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Servo Gearbox

October 28th, 2012 Leave a comment Go to comments

3-speed gearbox controlled remotely with the PF Servo motor.  

A while ago I created an orbital gearbox, controlled by simply rotating an axle. It was limited to just two speeds and not so easy to control. With the advent of the new PF Servo motor, it occurred to me that it can both make controlling such a gearbox easy and expand the number of speeds to three.

The gearbox is relatively small, very easily controlled, and with short shifting time. It’s primary advantage is seamless engaging and disengaging at any speed – while most other gearbox shift with the risk that gears’ teeth won’t mesh properly, this design is completely free of such risk. The disadvantage is that high gears are torque-sensitive, causing the Servo motor to yield under load. However, the 1st gear handles torque fairly well, and even if the gearbox disengages under torque, it can easily engage back the moment the load is gone.


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  1. Artwodeetwo
    October 12th, 2014 at 19:45 | #1


  2. Sariel
    October 12th, 2014 at 19:44 | #2
  3. Artwodeetwo
    October 12th, 2014 at 19:37 | #3

    Hi Sariel, I want to publish an MOC (sometime in the future), and it contains this Servo Gearbox, would it be alright for me just to call it “Servo Gearbox by Sariel”? Thanks very much for this design!

  4. Sariel
    June 23rd, 2014 at 20:12 | #4

    Great. Go share it on some community website, like Eurobricks.

  5. EV3fan
    June 23rd, 2014 at 18:29 | #5

    I just made a 4 speed box operated with a worm gear

  6. Arthur
    December 11th, 2013 at 01:48 | #6

    I think that it would be cool if you could make another one on the other side, so that the one servo could power two axles.

  7. eberkain
    December 9th, 2013 at 17:33 | #7

    Lets say I wanted to add this to my 4×4 crawler, with a single servo controlling two gearboxes… Put this servo right on top of the other one, put the gearboxes on each end, just got to figure out how to link them since the distance from servo to gearbox would not remain static when the suspension shifts.

  8. Sariel
    January 23rd, 2013 at 13:57 | #8


  9. bob
    January 23rd, 2013 at 13:26 | #9

    whats your best truck trial gearbox except your 2 speed LA heavy duty gearbox?

  10. Sariel
    January 11th, 2013 at 23:39 | #10

    I doubt it.

  11. Lamboguy59
    January 11th, 2013 at 22:57 | #11

    Would this handle the torque of a nxt motor and could i use a nxt motor for the gear change too?

  12. Mike
    January 6th, 2013 at 10:37 | #12


    wow, im blind!!!

    thank you!

  13. Sariel
    January 6th, 2013 at 10:22 | #13

    Mike, look above the video.

  14. Mike
    January 6th, 2013 at 05:39 | #14

    in the video it says building instructions available. but i do not see them in the downloads section, are they elsewhere? or were they taken down?

    can you help me please.

    btw. i love your work, keep it up!!

  15. mraudisubaru
    December 2nd, 2012 at 22:26 | #15

    thanks for saying that really helped a lot.keep on with building legos you re the best

  16. Sariel
    December 2nd, 2012 at 22:08 | #16

    No, it won’t handle the torque.

  17. mraudisubaru
    December 2nd, 2012 at 21:49 | #17

    i am buildng tatra 8×8 do you think that this gearbox would handle the torurq or is that wich you hald in monster truck

  18. Sariel
    December 1st, 2012 at 21:45 | #18

    @a guy
    No idea. You need this motor, that’s the point.

  19. a guy
    December 1st, 2012 at 18:41 | #19

    i dont happen to have those motors so how would i manually power it?

  20. Sariel
    November 29th, 2012 at 23:18 | #20

    In theory, yes, but I suppose it would be troublesome to control it accurately. Besides, 3 speeds is enough in most cases.

  21. Matt
    November 29th, 2012 at 19:28 | #21

    Could you use a speed control remote to have it turn at increments, thus having more gears as the servo would work proportionally?

  22. jimmy3
    November 18th, 2012 at 18:50 | #22

    wow that is some gearbox

  23. jan
    November 16th, 2012 at 13:45 | #23

    okay , thanks anyway

  24. Sidster
    November 15th, 2012 at 00:38 | #24

    Paul, I tried to apply the 2spd gearbox in my version of the Tumbler, but my tumbler is too heavy too handle it correct. Therefor I think your servo gearbox will be better to use. Fantastic job man!!!

  25. Sariel
    November 12th, 2012 at 18:13 | #25

    Not possible. Best ratios are different for every model. You have to figure it out yourself.

  26. jan
    November 12th, 2012 at 17:06 | #26

    help on the ratios best used in it

  27. Sariel
    November 9th, 2012 at 20:23 | #27

    What kind of help exactly?

  28. jan
    November 9th, 2012 at 19:31 | #28

    @Sariel i need an high torque /low speed / med speed&torque gearbox for a tractor, i thought of an 2-speed gearbox (hi-lo gearbox , used next to a normal gearbox in tractors) but i don’t know about the ratios any help on this?

  29. November 7th, 2012 at 10:38 | #29


    I developed a gearbox That has motorized gear shifting and uses the power of both motors to drive the output shaft at the same time. As such you don’t have an idle motor (dead weight) most of the time.
    See my blog for an explanation: http://nxttime.wordpress.com/2012/10/15/a-motorized-gearbox/
    I recommend the instructional video.

  30. Sariel
    November 5th, 2012 at 20:33 | #30

    @Zack trapp
    It doesn’t take much experience to know that XL motor has more torque.

  31. Zack trapp
    November 5th, 2012 at 17:38 | #31

    @Sariel perhaps for some it’s not as obvious as others. Not everyone has as much experience in this field

  32. Sariel
    November 5th, 2012 at 13:42 | #32

    @Zack trapp
    Yes, obviously.

  33. Zack trapp
    November 5th, 2012 at 13:39 | #33

    Would the presence of a large motor instead of a medium make the difference with torque?

  34. Sariel
    October 31st, 2012 at 13:55 | #34

    That depends on what mechanism you drive with it.

  35. rastwo
    October 31st, 2012 at 11:56 | #35

    Awesome! Can it handle XL-motors torque?

  36. Sariel
    October 31st, 2012 at 10:35 | #36
  37. That_LEGO_Guy
    October 31st, 2012 at 09:20 | #37

    Brilliant idea yet again, but, will it work with more gears and have the servo controlled by a speed controller and have it running through a screw drive then back to 1:1 to remove motor yield?

  38. Mike
    October 28th, 2012 at 19:34 | #38

    Not sure if there are any published designs but a few months back I made a free-wheeling mechanism that worked both ways, ie you reversed the engine and it would change direction – but it could still free-wheel in reverse (like if the vehicle’s speed exceeded the drive train or you stopped the engine (without re-reversing it)… perhaps that could be put to use for your dilemma. It was basically the same mechanism as in the new helicopter rotor safety, but double-sided.

  39. Sidster
    October 28th, 2012 at 12:37 | #39

    This is awesome! Would be perfect if Lego sold 2 servo’s in one packet. One for the steering part and one for the gearbox.

  40. October 28th, 2012 at 11:34 | #40

    Nice build only sad that you have press 2 buttons when you want an another gear than 1:1, so I’ve tried to build one with a free (I don’t know how to call it in English) (so there is no transmission). but that was quit hard en it took a lot of space.
    Why I wanted a free, is because you only have to press 1 button to start the transmission and so to let your car drive. There is only one problem you have to make sure that you can go in reverse. (and that was exactly my problem with my free gear (and the motor was struggling to move all the gears)).

    Someone an idea?


  41. gabry
    October 28th, 2012 at 09:12 | #41


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