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Hardness of splined shaft

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  • Hardness of splined shaft

    I've been asked to make a replacement for a splined shaft. It's about 30cm long and has 6-tooth straight sided splines on both ends. Machining should not be a problem, but I am not sure about the hardness needed.
    The preliminary plan is to make it from heat treated 4140 which is around 30 HRC. The question is whether it needs to be harder than that.

  • #2
    Yes.
    Actually, nobody could say without a lot more information. It would help if you could explain a bit about the application, dimensions, estimated torque levels, vibration, reversal, spline fit/slop. Then a mechanical guy (not me :-) can try to give some kind of judgement.

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    • #3
      It is for some kind of agricultural earth equipment driven by tractor PTO, like a rototiller. I've only seen the shaft.
      No idea of the load on it except the tractor probably has more than 100hp available at the PTO.

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      • #4
        How hard was the original, and how did it break?

        allan

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        • #5
          This sounds like a standard 6 spline pto shaft, not saying you can't or should not attempt to build your own. I'm not even sure what the availability is for this item in your location, just wondering if it is cost effective for you and your customer for you to make one yourself.

          Here is one that I think that may be like what you are looking for, $46 USD, you may want to check local sources just to see if it is economically feasible for you to pursue this project.

          https://www.surpluscenter.com/Power-...S-1-2937-6.axd
          Home, down in the valley behind the Red Angus
          Bad Decisions Make Good Stories

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          • #6
            The original was very hard: It was used but the wear didn't even begin to take off the unevenness left from the milling of the splines. The break seemed very clean and with no twisting that I could see.
            According to the owner it broke when he bumped the PTO lever (accidentally) while the thing was down on hard ground. I was also told it's not available anymore. The owner got turned down at another place when he asked them to make a new shaft.
            It is different from the one in the link: The splines on each side is a different diameter, it has a bearing seat in the middle and some other features. This obviously isn't going to be cheap.

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            • #7
              It obviously failed due operator error but I suspect hard on the outside to combat wear and tough on the inside for durability would be the desired goals.
              Home, down in the valley behind the Red Angus
              Bad Decisions Make Good Stories

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              • #8
                All the ones I've messed with were Very Hard. Like Rockwell 60 -65. A file would hardly touch them. Of course this was all crane service, so maybe your requirement would be a bit less... weed make the replacements in the Inside Machine Shop & then send them out for hardening.
                HTH,
                David

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                • #9
                  I'm with Willy on this one, and possibly the original was a bit to hard, and not tough and springy enough?

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                  • #10
                    If the broken bits are hardened to where a file barely touches them then I'd think that's your answer. That indicates something a lot harder than Rc30. So

                    There's bound to be a lot of impact torque loads on this thing as well. I'd think you want the connecting shaft to be as durable as something like impact wrench extensions. What is something like that made from and what is the hardness?

                    Also it's not like PTO shafts are uncommon. There should be information out there on what alloy they are made from. And since it's highly likely that the replacement will require a proper commercial heat treatment operation taking the stub end along with the new one and asking that it be made to the same hardness or for suggestions as to a suitable treatment aimed at toughness seems like the way to go. I can't imagine a heat treat outfit not having a hardness tester to test their own work.

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                    • #11
                      Sounds like a candidate for S7 steel, can be very hard and very tough.

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                      • #12
                        Or just make it from that 4140 and have it case hardened after machining. PTO shafts are usually made from prehardened stuff or in some cases just "basic" steel and then case hardened to get a tough core with durable OD.
                        Amount of experience is in direct proportion to the value of broken equipment.

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                        • #13
                          Hi,

                          I agree with Jaako, the PTO shafts I've seen broken were not through hardened. Always cased. They need the tough core to prevent just that kind of failure your friend had.

                          That said, there is supposed to be a "failure" point before the PTO shaft to prevent such a failure from happening in the first place. Like a shear pin or clutch.

                          I would choose either a 1045/1050 plain carbon or 4140 for cost effectiveness and have them case hardened for wear.

                          Dalee
                          If you think you understand what is going on, you haven't been paying attention.

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                          • #14
                            I'd rather have the shaft break than a gearbox so I'd case harden it so the outside doesn't wear but the shaft will shear if needed.
                            "Let me recommend the best medicine in the
                            world: a long journey, at a mild season, through a pleasant
                            country, in easy stages."
                            ~ James Madison

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