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Bachmann K-27 New Gear Head Motor

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  • Bachmann K-27 New Gear Head Motor

    No it's not a K-27 upgraded with a small block Chevy 327, it's an attempt to replace the existing motor in my Bachmann K-27 with an off the shelf Pittman gearhead motor so that it will run better up hills and around curves. Here is a little back ground on my railroad. I have basically an elongated loop of track that climbs a hill in the back of the yard. In order to transverse the creek in the hill as well as not dig out fifty million tons of dirt I needed to climb up about a foot and a half. Only way to do this was a 4% grade. Now I know some of you are thinking I am crazy however, don't criticize unless you have tried it. My curves are 10.5 diameter as well as a 9 on the other end. I use Airwire G3's to control the engines. This is awesome because of the cruz control feature. With my C-19's I can pull 7-8 cars without even a whimper from the engine. The K-27s will pull 12 however they will slow on the 9 foot curve and have a little convulsing as it runs along trying to maintain a constant speed. It isn't that noticeable but drives me nutty! I came into the game of large scale a little late so the Rodney Gear reduction was no where to be found, well that is what I thought until one day my neighbor said to me, "Hey Bill, I got this motor that I think is suppose to go in a K-27 and I don't need it. You want It?" Are you kidding me, Yes!!! So I proceeded to replace the stock motor with the new gear reduction one. Sorry to say, it was the nosiest motor I have every heard. Some people have said it is noisy and that if you turn the sound up you cant hear it. Bull crap! this thing was so load I don't even think the Winter National Drags could hide the sound. Out it came and on the shelf the engine went.

    That is until know. I like to solve problems and this is a problem I felt needs to be solved so I think I have come up with a low cost backyard solution to the gearing issue. In this post I will walk through what I have done as well as what I am going to do and well see if it works. If it does, I hope those who are out there like me will give it a try. If it doesn't work, no biggy, I always try to do these things so as to easily return to original. Ok off to take some pictures, Ill return soon!!

  • #2
    All right lets get started. Here is the type of motor that I will be using. It is from an Accucraft K-27. I replaced the original with one form the K-28 so Ill use this as the experimental one until I receive the 38.3:1 motor that I bought from Ebay. The part number is PITTMAN 24V DC MOTOR GM9434K043 38.3:1. You can get the very same one form Accucraft but you will pay about 100 bucks, Ebay was 50. A few things to notice here, The shaft is too large and the gear head makes the shaft off center. Both of these we will address.

    After taking the K-27 apart and removing the motor and cradle I made a template where the original motor mounted.

    I placed the motor shaft without gear into the gear box and rotated the motor against the gear box on the K-27 until the motor was centered in the chassis. I then made note of this as to where I would be placing the template on the gear head cover plate. The original was off to one side. I will also need to make some modifications to the cradle but that will be later.

    I then dissembled the gear head and removed the plate. I marked, drilled and tapped the top plate making sure that I was in line with the center hole perfectly. I used a caliper to help.


    • #3
      A washer the thickness of the raised portion of the plate needs to be added to make the attachment surface level. I did this by cleaning the area and super gluing the washer in place. You will notice a notch so that it will rest over the hole correctly

      I also needed to make some sort of guide so that the worm gear on the motor would be in center of the hole as with the stock motor. A couple of washers stacked and placed using a Gel Super glue and caliper. The Gel gives you some time to place it properly. Note, you need to find a washer the same size as the hole in the gear block on the loco. I had to drill out the center so that the shaft would fit. I did this by super gluing the washer to a piece of plastic then drilling with my drill press. After drilled, broke off the plastic cleaned the washers.

      next I measured the height of the worm gear off the motor.

      Then removed the worm gear from the original motor. This was a pain in the rear. I think they used some glue or something. My poor gear puller bent all out of shape and the end result was a bent shaft on the original. Not worried, if this doesn't work I'll buy a replacement motor form Bachmann. If you look closely you will see a thrust washer. This will not be needed because the Gear head motor shaft does not have play in and out as does the original motor.

      So far for now, it hasn't really been difficult. Now comes the experimental stage. A Big Note Here; I do not have a machine shop and the tools I have are very limited to the basic tools so my way of creating can and will seem odd. However, I have been able to work this way for many years and get the results I am after just takes a little longer to achieve.


      • #4
        I received the new motor today and got some time to tinker. I have good news and bad news, First the good news. My concept works, Bad news and this is a bummer but wont stop me, the gear ratio is way off. I didn't take into consideration the gearing in the gear box of the loco. Well Edison didn't give up so neither will I. Now I am not good at math so without going back to my old math books, any one out there know what gear ratio I would need in the motor to go with a 15 to 1 ratio to get me around 38-1? Any One? Anyone one? Bueler? Bueler?

        Once I get this info I will show how I attached the worm gear on the motor and will continue the build.


        • #5
          2.5333 : 1

          Take the red pill


          • #6
            Hey Bill...Can I help???



            • #7
              Thank you Jim Burley, From that info I would guess that in order to figure out the gear ratio by adding another ratio you would multiply the two ratio's, Correct? If that is the case I have another question. Correct me if I am wrong with this next assumption, assuming I have a motor that is 24 volts and I am running it at 12 volts it would run at half the speed correct? Therefore running the engine slower than if at 24 volts. Maybe the final combined gear ratio I want needs to be Higher such as 76-1? would I get closer to the speed of a gear ratio of 38 at 24 volts? Trying to think of it as a car with 4:11's verses 3:73. At 50 MPH the engine is running at 2500 RPM's. With 3:73's it runs at 2000 RPM's so with 4:11's if I go 25 MPH Ill get closer to the 2000 RPM mark. Am I even in the same ballpark here? Reason I ask is I have found a 5.9:1, would that come to somewhere in the 88:1 range and if at half speed be closer to the 38:1 I am looking for at 12-14 volts?


              • #8

                Your rear end analogy is making my head explode. At 25 mph with a 4.11 rear, the engine would be turning 1250 rpm. In theory the 3.73 would run 1000 rpm at 25 mph. You might be confusing yourself using 4:11 instead of 4.11 and 3:73 vs 3.73. A 4.11 is NOT a 4 to 11 ratio, it is 4.11 to 1 as a 3.73 is 3.73 to 1 ratio. The ":" is used for indicating ratio.

                Your rpm numbers are wrong. I'll show you why... 4.11 / 3.73 = 1.1018

                2000 rpm (3.73) X 1.1018 = 2216 rpm (4.11)

                So ditch the car analogy.

                Cumulative ratio is determined by either multiplying or dividing two ratios.

                The motor speed MAY be determined by applied voltage. Can you determine the "actual" motor rpm? How about using something like an optical tach?

                What top end rail speed are you shooting for? Are you thinking actual or scale speed?

                How many turns (like 3 and a half, or 4 and a quarter or whatever) of your driveshaft does it take to turn the driver exactly 1 revolution? This will give us one of the actual ratios to deal with.

                What is the diameter of the driver wheel?

                Take the red pill


                • #9
                  Hi Jim, Yes I was mixing apples and oranges, it was late and the only thing I could think about that could explain what I wanted to achieve was a very poor explanation using the wrong numbers dots and dashes. Here is what I have; Loco gear box is 15:1. The gear head motor has a rating of 24 volts, 38.3:1, 127 RPM. The loco wheels are roughly 1.89" diameter. My max voltage to run is 14 volts and the loco wheels will rotate at 5 RPM's, Not very useful. What I am trying to achieve is lots of torque and a wheel rotation of 240 RPMs at 14 volts. I hope I am not going down the wrong street again, I appreciate your help.


                  • #10

                    Now you're giving some good info. If you want 240 RPMS on the loco wheels, you'll have to have a motor speed of 3600 RPM at your 14 volts. 3600 / by that 15:1 ratio = 240RPM. If I'm reading it right, 127 RPM is what the output shaft is providing. The 127 is the motor speed (X) * 38.3. So the motor speed at 24 volts is 4864.1. At 14 volts, would be 2837.4. So here's the problem, if you direct drive the unreduced motor to the Loco's gear box, you will get 2837.4 / 15 = 189.16 RPM. Any further reduction will of course only make it run slower. DIRECT DRIVE IT.

                    How did we arrive at the 2837.4 number? We determined the 4864.1 number by multiplying the 127 RPM X 38.3.

                    Then we took that 4864.1 number and divided it by 24 (24 volts on your rating plate) which gives approximately 202.67 rpm per volt.

                    Multiply the 202.67 * 14 and that's where the 2837.4 came from. Divide it by the 15:1 ratio and you get the 189.16 RPM final output.

                    It's OK to use Apples and Oranges as long as you don't go Bananas!

                    Fun exercise.

                    Take the red pill


                    • #11
                      Thanks Jim, Makes perfect sense except for a big error on my part. My 240 RPM's at the loco wheels was a brain fart and should have been 60 RPM's. I was counting cuff's not wheel rotation, DUH! In light of the new information and if I am understanding it correctly, again it is late here, it would look something like this;

                      If I want 60 RPM's on the loco wheels I will need to have a motor speed of 900 RPM's. 900/15:1 = 60 rpm's. With the ratings I gave on the motor we know at 14 volts we would have 2837 RPM's. 2837/15 = 189 RPM's, 3 x too fast so I need to slow the motor down. I hope?


                      • #12

                        Yep, you got it! Motor reduction would be 2837 /900 = 3.15, so 3 to 1 will get you about right where you want to be. What a fun exercise.

                        Good luck!

                        Take the red pill


                        • #13
                          Ok, now that we have some good info at hand we can move forward, My experimenting would not have worked without it. As I mentioned above I had some motors that I had replaced on an Accucraft K-27. The original motor of 19:7.1 as well as my first attempt to make that engine run better with a 65:5.1. This was before they came out with the K-28 and a better motor at 38:3.1. I'm telling you this not to drive you away but to explain how I got to where I am now. With the info given by Jim, I decided to use the 19:7.1 motor Part # GM9413-2, 24V DC, 284 RPM's and will explain why later.

                          I dismantled the gear head into pieces. The output shaft has a large gear on it with the size of the output shaft roughly .25"

                          This shaft will not fit on the stock Bachmann worm gear so it will need to be turned down. I do not have a metal lathe so the drill press will have to do. I placed it in the chuck and used two new files to cut down the size. you have to do this very slowly checking the size with a caliper as you go. I stopped just shy of what it read on the old shaft the worm gear came from.

                          before you say , hey you shouldn't have placed it in the chuck on that shaft, I know but I didn't want to damage the splines above. I was able to clean it up with no ill affects.

                          Remember all those wonderful gears in the head?

                          Well I found that by removing all except the main gear on the motor and the gear running against it and adding the shaft gear gave me pretty close to what I wanted for RPM's at the wheels. This was all trial and error. If I tried to do the math again I would still be trying to figure it out. In order for me to use the shaft gear to mate with the lower gear I needed to move it down the shaft. You can see it in the back ground. These are the gears left after I removed the extra ones.

                          Here is a picture with the shaft gear back in place.

                          There is a sleeve and a Teflon grease deflector that still needs to be installed. Ill put it back together and install. once I do that I will record a video and put the link up. I am thinking I will be upgrading to an 18 volt battery just to give a little more top end speed.


                          • #14
                            Here is an update on the removing the gears. I have figured out that it is now a 5.9:1 ratio. With that info as well as the correct RPM's for this motor of 284 at 24VDC we can use the math above to figure the RPM's at the wheels.

                            60 x 15:1= 900 This is the target RPMs of the motor.

                            284 RPM's x 5.9:1 Gear ratio = 1675.60 RPM's This is how fast the motor runs at 24 volts.

                            We then divide 1675.60 RPM's by 24 volts to get RPM's at one volt, 69.82 RPM's.

                            To find out what RPM's at 14 we multiply,

                            69.82 RPM's x 14 volts = 977.48 Motor RPM's!!!!!

                            Walla!!! Roughly 60+ RPM's of wheel rotation.

                            Conclusion, It is possible to use an off the shelf gear head motor to re-gear and motor the Bachmann K-27.

                            Now the fun part making it all fit.


                            • #15
                              Hummm Where to start?

                              In order for the motor to fit in the cradle, one side needs to be cut off. I cut at an angle just at the mold line.

                              I then reinstalled the cradle and then the motor. Once I did that I took the cut piece and welded it with super glue back to the cradle next to the motor and on the edge just like the opposite side.

                              Next the gear box strap and a fabricated motor strap out of styrene.

                              Ready for a test run.