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Gear OD, how do I cheat?

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  • Gear OD, how do I cheat?

    Need a 15 tooth 24 DP spur gear with a 7/16” bore and a 3/32 keyway. The gear is for an old dividing device. When the gear is cut the keyway is too close to bottom of the teeth and easily breaks.

    How does one cheat and make a 15 tooth gear that will mate with other 24 DP gears and be a larger diameter or less tooth depth?? Center distance of gears is adjustable and there is no way to make bore or keyway shallower and still work.


  • #2
    Make the keyway in the gear a half round slot and radius the top half of the key.No corners=no stress risers.
    I just need one more tool,just one!


    • #3
      How does one cheat and make a 15 tooth gear that will mate with other 24 DP gears and be a larger diameter or less tooth depth??
      Assuming we're talking standard involute spur gears, you can't.

      Can you braze/weld/solder/pin/glue on a collar to the 15 tooth gear, and put the keyway in the collar?
      Last edited by tony ennis; 02-05-2009, 12:35 AM.


      • #4
        Can't cheat on the keyway, It has to inner change with the other 23 gears that fit on a keyed shaft.


        • #5
          Is this a new gear, or are you replacing an old one?


          • #6
            better metal? only option i see.


            • #7
              The gears that break...was the keyway cut right under the center of the tooth or was it off to one side or the other a bit?
              I have tools I don't even know I own...


              • #8
                The keyway was centered under a tooth gullet so the sharp corners were under the teeth.

                Steel used was stress proof.


                • #9

                  Drill the gear and the shaft as an assembly - about mid-way across the gear width - and insert a roll-pin. That should not only do the job quite well but it will eliminate the need for a key and a key-way.


                  • #10
                    It has to be removable and one of the two shafts that it can be used on does not have a removeable key. This is a dividing device that uses gears to get the different divisions.


                    • #11
                      Make a gear slightly longer than you need, and shrink a collar around the end of the gear on the unused portion of the teeth.

                      The collar will reinforce the gear, and it probably won't interfere with anything.

                      Any products mentioned in my posts have been endorsed by their manufacturer.


                      • #12
                        Pics please


                        it would be a lot easier and a lot less guess-work if you were to post some pics to assist.


                        • #13
                          Roger, maybe collar might work. Won't be able to try tomorrow.
                          Don't pic of broken gear but here is dividing device.


                          • #14
                            You can cheat slightly, probably enough to make the difference you need. Involute gears have the property of producing the same ratio even with changes from the nominal center distance.

                            Make the gear the same OD and pitch circle but change the tip profile of the cutter. The way to do this is to modify the cutter so that it still produces the same depth but cuts the bottom of the tooth profile as round. By grinding the outer tip of the cutter with a rounded tip profile it will produce a gear that will still mesh just fine at a slightly increased center distance. It won't take much to strengthen the gear enough to prevent breakage.

                            It also looks like you can make the gear at least 25 percent wider than needed. That will increase the strength of the gear as well.

                            If this doesn't seem like a good idea then try making the keyway so the top is actually round. Even though it will remove more material than a squared keyway it will be stronger as long as it is centered under the root of a tooth.
                            Free software for calculating bolt circles and similar: Click Here


                            • #15
                              First thing is file the key down to 1/2 hight and make a shallower keyway in the gear.
                              The other gears will still fit the shaft.

                              Next kludge is to make a new gear with the OD equal to a 16 or even 17 tooth gear and then have 15 teeth HOBBED onto it.
                              It will look weird but it will be geometrically correct and give a stronger tooth form.

                              Three gears, all the same OD and only the 22 tooth is correct but they will all run in any order.


                              Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.