bulkhead blues

Topic: bulkhead blues (Read 12047 times) previous topic - next topic

Re: bulkhead blues

Reply #25 – July 06, 2012, 02:15:15 pm

Quote from: acousticart – July 03, 2012, 12:59:51 am

Regarding the vertical supports you mention for the engine side of the bulkhead... when I am done with the re-skinning and I close the gap for the last time, I am welding some 3"X5"X3/16" rectangular tubing to the vertical angle iron bulkhead supports which go on either side of the two horizontal suspension/powertrain back bones with added 1/4" gussets between same. This will support the push/pull of the drive train and suspension so that the bulkhead is not the (nearly) sole support for the rear axle location. Attached is a mockup made out of the two supports and some triangular gussets. There will be two of these just inside of where the shackles meet the suspension. I believe this will offer adequate backing for the bulkhead joint, at least more than what it came with...

Edit for added content:
Don

Hi Don,

You know how this goes... strengthen it in one place and concentrate the stress in another. I find it interesting that Rolok fasteners have lasted as long as they have in the Foretravel bulkhead application. Without doing a complete analysis, one reason might be the flexibility built in to the bulkhead where it meets the basement floor at the Rolok fastening point and also where it meets the horizontal suspension/powertrain backbone at the top of the bulkhead. In addition to these attachment points, there's flexibility of the bulkhead itself. Adding 3"x5" vertical stiffeners to the bulkhead may create stress concentrations elsewhere. Since the Roloks have lasted as long as they have, I'd be tempted to fix the fastener issue and leave the rest of the design more or less unchanged. Just my $0.02

David

Re: bulkhead blues

Reply #26 – July 06, 2012, 05:11:56 pm

Are the loads imposed by towing a car or trailer transmitted through the rear bulkhead? I am a little unlcear on the way it works for the unihome and unicoach designs.

Craig

Re: bulkhead blues

Reply #27 – July 06, 2012, 05:55:25 pm

Craig,
I fail to see how the loads could be imposed on the bulkhead joint, unless our coaches were front wheel drive

To me, it seems that that load is carried entirely by the power train frame work, which is very, very robust on our coaches. Most the tubing is 3/8" rectangular tubing, whereas on many SOB's they are largely made up of 1/4" or less "C" channel... And mostly bolted instead of welded. I think that the towing limits on our coaches are mostly just dependent on the hitch and power train's ability to move the loads... But then again, I am just a musician... LOL
Don

Quote from: wa_desert_rat – July 06, 2012, 05:11:56 pm

Are the loads imposed by towing a car or trailer transmitted through the rear bulkhead? I am a little unlcear on the way it works for the unihome and unicoach designs.

Craig

Re: bulkhead blues

Reply #28 – July 06, 2012, 07:06:29 pm

I've heard that the retarder may be a compounding issue to the bulkhead separation. I suppose this may be true, but I do not know the design of the chassis in enough detail to support that theory.
Peter

Re: bulkhead blues

Reply #29 – July 06, 2012, 11:21:37 pm

Craig, a while back someone posted a drawing of the rear substructure; I've attached a copy. I believe it came from Ray Stoltz, so it would apply to a 1993 U280.

David

Re: bulkhead blues

Reply #30 – July 07, 2012, 01:36:19 pm

Quote from: travelite – July 06, 2012, 11:21:37 pm

Craig, a while back someone posted a drawing of the rear substructure; I've attached a copy. I believe it came from Ray Stoltz, so it would apply to a 1993 U280.

David

David... thanks for that drawing. My stress analysis skills aren't what they used to be but that drawing makes me believe that there is some stress on the Rolocks on acceleration and especially when towing a vehicle. There would also be some load transmitted through the bulkhead if the towed vehicle applies brakes.

Most of the load would be carried on the top frame member but there would have to be at least some load on the Rolocks.

Unless, of course, my ME skills - never all that great anyway - are worse than I remember.

Craig

Re: bulkhead blues

Reply #31 – July 07, 2012, 01:56:40 pm

Looks like some of the load from acceleration or, I suspect more significantly, braking differential between front and rear could be on those bolted connections. Noting the broken verticals in the drawing it almost looks like it they could stand to be gusseted to the main frame, speaking strictly from the shade tree school of mechanical design.

Re: bulkhead blues

Reply #32 – July 07, 2012, 02:16:30 pm

Edit: To add close up picture of cracked vertical angle iron bulkhead support. This one was taken when I first discovered it and had just cleaned the crud off so I could view it more clearly. There was no rust whatsoever involved in this failure. In my view, it was the stress of the bulkhead joint opening and closing after the failure of all or most of the Roloks.

My take on it is that when accelerating, the trailing arms which are attached to the drive axle are pushing on the bulkhead joint tending to close it. When braking with the rear wheels (exhaust brake or retarder), the joint is being pulled tending to open it. I don't think toad braking would have an much of an effect, unless you were dragging a sled or were towing something closer to the mass of the coach. Not that there wouldn't be some effect from toad braking, just not much... But in my opinion, accelerating with or without the toad would always be pushing the joint closer together because the drive wheels are pushing on the trailing arms which are attached to the angle iron and are what actually what locates the axel front to back. But this would probably never be an issue if moisture and or corrosive elements are kept away from the Roloks...

Those broken vertical pieces of angle iron are cracked or broken on mine as well and the 3x5" pieces of rectangular tubing I cut to fit are going to be welded right between them giving more surface are to weld gussets to join the horizontal and vertical main framing. As it is, I can open and close the bulkhead joint at will by removing the forward frame blocks and backing off the bulkhead joint fasteners because of the cracked or broken vertical angle iron pieces. I am attaching picture of the cracked supports... the rust visible in the picture was not a contributing factor. It is only surface rust which appeared after I cleaned the area with an angle grinder to get a better look at it.
Don

Quote from: wa_desert_rat – July 07, 2012, 01:36:19 pm

David... thanks for that drawing. My stress analysis skills aren't what they used to be but that drawing makes me believe that there is some stress on the Rolocks on acceleration and especially when towing a vehicle. There would also be some load transmitted through the bulkhead if the towed vehicle applies brakes.

Most of the load would be carried on the top frame member but there would have to be at least some load on the Rolocks.

Unless, of course, my ME skills - never all that great anyway - are worse than I remember.

Craig

Re: bulkhead blues

Reply #33 – July 07, 2012, 02:49:43 pm

Today is a month since the pour so I drove out over the pit and did a quick inspection. Our U300 was always in a pretty dry area and when we bought it, the ODO was at 62,000 with never any winter or northern state trips. Underneath, there is a minimum amount of rust/corrosion, surprising since the front and rear subframes appear to have never had any kind of primer. Using a torque wrench, the first six bolts up front on the passenger's side and the first 5 in the rear, also on the passenger's side were broken, about a quarter inch from the head. The big angle iron is very clean. They have been broken for quite a while as none show any sign of a recent break. Rust jacking would seem to be the culprit with the angle iron forced out perhaps only a 32nd of an inch. They all line up exactly with the spray pattern off the tires when driving in the rain. Can't really figure out why only the passenger's side is affected. Ideas?

After viewing the drawing of the rear substructure and taking a close look at tube thickness, I am going to bolt through the rectangular tubing and make a conventional repair. Don't think the failure has anything to do with retarder (I don't have one) or towing, rather the design where the water and road chemicals can easily collect on top of the angle iron and also wick up from the bottom damaging both the bolts as well as the tubing. The catastrophic failures experienced by several forum members are the result of gradual breakage from rust jacking of the bulkhead bolts until there were not enough left to transfer the load to the rest of the possibly rust compromised steel structure forward of the substructure in my opinion. After the last one failed, some kind of momentary load caused the larger failure. Remember the failure of the EL AL 747 cargo plane engine bolts? Corrosion weakened them until one failed with the rest taking the increased load until they also failed and both engines fell off. Bad deal for the apartment complex.

Tank leakage may have also damaged the tubing in the cargo/tank floor until weakened members forced others to take loads from the rear substructure they were not designed for.

So, why just the right side both front and back? Anyone ever experience driver's side failures? Even if your coach has been stored in the desert, good idea to take a torque wrench to check.

Don't think with healthy bolts and rust free tubing, any normal RV related stress could possibly result in a problem.

Pierce

Re: bulkhead blues

Reply #34 – July 07, 2012, 02:57:08 pm

Pierc,
Consider there is more water on the passenger side of road due to road crown, so I would guess there would be a little more spray from the right side wheels. ??
Dave M

Re: bulkhead blues

Reply #35 – July 07, 2012, 03:08:40 pm

Quote from: Dave M – July 07, 2012, 02:57:08 pm

Pierc,
Consider there is more water on the passenger side of road due to road crown, so I would guess there would be a little more spray from the right side wheels. ??
Dave M

I was thinking the same thing...Crazy, eh? Donno. Who knows? Maybe even the road chemicals collect more so on the right side of the right lane due to the same crown!

Re: bulkhead blues

Reply #36 – July 07, 2012, 03:47:09 pm

Quote from: Peter & Beth – July 07, 2012, 03:08:40 pm

I was thinking the same thing...Crazy, eh? Donno. Who knows? Maybe even the road chemicals collect more so on the right side of the right lane due to the same crown!

Good logic guys! Perhaps the right side tires run just to the right of passenger cars and collect more all the way from the passing/left lane.

How about the coach sits slightly low on the right side when parked from street crown and any water in the tube runs downhill and collects there?

Which side is the heavy side? Just guessing, would say it is the driver's side with generator, fridge, sink vs. fuel and radiator on right.

Pierce

Re: bulkhead blues

Reply #37 – July 07, 2012, 05:02:51 pm

From the coach inspections I have done, there appear to be the three primary causes of bulkhead issues.

1. Leaks in the wet bay. In fact 2 to recent Foretravels had leaks for so long that the beams around the sewer door in the basement floor were rotted away. IMO, basically neglect.

2. Migration of water around the Rolock bolt threads because the 1/4" angle is (and has to be) over-drilled so the Rolock will thread into the box beam. Sealing around the head of the bolt (I mask off the part of the bolt where you would use a wrench) with undercoating, etc will seal that area to water intrusion.

3. Salt corrosion from up north of from parking on the beach without washing off the salt. Very rare on a coach from the south, unless they were beach-driven.

Brett

Re: bulkhead blues

Reply #38 – July 07, 2012, 05:58:11 pm

Quote from: Pierce & Gaylie Stewart – July 07, 2012, 02:49:43 pm

Today is a month since the pour so I drove out over the pit and did a quick inspection. Our U300 was always in a pretty dry area and when we bought it, the ODO was at 62,000 with never any winter or northern state trips. Underneath, there is a minimum amount of rust/corrosion, surprising since the front and rear subframes appear to have never had any kind of primer. Using a torque wrench, the first six bolts up front on the passenger's side and the first 5 in the rear, also on the passenger's side were broken, about a quarter inch from the head. The big angle iron is very clean. They have been broken for quite a while as none show any sign of a recent break. Rust jacking would seem to be the culprit with the angle iron forced out perhaps only a 32nd of an inch. They all line up exactly with the spray pattern off the tires when driving in the rain. Can't really figure out why only the passenger's side is affected. Ideas? Pierce

Pierce, It'd be really interesting to get a macro lens close-up of the sheared end of those broken bolts. Does anyone know if these bolts were plated or what exterior coating treatment was applied?

David

Re: bulkhead blues

Reply #39 – July 07, 2012, 08:59:24 pm

Quote from: travelite – July 07, 2012, 05:58:11 pm

Pierce, It'd be really interesting to get a macro lens close-up of the sheared end of those broken bolts. Does anyone know if these bolts were plated or what exterior coating treatment was applied?

David

David,

Here is an example. They pretty much look all the same with the fracture at about a quarter inch.

Pierce

Re: bulkhead blues

Reply #40 – July 07, 2012, 09:07:39 pm

David,

I played with the focus, definition and sharpness on this one.

Pierce

Re: bulkhead blues

Reply #41 – July 07, 2012, 09:21:02 pm

The reason Pearce that they are broken 1/4" from head is because that part is not into anything(holding)that is the angle iron thickness, so I would guess then that the bolts were holding fine in the tube, but broke because of the rust jacking. It tells me that the tube strength is fine, what do you think??
john

Re: bulkhead blues

Reply #42 – July 07, 2012, 09:36:17 pm

I know I'm in the minority here but really question the idea that these bolts are pulled apart due to pressures from rust jacking. I can see the angle between them swelling and being bent from rust jacking effect, can accept that the bolt can be weakened enough by corrosion and subsequent electroyitic action to break, but suspect an applied force is the culprit.

You've got a high tensile strength bolt with minimal threads in thin mild steel angle and tubing. I would think those threads would fail before the bolt would fail due to jacking tension. In looking at failed joints, is the angle iron straight between bolts or wavy?

Chuck

Re: bulkhead blues

Reply #43 – July 07, 2012, 09:43:42 pm

The Rolock bolts break exactly where they torque into the "near side" of the box beam.

And rust jacking will break bolts on 18 wheelers with HUGE frame rails. Do a little research on rust jacking.

Brett

Re: bulkhead blues

Reply #44 – July 08, 2012, 12:37:23 am

Quote from: wolfe10 – July 07, 2012, 09:43:42 pm

The Rolock bolts break exactly where they torque into the "near side" of the box beam.

And rust jacking will break bolts on 18 wheelers with HUGE frame rails. Do a little research on rust jacking.

Brett

No need. Worked on rotating machinery pumping millions of gallons of saltwater a day for many years . The break location is at the most likely point of failure, obviously, as the angle is clearance drilled. These bolts corroded due to moisture intrusion around the head, intrusion exasperated by the locking serrations. Exactly what caused them to fail in their weakened condition is the issue.

Research torsional shear of radially loaded fasteners.

Chuck

Re: bulkhead blues

Reply #45 – July 08, 2012, 01:15:48 am

Quote from: Chuck Pearson – July 07, 2012, 09:36:17 pm

...................I know I'm in the minority here but really question the idea that these bolts are pulled apart due to pressures from rust jacking. ......................................

Chuck,
I've always considered our bulkhead issue to be one of galvanic corrosion and tensile stress fracture/failure of the fasteners due to the forces imposed by rust jacking. Pierce's pictures of a failed fastener look just like many samples that I've seen and examined closely and they don't show evidence of radial or shear loading failures..............only tensile and galvanic corrosion failures.

"Rust-jacking" can occur with any joined-components that are made up of iron or steel. The only requirements are iron (Fe), oxygen (O₂) and a physical connection (bolt, spot weld, etc).

The "jacking" is the result of the chemical change that takes place when iron oxidizes (corrodes or rusts). As the iron rusts, it changes from iron to iron oxide and the resulting iron oxide occupies more volume than the original iron. It will exert tremendous force in doing so, if it is confined and the oxidation continues.

Depending upon a number of factors, the "swelling" of the iron oxide, can surpass the tensile, yield and fracture strengths of the physical connection (bolt or spot weld, etc.). Dependencies include the presence and concentration of things that aid in the oxidation process (presence of water or moisture, chlorides, heat, affected surface areas vs. bolting cross sectional area, affected surface preservatives, strength of the bolting, corrosion resistance of the bolting, galvanic cell values, etc.).

The nature of this volumetric expansion of the iron oxide is more than strong enough to force iron and/or steel joined-components apart and cause stress fractures of bolts in bridgework, skyscraper foundations, utility structures (and the frames of over the road vehicles) to the tune of multimillion's of dollars of annual damage nationwide. It is of particular significance to the trucking, utility transmission and distribution and salt water marine industries.

The accelerants that influence us FT owners the most, with our bulkhead joint design, are:
1. moisture/water
2. hygroscopic ("wicks" in water) liquid calcium chloride ions (from the ice preventative liquid highway treatments, rock salt residue to a lesser extent and salt air environments to a lesser extent yet.)
3. corrosion resistance of the fasteners that we use.

Thus, if we:
1. neutralize any corrosion (if present)
2. replace with corrosion resistant bolting (that is as galvanically neutral as possible in the joint)
3. exclude water from reaching the joint and fastener in the future

We will be on the clear success path.

Pierce,
Many, if not most, of us with bulkhead bolting failures have seen more failures in the vicinity of the FW tank overflow (or other long duration wet bay leaks). Where is your overflow directed? If it is on the right side, don't forget that even though you may not overfill the tank, if it is anywhere near full, it will wet that side down each time you turn in that direction, until the volume is reduced sufficiently to prevent re occurrence.
FWIW,
Neal

Re: bulkhead blues

Reply #46 – July 08, 2012, 03:49:46 am

i agree with Neal. That is why I use SS bolts and now only fill the tank half way.

Re: bulkhead blues

Reply #47 – July 08, 2012, 08:30:43 am

Quote from: Pierce & Gaylie Stewart – July 07, 2012, 09:07:39 pm

David,

I played with the focus, definition and sharpness on this one.

Pierce

Thanks Pierce for the pics, and thank you Neal for the explanation.

Pierce, have you removed any bolts that aren't broken? If so, it'd be interesting to take a close-up look at these.

Felix, what grade stainless?

David

Re: bulkhead blues

Reply #48 – July 08, 2012, 09:44:15 am

Nice explanation Neal, appreciate the detailed description. I'm not disagreeing with the concept of rust jacking, it was a common problem with saltwater pumps and mounts though the usual effect of it was seen in coupling misalignment over time. I've seen heavy flanges that were distorted between fasteners from rust pressure but can't say I've noted it pop bolts apart. It was pretty typical to see a splash zone centrifugal pump, motor and mount with so much active rust on it you really had to dig through it to even see the bolt heads. Many times these would break when attempting to loosen, and it was common to see a reduced diameter of the shank. It was common to see bolt heads so eroded by rust that flats were gone.

How many mild steel bolted connections do you have around the house and how many of them are falling apart due to rust jacking? I realize road salt spray is a factor, but living in Port Aransas where everything would be covered in a fine white coating of salt that would coalesce daily during the summer due to wind over a hypersaline water environment provided a pretty corrosive environment. My truck endured eight years of this, parked outside, and is still intact though more rusty than I would like on the underside. No sign of rust jacking failure in bolted, riveted or welded connections. Where it was a very real problem, however was not bolted connections at all but rebar improperly embedded in concrete. Now there's a good example of rust jacking.

Recognizing that pretty much everyone than me is accepting that this is the culprit, I'll state my thoughts and then shut up. Here are the factors I see involved in failure:

High tensile bolt with threads rolled into mild steel,water intrusion around bolt head=galvanic potential.

High tensile bolt held rigidly along the majority of its length due to these same rolled threads where it passes through members other than the clearance drilled angle iron=stress concentration.

Washer head bolt with gripping serrations held tightly against angle iron=increased stress concentration.

30K lb motorhome going over irregular surfaces with huge torque forces applied on the above=puny bolt failure.

Maybe shear isn't the right term as I tend to think of it as a guillotine type action and this is more like flex of the (extremely flex intolerant) bolt shank between the gripping washer head and the rigidly help portion of the bolt, in the area of the clearance drilled shank. I think this is what breaks these bolts, weakened in this area by corrosion. I suspect that if the number of fasteners had been tripled initially we wouldn't be seeing these failures. Take a look at a riveted wing spar, or for that matter the bridge over the Corpus Christi ship channel and note the concentration of fasteners for a very general comparison.

Anyhow, that's my take on it and I'm sticking to it until convinced otherwise. I appreciate everyone's willingness to discuss this stuff and hope my unwillingness to accept the rust jacking blanket explanation didn't get anyone's panties in a knot. It's in my nature to be intrigued by discussions of this nature and I do not mind being proven wrong.

Chuck

Re: bulkhead blues

Reply #49 – July 08, 2012, 10:18:30 am

Chuck,

Thanks for the candidness. What troubles me is that if there's no consensus on the cause, then can be no consensus on the fix. The foretravel community ends up with as many fixes as there are opinions. If the community erred on the conservative side relative to the cause and put together a universal fix based on this, then everyone could implement one fix. This fix would specify among other things: fastener count, fastener material, fastener size, fastener grade, fastener type (thru hole, blind), fastener spacing, joint treatment (anti-rust coatings, undercoats), etc.

David

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