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]

Mike,

As Desertfox mentioned, any force over 6113 lbs on our perimeter bolts will yield the joint material, then aren’t we yielding the paint too?

Any way I will try to post the pics then.

Desertfox here are the thread engagement lengths for all bolts

3/8 center bolt -0.596"
Perimeter 3/8 bolts - 0.707"
7/16 center bolt - 0.794"

 

[boo1]

If the bond strength of the paint system is exceeded then you will see delamination between the housing and the paint.

 

[preload]

boo1,

how do I know if my clamp force is exceeding the bond strength of the paint system?

so are you saying that even if my clamp force is good enough to yield the die cast aluminum, but less than the bond strength of paint system, then I will not have embedding problem?

 

[desertfox]

hi preload

Thanks for the info I will get back to you.
Regarding the sketch I was meaning just a hand drawn sketch with some dimensions on would suffice.

regards

desertfox

 

[dimjim]

Helicoils work in space applications
and also in military tank bearings.
Why are they not used here as a benefit.
They are used on the hummer machine gun
bearing mounts. These also have low
l/d ratios and see heavy vibration
as you can imagine.

How many mils is the paint thickness that
you are loosing by embedment. We had special
washers that we used around holes on the mounting
surface sides to keep the paint off these surfaces.
Are the aluminum surfaces anodized and then sealed
before painting?

 

[preload]

Dimjim,

We don’t do any anodizing and sealing before paint.

And how do I know how many mills of paint we are losing? By microscopic testing in the lab?

 

[dimjim]

You must have some specification for applying the
paint. Are there two coats? First a primer, then
the topcoat?

 

[preload]

Dimjim,

yes first a primer and then topcoat. I can get you the paint thickness value

Thanks

 

[desertfox]

Hi preload

Will do calcs shortly however I was looking at your thread engagement which doesn't seem an awful lot, the failures your having you said doesn't break the bolt but what about the internal thread is there any damage to that area?
Have you got any of the failed parts? have you actually got
any of the bolts which have come from one of these failures
I assume you must have otherwise you wouldn't know whether the bolts are breaking or not.

regards

desertfox

 

[dimjim]

Preload,
as to:
Dimjim yes, first we tighten center ones then go to perimeter ones. how does this effect?

I assume you are tightening the 7/16, then the
3/8 in the center, then the 3/8 perimeter bolts.
Each bolt would lessen the clamp load of the
previous bolt clamp load. Sounds like you
are ok.

 

[preload]

Desertfox,

No I dont have any failed parts.But the warranty document says they dont see any signs of damged threads. Yes I agree thread engagement length is not really good.This is what I meant by L/D ratio is very less on especially those 3/8 perimeter bolts.

 

[preload]

Dimjim.

Paint thickness (paint+primer+top coat) = 0.0037 in
By the time we put the fastener it's only half cured. We can loose 0.001-0.002 in of paint thickness in between the fastener installation and the paint is fully cured.

 

[desertfox]

Hi preload

I did some calculations on thread engagement for the 3/8" bolt with the 0.596" and found that the thread engagement
should actually be about 2.6 x bolt dia,calculations were based on the Aluminium tensile strength of 42000 psi and I couldn't find exactly your bolt spec but assumed a tensile strength of 100000 psi, perhaps you can let me know the actual bolt value.This means that the internal thread is the weakest part of the joint and internal thread failure
would occur before the bolt breaks, so I wondered under service conditions if the internal thread failed that it might render the locking patch useless and ultimately the bolt disappears.
If you have no failed parts, what as happened to them have the bolts been replaced?, gear case scrapped?

http://www.steel-stainless.org/TechRef/ViewTech.pasp?tre_id=10&section=section&sequence=13

http://www.geocities.com/dtmcbride/home_garden/bolts.html

The above sites I looked at for fastener info the first site
mentions grade A4 for marine use and A2 for light industrial use are you using the right grade?
The second site lists A2 (304) as being 700N/mm^2 as the tensile strength which is close to 100000psi.


I would say that with such short engagement lengths then helicoil inserts would have helped if only from the point of view of increasing the shear stress area.
We really need some more info now to be of any further help
eg:- dynamic loading, evidence of failed parts,sketch of gear case etc..

What is happening in respect of your 30pc clamping loads
and working a new K value.

One last question whats happened with the other failed bolts
in the engine as the problem been resolved?

regards

desertfox

 

[desertfox]

Hi preload

What did engineering dept say about embedding issue?

regards

desertfox

 

[preload]

Desrtfox,

1)Thanks for the calculations. Yes I agree the thread engagement is less.But could u explain how does it help if the thread engagement is more?

2)we are using A2 grade bolt, not A4.I will confirm the tensine strength numbers of the bolt on monday for sure.

3)when u say u need dynamic loading, do u mean the dynamic loads the joint sees in the service? id yes , I am in process of doing the field test, I will let you guys know the loads as soon as I am done with the testing.The documented failures occured 3 yrs back, so we dont have those bolts now, but the docunemtation says bolts missing/loose almost all. one case bolts broken.And on the loose bolts they dint see any thread damage.regarding the sketch I will get the sketch link too on monday for sure.

4)when u say helicoils would help, how does ncreasing shear stress area help in my case?

5) 30 pc sample testing is not done yet.It should be done next week for sure.

6)regarding embedment issue, I dint hear anything from them , but I got a meeting on this issue on monday. I will let you know thier views.

7)reg other failed bolts in engine- I thought of updating that thread today.I think I got the minimum clamp load spec u were asking.the nest thing is to develop angle strategy.I will let you know the minimm clamp load number ASAP.

 

[desertfox]

Hi preload

Basically the shear area is increased if either the thread engagement is increased or a bigger thread diameter is used.
Remember stress equals force/area, then if I increase the area I decrease the stress for a given load.
Now if you fit an insert to take a 3/8" bolt then the shear area of the internal thread increases because you have to tap a larger thread in the Alumium to fit the insert.
Now also the insert according to the list given in dimjim's
post gives other advantages such as spreading the load more evenly etc.Further when a joint is designed it should be designed so that the bolt fails in preference to the internal thread as the bolt is easier to replace than the tapped component. This is not to say that the insert will solve the problem, in order to do that the joint needs to be designed correctly in terms of its pre-load to keep the joint together in service and a correct installation method that ensures the pre-load is achieved within sensible tolerances.
If you wish to understand more about thread strength have a look in the Machinery's Handbook.
Interesting you say one bolt broke was it the 7/16" bolt or a 3/8" bolt?

regards

desertfox

 

[dimjim]

Desertfox,
I too am going to check out the length of engagement.
I have never seen a factor greater than 2.
Which formula are you using. I have seen one in
Machineries Handbook under the bolts and nuts section.
It starts out by defining a J factor as
J= As x tensile strength of external thread divided by
An x tensile strength of internal thread.
They then define As and An. Is that the basic formula
that you are using?

 

[desertfox]

Hi dimjim

Yes thats the formula I used, bearing in mind I used 100000psi for the bolt.
Glad your working it out too, let me know if I have made a mistake.


regards

desertfox

 

[dimjim]

Desertfox,
Since the strength is just a tad over 100000, I would use
the At value for steels over 100000 psi just to be conservative. It will be good to see the actual value of
these bolts that preload will eventually define.
I have seen proof loads equal to 125000 psi from one source that had a value for strained hardening. Interesting.

 

[dimjim]

Desertfox,
I calculated 1.689D. I used 102000 and 42000 psi
for the tensile strengths. I also assumed 2A and
2B threads for the calculation.
Since pi x n x Le is in both As and An formulas,
I simply removed them from my calculations as we
are only looking for a ratio.
I ended up having .69552 x 102000/42000 yielding the
1.689D value.
This was for the 3/8-16unc bolts.
I assume the 7/16-14unc will have a similar ratio.