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Looking for small plastic gear

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  • Looking for small plastic gear

    Help - I have an automatic door for my Dog (he has the easy life) and it has a motor that picks up a lexan door when he approaches. He has a sensor in his color that triggers the door. Over the weekend, I was out of town and my wife called and said the door does not work. Well, it probably has 10,000 cycles on it so when I got home tonight I went to explore the problem. I pulled out the gear motor and I found the problem. My guess is that this part if could be found to purchase couldn't cost a buck or two. Here are the details and maybe someone can point me in the right directions.

    Looks to be a standard SPUR gear and my guess it is actel or delrin. White in color - has 12 teeth. OD of the gear is .283 and .243 thick. It presses on the motor armature and my guess is the ID of the gear is .125. If I could buy something with the correct teeth, OD (which I think if it has 12 teeth it needs to be that OD) I could machine the rest of it to work.

    The replacement motor from the Dog Door people is $86.00 + $22 shipping.

    So - I hope you can point me in the correct direction.

    Thanks in advance.


  • #2
    If you are patient and lucky, dozens of printers are being discarded daily, and they are loaded with plastic gears - you might find one that fits, or might adapt things to make it work. (Swap both the bad gear and the one it meshes with.) Try your local FreeCycle.

    Or perhaps these people, for a new one:
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979


    • #3
      Does this look right? In particular is the tooth profile correct? Note the angle between the sides of the teeth near the root. This is a 20 degree pressure angle and is most likely the correct angle since it isn't undercut.

      If so I will make you a few, your choice of material. Brass, nylon, acetal, aluminum, or whatever.

      Free software for calculating bolt circles and similar: Click Here


      • #4
        If Evan's offer,most generous by the way, doesn't work out for you, try PIC products. They carry a wide variety of that sort of thing. Bob.


        • #5


          By George I think you got it. That would be real nice of you. I will try to locate on tomorrow but I'd be happy with a spare of two. It is a shame that the failure was a simple part.

          Here is a picture of the gear and the failure (crack)

          As for material - it must press on the armature - the armature diameter checks .124 and over the splines .134-.135. What would be your guess on material?



          • #6
            Not to highjack the thread, but I had a similar problem some years ago. I mounted a small pulley in the place of eacy of the gears and anchored a piece of string in each one, so the driven one acted like a winch and turned the other one. In my case, weight returned the thing to down when the motor was reversed. A timer determined how long the motor would run. That worked fairly well for several years.



            • #7
              The material is almost certainly acetal or Delrin. It is tough and unlike nylon it is dimensionally stable within a wide range of humidity.

              Brass would be difficult to press on that shaft so plastic is probably the best bet. I may even have the right part in my junk box so I will check that first.

              PM me your details and I will give you my e-mail.
              Free software for calculating bolt circles and similar: Click Here


              • #8
                Years ago, I had some sort of little Radio Shack printer that had a similar setup and identical problem. IIRC, I bored out the hole a bit and reinstalled the gear with a dab of glue. (Not sure about that, but

                I got a replacement gear from RS, only a couple of bucks, but bored/drilled it out before installation, then used some glue when I installed it. The whole trouble was due to a too-tight press for that tiny gear. It looks to me like your problem is the same.


                • #9
                  I have seen that failure so often it must be the most common failure mode for those types of drives. They are widely used in small printers and copiers as well as many other similar uses. As usual, even inside Xerox the gear was not available even though it would have saved a lot of money for the company.
                  Free software for calculating bolt circles and similar: Click Here


                  • #10
                    Small parts carry small gears and so do most gears makers
                    The shortest distance between two points is a circle of infinite diameter.

                    Bluewater Model Engineering Society at

                    Southwestern Ontario. Canada


                    • #11
                      I was surprised that RS not only sold the gear as an individual part, but that it was so cheap. Maybe it was because they had such a high failure rate. IIRC, I think I had 2 of those printers, and both failed in the same manner. Easy fix, though.


                      • #12
                        Well, I just finished supper so I'm going downstairs where it is cool and make a length of pinion stock the right size.
                        Free software for calculating bolt circles and similar: Click Here


                        • #13
                          I used to see that problem all the time as well. I had two sizes of gear that would split, so my solution was to make a holder in the lathe that the gear would press into, closing it back up, then I'd press on a previously made ring to occupy about a third of the width of the gear. The ring would keep it together and tight on the shaft at the same time. The holder made it a quick and easy job to press the ring on straight and to the correct placement. The gear was then pushed out of the holder and pressed back onto the motor shaft. Seldom if ever did the mating gear use the full width of the teeth on the motor gear, so it worked well.

                          The rings were made by drilling and boring a length of steel, then parting off. I could get about five rings per length of bore, so I'd make about 15 rings at a time.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                          • #14
                            Update - I contacted the manufacturer of this motor and asked them it they would sell or send this small gear. They said they do not sell parts and I must contact the door supplier to get a replacement motor.

                            Here is what ticks me off, all of the gears in this motor/gearbox are either brass or metal. The only one that is plastic it the one that attaches to the armature. If they designed this to be the weak link than it did what they expected but if they don't sell the replacement part, why do they care where the weak link is. I think this is just crazy.

                            Oh well, I guess I will wait until Evan sends a replacement gear from plastic. I even went to the RC hobby shop in town and they didn't have this gear. They could only order them in metal.


                            • #15
                              I will have it ready in a couple of days. I am in the middle of some medical business. I have it all ready to machine but then realized that the 4th axis driver on my CNC blew out last fall. I haven't been doing much machining lately and I totally forgot the 4th axis was dead. I have the parts to repair it and worked on it last night, will have it ready to go this evening or tomorrow. Once that is done the rest is simple. I have already made the form cutter for the teeth.

                              Is there room to add some strength to the gear?

                              Like this:

                              Note: 4th axis problem due to personal stupidity. Some chips got into a connector that had been untaped to try another motor and didn't take the time to retape it to prevent FOD.
                              Last edited by Evan; 07-18-2012, 01:48 PM.
                              Free software for calculating bolt circles and similar: Click Here