| Author |
Message |
Larry Baird (Airhog)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 2:30 am: | |
I am in the process of changing out the old 8V71 and 4 speed for a 8V71T (rebuilt) and 10 speed RTO(rebuilt). After we cleaned the engine area so it looks like new we found that almost all the supports are cracked on the frame (small tubes and braces under the area the engine sets) so we are grinding out all the welds and welding them up again with 309 stainless or would it be OK to use mild steel wire? Should I add any other supports or plates? I am also removing the oil air cleaners and adding a dry filter, cutting the floor out for the turbo and trying to find a place for the muffler. This all should give new life to my old MC-7. |
Jon W.
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 8:13 am: | |
I am not an expert, but I would use steel wire if the base metal is steel.
For what it is worth, I would suspect the SS wire, being harder than the base material will contribute to more cracking in the same spot. In another life I designed shipping skids of steel for one of the big diesel engine manufacturers. They had to go through vibration testing and impact testing because they were returnable skids and their life was to be measured in decades.
We found with certain vibration frequencies anything (and I mean anything) can be destroyed. We saw the engines being vibrated apart. Fortunately our skid design (which was relatively light sheetmetal) handled the vibration because it was strong enough to support the engine and withstand 12G impacts, but was compliant enough so the welds and joints did not fail.
Your engine mounts are subjected to the exact same conditions, and I would suggest you stay with steel wire, and if you use plates or doublers you extend them to an area that might not be subject to vibration damage.
As I said, I'm no expert so take this opinion with a grain of salt. |
Marc Bourget
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 10:09 am: | |
Everybody,
While some stress concentration may occur by using SS wire on mild steel, the recommendation I recall from my GTAW ("TIG" or Heliarc) welding class with SS to mild steel joining was to use SS wire. Explained by the fact that the SS, in the "melt zone" will precipitate out and "dilute" gradually into the mild steel.
Also, it's prudent to keep in mind that selective installation of gussets and doubler plates, unless intelligently implemented, may focus stresses in concentrated areas causing failure where it otherwise might never occur.
This might result in outright, complete failure rather than the widespread cracking mentioned.
Onward and Upward
Marc Bourget |
Richard Bowyer (Drivingmisslazy)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 11:04 am: | |
Marc, please clarify for us dummies. Are you talking about welding SS to mild steel using SS wire, or welding mild steel to mild steel using SS wire?
Richard |
Larry Baird (Airhog)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 11:11 am: | |
This is an area that has stainless and mild steel and in the past I have used steel wire but in this case I think stainless wire would be better. |
Kyle Brandt (Kyle4501)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 2:13 pm: | |
Here at work, we weld ss sheetmetal to mild steel all the time. We make large washers & steamers for what is left of the textile industry.
Our shop practice is to use ss309 filler wire. The only problems that we have had with weld cracking have been traced back to use of 'regular' filler wire instead of the ss309.
I too would be carefull of adding gussets. If you do a good job re-welding the cracked joints, you should be done unless you are pulling a toad or trailer. Greyhound put millions of miles on her to get it to crack. How many miles are you gonna put on her?
I'm guessing that since you knew about 309 wire that you know more than you're letting on :^)
Have fun with your nice MC-7,
kyle4501 |
Marc Bourget
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 5:09 pm: | |
Richard,
You, Sir, are not a "dummy". I guess I am for not making myself more clear.
Stainless to stainless or Stainless to mild steel use the SS wire. I don't know from memory (fer sure) if it's 309, that's in the little black book I keep with the welders.
I would be concerned using SS (309?) on mild to mild as it would present a stress concentration, (metalurgical, rather than conformational - but a stress concentration nonetheless).
BTW, just upgraded to a Miller Synchrowave GTAW with the "toys and whistles" from an older Airco that worked "just fine" Now have two GTAW's 1 MIG, and one oxy-acetylene rig - welder rich and "finished" conversion poor! LOL! |
R Jensen
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 9:24 pm: | |
So Larry
The condition you find your engine supports is common with all mci 7, 8 & 9's. The late 9's 83 & on have some extra reenforcement in these areas. Your cracks are likely at the ends of the digonal braces on the truss that supports the engine. The solution to this problems is to fab plates that will be welded to each side of the engine support and make it a solid beam. The plates can have large holes for access but I have seen many that are simply solid plates. The weld doesn't need to be continous. If you look you will see many MCI's with this modification and if you call MCI support they may remember this solution and give you the details you will need. Much easier done with the engine out but people have done it with the engine in. (didn't look very good)
R Jensen |
Larry Baird (Airhog)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 9:41 pm: | |
I ordered a roll of 309 today and will grind and weld them with that. As for the plates do you think it is necessary and if so how thick? 16 or 10 ga. or not necessary? I did have a welding business for years and am AWS certified for structural steel, but you can never have to much information and this is all new to me. Thank you all for the help.
Larry |
mike lutestanski (Mikelut)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2005 - 9:47 pm: | |
Hello:
I have a 72 7 and went through the whole rear area and replaced all the tubes with 1" square tubes approx .125 thick mitered and welded in place. I had to replace one end of the top of the V rail because the salt had gotten through a hole in the inner fender and destroyed it. Also replaced the bottom V below with new stainless. Have not seen any new cracking with the new motor tranny in 45K miles which is not much but will review this area in the next several monthes as we are planning to switch engines. Another fix fostered by MCI in the past was to provide a plate .125 inch thick which went from top to bottom and front to back (along the frame); stitch welded one plate on each side of the motor. I did not use that option but two of us spent 44 hours welding and fitting replacement square tubes to repair the whole rear end frame system. Good painting to finish the job helps to monitor any changes and is easy to inspect. We had the motor out for rebuild and were replacing the four speed with a 740. At the same time we removed the inner fenders and replaced the tag axle as that was shot. Thanks mike |
Marc Bourget
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2005 - 1:15 am: | |
If you want to replace or "re-engineer" the motor mount beams (whether they be solid or built up versions) you have to consider the physics or mechanical engineering properties of beams. If you don't you're just taking "shot in the dark@"
Onward and upward. |
ray jensen
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2005 - 1:46 am: | |
So Larry
My first bus was an MCI8. After removing the drive train and cleaning I got the same surprise you had. The severity of the cracks has alot to do with amount of use the coach has seen and so in my case it was light enough to just tig weld the cracks. Do I think reenforcement is necessary. Yes. Mike's solution is slick...,removing the round tube digonals and replacing it with square tube to square tube joints would be Atlas strong. The MCI solution is simpler but not as clean. It was for Bus lines who needed to get the bus fixed and back on the road, but it is effective. My present MCI 102c3 actually has thin plate on on each side of the engine support right from the factory. These plates have large dia holes so the plates are more like gussets than enclosures. I can't tell you what thickness or alloy the plate should be because I have never had to do it. Somewhere near you will be a charter line or Grayhound yard that can give you the whole skinny on this...,it was commonly done. Don't forget, you can call MCI and they they may still have the service bulletin which fully describs this.
Ray Jensen |
Jim Shepherd (Rv_safetyman)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2005 - 1:46 pm: | |
First of all, as an Eagle owner, it is interesting to see MCI folks talking about rust and cracking.
Marc, I don’t want to start some esoteric engineering discussion, but I can’t for the life of me see how reinforcing the tubes in the engine cradle area would cause excessive stresses. It has been a looooong time since I did any engineering calculations on structures, but I believe that changing the structure with gussets and some double tubing would not make any significant difference in the engine area, since is was not designed for flexure and the strains would be very small. If one changes the structure of, say an Eagle frame over the length of the frame, I can see where that could be an issue since that frame does flex a bit. OK, so much for engineering discussion.
On my Eagle, I double up the tubing anywhere there was corrosion, plus I doubled the tubes that were the main supports of the engine “rails” -- to both support the Series 60 engine and to give me more structure for the trailer hitch in case I ever choose to haul a trailer.
I am not a fan of welding cracks. The cracks are there because of both corrosion and fatigue. The cracks are fatigue failures (sometimes accelerated by corroded members) and the tube could be compromised in the area surrounding the crack. A good welder can probably produce a weld that will be much less prone to cracking in the future. Merely squiring some wire or rod at the crack is not going to do much good. A good TIG weld with properly cleaned parts might be OK. Remember that paint and oil contain carbon and that will be introduced into the weld if the parts are not properly prepared. Rust will introduce oxides in the welds and it is almost impossible to clean the rust out of the inside of the tube. Either carbon or oxygen will compromise the weld.
In my opinion, you should either weld a second tube onto the existing tube (doubling the tube) and/or reinforce the area with a gusset. That will give you a big welding area and the welding is not nearly as critical.
If MCI published a “fix” for this problem, it would be best to follow their methods. This would avoid any structure issues. If someone has found that document, I think a link should be posted. Ray Drummond sent me a PDF that contains a great deal of information on Prevost Le Mirage/XL buses. It briefly touches on where frame cracking can occur. It is not an official Prevost document, but the information is valuable and we should all be sharing anything we find relating to our buses and what can happen to them after many years of service.
Lastly, permit me get on my soap box again and say that discussions of these kinds of problems just reinforce in my mind why large trailers should not be pulled behind our buses without a significant consideration given to reinforcing the frame or using a dolly to reduce the vertical load on the engine cradle.
Jim Shepherd
Evergreen, CO
’85 Eagle 10
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Marc Bourget
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2005 - 5:24 pm: | |
Jim, I was talking about gussets causing stress concentrations.
It doesn't have to be esoteric.
I'll try a simple example - 10' pipe with a rope on the end. When a load is applied it sags too much so you stiffen it with 3 two foot sections of pipe sized to slip over the first and spaced out with 1' in between each doubler section and the ends.
Since the doubled areas have greater stiffness those areas won't flex the same as the open sections. The loads are concentrated at the junctions of the doubler tube with the main tube. Without the doubler tubes the bending is uniform and the tube's max or yield limit isn't reached and the tube remains un damaged.
With the spaced doublers, the loads are concentrated at the end of the doublers and the pipe will "fail" there first.
Moral of the story is to plan your gussets and doublers to avoid concentrations which might accelerate a failure.
Onward and Upward |
Jim Shepherd (Rv_safetyman)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, October 21, 2005 - 11:34 am: | |
Marc, as is usually the case, we are on the same page, although it may not seem like it.
Let me say it a different way and see if we agree.
On beams which are designed to flex/bend uniformly, reinforcing can cause stress raisers (reinforcements cause non-uniform bending). That would be the case on an Eagle chassis where the mid-section (between the front and rear axles) is designed to flex slightly. Incidentally that is the reason why installing slide-out framing structure can cause other issues.
In the case of engine cradles (at least in the case of Eagles), the structure is quite rigid with lots of triangulation. In my opinion, none of this structure is designed to flex. As a result, reasonable gusseting and “double tubing” should not be a problem.
Jim |
Marc Bourget
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, October 21, 2005 - 1:02 pm: | |
Agreed, but I haven't analyzed the cradles.
One point remains undeniably true, the "bean counters" won't allow structure to be built any stronger than necessary, giving full regard to the Corporation's liability defense counsel.
In such cases, any irresponsible gusset/doubling raises the potential for problems.
Doubling the chord as you referenced is the safest layperson approach as the strength and stiffness added is hopefully uniform.
I'd like your views on stitch vs continuous welds on the chords, |
Larry Baird (Airhog)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, October 21, 2005 - 3:06 pm: | |
OK let me sort this out so I can finish this job and get back to bussing. Welding with 309 is correct. Gussets, plates and glue are all ok because when that big stiff rigid 8V71 is bolted back in nothing will flex under it, in front maybe but look before you speak, no flex designed there either and if it does the current gussets are short and not designed to flex. The current cracks are due to fatigue from vibration not flexing and this is why MCI went with the plates as a repair. Did I get this all correct? |
Marc Bourget
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, October 21, 2005 - 4:44 pm: | |
Meaning to clarify,not "harp"
Vibration, with one end secured "is" flexing.
Even diamonds will flex - a little, nothing is perfectly rigid
Engine, through the engine mounts, contributes little to stiffness. But, being the major load, do contribute alot to flexing.
Plates are intended to reduce flexing to a point it's below the failure or yield point.
Re-weld, as suggested above with a good TIG and maybe add plates. I think most converters will park rather than subject themselves to the demands of a common carrier with a schedule to keep, bad weather plus potholes, etc. Stay away from too heavy trailers and probably will be ok! |