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  • Round keyway question.

    In a search I found one mention of doing this, I was just wondering if this is mechanically sound, relatively speaking. The shaft is 3/8", the threaded stud is 1/4-20 and the key is 3/32" x 3/8", those sprocket looking things are 1/2" thick . While there will be torque its not much. I drilled these using a jig and drill guide , I thought for sure the holes would be big enough for a dowel pin which is usually the case unless I drill a pilot hole first but it was too snug, luckily I had some needle roller bearings that fit.


  • #2
    Its fine, even square keys fail.

    I would not recommend doing that on a big ol tractor PTO though..

    Comment


    • #3
      Scotch key

      Yep. AKA "scotch key" and has been around - and very effective almost for-ever. Some were/are drilled/reamed and others ("Scotch") were just drilled and tapped. "Roll-pins" work pretty well too. The nut or washer on the thread will hold the key in against any tendency to "walk" or "work" out.

      It beats the hell out of making internal and external key-ways!!!

      Comment


      • #4
        Tiffie.. I always wondered if there was a proper name for round keys.. thanks!
        Dewat... what the heck are those fancy lookin gizmos for?
        Russ
        I have tools I don't even know I own...

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        • #5
          A square key transmits energy tangentially to the next component. Round keys exert a force away from the center to the next component. It probably won't matter right away if the key is square or round. I do wonder what would happen with an oscillating shaft over time, though.

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          • #6
            We used the same in the Westinghouse for some pretty damned big couplings.

            Inch and a half on the big end of tapered pins, holes cleanly reamed, and the pins soaked in liquid nitrogen, driven home, and allowed to warm up.

            Don't think there was a one that ever let go. 1,000 HP and up motors, to boot.

            Small couplings, it works just as well.

            The fact that we used round, tapered pins for larger HP motors/generators, and square keys for lower HP, makes me think that the round pins are better.

            Of course, million KW generators, did use square keys. They also speed up quite a bit slower than a 10,000 HP motor does. 0 to 3600 in a couple seconds, versus 10 minutes, mebbe, to get 200 tons of rotor turning in a generator.

            Cheers,

            George

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            • #7
              I could be wrong, but I believe I read that round keys are stronger because they avoid the stress risers that the square key slots create in both the shaft and the wheel/knob at the corners. As for the keys themselves, size for size, the shear strength should be close to the same.
              Paul A.

              Make it fit.
              You can't win and there is a penalty for trying!

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              • #8
                The key to it all??

                Originally posted by dp
                A square key transmits energy tangentially to the next component. Round keys exert a force away from the center to the next component. It probably won't matter right away if the key is square or round. I do wonder what would happen with an oscillating shaft over time, though.
                Hi Dennis.

                If a square key is "male" to both the "driver" and "driven" the square key can develop a "rocking" motion in its "slot/s" (keyways) if the shaft/coupling is oscillating. A Woodruff key is often better because of its depth of embedment (usually) in the "shaft". Multiple keys are often better than one. The optimum perhaps is a spline with perhaps the "involute" spline/d shaft or its derivatives being best and its mating (usually) "broached" mating (female) coupling/gear etc.

                As said, a round key is surprisingly adequate. They are often designed/meant as a "weak link" (mechanical "fuse"?) to ensure shearing-off before the gear train or what ever is over-loaded.

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                • #9
                  Works very good. Use it (among other things) to repair a stripped threaded hole with a threaded plug of a larger size and a round key to hold the threaded plug in place.

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                  • #10
                    Originally posted by oldtiffie
                    Yep. AKA "scotch key"
                    What's the difference between a Scotch Key and a Dutch Key?

                    By the way, there was a thread over at PM awhile ago, and a Scottish member was quite offended at the reason given by an Aussie for why they're called Scotch Keys
                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                    • #11
                      Thanks for the info guys, I thought it would work but hadn't seen anything other than the one brief mention. I managed to get a dowel pin in it, its nice and tight just difficult to work with such a small pin.

                      Oldtiffie "It beats the hell out of making internal and external key-ways!!!"

                      LOL that exactly what I was trying to avoid, plus I have to make sure its indexed to the ratchet mechanism on the other end.

                      Thanks, Jim

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                      • #12
                        Originally posted by oldtiffie
                        Hi Dennis.

                        If a square key is "male" to both the "driver" and "driven" the square key can develop a "rocking" motion in its "slot/s" (keyways) if the shaft/coupling is oscillating.
                        Quite so, Tiffie - on every large system I've worked with there's always been a set screw on the key to prevent that problem. Even so you can very often see the tell-tail line on an old key that indicates it's been moving around. The old shaper I'm restoring has an interesting combiination of woodruff and square keys. Mostly because of the conversion from belt drive to v-belt drive. The woodruff key is left floating in an adapter collar and the setscrew sets against the shaft. I think I'll rework that so it doesn't bugger up the shaft any more than it has. The babbit bearings will appreciate it when it comes time to yank out the bull gear.

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                        • #13
                          "Touchy ain't they?

                          Originally posted by Originally Posted by oldtiffie
                          Yep. AKA "scotch key"
                          Originally posted by lazlo
                          What's the difference between a Scotch Key and a Dutch Key?

                          By the way, there was a thread over at PM awhile ago, and a Scottish member was quite offended at the reason given by an Aussie for why they're called Scotch Keys
                          This one?
                          http://www.practicalmachinist.com/vb...3&postcount=29

                          Comment


                          • #14
                            Well I sure don't see anything wrong with Dutch pins.

                            1. Because I have Dutch heritage.
                            2. Because I'm repairing a cracked varidrive sheave and used them.



                            Clutch

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                            • #15
                              "Pinned"

                              I am quite unbiased.

                              I appreciate a good pair of pins no matter what the nationality.

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