Deleted gearcase thread issue 8

[preload]

On Mike's suggestion, I have asked admin of the forum to delete the thread. But we can discuss that here

Desert forx post
"What grade of bolt is it ie whats its proof load? img511 and 514 don't work.
I assume the bolts are in tapped holes?
Could be a number of things like you have already mentioned
vibration;cyclic loading.
If the bolt pre-load is not high enough the bolts can loosen
off under vibration and subsquently fail in fatigue.
Have you any of the failed screws? if so post a pic.
In addition if your truely getting 80% - 90% of proof load on all bolts it doesn't give you much room for increasing
bolt tension unless you use a higher strength material.
Finally the bolts will not see equal tension , shear etc
for a given engine position: but depending on engine position and external force position at a particular point in time the bolts will share the load unequally.
"

Yes the bolts are in tapped holes and the holes are blind. Bolts dont break, they just come loose or missing.

6 Bolts,joint and proof load info

4 perimeter bolts (Bolt 1-4) – 3/8-16 (1.75 in) unc-2A stainless steel plated(tq spec – 27 ft-lb target)-min proof load - 8370 lbs
1 bolt in center- 3/8-16 (3.5 in) unc-2A stainless steel(tq spec – 27 ft-lb target)-min proof load -8716
1 bolt in center– 7/16-14 (3.5 in) unc-2A stainless steel(tq spec – 47 ft-lb target)-min proof load - 9567


Desertfox you saw one of the pic which is been deleted now right.Did that give u an idea about the joint?

 

[preload]

Desertfox,

But when I referred to a bolted joint book, it says unless the external service load is more than the joint clamp loads, the joint wont come loose. It says, for example if the joint clamp load is 12000 lbs and the external service loads acting on the joint is 10,000 lbs then the joint wont come loose. Is this principle only good for vibrations?

And when there is transverse loading(transverse slip), even smaller loads than clamp load would make the joint loose?

 

[dimjim]

Desertfox,
from your earlier posting,

9300lb/0.0775 = 120000 psi the 0.0775 square inches is the stress area for your bolt.
For steels up to 100000 psi .0775 square inches is correct. As = pi/4 (D - .9743/n)^2
For steels over 100000 psi .0678 square inches is correct. As = pi (Dpe/2 - .162/n)^2
Dpe = Min. Pitch Dia. n = No. of threads per inch. Yields 8136lb/0.0678 = 120000 psi.

if we are now talking (4500lb pre-load) with the infomation we now have, I calculate that to achieve this pre-load you would need to stretch a 3.5 inch length bolt 0.0047534”

Going back to the earlier calculations which I uploaded, I now calculate that to achieve a 4500lb pre-load you will need about 21ft-lbs of torque with an assumed K of 0.1497
which is unlubricated.

?*L/E = x (deflection)
where ?=stress= F/A approx (4500*4)/(3.142*0.375^2) = 40744 psi
L= bolt length 3.500
E = modulus elasticity Bolt= 30*10^6 psi
E = modulus elasticity Casting= 10.3*10^6 psi
Punching this through the calc gives: x= 0.0047534" assumes steel & steel.

Should x be almost triple that value to agree with the
different modulus values or are experiments necessary
to confirm those values?

 

[preload]

John Bickford says, self loosening occurs when there are vibrations, so when a force (transverse) acts at 90 deg to the axis of the bolt, and if that force is high enough to break the frictional forces in the joint (under head friction force and thread friction force) then the bolt will come loose.

One thing got cleared to me now ,

If an external load is tensile or axial load, then the external load should be more than clamp load to make the joint loose.

But if the external load is transverse vibration load, then to make the joint loose, the external load need not be more than clamp load of the joint

If I understand the whole thing right, then

If I have a clamp force of 1000 lb in the joint and my frictional force in joint is for example 0.25*1000 = 250 lbs.
A transverse load of anything more than 250 lbs can make my joint loose, even though I have a 1000 lb of clamp force in the joint.

Am I right???????

 

[preload]

Dimjim,

Our production clamp load studies says K = 0.11 and torque is 27 ft-lb.The length of bolt is 1.75 in.

not to get confused, though we have
3/8 x 1.75 in - 4 bolts used in assembly
3/8 x 3.5 in - 1 bolt used in assembly in the center

we are here talking about 3/8 x 1.75 in bolt because these bolts are used on perimter of the joint and are the weakest link in the joint.

 

[dimjim]

Preload,
Sorry, looks like I jumped threads.

 

[desertfox]

Hi preload

Yes your right with your post ref J.bickford.

Hi dimjim will respond later just starting work now.

desertfox

 

[preload]

Desrtfox,

According is Bickford (chapter 20 page 735, an intro to the design and behavior of bolted joints) joint slip can be prevented if
F is greater than or equal to Mu * L

F= clamp force in joint
Mu – friction coef
L – external "transverse" load (not the whole external load)

Now if we go by an example,

External transverse loads = 7500 lbs
Mu = 0.2
Then clamp force should be greater than or equal to 1500 lbs

How come external loads are more than joint clamp load?

 

[desertfox]

Hi preload

I haven't reference to Bickford I only mentioned it earlier so you could see which of your posts I was refering too.
I think you have your numbers in your last post the wrong way round ie:-

F/N =mu

N= normal reaction (clamp load)= 1500lbs

F= sliding force or transverse force

mu= coeff of friction = 0.25


F= N*mu using your figures = 0.25*1500 = 375lb

therefore a transverse force of 375lb or greater will cause slip of a joint.

The idea of a designed joint is that you know what your external loads are at the beginning, you then calculate the correct size and number to ensure the joint doesn't fail in service.

regards

desertfox

 

[preload]

Ohhh got ya. Thanks a ton for explanation.

So now I need to know my transverse service loads. But if I strain guage the bolts and do some field testing and get the service loads figured out, how will I know how much of that service load is coming from transverse loads?

 

[desertfox]

Hi preload

I am not sure how you would measure the transverse load, its not an area I am familiar with to be fair, however I think it maybe difficult for you to do in practice as you need to know the vibrations the engine is subject to during service ie:-
the tilting of the engine as you described previously which
eventually causes failure.
I thought the engineering team were going to do an dynamic analysis for you according to one of your earlier posts.
Have a look at the site and video I posted earlier
junkers test video at the bolt science site.

regards

desertfox

 

[desertfox]

Hi dimjim

The deflections within the bolted joint would be different
for the bolt and casting material and these will depend on the relative stiffness of each part.
By way of example consider a 20mm bolt passing through a copper tube having a 25mm bore and 45mm o.d. and a length of 0.75m.
assume a nut is wound onto the bolt and just touches the tube end ie no stress induced in tube.
Now tighten the nut by half a turn and assume pitch of thread is 2.5mm.

therefore F*Lb/(Eb*Ab) + F*Lc/(Ec*Ac) = 2.5mm/2

WHERE Lb= BOLT LENGTH
Eb= MOD ELASTIC OF BOLT
Ab= AREA OF BOLT

The other symbols are the same but represent the copper tube. Working through this assuming 210GN/m^2 = Eb and
100GN/m^2=Ec

F= 68723N

defl for copper tube is 4.6875*10^-4m

defl for bolt is 7.81257*10^-4m

In this case the tube is stiffer than the bolt and adding the two deflections together roughly gives 1.25mm the half pitch of thread.

regards

desertfox

 

[preload]

Desertfox,

Yes engineering will do my testing. I got to ask them if they have any other way to find transverse forces on the joint exclusively.

Yes I loked at your link about junkers test. I think junkers test is for experimental set up with small vibration loads and here in my case will be larger loads and also I need to find those loads to check if our joint is designed properly or not.

 

[MikeHalloran]

Unless I missed a meeting, ...

whatever you've been doing on the floor is sort of covered by your documentation:

<<<<One of our lube doc says,

All IFI124 (non-metallic lock patch fasteners) shall be coated with wool grease heated to 200 F. But the mating parts, holes, nuts should be oil and residue free.

All IFI125 (chemical lock fasteners) shall be free of oil and residue and wool grease.>>>>

It appears that the root cause of bolts loosening in extreme service is that the "lube doc" does not specifically identify epoxy-lock as a chemical locker, or refer to a document that does, but leaves it up to the production floor to guess which part of the spec covers it, and they guessed wrong.

In addition to cleaning the tapped holes and not greasing the epoxy-lock bolts, you need to get the spec fixed, to prevent recurrence.

You have a data point, sort of. It seems that normal customer applied loads are not large enough to dislodge the joint, but the loads associated with running the o/b as a surface piercing prop, are large enough to do it. So you need to get the epoxy-lock functioning correctly.





Mike Halloran
Pembroke Pines, FL, USA

 

[preload]

Mike,

The info I got from the dept about the lock patch is wrong. It is not a epoxy lock, they minsunderstood a non-metallic hard plastic lock patch as epoxy lock.

I then found out with my loctite expert that a non-metallic lock patch is like amechanical lock patch and woll greasing of bolts wouldn't effect them.But wool grease will effect a chemcal lock patch as you guys already said.

so now we shorted listed the possible root causes to
1) transverse loads more than frictional forces in the joint and joint slip is occuring, which makes the bolt loose
2)very low friction in the joint because of cad plate+wool grease

 

[desertfox]

Hi preload

I have a couple of questions:-

Are you using a locking patch (mechanical) on the
fasteners you are using? if so why are you lubricating
them before assembly?
Looking at MintJulep's link to longlok and reading the installation data there is no suggestion of lubricating the
fasteners prior to assembly,they clearly state that the bolts have to be installed correctly and the technical data
provided is for dry threads with no special cleaning.
Taking a step back here my final question before go off on another tangent is "are you installing the fasteners correctly"?
Oops its not my final question I would like to refer back to the clamp loads you posted the other day ie:- 4300lb-9400lb how have these figures been derived? and what is the tolerance your tightening specification allows for this application? as previously pointed out you are outside the
80% to 90% of proof load.

regards

desertfox

 

[desertfox]

Hi preload

Your last post states two possible causes of failure:-

"so now we shorted listed the possible root causes to
1) transverse loads more than frictional forces in the joint and joint slip is occuring, which makes the bolt loose
2)very low friction in the joint because of cad plate+wool grease"

Can you expand on the second one and why you think that is the cause.

regards

desertfox

 

[preload]

Desertfox, you asked me very good questions.Luckily I got answers for them.

Regarding why we are using wool grease? the answer is the lock patch we are using is mechanical non metallic lock patch which will not interact with the grease at the time of assembly.But before applying lock pach to bolt, the bolt should be free of oil or grease and we are doing that.

longlok link u got is a chemical based lock and wool grease interacts with this kinda lock patches.

for your question "are you installing the fasteners correctly?" . after the installation process we check the torques again with clicker wrench seat at target installation torque at the end of the line to make sure all fasteners reached the torque specification.

But the numbers 4300-9400 lbs are collected before checking them with clicker wrench and this shows that friction is not consistent in our case.tightening tolerance is 26-28 ft-lb, 27 ft-lb being target.

reg your last post,

Acc to Bickford,low friction coefficient in the joint is one of the major cause for vibration loosesning. He suggested no use of lubrication to avoid self loosening

 

[desertfox]

Hi preload

Not the answers I was looking for what I really wanted to know was how you measured those clamping loads of 4300lb etc. Your tolerance on torque equates to +/- 3.7% if you expect the clamping load to fall within that region you need
to change your method of tightening control ie:-

Hydraulic Bolt pretension +/- (1% to 10%)
Strain Gauges / Ultrasonics +/- 1%

The locking patches covered in the longlok site covers
plastic mechanical patches too and again it does not refer to lubrication in the installation info.
Regarding friction on the threads the higher the K factor
the less pre-load or clamp force you obtain for a given torque and the converse is true for a lower K factor.
Low friction in the joint would probably refer to joint face
with bolt head or nut and the joint mating faces themselves
and not neccesarily the threads.
I think you need to look at the installation method your using, your bolt pre-loads are all over the place, if it only takes one bolt to come loose and the rest follow then you need to get the pre-load consistant, or more consistant
then you currently have.
In addition the longlok locking fasteners claim they prevent the joint coming apart even when the intial pre-load
is lost, see page 3 of there handbook but to achieve this they need to be installed correctly hence my question in my last post. What info have the engineering dept come up with so far for this problem and for that matter what about the
other engine bolt failures that the designer promised clamping loads for about 4 weeks ago.
Let me put this installation matter to you another way if
all the bolts in the engine for this particular problem achieved a pre-load of 9400lbs would this failure still occur? now if only all but one bolt had 9400lbs pre-load
would the failure occur and finally what happens if all the bolts in the joint only have 4300lbs pre-load?

regards

desertfox

 

[preload]

\\\
Not the answers I was looking for what I really wanted to know was how you measured those clamping loads of 4300lb etc.\\\

we used ultrasonics to measure the clamp load.

\\Your tolerance on torque equates to +/- 3.7% if you expect the clamping load to fall within that region you need
to change your method of tightening control ie:-

Hydraulic Bolt pretension +/- (1% to 10%)
Strain Gauges / Ultrasonics +/- 1% \\
if lubrication is not in control, then even if we use precise tightening stategies for a given torque, clamp load can be different right? so I need to make sure the lube is consistent.

\\The locking patches covered in the longlok site covers
plastic mechanical patches too and again it does not refer to lubrication in the installation info.\\
Then I got to recheck with our loc patch expert. he assure me that wool grease wont interact with mechanical lock patches. Any way I will confirm about this again.

\\Low friction in the joint would probably refer to joint face with bolt head or nut and the joint mating faces themselves and not neccesarily the threads.\\
so when we say F=N*mu, is Mu = frictional coeff of bolt under head surface + joints mating face?

\\
if all the bolts in the engine for this particular problem achieved a pre-load of 9400lbs would this failure still occur? \\
This is what I gotta find out doing field test. To answer this question, I need to know my external loads in the worst possible condition.

 

[preload]

I take back my previous post

"if lubrication is not in control, then even if we use precise tightening stategies for a given torque, clamp load can be different right? so I need to make sure the lube is consistent."

I agree if we go to precise control, then lube will not effect as we are controlling the bolt tightening with stretch.