Repairing cracks:

Discussion in '<a href=http://www.lawnsite.com/buttons/jump.php?i' started by 75, Mar 14, 2002.

  1. 75

    75 LawnSite Senior Member
    Posts: 992

    For the next couple of weeks anyway, I know what I'm going to be up to - repairing cracks on the dump box of a rock truck which is down for a major overhaul right now.

    Took a few photos today to illustrate the process of repairing cracks, although the truck is a bit bigger than what you would probably find in most workshops the basics are the same. Here's one of the cracks before anything is done, the two black marker lines show where the crack starts and stops:

    t-rex dump box repair 1.jpg
     
  2. 75

    75 LawnSite Senior Member
    Posts: 992

    That's important because the next step is to gouge ("vee") the crack out, making sure the gouging extends beyond the ends of the crack. The reason for gouging in the first place is to get the weld to penetrate the full thickness of the metal being repaired: a quick pass over top of the crack won't last. Unless the crack "disappears" completely, meaning none of it is left inside the repair, it will "re-appear" soon.

    NOTE: at the bottom right of the pic is a pin through the dump box hinge. There's one each side, they are safety pins to keep the box up if the hydraulic hoist cylinders fail. Before crawling into or under anything like this that could move/lower or otherwise kill you, make sure it is secured/blocked/pinned etc as required - never trust the hydraulics alone.

    I'm using arc-air (air/carbon-arc) for gouging here since there is a lot to do and most of it in tight places, but a grinder will work fine for most applications. Arc-air requires a fairly large machine - the one on the work truck puts out 400 amps peak - and a good supply of compressed air so it isn't really feasible for a home workshop. Too bad, otherwise I'd have it.............:D

    t-rex dump box repair 2.jpg
     
  3. Bill c

    Bill c LawnSite Member
    Posts: 114

    Hey 75 I also favor the arc air for "heavy metal" removal and it saves alot of time.I have a 400 amp miller on the work truck and couldn't imagine having anything smaller and not being able to run my 3/8 gouging rods.
     
  4. 75

    75 LawnSite Senior Member
    Posts: 992

    Miller "Big40"? That's what I have on "my" welding truck, arc-air is definitely the way to go. In addition to being an efficient way to gouge, it also keeps annoying supervisors from getting too close..................... :D

    Once the gouging is done and any slag cleaned up, time to weld. First pass, known as the "root" pass, goes right down at the base of the V and additional passes are added as required. Wasn't counting, but there's at least a dozen here. This was one of the easy welds to do, most of them are either vertical or overhead and a lot of 'em are full of dirt.
     
  5. 75

    75 LawnSite Senior Member
    Posts: 992

    Had a bit of "attitude" from this photo, but the problem seems to be solved now. This is the completed weld from the previous photos:

    t-rex dump box repair 3, completed weld.jpg
     
  6. 75

    75 LawnSite Senior Member
    Posts: 992

    Here's a shot of the truck itself, the area shown in the photos is at the end of the frame, between the hoist cylinders and above the rear axle. Because the truck has always been loaded from the driver side, the passenger side of the dump box is in a lot worse shape - most of the cracks to be repaired on on that side.

    At least I don't have to worry about anyone starting the engine on me.......................... :p

    t-rex disassembled for overhaul.jpg
     
  7. Bill c

    Bill c LawnSite Member
    Posts: 114

    Actually 75 our company bought us the miller airpaks because
    we where burning up the 300 ampers.I love the duty cycle with the 400 amp machines and the built in air compressor is a great feature.
     
  8. chip hayden

    chip hayden LawnSite Member
    Posts: 29

    nice pictures! how do you feel about piercing thru at the ends of the crack- using a back-up bar-weld repair- then cutting off the back-up bar? also, would you consider short stitches 90 degrees to the crack at the ends and every few inches along the weld repair? overkill? stress risers? i'm not being critical, i'm just looking for different ideas/
    thanks, chip.
     
  9. 75

    75 LawnSite Senior Member
    Posts: 992

    Chip, ideas are always good! :)

    I haven't seen the short stitches done, I don't really think they are necessary (at the same time of course, I admit to not having the engineering knowledge to say for sure) but what I have seen on heavy equipment - from the factory - is a fishplate over top of a high stress area: the plate tapers down at the ends (basically a diamond shape) and the welds run past the end of the plate a couple inches. This helps distribute the stress. No photo at this time :( but I'll try and get one when I have a chance.

    One thing I do try and avoid is 90 degree corners when possible, the abrupt change in direction could be a stress riser IMO. In some spots where it was necessary to plate sections of the dump box I put a radius on the corners.

    In the case of the dump truck a backup bar wasn't feasible because the crack in the photos is not accessible from the other side due to wear plate liners in the box. The other cracks could only be done from one side due to the dump box being of tubular construction.

    For all the apparent mass of the truck, the box is surprisingly light material - 1/4" to 5/16", tops. Granted, it isn't that big as haul trucks go, it's about a 35-tonner.
     
  10. chip hayden

    chip hayden LawnSite Member
    Posts: 29

    thanks, i wholeheartedly agree. smooth transiton from one plane to the next is good practice to avoid stress concentration. i always try radius any sharp corner on doubler plates. there still seems to be much debate as to whether transverse welds as stiffeners cause more stress than strength. also, as with most repairs, time vs. cost is a considersation.
    thanks again, chip.
     

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