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Thread: Case hardening a bolt

  1. #1
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    Default Case hardening a bolt

    A farmer friend of mine came by and asked if I could harden a bolt for him that is used as a pin in a mowing machine. These bolts tend to wear rapidly even when using a grade 8 bolt. I am not sure what material is used for a grade 8 bolt but I was thinking maybe I could harden one of these (3/4" bolt BTW) with the "Cherry Red" compound I got recently and hopefully not reduce it's shear strength too much. The hole through the parts the bolt fits has work hardened and appears to be wearing very little but the bolt wears fairly rapidly and then shears when it gets reduced to about 50% or 60% of it's original size. A hardened pin could be used I guess with some modification but in this case a bolt with a nut is the ideal set-up if we can solve or even just slow the wear process.

  2. #2
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    Real grade 8's (not the Home Depot/Tractor Supply Company imports) are heat treated chromoly. You'll be hard pressed to surpass the mechanical qualities.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  3. #3
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    Quote Originally Posted by lazlo
    Real grade 8's (not the Home Depot/Tractor Supply Company imports) are heat treated chromoly. You'll be hard pressed to surpass the mechanical qualities.
    agreed, even if you could, would you want the bolt to last and the hole not? if its in a easily replaced wear part who cares, but if its the frame or whatnot I'd want to keep buying bolts

    if its wearing that fast there has to be a bunch of force & friction, kind of amazing that the grade 8 wears away but not the frame (or whatever material)? if work hardening produces some thing that much tougher than a grade 8 bolt, sounds like we need to figure out how to bottle that!

    maybe a plate with a bush 4 or 5x as long as the existing mate welded on with a zerk for grease is the way to go....correct the failure of the original design?
    .

  4. #4
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    Without knowing the exact application, I too would just treat this bolt as a consumable and save the framework. Is it very hard to change the bolt?

    A true grade 8 bolt is pretty darn tough. Hard to imagine the framework taking the abuse better than a good bolt or pin.

    Mark

  5. #5
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    As others said, the 8 grade bolt is harder than anything you will make. What is wrong with replacing the bolt on a regular schedule. Nothing lasts for ever and it sounds like he doesn't want to do any preventive maintenance. Does he even lube and service the tractor on a regular schedule?

    There is no way your not going to have to fix or repair a tractor and it's attachments on a regular basis. He needs to replace the bolts as they wear.

    Another thought is, which would he rather replace, the bolt or rebuild the holes where the bolt goes through? He will have to do that soon enough without making it happen sooner.

    Since I own and use a tractor and many attachments quite often I fully know what is involved and would much prefer replacing bolts than rebuilding the holes the bolts go through.
    It's only ink and paper

  6. #6
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    This is not a lubed connection and runs dry. Since posting this I went back and looked at the thing and there are hardened inserts where the bolt goes through, I had to scrape away several layers of paint to see this. Also I think maybe a better solution will be to add another link to this outfit and distribute the load instead of trying to increase the wear resistance at an obviously over-loaded point, in any case I will take everyone's advice and not be attempting to try and harden any bolts for this thing.

  7. #7
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    The bolts are supposed to be somewhat shearable. This allows a blade to come off before you break something expensive, like the gearbox.

  8. #8
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    What about replacing it with something like a shoulder bolt, or one thats not threaded the whole length, so that the wear area will be on a 'solid' shaft of the bolt, and not the threaded section. (assuming its not like that allready)

  9. #9
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    Quote Originally Posted by radkins
    I went back and looked at the thing and there are hardened inserts where the bolt goes through, I had to scrape away several layers of paint to see this. Also I think maybe a better solution will be to add another link to this outfit and distribute the load
    Is the bushing worn, and the bolt is slapping around inside it?
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  10. #10
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    Any good mechanical design has an intentional weak point that saves the rest of the system from damage. Bolts and pins are often that weak point. I designed my Land Rover snowplow attachment that way and I have sheared the several bolts that serve that purpose more than once. Make that weak point stronger and something else will become the weakest link.
    Free software for calculating bolt circles and similar: Origin now settable to bottom left! All values positive. Click Here

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