bulkhead blues

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

Re: bulkhead blues

Reply #75 – July 09, 2012, 08:46:10 am

Quote from: acousticart – 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

Don, I'm not following you. Maybe you can draw a picture of what the bolted Rolok joint looks like?

David

Re: bulkhead blues

Reply #76 – July 09, 2012, 01:50:44 pm

David,
I don't have the time just now, but maybe this picture will suffice. This is taken from the street side with the bulkhead on the right hand side of the picture. The first 18" or so of the transverse 1&1/2 X 1&1/2 X 1/8" is doubled up with 16 gauge tubing. Just past that, you can hopefully see the end of a broken Rolok sticking through the eighth inch transverse tube. So the 3" Rolok bolt passes through the 1/4" angle iron, some 16 gage (or thereabouts) sheet metal which is the actual bulkhead partition, then the near side of the square inch and a half eighth inch tube, and finally the opposite side of the transverse tube. For that first section which has the sistered 16 gage tubing, the 3" Rolok also passes through and threads into one wall of that tubing. Thus the nut thickness is two cross sections of the tube and because the threads are cut when the Rolok is installed, it is applying clamping force to both vertical walls of the square tubing at the same time, eliminating crushing tendencies. and possibly adding some binding component to secure the bolt better than would be the case with one thicker wall, but not really sure about that.
I hope that helps clarify things...
Don

Quote from: travelite – July 09, 2012, 08:46:10 am

Don, I'm not following you. Maybe you can draw a picture of what the bolted Rolok joint looks like?

David

Re: bulkhead blues

Reply #77 – July 09, 2012, 02:50:54 pm

Can't find my Bendix-Besly tap book right now, but if I recall, approximately 1-1/2 times the thread dia for length engagement, at 72% of thread form in the "nut" the bolt will break before the threads strip in the female part (nut). I can't remember whether it was for grade 8 bolts or not. I think aluminum length of engagement had to be 2+times the thread dia.

The above info is from Bendix-Besly Tap Mfg Co. technical book. Usually a pretty good source of information and something for everyone to consider when determining if they have enough holding power for an object that goes down the road subject to bouncing, heat, rust jacking, and all the other stresses put on our coaches.

Re: bulkhead blues

Reply #78 – July 09, 2012, 05:22:38 pm

Thanks Don, I see what you're talking about.

There's no bolt stretch at the center of the box section. All the preload clamping force is between the bolt head and the 'nut' at the box section front face, and it's mostly at the first two threads of the 'nut'. We need material there to help support the nut so it doesn't deform and strip. Because the nut deforms we live with a lower preload force. Having a threaded back face does give us redundancy - if the front face threads fail, then we can preload with the 'nut' at the back face. Now we have bolt stretch at the center of the box section, and we again have preload.

As a thought experiment you can take this to extremes. Say you have 8 thin walls of 1/32" that are spaced evenly in the 1 1/2" box section. Is this equivalent to one 1/4" nut? David

Re: bulkhead blues

Reply #79 – July 09, 2012, 06:04:55 pm

David,

With VERY rare exception, when I inspect bulkheads with a torque wrench, the Rolock will either torque up to 250 inch-lbs or be broken. Very few are loose and will torque up.

And as most have found they break where the Rolock threads into the "nearer" side of the box beam.

I have yet to break a Rolock by torquing them-- easy to determine, as 100% of the breaks were rusted, not fresh breaks.

Best advice, as I posted 3 years ago, is to put a torque wrench on the bulkhead bolts once a year. View it a routine maintenance (which it is) and it is a lot less intimidating that letting it go until it is a big problem.

Brett

Re: bulkhead blues

Reply #80 – July 09, 2012, 06:13:29 pm

Brett,

I second your advice. Everyone should have their Roloks periodically torqued. I'd add to the protocol by suggesting that folks first back off the Rolok and then torque it. This will help detect bolts that are rusted in place. A bolt could be rusted solid and it will pass the torque test, but it may have long ago given up it's preload. By backing it off first, we can then torque it to the proper spec and rest assured that the preload is restored.

Also, I wouldn't use Loctite. Loctite can mask preload issues. I'd put anti-seize on the bolts. I'd want them in a condition where the can readily and routinely be checked. David

Re: bulkhead blues

Reply #81 – July 09, 2012, 06:31:41 pm

David,

Particularly after sealing around the shoulder of the Rolock (I used regular automotive undercoating) I would not back off the Rolock. Again, in the hundreds I have checked, they are either broken or will torque up. I would not break them loose just to test. Loosening first would open up the potential to break a perfectly good bolt with good clamping force if rust/corrosion between threads and the two sides of the box beam increased torque needed to break it loose to the point of bolt failure.

Brett

Re: bulkhead blues

Reply #82 – July 09, 2012, 06:42:11 pm

I defer to your knowledge and experience Brett.

David

Re: bulkhead blues

Reply #83 – July 09, 2012, 07:09:50 pm

I think if the 1/8" walls take the torque load one at a time . The joint only has the tensile strength of the threads in the first wall. When the first wall fails the screw must be re torqued to restore the clamping force. The rolled threads made by a 5/16" rolock and a .277 pilot drill have 100% thread contact and are much stronger then cut threads. After taking a torque load a screw with that much thread contact tends to seize up and can not be backed off,only tightened.The failed rolocks used in the bulkhead have the heads broken off not the threads stripped. The exception to this may be over torqued. 5/16" Rolocks should not be used with a wall thickness of less the .109". I agree that the way to go is torque check ,clean rust, seal joint, and replace failed Rolocks with through bolts and I wouldn't use any penetrating oil on the joint. Gam

Re: bulkhead blues

Reply #84 – July 09, 2012, 07:20:41 pm

Thanks Joseph, I agree, though I'd add that nut failure doesn't need to take the form of stripped threads. All that's required is that the nut deform such that preload is lost. If this happens then the lower bulkhead can experience joint separation under load, meaning that the external applied forces overwhelm the remaining bolt clamping preload. Once this occurs the joint has failed. Now all the load is taken by the bolts with none supported by the bolted joint. At this point failures can occur in many ways including what we've seen to date. Couple this phenomenon with stress corrosion and you may have what we're seeing today. David

Re: bulkhead blues

Reply #85 – July 09, 2012, 08:12:05 pm

Well I guess you can beat a dead horse. We each just have to deal with what we have .But I do think we should as a group standardize on one or two types and sizes of bolting as a repair or replacement for the rolocks. I think this would be best for everyone and for Foretravel and I do think that Wolfe is on the right track. Gam

Re: bulkhead blues

Reply #86 – July 09, 2012, 11:09:52 pm

Quote from: gam – July 09, 2012, 08:12:05 pm

Well I guess you can beat a dead horse.

I'll stop if someone Karma's me up a notch.

Quote from: gam – July 09, 2012, 08:12:05 pm

I do think we should as a group standardize on one or two types and sizes of bolting as a repair or replacement for the rolocks. I think this would be best for everyone and for Foretravel and I do think that Wolfe is on the right track.

You and me both. I think this would be a great outcome.

David

Re: bulkhead blues

Reply #87 – July 09, 2012, 11:13:09 pm

Gam,

Beating a dead horse is better than beating a dead coach by the side of the road. While our rigs have few flaws, the bulkhead problem along with rust damage to the floor tubing are major issues with different solutions to different areas of damage. Don's damage was extensive and required major work (that he did an outstanding job on) while others will only need to check on a regular basis with a possible replacement of a few bolts. The more we read about others problems, the better understanding we have as to the causes, what to watch for on our coaches as well as what to check when we survey a potential new acquisition. Very important to make this inspection a part of a regular maintenance schedule for new owners and members of the forum.

OK, out to check on my slightly limp horse and make sure a new shoe takes care of the problem.

Pierce

Re: bulkhead blues

Reply #88 – July 09, 2012, 11:38:32 pm

Pierce What you wrote is completely correct. But I was only talking about the Rolock repairs or replacement. Foretravel has a repair kit ,but no instructions with it. I just hate to think of a foretravel going down the road with slick headed bolts holding it together. Gam

Re: bulkhead blues

Reply #89 – July 09, 2012, 11:48:25 pm

Quote from: gam – July 09, 2012, 11:38:32 pm

Pierce What you wrote is completely correct. But I was only talking about the Rolock repairs or replacement. Foretravel has a repair kit ,but no instructions with it. I just hate to think of a foretravel going down the road with slick headed bolts holding it together. Gam

Gam,

Not a dig at all. I was just so surprised when my apparently almost rust free U300 had 5 and 6 bolts in a row break off on the one side. Just had to ramble a bit for any new members who might be fat and happy like I was BTW (before torque wrench).

Pierce

Re: bulkhead blues

Reply #90 – July 10, 2012, 12:18:59 am

It's fortunate that re securing the bulkheads is a pretty simple task provided it's caught before separation. In the grand scheme of motorhome ownership this is a trivial fix...provided it's caught early on. It's also fortunate that the procedure for checking is well documented.

There are a number of ideas on what causes these failures, the most accepted and popular of which I seriously doubt. I think what it comes down to is this. The connection is marginal, even without the environmental degradation that comes with use. The joint needs to be stronger. More fasteners. Travelite laid it out pretty well with his calculations.

More Rolocks and protection for them seems to be the best choice. They have some strong advantages over drilled and bolted connections. I see Rolocks as studs, bolts as compression fasteners. I suspect that's why they were chosen originally rather than for ease of production.

Chuck

Re: bulkhead blues

Reply #91 – July 10, 2012, 08:59:08 am

it appears we have gone full circle on this issue, and not understanding all the engineerspeak, the question remains. if one finds a few broken bolts, is it better to through bolt with nuts and a heavy washer or remove the old bolt and replace with larger rolok? I like gam's idea of coming to a general concensus of the group for the best fix. at first I liked the idea of the bolts and nuts, now i'm not so sure. back to square one.

Re: bulkhead blues

Reply #92 – July 10, 2012, 12:09:37 pm

If you sister with additional Roloks, how do you make sure that they're installed straight and true? If the bolt flange doesn't seat flush with the clamped joint then the bolt shank is both under tension and bending which means, guess what? More preload derating... For the shade tree mechanic, nuts and bolts are more foolproof - but even so, one needs to be careful of bending which is easier to avoid if you're only bolting the front two faces together, not thru bolted.

Re: bulkhead blues

Reply #93 – July 10, 2012, 01:18:15 pm

Quote from: travelite – July 10, 2012, 12:09:37 pm

If you sister with additional Roloks, how do you make sure that they're installed straight and true

Good point. Short of using a drilling jig or just being lucky/good, don't know. Of course if the flanges aren't tight when drilled they will change alignment a bit also. Maybe flanged nuts and bolts are the best way to go. At least you can understand what's happening when you tighten them up.

Re: bulkhead blues

Reply #94 – July 10, 2012, 02:30:32 pm

Something for those with torque wrenches. When I had a job{before SS } every year new or old I had to send out for calibration the 20 or 25 torque wrenches we used during our plant outages. The calibration of the wrench can be checked at home very easily. Check on line. Of the ones sent out 5 or more would be out by more then 10%.when not in use the scale on the snap wrench's should be turned back to zero to prevent the internal spring from taking a set. Gam

Re: bulkhead blues

Reply #95 – July 10, 2012, 03:45:06 pm

I was working on my coach today getting ready to repair my forward bulkhead. As described in a previous post I had major bulkhead failure in Louisanna. In stripped the Filon and cleared out the blue insulation getting ready to drill holes for the throughbolts. As I was looking at the assembly I stated wondering why during the initial build did they not weld the angle iron and the box beam together? Seems to me a welded joint would be a bit stronger, and require less specialized equipment than the rolok bolts? Wouldn't be any spray leaching up between the joint to cause rust jacking and snapping bolts. The only reason that I bolt things together is to be able to take them apart in the future. This does not look like something you would want to dis-assemble. Inquiring minds would like to know.

Roland

Re: bulkhead blues

Reply #96 – July 10, 2012, 06:43:53 pm

Quote from: Roland Begin – July 10, 2012, 03:45:06 pm

I was working on my coach today getting ready to repair my forward bulkhead. As described in a previous post I had major bulkhead failure in Louisanna. In stripped the Filon and cleared out the blue insulation getting ready to drill holes for the throughbolts. As I was looking at the assembly I stated wondering why during the initial build did they not weld the angle iron and the box beam together? Seems to me a welded joint would be a bit stronger, and require less specialized equipment than the rolok bolts? Wouldn't be any spray leaching up between the joint to cause rust jacking and snapping bolts. The only reason that I bolt things together is to be able to take them apart in the future. This does not look like something you would want to dis-assemble. Inquiring minds would like to know.

Roland

I'm no welding expert, but I think it's cause it's bad form to put a weld in bending: Don't Put Welds in Bending. To keep the weld bead (root, toe, and face) out of the compression and tension of bending will require a connection redesign.
David

Re: bulkhead blues

Reply #97 – July 10, 2012, 07:15:32 pm

To assure alignment of any new bolt holes two very simple fixtures can be fabricated by anyone with a lath and welder available to them. The first would be a plate about 4" in dia with a 1/2" dia shaft about 1 1/2" long welded to the center of it. A hole the dia of the new Roloks would be drilled through the center of the shaft and through the plate. the under side of the plate would be machined to make it flat and 90 to the drilled hole. This fixture can be used as a drill guide to drill the hole through the angle iron for the new screw. A collar stop on the drill will prevent over drilling in depth . A fixture for the pilot drill would be fabricated in the same way ,except it would have an embossment the dia of the first brill hole on its under side. this will center the pilot drill in the first hole. I will be picking up a torque wrench at one of the plants I worked at in the next few days . I hope I will have to do none of this . But if I do I'm thinking of replacing all the 5/16' roloks with 3/8" roloks. The install or drive torque is about the same at 130 inch pounds. So I'm thinking of a finish torque of about 270 inch pounds . Gam

Re: bulkhead blues

Reply #98 – July 10, 2012, 08:45:10 pm

It would be interesting to know how the factory does, or did the drilling. Suspect the angle was predrilled, the rest field drilled. Redundancy can make up for derating.

Re: bulkhead blues

Reply #99 – July 10, 2012, 09:42:40 pm

Quote from: gam – July 10, 2012, 07:15:32 pm

To assure alignment of any new bolt holes two very simple fixtures can be fabricated by anyone with a lath and welder available to them. The first would be a plate about 4" in dia with a 1/2" dia shaft about 1 1/2" long welded to the center of it. A hole the dia of the new Roloks would be drilled through the center of the shaft and through the plate. the under side of the plate would be machined to make it flat and 90 to the drilled hole. This fixture can be used as a drill guide to drill the hole through the angle iron for the new screw. A collar stop on the drill will prevent over drilling in depth . A fixture for the pilot drill would be fabricated in the same way ,except it would have an embossment the dia of the first brill hole on its under side. this will center the pilot drill in the first hole. I will be picking up a torque wrench at one of the plants I worked at in the next few days . I hope I will have to do none of this . But if I do I'm thinking of replacing all the 5/16' roloks with 3/8" roloks. The install or drive torque is about the same at 130 inch pounds. So I'm thinking of a finish torque of about 270 inch pounds . Gam

Joseph, I understand that if the 5/16" Rolok is broken it can be fixed by putting in a 3/8", assuming a clearance hole is drilled into the 1/4" angle iron. In either case, you're still limited by the 0.125" nut size. Your finish torque will need to be adjusted based on the nut. I think you'll find the clamping force will be essentially unchanged. I'm with Chuck, redundancy - multiple load paths. Chuck said earlier that if Foretravel had used 3x as many Roloks we wouldn't be seeing this problem. I like your plan - I'm not fond of mixing fastener sizes either. Make 'em all 5/16th's or make 'em all 3/8th's. David

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