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?

 

[dimjim]

Earlier you said:
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"

then you said:
for the perimeter 3/8 bolts we have thread engagement length of 0.707 in
for center 3/8 bolt we have 0.79 in
if 0.75 inch is the required LE then I am good except for the perimeter 3/8 right?

So which values are correct?

 

[preload]

Dimjim & Desertfox,

Thnaks for correcting me.

"for the perimeter 3/8 bolts we have thread engagement length of 0.707 in
for center 3/8 bolt we have 0.79 in"

this is correct. I am sorry about that confusion.

 

[desertfox]

Hi preload

dimjim beat me to that one.
I get just over 0.8" for 7/16" screw engagement.
so your still short on engagement, which means that the thread will fall before bolt thread ultimately

 

[dimjim]

I have been reviewing the H28 handbook to see where
the early equations came from and get a better explanation.
If you can get a copy, I think you will agree that
the equations are over conservative.
It looks like .75D would be the normal Le length
for steel on steel and somewhere it says that even
this could be reduced to somewhat like 17 percent.
So I think the value of J times .75D would be realistic.
You could have 2.00D in the joint and it may be a false
assumption that you can ever get that many threads sharing
the load. So I feel very confident that 1.5D lengths of
thread is sufficient assuming these all share the load
which again is probably not happening. Remember that
this has a factor of 2 built in to assure that the bolt
fails before stripping of the threads. I would like to
see the root diameter of the bolt be used for the
min. bolt cross section for high strength bolts ie those
having a tensile greater than 100000psi. I did find a
source of the H28 handbook but it is a pdf file and
some of the equations are hard to read.

 

[preload]

Dimjim and Desertfox,

You guys are talking about the formula in the link below right?

http://img514.imageshack.us/img514/2563/untitledfu0.png

 

[dimjim]

Yes,
What is different is that they define Le = Length
of thread engagement required to develop strength.
This is absent in my Machinery's Handbook.
Le can be interpreted to be the actual Length of
engagement in the clamped parts.
Thanks.
Another source uses the Nut example that the normal
nut thickness is .8xD and suggests using .75D as
the value of Le.

 

[preload]

Dimjim,
So what do you think from your calculations? We are short of Le on 7/16th bolt and perimeter 3/8 th bolts? If so how much Le do I need on both of them?

By any chance could you please upload your calculation because I wanna see the numbers you guys used for minimum OD of bolt threads and maximum PD for nut and basic (minimum) PD for the nut? I couldn't find them on net.

 

[preload]

So you guys are saying that from design point of view we got 2 issues already on this joint

1) embedment issue
2) insufficient LE issue

These two "might" be the causes for my loose/missing bolts.How can I prove embedment in our lab? Any easy and quick method?

I think after our dynamic testing and getting the service loads we can get more info which should guide us in right direction of solving the issue.

P.S: on this issue, we cannot validate the joint after making any changes because failure rate is very low. The only thing I can do is make some changes to the design and process which I believe might be the causes and that’s it I am done.

 

[dimjim]

D Bolt Dia. 3/8 Dia. 7/16 Dia.
Ds Major Dia. Min .3643 .4258
Ds Major Dia. Max .3743 .4361

Es Pitch Dia. Min .3287 .3850
Es Pitch Dia. Max .3331 .3897

Minor Dia. .2970 .3485

Kn Minor Dia. Min .307 .360
Kn Minor Dia. Max .321 .376

En Pitch Dia. Min .3344 .3911
En Pitch Dia. Max .3401 .3972

from Machinery's Handbook 17th Edition

Where did your page come from for future
reference?

 

[preload]

Dimjim Thanks for the quick response, I got the page from "An introduction to the design and behaviour of bolted joints" by Bickford. You can also refer "The handbook of bolts and bolted joints" by Sayed Nassar and Bickford.
What did you take for tensile strength of the bolt material? Ultimate tensile strength of bolt is 120-160 ksi
And tensile area for bolt is 0.0747 in^2 or 0.0775 in^2?

 

[dimjim]

120000psi and .0747 square inches

42000psi for the casting

 

[preload]

Great thanks a ton Dimjim

 

[preload]

Dimjim and Desertfox,

My calclulations using the formula in the link I posted gave

Le = 0.515 in for 3/8
Le = 0.621 in for 7/16

I used

tensile strength of bolt = 120ksi for both bolts
tensile strength of nut = 42ksi for both
n = 16 for 3/8, n = 14 for 7/16
minimum OD of bolt threads = 0.3643 in for 3/8
minimum OD of bolt threads = 0.4258 in for 7/16
maximum PD of nut = 0.3401 for 3/8
maximum PD of nut = 0.3972 for 7/16
tensile stress area of bolt = 0.0747 in^2 for 3/8
tensile stress area of bolt = 0.1063 in^2 for 7/16

 

[desertfox]

Hi preload

You need to use 160000 for the bolts to give you worst case.
If you have embedding you should see a witness mark were the washer was situated.
In an earlier post I said if you change the washers you can
eliminate embedding ie use plain washer's series W.


regards

desertfox

 

[dimjim]

Desertfox,
You are right about the 160000psi.
I missed that. I did get Preload's
values using the 120000psi value.
Have we lost Cory and Mike?
Have not heard from them for awhile.
It would have been nice to have gotten
all of the correct data required at the
beginning.

Ultimate Tensile strength of the bolts is 120-160 Ksi
That is quite a range. I wonder if they have been
tested?

 

[preload]

Dimjim,

Yes they are tested, but I need to rquest our supplier for the data.

but even with 160ksi for a 3/8 bolt we are safe. I got LE of 0.68 in for 160ksi which is still less than our current le values for 3/8 bolts

the problem is only with the 7/16 bolt regarding the LE

 

[desertfox]

hi preload,

if you refer to my posting of 5 june i calculated, using the 160psi bolt strength, that you need 0.75in thread engagements. the thread engagement given by you was 0.707 which means, according to my figures, you are marginally short on thread engagement.

referring to my earlier post today, the thread engagement for the 7/16 bolt should be over 0.8, therefore again the thread engagement is slightly short.

i will re-check my calculations, can you re-check yours and post again?

regards,

desertfox

 

[CoryPad]

dimjim,

I am following this thread on a part-time basis. Trading 100+ posts on detail engineering is not really within the scope of this site. It is Eng-Tips, not Eng-Solutions. Also, it has been hard to follow some of these threads due to missing information, lack of images, etc.

Regards,

Cory

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

 

[MikeHalloran]

I'm watching, but I'm not checking numbers.

I like doing this kind of stuff, but I like getting paid for doing it, too.



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi preload

The issue of thread engagement will go away if you fit helicoil inserts.


regards

desertfox