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Old 06-13-2020, 01:59 PM   #1
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Bolt/Weld 2 Frame?

Ive heard that when it comes to the bus frame,neither bolting nor welding is a good idea. is there a concensus about this issue?

Could someone briefly explain the logic behind these beliefs... unless they both merely "compromise the integrity (needlessly)"

If you needed to mount something heavy, just bolt/weld it all through/to the floor members?

My specefic issue is a 420 lb generator. I was hoping to avoid drilling up through the floor by mounting the shelf on the frame.

Thanks

Evan

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Old 06-13-2020, 02:44 PM   #2
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So many variables...

1) What is the frame construction? "box", I-beam, C - channel
2) WHERE are you wanting to drill a hole? On the side, top, or bottom?

Search "structural beams" or similar. Understand their shape and how that shape gives strength and then you'll see where drilling a hole makes (almost) no difference vs huge difference. Will you sleeve the hole? That could make drilling holes in the frame make the frame stronger -- amazing.

Welding will be ALL ABOUT the quality of the welding...
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Old 06-13-2020, 03:14 PM   #3
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Quote:
Originally Posted by stlthe View Post
Ive heard that when it comes to the bus frame,neither bolting nor welding is a good idea. is there a concensus about this issue?

Could someone briefly explain the logic behind these beliefs... unless they both merely "compromise the integrity (needlessly)"

If you needed to mount something heavy, just bolt/weld it all through/to the floor members?

My specefic issue is a 420 lb generator. I was hoping to avoid drilling up through the floor by mounting the shelf on the frame.

Thanks

Evan
Always bolt to the frame. Most of my buses have had a tag somewhere that warns never to weld to the frame.
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Old 06-13-2020, 03:50 PM   #4
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Quote:
Originally Posted by stlthe View Post
Ive heard that when it comes to the bus frame,neither bolting nor welding is a good idea. is there a concensus about this issue?

Could someone briefly explain the logic behind these beliefs... unless they both merely "compromise the integrity (needlessly)"

If you needed to mount something heavy, just bolt/weld it all through/to the floor members?

My specefic issue is a 420 lb generator. I was hoping to avoid drilling up through the floor by mounting the shelf on the frame.

Thanks

Evan
What are the dimensions of this generator? There are some space constraints as to where something can go underneath a bus.
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Old 06-13-2020, 04:32 PM   #5
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Gen dimensions are 34.5L x22.4W x 20.3H inches. It weighs about 420lbs

Between the wheels it will be exposed about 1" below skirt, which is a few inches above axle.

My frame is I-Beam and i hadnt thought too much about different positions to bolt to, i was hoping my (eventually) welder would help design it. I will read up on structural shapes to try and get some ideas.
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Old 06-13-2020, 04:44 PM   #6
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Quote:
Originally Posted by stlthe View Post
Gen dimensions are 34.5L x22.4W x 20.3H inches. It weighs about 420lbs

Between the wheels it will be exposed about 1" below skirt, which is a few inches above axle.

My frame is I-Beam and i hadnt thought too much about different positions to bolt to, i was hoping my (eventually) welder would help design it. I will read up on structural shapes to try and get some ideas.

With an I beam frame you can do this with now welding.


You sandwich the frame members with angle or unistrut and then draw those together on each end with 3/8 allthread.
Very easy peasy and cheap to do, can easily support that gen.
Check how much clearance from the top of the frame to the highest part of the floor corrigation, ie, in the web of the metal. 1 1/2" -2"?

I don't understand this, "Between the wheels it will be exposed about 1" below skirt," Are you talking from a side viewpoint or?



John



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Old 06-13-2020, 04:59 PM   #7
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thanks for tip on construction

sorry, by skirt yes i meant a side view, mounted under the left side of bus between the battery compartment and the rear wheel

i'd say the (i think c-channel) floor is 1.5 inches, maybe 2 at the furthest from the frame..bus is not here now for me to measure.
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Old 06-13-2020, 07:50 PM   #8
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Old 06-13-2020, 09:31 PM   #9
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Might want to check the shear load on any bolt-through mounting. Of course you have the static load of the generator and frame weight, but you'll have quite a bit more dynamic load from bouncing down the road.

And this too, you should drill and ream your bolt holes in the frame. This is to avoid stress risers in the drilled hole. I'm sure people just drill the hole and don't have a problem. After all, the duty-cycle on most our buses' is light.
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Old 06-14-2020, 12:15 PM   #10
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Do not weld to the frame, never,ever,anywhere! Don't even say the word "weld" where the frame can hear you. The reason is the high alloy that the frame is made out of. When it is liquified in the welding process, the molecular structure is modified and when it cools, high concentrations of brittle elements occur in the weld zone, essentially marking the place where it will brake. Nor would I recomend drilling the frame. And even if you
attached to the frame by clamping, (best practice) I would not hang 420 pounds off to one side without supporting the far end. Hanging from the floor is the best solution, and like the feller says-easy squeezy. I was going to install my generator in the same place, but my wife put a stopper on it, arguing it would be too noisy. So we just stow it in the back and haul it out and set it on the ground off a ways (it doesn't way 420lbs.). I am not sure you will find the concensus you're looking for.
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Old 06-14-2020, 02:53 PM   #11
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Quote:
Originally Posted by obrien creek farm View Post
Do not weld to the frame, never,ever,anywhere! Don't even say the word "weld" where the frame can hear you. The reason is the high alloy that the frame is made out of. When it is liquified in the welding process, the molecular structure is modified and when it cools, high concentrations of brittle elements occur in the weld zone, essentially marking the place where it will brake. Nor would I recomend drilling the frame. And even if you
attached to the frame by clamping, (best practice) I would not hang 420 pounds off to one side without supporting the far end. Hanging from the floor is the best solution, and like the feller says-easy squeezy. I was going to install my generator in the same place, but my wife put a stopper on it, arguing it would be too noisy. So we just stow it in the back and haul it out and set it on the ground off a ways (it doesn't way 420lbs.). I am not sure you will find the concensus you're looking for.
It's as if "Truck body upfitting" wasn't a thing...
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Old 06-14-2020, 07:10 PM   #12
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welding vs bolts.... and brackets..

Many truck frames these days are of "high strength" alloys.. another way of thinking is medium or high carbon steels. This type of steel can be crack prone after welding .. If it is not heat treated... In the world of airplanes, gas welding was preferred cause the gas welding itself was a kind of heat treatment -- annealing the weld zone.

As an over all precaution to prevent cracks, dont weld it. You will not often see welds on big rig frames because of the crack problems.... that being said..

I have seen more than one rig with a cracked frame that was welded and then the area of the cracks had plates bolted to each side of the frame making three layers..... this was done as a repair.

I would bolt it. the smooth shoulder of the bolt should go through the frame and the part carrying the load, not any of the threaded part of the bolt. size the bolts for shear load in single shear. For shear strength and toughness I use AN bolt, AN stands for Army Navy. You will pay more. but you can get the bolts in different length shoulders. Lots and lots of ways to mount this thing. I would make the frame carry most of the load an small drop down from the floor to carry the outer weight, Figure every thing has to carry 10 times the weight... so little over 4,000 lbs. A 10 g load like driving off over a 8" curb is not hard to do.

hope this helps more than it complicates

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Old 06-14-2020, 07:23 PM   #13
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magnakansas-ah, an A&P amongst us. I too am an A&P. Good to meet you, more or less.
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Old 06-20-2020, 05:31 PM   #14
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Bolt- most trucks have warnings against welding on the frame.
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Old 06-20-2020, 07:10 PM   #15
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Welding on frame

The warning on welding on a frame depends to some extent where on the frame, how much welding. Usually welding and drilling is to be avoided on the frame on the top and bottom flanges because that is where the load is carried. Holes are able to be drilled through the sides of the frame given adequate distance from top or bottom, proper sizing and spacing and proper load per hole size. Many frames come from the factory with a lot of extra unfilled holes so it is preferable to use one of those if possible. I have been told welding on a frame outside of a load bearing area is ok. This is an area not between the axles and well beyond the suspension mounting area or beyond the last cross member. This way if there is any problem it will not impact the integrity of the frame. If any welding is done it should only affect one side of the frame rail not box in both sides. The frame is meant to flex and the cross members are huck bolted in to allow flexing. If you weld the back together it can stop the flexing. I had a conversation with a guy who installs concrete mixers as an oem and he insisted he was allowed to weld to the frame anywhere if done properly. I don't know what that means exactly but apparently there are exceptions to the no welding rule.
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Old 06-20-2020, 08:38 PM   #16
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Lots of semi tractors have had the frames stretched to allow a tag axle to be added, normally with an insert inside the C frame.
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Old 06-20-2020, 10:00 PM   #17
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Frame stretch

Quote:
Originally Posted by shorthair View Post
Lots of semi tractors have had the frames stretched to allow a tag axle to be added, normally with an insert inside the C frame.
But you have to be sure it was done right. Years ago before I knew better I bought a semi truck from a welding shop. ( yes for the pedantic it was a whole truck but just using conventional wording).
They stretched the frame by cutting the truck behind the cab, inserting an extra four feet of frame cut at a bias, extended the driveshaft and wires etc, and welded it up with little round cutouts bent in half welded over the top and bottom if the webbing. Looked good to me right? I took it to my mechanic to have a wet kit put in and he about had a fit. No reinforcements. I learned they should have extended and fishplated the frame first out the back them slid the axle assembly where needed. That way only one joint needs to be made. And the frame doubler plate is bolted in not welded. Live and learn. No harm but the cost this time.
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Old 06-20-2020, 10:10 PM   #18
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I'm surprised your frame is an I beam. Most commercial vehicles use a c-channel frames.

I forget what the exact spec is (an inch from the flange sounds right), but you can drill holes through the web part of the frame and it won't cause structural issues. That's how they mount nearly everything on the frame. Tanks, axles, cross members, etc.

I mounted my generator off my c channel frame. I drilled 3/4" holes through the web and used 4 3/4 inch grade 8 bolts to hold it on there. No idea how heavy the generator is but I'd bet big money that it isn't going anywhere, and it's not going to damage the frame either.

You don't want to weld something to the frame for the reasons stated above. However, numerous commercial vehicles have their frames lengthened and that's done with a welding process. Typically it's a diamond pattern that is covered with reinforcement plates that are welded in so as to spread the stress over a bigger area. You don't want to weld it in a vertical manner because that will cause it to be the weakest.
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Old 06-20-2020, 10:14 PM   #19
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A friend had one of his done and it took about 6 months because the cut rate shop couldn't find the right size drill bit and wouldn't buy a new one because the owner just knew the bit was around somewhere.
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Old 06-20-2020, 10:29 PM   #20
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For some reason this web-site was down last week when this thread started --
I just checked the link and it's back up...
And that's why I still print info I really need...

Anyway -- it's a good guide to give some ideas...

https://www.gmupfitter.com/files/med...hss_041113.pdf
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