bulkhead blues

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

Re: bulkhead blues

Reply #50 – July 08, 2012, 10:40:49 am

David,

We may be "overthinking" this.

I have found very few failed bolts/bulkheads where the coach shows sign of previous bulkhead repair or just broken bolt replacement/sistering-- and it is something I check carefully on each coach I inspect.

Not sure there is one best fix, as the amount of failure varies so widely, as do the causes.

Brett

Re: bulkhead blues

Reply #51 – July 08, 2012, 10:47:29 am

Quote from: travelite – 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

Don't forget process!
Peter

Re: bulkhead blues

Reply #52 – July 08, 2012, 11:25:22 am

Has anyone ever sistered the joint with more 5/16" rolok screws? gam

Re: bulkhead blues

Reply #53 – July 08, 2012, 12:09:04 pm

I agree with Don that the towing capacity is only limited to what the driver is stupid enough to hook up to. The rear substructure is massive and would do all the pulling. A couple of months ago, I saw a U300 pulling an equipment trailer larger than the coach. It had a large backhoe plus other construction equipment and was heading down the road at 55 plus.

No, will have to wait until next week to pull bolts. Will drill a hole, stick my borescope in and post some interior tubing photos.

Decided to do conventional bolt through modification with washer/nut combination so I could examine the far side of the rectangular tubing at each location.

No, don't have a tank leak problem. The largest number of broken bolts were on the passenger's side up in front of the fuel tank (at the front of the coach). That is why I suspect the water,chemical and road debris throw back is responsible for the rust jacking.

Here is an excellent source of fasteners with all sizes of grade 8, zinc, hot dipped galvanized and stainless available with no minimum order: Bolt Depot - Nuts and Bolts, Screws and Fasteners online

Pierce

Re: bulkhead blues

Reply #54 – July 08, 2012, 12:09:47 pm

Quote from: gam – July 08, 2012, 11:25:22 am

Has anyone ever sistered the joint with more 5/16" rolok screws? gam

No, but have done and have seen others that have sistered with thru bolts. I understand others have done it, though.

I prefer the thru-bolts with large steel backing "washers" on the box beam, as it gives better holding on the box beam. Instead of threads gripping in each of the 1/8" box beam walls, the load is spread over a larger area side to side and to the horizontal section of the box beam by the steel backing "washers". Would be particularly important if there has been any degradation of the beam due to water intrusion.

Brett

Re: bulkhead blues

Reply #55 – July 08, 2012, 12:22:13 pm

Why not take a scientific approach, I'm sure there are engineers on the forum that would find this a piece of cake. I am not one of the lettered ones but this is the approach I would take.

Compressive strength of mild steel X Area of bolt head.

Tensile strength of bolt.

If the compressive strength exceeds the tensile strength then rust jacking can break the bolt.

Forces involved in torsion are more complex to figure but suspect that taking the area of the belly between bulkheads, weight at each corner, length of bulkheads would give a starting point. If the twisting force exceeds the shear or bending strength of one furthest outboard bolt, then the stage is set for torsional failure of successive bolts.

The truth is out there and I personally doubt there is any one force that causes failure, rather a compounding of issues and events. I haven't checked the bolts in the bulkheads of my coach, it's quite pristine underside having spent most of it's time in Tucson. Except for four months in Port A during which time I saw the rust/corrosion level double what it had acquired during its prior 14 years in Tucson. And then there was the time I dropped a front wheel off a steep shoulder at speed, producing enough body twist to unseat a windshield. Then there was the time before I owned it when it blew a tire and went offroad in Mexico, wiping out much of the front end. So, thinking about it, this is a pretty good test bed for the failure theories. This coach has taken some hard hits. There is almost zero rust underside. If there are no failed boats then I'll eat my words and accept that rust jacking is the primary culprit.

If I find a few broken bolts I will install more Rolocks then attempt to seal them and the seam to prevent moisture intrusion from below. If I find a lot of broken ones, assuming I can see through the tears, I'll affect a bolted fix with backing washers as suggested and attempt to inject an adhesive/sealant into the seam. I suspect I will own this coach for the duration so whatever it takes. Just last week I subtly hinted to my lovely wife that we might consider a newer Foretravel with slide when we are able to continue our travels. Her answer was quick and succinct. "Why? No. I like this one, a bigger one would just be more effort to clean."

Chuck

Re: bulkhead blues

Reply #56 – July 08, 2012, 12:27:53 pm

Check out Lowes Part number ZAB-241-3/8 Superstrut these are 1/4" thick steel plates 1&1/2" square goldguard 3/8" bolt hole plates by Thomas&Betts located in the electrical dept. bag of 5, this is not junk stuff. UPC code 75114 01390. Made in the USA. May want to put caulk between plate and tubing. thats it jc

Re: bulkhead blues

Reply #57 – July 08, 2012, 12:40:14 pm

Quote

No, but have done and have seen others that have sistered with thru bolts. I understand others have done it, though.

I prefer the thru-bolts with large steel backing "washers" on the box beam, as it gives better holding on the box beam. Instead of threads gripping in each of the 1/8" box beam walls, the load is spread over a larger area side to side and to the horizontal section of the box beam by the steel backing "washers". Would be particularly important if there has been any degradation of the beam due to water intrusion.

Brett

Brett, A question about ROLOKS, Do I understand correctly that these are THREADED only thru the box beam ? If so this is very few threads due to thickness of boxbeam.
Gary B

Re: bulkhead blues

Reply #58 – July 08, 2012, 01:58:06 pm

This part is potentially usable, but it is 1 5/8" square and so you would have to grind a (at least) 16th" off of two opposing sides in order to leave the hole centered to fit the inch and half square tubing. Probably easier to buy 1/4" by 1&1/2" flat stock and make your own, especially if you have a drill press. That way you can spread the force laterally as well...
Don

Quote from: hotonthetrail – July 08, 2012, 12:27:53 pm

Check out Lowes Part number ZAB-241-3/8 Superstrut these are 1/4" thick steel plates 1&1/2" square goldguard 3/8" bolt hole plates by Thomas&Betts located in the electrical dept. bag of 5, this is not junk stuff. UPC code 75114 01390. Made in the USA. May want to put caulk between plate and tubing. thats it jc

Re: bulkhead blues

Reply #59 – July 08, 2012, 02:05:07 pm

Quote from: Gary Bouland – July 08, 2012, 12:40:14 pm

Brett, A question about ROLOKS, Do I understand correctly that these are THREADED only thru the box beam ? If so this is very few threads due to thickness of box beam.
Gary B

Gary,

You are correct-- they are threaded through both 1/8" walls ONLY. Obivously, the 1/4" is over-drilled so that the Rolocks will torque into the box beam.

That is why I like the thru bolts with large flat metal "washer". I had mine made by a machine shop from 1/4" steel very inexpensively. They cut it into strips, I used my drill press to make the hole and then I cut the "washers" to length with a hack saw.

Pierce-- yes, please post after you have put a torque wrench on your bulkhead bolts. Thanks.

Brett

Re: bulkhead blues

Reply #60 – July 08, 2012, 02:19:14 pm

I find it int erasting that there are 3 roloks close together on each end of the bulkhead. Did Foretravel expect a twisting action of the bulkhead when breaking? Like a chain a number of small fasteners holding something together is only as strong as the one taking the load. As for holding power in thin wall tubing ,I have never seen a rolock pull out only break.I have seen rust jacking in masonry and concrete construction and in some structural fabrication of heavy steel. But not to bolt failure in light weight fabrications like the tubing angel iron of the bulkhead. I thing you will find very little rust in the area of metal to metal contact under the screw head and between the parts in that area. Most of the joint opening will be between the screw holes and put little load on the roloks. But will this allow more twisting of the bulkhead and more load on one or two of the screws when breaking ? This is just me thinking out loud! As for a fix , grade 8 through bolts and rectangular washers' . But I just don"t like using a holesaw on my baby. Gam

Re: bulkhead blues

Reply #61 – July 08, 2012, 03:00:04 pm

Is there a reason why the treads could not be on the outside, easier to torque , protect and inspect etc.? jc

Re: bulkhead blues

Reply #62 – July 08, 2012, 03:34:03 pm

Quote from: hotonthetrail – July 08, 2012, 03:00:04 pm

Is there a reason why the treads could not be on the outside, easier to torque , protect and inspect etc.? jc

There is no access to the "back" (center of the coach) to install a bolt. You could do that as a repair, but have a minor preference for not having exposed threads, washer and nut outside in the elements. Would still need the large rectangular washer to spread the load on the box beam.

Brett

Re: bulkhead blues

Reply #63 – July 08, 2012, 04:12:42 pm

Is there a chance that the Roloxs might have been over torqued when installed? Might this help account for the number of broken bolts found on coaches without much rust jacking? Or the missing of just an occasional bolt head or two?

Re: bulkhead blues

Reply #64 – July 08, 2012, 04:40:23 pm

Dave,

Rare, but yes. I had one that I suspect was broken during installation. I am saying that because yellow 3M weather strip glue was used to hold the head of one of the Rolocks in place! That is the only one I have found like that on any coach that I have checked with a torque wrench. But it CAN happen.

Re: bulkhead blues

Reply #65 – July 08, 2012, 06:36:04 pm

David, I used 304 ss which is what my hardware store carried. I had used a adhesive caulk to seal the seam, 3M5200 but it is not holding up and am in the process of scraping it off. It came off fairly easy after using brake part cleaner and wire brush. I ordered a starter kit of POR 15 to seal the area. It seams to have the charecteristics to inhibit rust and form a good seal, although I have never used it before.

Re: bulkhead blues

Reply #66 – July 08, 2012, 08:43:11 pm

3/8" 304 stainless steel bolts with 304 nuts have a torque rating of only 216 in=lbs or just under 18 ft-lbs. Gam

Re: bulkhead blues

Reply #67 – July 08, 2012, 10:25:47 pm

My (& Tys's) 400th post! You guys with several thousand... Don't laugh! Oh, go ahead and laugh

Now to the point... The ARP 300 series 3/8" fasteners that I am using have a lubed torque spec of 45ft lbs... I have no intention at this point of torquing them that tight, rather I plan to use loctite to lube and secure the bolts at around 30ft lbs. My experiments show that 1.5" Square X 1/8" thick walled tubing crushes at about 33ft lbs. when using ARPS ultra torque lube. I could post a YouTube video of that if anyone cares to see it... Now my particular installation has a sistered 1/8" square tubing along the bulkhead, backed in most places with welded 3/16ths angle iron and in a few spots with 1/8" angle iron. The angle iron also serves to give me a way to mount the aluminum plates that I am using in place of the original plywood which destroyed the integrity of most of the frame in contact with it because was so saturated with moisture from freshwater leaks. Weld nuts are welded to the angle iron, so I will be able to remove the bolts if it is ever necessary for some reason. Because I was able to reuse all but one of the original holes (because one of them went into a longitudinal part of the frame, that one I drilled a fresh hole for), there are 17 of these fasteners across. The outer ones are 3" long because they only go through the 1/4" bulkhead angle iron and one cross section of the the transverse frame member, the other 15 fasteners which have to go through 3" worth of square tubing, a 1/4" angle iron and a 3/16" angle iron are 4" long. I was able to use even the outer most holes because I replaced the outer longitudinal frame members on both sides, each of which I welded into place after welding a nut on the transverse tube that is inside the outer longitudinal frame members. As you might imagine, I never want to have to do this again! These ARP fasteners are expensive at about $5 apiece, but compared to the totality of the project, it doesn't amount to much. Here is picture of the fasteners... pity no one will ever see them as they are pretty things.

Quote from: gam – July 08, 2012, 08:43:11 pm

3/8" 304 stainless steel bolts with 304 nuts have a torque rating of only 216 in=lbs or just under 18 ft-lbs. Gam

Re: bulkhead blues

Reply #68 – July 08, 2012, 10:43:06 pm

Art,

First class work, I suspect you will have the highest structural integrity possible as a result. Nice selection of materials, innovative modifications. Hats off to you and Tys.

Chuck

Re: bulkhead blues

Reply #69 – July 08, 2012, 11:34:47 pm

Just curious...how much does FOT charge to fix this very poor design flaw?

Re: bulkhead blues

Reply #70 – July 08, 2012, 11:38:30 pm

Thanks Chuck! Structural integrity is what I am most concerned about, but the fluff (such as electronics upgrades and the like) are definitely more fun... can't wait to get to that part

Don

Quote from: Chuck Pearson – July 08, 2012, 10:43:06 pm

Art,

First class work, I suspect you will have the highest structural integrity possible as a result. Nice selection of materials, innovative modifications. Hats off to you and Tys.

Chuck

Re: bulkhead blues

Reply #71 – July 09, 2012, 12:14:38 am

As to cost, Foretravel repaired my rear bulkhead. Wayne removed all the old bolts, or cut them, treated the area, installed two thru bolts to hold the floors up while working on the open areas between the angle and the box steel. He then installed 16 Hucks, resealed, and added a cover strip and sealed that along with the two plugs to cover the cut outs in the fiberglass belly pan. Total cost was around $900 dollars.

Seeing him doing the work, over the course of two days, with all the correct equipment and experience, after reading, gathering some of the parts, and discussions with others that had done the repair, I think it a bargain. Plus several "free" night at Camp Foretravel were included as well.

Re: bulkhead blues

Reply #72 – July 09, 2012, 01:23:51 am

What we don't want is joint separation at the bulkhead angle iron to basement floor interface as we're driving down a bumpy road. There should be enough Rolok clamping force preload to prevent this. Fasteners aren't my field but I managed to dust off some formulas.

The Rolok spec says that the 5/16th inch part should be torqued to 250 in-lbs. The clamping force per bolt would be F(bolt)=T/(K*d) where F(bolt) is the clamping force, T is the tightening torque applied (250), d is the nominal bolt diameter (0.3125), and K is a correction factor that depends on the material, size, friction, and thread of the bolt. For small diameter zinc plated bolts K would be around (0.2).

So the clamping force per bolt would be: 250/(0.3125*0.2)=4000 lbs. If we assume there are 17 Rolok fasteners then the total clamping force would be 68000 lbs. Now we need to correct for the 1/8" wall thickness of the basement box tube member, or what I call the 'nut'. This nut thickness allows for only 2.25 threads of engagement instead of the optimal 5.6 threads of engagement.

Dusting off more formulae. The clamping force of the nut F(nut)=t*A*C1*C2, where t is the nut material shear strength (20000 psi, or 60% of proof), A is the nut shear area, C1 is the nut dilation factor (1.0 for this case), and C2 is the nut thread bending factor (0.897).

The nut shear area A=((pi*d*m)/P)*(P/2+(d-D)/sqrt(3)), where d is the thread major diameter (0.3125), P is the thread pitch (0.0556), m is the thread engagement length (0.125), and D is the nut pitch diameter (0.2817). The nut shear area, A, is then 0.1006 and the clamping force provided by each nut, F(nut), is 1800 lb. If there are 17 Roloks then our clamping force is reduced from 68000 lbs, calculated based on the bolts, to (1800*17) = 30600 lb when we consider the nut.

Then we need to derate for installation error. If the installer over or under torqued a bolt then the other bolts need to carry a higher load. Then there's the issue of rust developing inside the box tubing over time degrading the rolled threads. Then there's the issue of bolt alignment - is the Rolok reasonably true and perpendicular to the clamped surface. Then there's the issue with the Rolok's being of a much harder material than the nut; i.e., there's no hardened washer bearing surface between the Rolok and the 1/4" transverse angle member. Cyclic loads and micro-vibrations may allow the Rolok to wear into the transverse member in which case one or more Roloks will lose their preload.

Then we need to consider the load. Some number crunching I did some time ago shows that static load on the lower bulkhead may be as high as 15000 lb. I believe typically you'd design for a safety factor of 3 to handle dynamic loads which would put us at a desired design strength of 45000 lbs. An example of dynamic loading is braking. Say you're braking at 1/2g. The drive axle can carry as high as 20000 lbs of load. One half g of braking exerts a force of 10000 lbs at the tire/road contact area of the drive axle tires which translates into as high as a 15000 lb force at the lower control arms. This puts us well into our F(nut) strength territory of 30600 lb.

The picture I'm painting is that the Roloks may suffer joint separation, that is, the external loads may overwhelm the Rolok preload. If the joint separates, then all of the load falls onto the bolts - the joint no longer supports any load whether torsion, bending, tensile, or shear.

It's late. I'll check my numbers in the morning. David

Re: bulkhead blues

Reply #73 – July 09, 2012, 01:47:18 am

I don't See where you accounted for the fact that the Roloks all go through at least two 1/8" thick walls of the tubing, which I think adds an interesting wrinkle to the calculations, especially when you consider that five or so street side Roloks go through an additional 1/16" wall of tubing where it is doubled up. Also the fact that the Roloks are engaging both front and back walls of the eighth inch tubing means that there is no crushing force exerted on the tube. About five or six of mine were broken off inside the transverse tube (well past the usual 1/4" mark and still had a grip on the near side. None of the bolts had a "nut" failure except one, which in which I believe was stripped on installation. It was the only completely intact fastener!
Don

Quote from: travelite – July 09, 2012, 01:23:51 am

What we don't want is joint separation at the bulkhead angle iron to basement floor interface as we're driving down a bumpy road. There should be enough Rolok clamping force preload to prevent this. Fasteners aren't my field but I managed to dust off some formulas.

The Rolok spec says that the 5/16th inch part should be torqued to 250 in-lbs. The clamping force per bolt would be F=T/(K*d) where F is the clamping force, T is the tightening torque applied (250), d is the nominal bolt diameter (0.3125), and K is a correction factor that depends on the material, size, friction, and thread of the bolt. For small diameter zinc plated bolts K would be around (0.2).

So the clamping force per bolt would be: 250/(0.3125*0.2)= 4000 lbs. If we assume there are 17 Rolok fasteners then the total clamping force would be 68000 lbs. Now we need to correct for the 1/8" wall thickness of the basement box tube member, or what I call the 'nut'. This nut thickness allows for only 2.25 threads of engagement instead of the optimal 5.6 threads of engagement.

Dusting off more formulae. The clamping force of the nut F(nut)=t*A*C1*C3, where t is the nut material shear strength (20000 psi, or 60% of proof), A is the nut shear area, C1 is the nut dilation factor (1.0 for this case), and C3 is the nut thread bending factor (0.897).

The nut shear area A=((pi*d*m)/P)*(P/2+(d-D)/sqrt(3)), where d is the thread major diameter (0.3125), P is the thread pitch (0.0556), m is the thread engagement length (0.125), and D is the nut pitch diameter (0.2817). The nut shear area, A, is then 0.1006 and the clamping force provided by each nut, F(nut), is 1800 lb. If there are 17 Roloks then our clamping force is reduced from 68000 lbs, calculated based on the bolts, to (1800*17) = 30600 lb when we consider the nut.

Then we need to derate for installation error. If the installer under-torqued a bolt then the other bolts need to carry a higher load. Then there's the issue with the Rolok's being of a much harder material than the nut; i.e., there's no hardened washer bearing surface between the Rolok and the 1/4" transverse angle member. Cyclic loads and micro-vibrations may allow the Rolok to wear into the transverse member in which case one or more Roloks will lose their preload.

Then we need to consider the load. Some number crunching I did some time ago shows that static load on the lower bulkhead may be as high as 15000 lb. I believe typically you'd design for a safety factor of 3 to handle dynamic loads which would put us at a desired design strength of 45000 lbs. An example of dynamic loading is braking. Say you're braking at 1/2g. The drive axle can carry as high as 20000 lbs of load. One half g of braking exerts a load of 10000 lbs at the tire/road contact area of the drive axle tires which translates into as high as a 15000 lb force at the lower control arms. This puts us well into our F(nut) strength territory of 30600 lb.

The picture I'm painting is that the Roloks may suffer joint separation, that is, the external loads may overwhelm the Rolok preload. If the joint separates, then all of the load falls onto the bolts - the joint no long supports any load whether torsion, bending, tensile, or shear.

It's late. I'll check my numbers in the morning. David

Re: bulkhead blues

Reply #74 – July 09, 2012, 02:02:50 am

I think I read that some are considering using steel backing plates that are pre-drilled, but that really doesn't work.
Drilling holes in angle iron and tubing for bolts & nuts ends up not being perfectly centered in the tubing.

We found that after drilling we placed the backing plate against the tube and 'marked' the backing plate with the drill bit.
Then the backing plates were removed and then the bolt hole is drilled where the mark is.

Painting the backing plates white will help see the drill bit mark.

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