted earlier:-

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

http://www.morbark.com/Service/belttorque.pdf

which makes me think the 4500lb might be a bit low for your application; especially when the dry torque of 33lbs-ft quoted in the above sites results in an estimated 6975lb pre-load.

regards

desertfox