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?

 

[desertfox]

Hi preload

What are the installation torque's or pre-loads given by
the locking bolt supplier and do they mention any lubrication or not? thats the important thing, if they give you a reccomended torque figure without lube the clamping force will be a lot smaller than the same torque figure with lube.
In addition I have just ran a calculation for the stress
under the washer of a 3/8" bolt on the aluminium casting ie:-

force req
to reach yield = area of washer * 19000lbs/in^2
in aluminium


= (.75^2-.391^2)*3.142 * 19000
--------------------
4


= 6113lbs force


It would appear that your exceeding the yield stress for the aluminium with any clamp load over 6000lbs which would lead to loss of bolt pre-load due to embedding of the fastener on assembly.
Your engineering department really need to start coming up
with design info for these joints.

regards

desertfox

 

[dimjim]

Desertfox,
the 7/16 bolts is even worse.
In what sequence are these bolts tightened?
Are the center bolts tightened first and then
the perimeter bolts tightened afterwards?

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
Washer ID is 0.391 +0.015/-0.005 and OD is 0.75 +0.015/-0.007
l/d ratio 4.667

1 bolt in center- 3/8-16 (3.5 in) unc-2A
stainless steel(tq spec – 27 ft-lb target)-min proof load -8716
Washer ID is 0.391 +0.015/-0.005 and OD is 0.75 +0.015/-0.007
l/d ratio 9.333


1 bolt in center– 7/16-14 (3.5 in) unc-2A
stainless steel(tq spec – 47 ft-lb target)-min proof load - 9567
washer ID is 0.453 +0.015/-0.005 and OD is 0.75 +0.015/-0.007
l/d ratio 8.000

Very complex system.

 

[preload]

Desertfox,

Great find. Thank you. So we are basically yielding the joint material (embedment issue). Our lock bolt supplier dosent mention any torque values. Our engineering dept mentioned the torque value as 26-18 ft-lb and clamp load should be 80-90% proof load. That is the whole info from our dept. Other than this they can help me in solving this issue.Now I am gonna take this embedment issue to them.

Dimjim,

Yes you are right, 7/16 is even worse, It can yield the cast aluminum for 5300 lb of force.
regarding the tightening sequence, we first do center boolts then do perimeter bolts.

 

[desertfox]

Hi preload

Just looking at the washers again if you changed the plain washers your using under the 3/8" bolts to type 'A' plain washer series W this would reduce the stress under the washer to 14,835 psi which is below the 19000psi you quoted
earlier.
The 7/16" bolt would need a similar washer again to reduce
the stress below 19000psi and as dimjim pointed out the material under this bolt is suffering more than the 3/8" bolts.
I was also thinking along the lines of the transverse loading would it be possible to fit dowels to take the transverse loading instead of the bolts but I am not sure
how often you need the joint to come apart.

Incidently the blind tapped holes in the joint are they fitted with threaded inserts?

regards

desertfox

 

[preload]

Desertfox,Is the difference between the current washer and type 'A' washer series W is ID and OD?

we use dowel pins on some of the other model gearcases. But I gotta find out if we are using dowel pins on these models.I will let you know on tuesday.

No the tapped holes are not fitted with threaded inserts.

 

[desertfox]

Hi preload

Yes the OD and ID are bigger but the main difference is the OD as this increases the surface area to dissipate the load.
Some more info if you can what grade is the stainless fastener and how much thread engagement have you got?
Whats the clearance hole in the component the bolts pass
through?
Are you saying from your last post the design department cannot give you any more technical info about the joint?
You need the dynamic analysis from them and also the original design info, if they can't give you this then somebody needs sort out your design dept as there's something terribly wrong.

regards

desertfox

 

[dimjim]

Desertfox,
I have long wondered why they have not used
helicoil inserts on these but assumed since
no threads were breaking they had at least
2 times the diameter length of engagement.
If no dowels, would a loctite product at the
mating surfaces help reduce the vibration
movement side to side? What are the hole
sizes in the castings?

 

[desertfox]

Hi dimjim

Yeah I thought they would have used inserts too but nevermind lets see what tuesday brings.

preload you quoted a K value of 0.11 for your bolts with cadmium plate my machinery's handbook says 0.12 for cadmium plate without lubrication your value seems high espcially as
the 0.12 is subject to +/- 20%.


regards

desertfox

 

[preload]

Desertfox,

the K value I quoted is calculated from our clamp load values. I will get you new set of clamp load values on tuesday(we are doing a 30 pc samle). we can see what is the K value. I bet as you said, my K values will be less than 0.09, because we use cad + wool grease.

how do inserts help in my situation?

 

[desertfox]

Hi preload

Very often when materials like alumium and copper are tapped it is common to use threaded inserts as it improves
the strength of the tapped hole by increasing the shear area
and distributing the axial load more evenly in the female threads and finally helps to prevent galling.
Have a look at this site and play the video :-

http://www.emhart.com/products/helicoil.asp?gclid=CIXq8O6rsYwCFRWOEgodoCMCSg

regards

desertfox

 

[dimjim]

Preload,
From the internet.

The assembled HeliCoil offers:
High tensile Stainless Steel threads
Flank hardness of 43-50 R.C.
Surface finish of 2 to 4 Microns.
Dimensional accuracy to extreme limits.
Reduce friction between bolt and female thread flanks.
Greater loading for an applied torque.
Minimal torsional strain in the bolt
Even distribution of load over ALL threads
Greater loading capacity
Maintains pre-load stretch in bolt.

Characteristics
Screw-Lock Inserts positively secure threaded members
against loosening caused by vibration and shock.
They have a high reusable factor due to the exclusive
HELI-COIL Resilient Screw-Lock which permits frequent
removal and reassembly of bolt without appreciable, loss of torque.

Positive self-locking torque, complying with MIL-I-8846 & MIL-N-25027.

Savings in space, weight and money, through the elimination of lock wiring, lock nuts, lock washers, chemical compounds,plastic pellets/patches and other locking mechanisms.

 

[desertfox]

Hi preload

Before you go rushing off to buy inserts there's a price to
pay in terms of installation, I obtained this passage from
this site:-

http://www.hillcliff-tools.com/helicoil.html

"To obtain the best performance from HeliCoil Screw-Lock
inserts it is recommended that only good quality close
limit fit, rolled thread, oil lubricated screws or bolts
are used. On no account should sub standard or "Black"
bolts (with heat treatment scale) be used with HeliCoil
Screw-Lock inserts.

"When using Unplated, Heat Treated or Stainless Steel
screws with HeliCoil Screw-Lock inserts and Anti Seize
Compound (such as Molibdenum Disulphide) should be applied
(preferably to the screw) to minimise galling and maximise
cycle life".

regards

desertfox

 

[MikeHalloran]

Helicoils are great, but they are the solution to none of preload's current problems.



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi Mike

I agree helicoils won't solve the problem.
Still to be resolved are the dynamic loads the unit is subject too, although it appears that it is only when the unit is abused this failure occurs.
The fastener pre-loads appear to be completely inconsistant
which surprises me that more failures have not occurred and
in addition if the bolts do manage to reach the figures quoted then there is a good chance of embedding.
In fact preloads biggest problem is getting any sense out the design dept.

regards

desertfox

 

[MikeHalloran]

Speaking of embedding, these units are typically coated with a very thick paint system.

Anybody know the mechanical properties of paint?



Mike Halloran
Pembroke Pines, FL, USA

 

[dimjim]

On critical joints, paint is not allowed.
Thanks for the info Mike.

 

[desertfox]

Hi preload

Is there any chance you can give a sketch of the outboard
showing the rough angle you think it is held in the water at and some dimesions overall to where it enters the water
and the bolts in question.

regards

desertfox

 

[boo1]

Paint system fail of there bond strength is exeeded by the fastener head embedment.

Epoxy systems on Aluminium have typically ~2000 PSI bond strength.

 

[preload]

Desertfox,

No we don’t use dowel pins.but we use alignment pins which we remove them after bolt tightening. So the answer is no, we don’t use dowel pins.

All are stainless steel A2 (304) grade strain hardened bolts per ASTM F593

For 4 perimeter holes, 2 holes dia 0.406 in (+0.002,-0.002), 2 other holes 0.441 (+.036,-.013). 2 holes dia is smaller than other 2 holes because we use alignment pins for better alignment.

Thread engagement, I can get you this info in 2 hrs from now

Regarding the sketch, I can provide that to you, but my only concern is according to Mike Halloran, its not advisable to copy images here. But I will try to draw a rough sketch and up load

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

Mike,

"Speaking of embedding, these units are typically coated with a very thick paint system. How does paint effect?"
Does thick paint coating helps me from embedment or does it worsens embedment? The only painted surface in this joint is under the bolt/washer head.

 

[MikeHalloran]

WE like to see images here, because they help to clarify what we are talking about. Your employer may not care to be identified, or identifiable, in a public forum where you discuss your problems. You should talk to your boss about it.

If the paint yields in compression, there goes some of your hard-won preload.



Mike Halloran
Pembroke Pines, FL, USA