Deleted gearcase thread issue 8

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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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Mike's post
"First thing that came to mind is Spiralok taps, which are alleged to produce (modified form) threads that are extremely vibration resistant.

Second thing that came to mind is, I wonder if the grease on the threads in interfering with the operation of the locking patch. I'm interpreting "... wool greased and also low strength (pink color) no metallic lock patch " to mean "well greased threads with nonmetallic lock patch".

Third thing that came to mind is Loctite PST instead of grease. Belt, suspenders, and more suspenders...
"
Thanks for the suggestion on the issue and also about posting on public forums. I will be more carefull hereafter

Mintjulep post

"I agree with Mike that there are probably some nasty cyclic loads imposed by running with only one blade in the water.

I would suspect loss of preload due to embedment (I assume the case is cast aluminum, yes?)"


Yes cast aluminum case

 

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I would like your help in improving the situation without much additional costs.

For example we are using low srength loctite, I can go to high strength as it is lil cost, but I cant change the bolt and use loct fasteners.It will cost us more and our dept will not buy off.

Initiall I thought the failure is because of low preload in the joint.. But what if I found out (we will be checking the joint preload tomm) that we are actually having 90% of proof load in the joint and still we find loose bolts? what might be the other reasons other than low preload for loose or missing bolts?

 

[desertfox]

Hi preload

If your setting the bolts by torque then your tensile load in fastener is quite open +/- 25% as I posted in the other thread about engine bolts yielding.
So you can be up or down by 25% before it even see's an external load.
When a joint is designed the pre-load is usually worked out on the basis of the various material strengths and the external loads the joint will see in service which is why we keep telling you to pester engineering dept.
It appears to me that your eng dept is just specifying 80% or 90% of the proof load for the bolt without consideration of anything else which is why your having so many problems
in the field.
To solve this problem somebody needs to do so serious stress
modeling or better still fit some strain gauges to the engine and run it like you say its being used and get some
practical data that can be used to calculate an accurate pre-load for the bolts.
Loss of pre-load as previously stated can be due to vibration, using torque to set pre-load, embedding of the head during tightening or in service, friction etc.
You say the material is Aluminium or do you mean the Alumium/Copper alloy as in the previous post?
Taking the 3/8" bolt and a regular plain washer form B I calculate the pressure under the washer 13288lbs/in^2 I have
no idea whether that would cause embedding unless you can tell us the yield stress for the connecting materials.
Finally as your using conventional washers the pre-load in the bolt can be affected greatly be relative lateral movement between bolt and washer ID which can change the friction surfaces from bolt head to washer, to washer and
joint surface; subsquently affecting the torque-tension relationship and hence the pre-load.
The bottom line is your engineering department have to take responsibility you cannot just do knee jerk quick fixes as you will find they have a habit of biting you in the bum.

regards

desertfox

 

[dimjim]

Preload,
Have you seen this?

http://www.nord-lock.com/video/nl_uk.wmv

Can you give us the material, grade etc. of
the stainless steel bolts. Have you tested
these?

 

[desertfox]

Hi preload

Look at this video

http://www.boltscience.com/pages/junkertestvideo.htm

desertfox

 

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Desertfox,

I agree with you 100% about pestering the engg dept.But the problem is there I will have no answer from them and as a quality guy from plant , its ultimately my work to do that.But I will get some help from eng like strain gauging the bolts in field and letting me know the possible service loads. I already asked them to do this and they will be working on it.
the joint materials here are die cast Aluminum only.
I will try to find out the yeild stress of material.

I am looking forward for our preliminary clamp load study on the joint on monday.I will let u guys know the values.I am eager to see if we are installing them at 80-90% of clampload or not.

 

[desertfox]

Hi preload

Have a look at this site it will give you some idea of whats involved doing an analsys for bolt fatigue.

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

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

regards

desertfox

 

[MikeHalloran]

I think you have to ask the design guys if they even covered the load cases associated with running at high speed with only one prop blade in the water.

I.e., it's conceivable that the gearcase flange is stretching the bolts while trying to bend and/or separate itself from the exhaust housing.




Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

This sounds like a difficult joint.

High vibration. High temperature swings due to exhaust. Relatively soft joint material with a relatively high coefficient of thermal expansion. Blind tapped holes (that very probably are contaminated with cutting oil).

My SWAG at what is going on:

The surface finish of the mating surfaces of the joint, and any spotfaces or whatever for the fastener heads is relatively rough (and of a soft material).

The finish of the (cut?) threads in the blind tapped holes is rough.

The cutting lube used when tapping the holes is not being throughly cleaned out, rendering the Loctite ineffictive.

The soft and rough surfaces result in significant relaxation shortly after assembly, and continue to allow further embedment in service due to loading from thermal expansion and vibration.

Very soon all bolt preload is gone and the bolts fall out because of vibration.

Although I generally detest them, properly selected and applied Belleville springs might help in this situation.

Or perhaps even easier is to advise your "special needs" customers to scrap the stock fasteners and install self-locking fasteners per MIL-DTL-18240, Type L, along with daily retorque.

 

[unclesyd]

Is there any material used for sealing between the two components at the flanges of the mating parts?

 

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Desertfox,

Thanks for the links.

Mike,

Yup, I requested them to run the field testwith worst conditions possible (single blade surfacing)

Mintjulep,

yes temperatures will be there in the exaust housing, but shoud be minimal, because it will get continous cooling in water. both housing and gearcase are die cast aluminum so theyare relatively soft. I am not sure about the un clean tapped threads and bolt underhead rough surfaces,but we use flat stainless steel washers under the bolt head. does it matter? But I will check for rough and unclean surfaces and letyou guys know.

when u say "The cutting lube used when tapping the holes is not being throughly cleaned out, rendering the Loctite ineffictive"

when loctite is ineffective,will loctite form as a gasket and comes off eventually and the boltgets loose?

Unclesyd,

we dontuse anyy sealing material between the two joint surfaces.

 

[desertfox]

Hi preload

What size are the washers ie-outer and inner dia?
With this joint flanged hex heads or flat washer hex head bolts maybe better provided that the above have heads big enough not to cause embedding.
Flat washers can cause variation in pre-load for the reasons I gave in yesterdays post.

regards

desertfox

 

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MintJulep,

Yes the tapped holes are not really clean and we have a smooth finish under the bolt head surface of the joint.

Desertfox,

For 3/8 bolt washer ID is 0.391 +0.015/-0.005 and OD is 0.75 +0.015/-0.007
For 7/16 bolt washer ID is 0.453 +0.015/-0.005 and OD is 0.75 +0.015/-0.007
I don’t know if I can get you the yield stress of the die cast Aluminum, but will try.

 

[desertfox]

Hi preload

Thanks for the response, can you not get the material grade for the aluminium of a drawing?
Also how can a quality department ensure the materials are
correct if you can't get material specifications.
Washer sizes noted.

regards

desertfox

 

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Desertfox, we dont do inhouse quality on some of parts. Supplier would do that. I emailed them. Thanks

 

[joebk]

We use Nord lock washers on a lot of our product and we have been very pleased with them. I think they make a wide series for SS washers for soft materials such as aluminum.

Without knowing the results of your study on the clamping load produced by your assembly procedures/tools and any issues with dimensional or material specifications, I would venture a guess you are dealing with an application that exceeds the intent and/or capabilities of the design.

If materials, pre-load, and dimensions are to spec I would tend to think you have done your part as QA. Maybe if you can confirm that everything meets spec this will give you (or your boss) enough ammo to send the problem on to the design department or at least get assistance from them?

Seems like you are stuck trouble shooting a lot of design issues without the assistance of the design department. Sorry to hear that, it sucks getting stuck in a situation like that.

 

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Small correction:

the lock patch we are using is a hard plastic kind of metallic/mechanical lock patch (yellow color)

 

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Our eng dept has got a theory for this failue.

The lock patch material can accumulate between the mating surfaces in the joint, almost like a gasket. After some time this build-up of patch material goes away and now you have a loose joint

do u guys think if the above cause is possible????
I as said the lock patch we are using is like a hard plastic lock patch (not liquid loctite)and they call it as mechanical lock patch

 

[MikeHalloran]

Yeah, I remember lock patches similar to that. It took three men and a boy just to squeeze them into the hole on the first try; forget torque readings.

You could check the accumulation/ false gasket assertion by disassembling and inspecting a few units on the line.



Mike Halloran
Pembroke Pines, FL, USA