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?

 

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Desertfox, I rechecked the number and I still got the same numbers. But still we are short of engagement length. Thanks for the helicoils suggestion.

 

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Cory,

I am really sorry for long threads.As I mentioned earlier I am brand new to this fasteneing field (for short term projects) and was seeking help.I hate to be new. Thanks for understanding and helping.

 

[dimjim]

Preload,
This forum is not for us to provide you with free
engineering but to give you guidance. The calculations
are your responsibility. I know the literature is
huge as to try to understand fasteners and their
applications. I think it is quite frustrating that
you come with a problem without having all of the
information. You have Bickford's book and it is a
good reference. There are always questions even with
having text information and examples of how to solve
some of the problems in a clamping situation. There
are even assumptions built into some of the equations
that expect your bearing surface areas be equal to at
least three times the area of the bolts. That comes
from the cross section of (2D^2 - D^2). We have not
even asked about your cross sectional area where the
parts interface. For a new guy, you seem to be able
to quickly pick up the thrust of the problems.

 

[preload]

Dimjim,

Thanks and reg the insufficient info, yes I agree it will be frustating for some one who is trying to help.

Here are the pics of assembly for better understanding. I can get different angle pics if you need.

http://img504.imageshack.us/img504/9900/view2vi6.png

http://img524.imageshack.us/img524/402/assypc9.jpg

I doubt if we have bearing area 3 times the bolt area on perimeter bolts (u can see it in pics-bolt 1,2)

 

[dimjim]

Thanks for the pictures. You probably have moments
putting compressive forces on some of these bolts
and tension forces on the others when the propeller
is driving. The pictures help a lot, but I would
like to see both the upper and lower surfaces before
they are assembled together to see the bearing surfaces
at the interfaces.

 

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Dimjim,

http://img403.imageshack.us/img403/5969/exhuastlm3.jpg

this is exhuast housing (tapped holes)

 

[preload]

I can get the pics of gearcase upper and lower surface (before assy)tomm. I dont have my digicam with me today.
Thanks Dimjim

 

[dimjim]

I wonder if you can get pictures before
painting. It is very difficult to see
after these are painted.

 

[preload]

Yes I can get those pics before painting.I will do that tomm. Thanks

 

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Dimjim&Desertfox,

More pix before paint

http://img254.imageshack.us/img254/118/untitledth3.png

http://img386.imageshack.us/img386/3924/untitled2ik5.jpg

http://img246.imageshack.us/img246/7451/untitled1nd5.png

http://img112.imageshack.us/img112/7981/untitled3pf9.png

 

[desertfox]

Hi preload

Thanks for the pic's they do help.
I'll study them a bit more and post later.

regards

desertfox

 

[dimjim]

Tell me about the draft angle of 1.5 to 2 degrees.
Can't they be spot faces to ensure parallel tightening?
Have not seen this mentioned before. Are the upper and
lower casting surface ground before painting and assembly?
How flat are they? It appears that you have a raised or
c'sunk surface area at the 3/8 perimeter bolt tapped holes.
What is this for?

 

[preload]

Thanks Desertfox.

Dimjim,

All the surfaces(exhaust housing tapped holed surface, gear case bottom surface) are really flat (no raise) except for the 3/8 perimeter 4 bolts on the top side of gear case where the bolt head/washer lands. That surface is little raised and has a draft angle of less than 2 deg, u can see that in this picture

http://img246.imageshack.us/img246/7451/untitled1nd5.png

which is a close up of

http://img386.imageshack.us/img386/3924/untitled2ik5.jpg

We thought a draft angle of less than 2deg is allowable and will not be a big problem. So I dint mention that before.

This raise is one of the reason I cannot introduce big washer to solve my embedment issue. If you see the pic again, there is no room to introduce a big OD washer to reduce the stress underhead.

 

[dimjim]

We have never painted interface surfaces before.
I agree it is pretty but ultimately acting as a
thin gasket. Something else to consider. I do not
know the acceptable allowable out of parallel surfaces
that is allowed. It only compounds the torque value
readings for the perimeter bolts. Need more input from
others about this. The unpainted surface pictures helped alot. It would be interesting to see a moment force diagram of the clamping bolts about whatever neutral axis you think opposes the thrust of the propellers.

 

[desertfox]

Hi preload

I can catch up a bit now I am at home, firstly I can see why we got different engagements lengths in our calcs, I used the 0.0775" area whereas you used 0.0747" for the 3/8"
bolts in my opinion the 0.0775" should be used as this gives worst case for engaged length, other than that I see no difference in our respective formula.

Looking at your pic's and assuming I am looking at them correctly I would say that the 3/8" bolts furthest away from the prop will see the most external tensile load and
those nearest the prop the least tensile load as the boat moves along. To clarify I am saying that bolt2 in this link

http://img504.imageshack.us/img504/9900/view2vi6.png

and its opposite one on the side we can't see carry most of the external load imposed as the boat moves forward; followed by the next pair ie bolt1 + opp bolt which will carry less and subsequent bolts carrying even less as we approach the prop. This is a vey simplified view of course assuming that the bolted flanges are rigid (which I very much doubt in practice) I am saying that the gearbox cover tips about the back edge ie:- pivots behind bolt6 given in this link:-

http://img524.imageshack.us/img524/402/assypc9.jpg

A good example of what I am saying can be seen in the link
below where an offset load on a right angle bracket is resisted by a line of bolts who's axis is 90 degrees to the plane of the offset load.

http://www.mech.uwa.edu.au/DANotes/threads/fatigue/exampleB.html#top

Of course in practice the gearbox cover gets pivoted to the right and left, up and down etc which will dramatically vary the forces on the bolts not to mention acceleration. So my view is only valid for the boat moving in a straight
line but I feel it gives an idea of how the bolts maybe loaded.
My view also leads me to think that pre-load in the bolts
particularly those furthest away from the prop are extremely
important as those bolts are working the hardest to resist slip and seperation of the joint faces.
Fatigue is another issue in any of the bolts if the joint
can seperate due to lack of pre-load.
On the 3/8" perimeter bolts do the bolt heads and washers sit on a tapered face so that only a portion of the washer
or bolt head are in contact with the case?

regards

desertfox

 

[dimjim]

Interesting.

Yet Stress = F/A

so if the force is constant like 9300 lbs and as A
decreases, S gets larger.

 

[preload]

Desertfox,

when u said bolt 2 and other side bolt would take the max external tensile load, are u sure about tensile load? I thought it would be more shear loading because the water flow or the water pressure is acting perpendicular to the bolts axis here right?

regarding the tapered face, if you look into this picture

http://img246.imageshack.us/img246/7451/untitled1nd5.png

the bolts underhead sit on this raised areas.you can clearly see the raised holes.those holes have a draft angle of 1.5 - deg. so they are not really parallel.but dooes a less than 2 deg tapered surface make much difference? but I agree we loose some clamp there because of that draft angle.

 

[desertfox]

Hi preload

There are shear forces acting too; in addition to the tensile loads, I was merely trying to convey that I see
the gear case as trying to pivot away from its mating face
like that bracket example from the site I posted.
You have a combination of shear and tension forces acting
together however the main point I am saying is that most
of the load is being taken by the first pair of bolts as the
gear case tries to seperate from its mating face.
The tapered faces can't be helping that bolt joint my feelings are it should be clamped square onto the joint face; by having that taper your bearing stress under the washer will certainly not be evenly distributed.
Regarding embedding is there any chance you can make some joints up ensuring that you obtain the proof load you were looking for by using ultrasonics;then stripping down the assembly and examing for witness marks on the joint surface?

regards

desertfox

 

[dimjim]

Preload,
Looking at 3pf9 picture for the 7/16 bolt hole.
How large is that diameter hole? I looks like
about 5/8 diameter at the interface surface.
The tapped hole surface looks great on the mating
part. How deep does this 5/8 diameter hole go
into the casting? It looks like about an inch
deep. Does this 7/16 bolt seem to be deflecting
the casting when you tighten it? Because that hole
is so large, you have very little surface area
mating between the parts for this bolt hole.
What is the od of that general area around the hole?

 

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Desertfox,

Yes I can do some testing to see any embedding witness marks. I will do that on monday.

Dimjim,

I will get you the dimensions of that hole on monday for sure. Thanks a lot.