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3 Studs Wide Differential Lock

March 11th, 2009 Leave a comment Go to comments

An idea on how to lock the new, 3 studs wide differential in a compact, reliable way.

The 8297 set came with a new, 3 studs wide differential, which is already a third generation of Lego differentials. It’s narrower than the previous ones, and much easier to transfer the drive to, but lacks any specific means to lock it (at least at the time of the 8297 set’s release). The solution shown here is the most compact locking mechanism I was able to develop so far. It relies on the transmission driving ring, which is a rare part but very easy to engage/disengage. It’s so compact that the driving ring actually touches the differential case, but the resulting friction is minimal thanks to its smooth surface. The 16-teeth and 20-teeth gears are well meshed and capable of handling a considerable torque, while their ratio reduces the torque transferred to the transmission driving ring by 20%.  I suppose the driving ring and the differential have similar torque resistance, but the driving ring is a much more expensive loss 🙂


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  1. Sariel
    April 17th, 2014 at 09:54 | #1

    Because it’s not needed.

  2. Ev3fan
    April 17th, 2014 at 09:41 | #2

    A good idea but I’d try to fit this around that LEGO diff case so often used with this diff to increase stability. Why Don’t you use this idea more often in your Models?

  3. Sariel
    March 13th, 2014 at 12:28 | #3

    It doesn’t really matter. Smooth just operates a little bit smoother.

  4. Witherley
    March 13th, 2014 at 11:13 | #4

    Another thing I though of is this: The axle connector that slots through the gear-changing element. Does the one you use have ridges or is it smooth? I had a play with it in LDD and noticed the ridged ones attach properly while the smooth ones do not.

  5. Sariel
    March 13th, 2014 at 09:06 | #5

    I would simply use the PF Medium motor.

  6. Witherley
    March 13th, 2014 at 07:14 | #6

    What would be the best motor to lock the differential? I want to make a 4×4 with remotely activated diff locks at both ends and your design here is hopefully compact enough todo the trick, so I’m gathering information before I buy the right parts.

  7. Sariel
  8. Santi claws
    July 18th, 2012 at 06:32 | #8

    Im sorry but i have no idea what differential do and how they work. Can someone please inform me

  9. Sariel
    December 31st, 2010 at 00:38 | #9

    No, there is no kind of movement you’re referring to. The gears that sit on both sides of the differential are exactly 1 stud wide both, so there is no space for backlash. If you used 8-teethed gears instead, that would be quite a different story.

  10. December 31st, 2010 at 00:10 | #10


    Consider the last question as obsolete, I found the hellbender model page with enough explanation

  11. December 31st, 2010 at 00:06 | #11


    play as in any movement of the lego diff other then the intended rotational movement.

    So in this case the diff is supposed to rotate around the axles. But due to the fact that there is not a 100% accurate fit there will be some degree of freedom of the differential in the structure (in this case the black L beams).
    This movement is called “play”.

    But forget about it if can’t make myself clear with this explanation.

    Just noticed a comment about a “hellbender”, how did this differential assembley held up under load?

  12. Sariel
    December 30th, 2010 at 23:45 | #12

    I’m afraid I fail to figure out what do you exactly mean by ‘play’. Not the angle, I presume?

  13. December 30th, 2010 at 23:23 | #13


    Thanks for the quick reply, but that was not what i meant.

    Imagine the following; you hold the Differential assembley in such way that you see the L beam as an “L” (sideview). Is the play (if any) of the lego diff a factor to take in account for.
    As far as text can be graphical here is the simplistic topview:


    G |
    E DIFF
    A DIFF
    R DIFF
    S |

    What i expect:

    G \
    E DIFF
    A DIFF
    R DIFF
    S /


  14. Sariel
    December 30th, 2010 at 23:07 | #14

    You can’t connect the gears outside here, because they have to engage / disengage the transmission driving ring located right next to the differential.

  15. December 30th, 2010 at 21:55 | #15


    Is there any play with this construction between the differential and the beams?
    Normally you would place the diff between a pair of beams and place any connected gears on the outside. I’m sorry for this crappy question but i don’t own any technic lego to build this (yet).
    Thanks for answering. regards and best wishes in advance for the future year.

  16. Sariel
    July 11th, 2010 at 10:29 | #16

    Yes. It’s called GOOGLE.

  17. Elijah
    July 11th, 2010 at 04:50 | #17

    do u know of a website?

  18. Sariel
    July 10th, 2010 at 09:14 | #18

    Yes, and you will find plenty of mechanisms like that in the internet.

  19. Elijah
    July 10th, 2010 at 07:49 | #19

    can u make a mechanism to switch from 2wd to 4wd?

  20. Sariel
    July 21st, 2009 at 13:37 | #20

    @Sanket D Shah
    You mean a bevel gear? The differential won’t work with less than three bevel gears.

  21. July 21st, 2009 at 12:56 | #21

    Wat will happen if i remove one of the spider gear from the differential assembly?

  22. Sariel
    July 15th, 2009 at 22:53 | #22

    Of course not.

  23. crawlerdude
    July 15th, 2009 at 22:29 | #23

    set 8297 has a locked diff??

  24. Sariel
    May 6th, 2009 at 14:27 | #24

    That would need additional mechanism.

  25. teunj
    May 6th, 2009 at 14:26 | #25

    Can you switch also from 4×4 to 2 wheel drive?

    Sorry for my bad english 🙂

  26. Sariel
    April 5th, 2009 at 19:57 | #26

    @PAtrick McShane
    Thank you Patrick, but I have to disappoint you. I am sharing my work, that’s the very purpose this website exists for, and I believe it’s a pretty sufficient form of sharing – a lot of people benefit from it without complaining. Also, I do not ship my parts. I understand your situation, but I simply can’t afford donating parts. Yes, I have plenty of them, but it’s because I need them and I’m using them intensively.

  27. April 5th, 2009 at 19:42 | #27

    TkX…Since I do not have any Lego Technik components, would it be possible for you to ship me examples (in a demo block, looking great, of course) so that I can video clip it for YouTube along with an intro of you, your work, and website ? I will do you justice, of that I can assure you. Examples of the Locking Differential, old and new. Examples of the 4 bevel working with lock Differential Pumpkin (real name that I love to say and the kids love to hear spoken…pumpkin) anything you can think of, please send ASAP. My UPS address is: Patrick McShane c/o Capitol Care Center (Room 207) 8211 Unstick Road, Boise, Idaho, 83704. UPS isn’t expensive and I am sure you have some spare parts. When I was told that I had 6 months to live, five years ago, I gave ALL of my Lego Technik to the first kid that came around saying that he would take good care of my Legos, after all you don’t just pass your Lego Technik to any kid that comes along, they have to promise to take good care of them and pass them along to their children and so on. I have seen Legos passed through 3 generations and headed for the fourth. TkX, Patrick McShane the original Lego Maniac.

  28. April 5th, 2009 at 19:21 | #28

    Great work with Lego Technik. Between 1983 and 1994 I founded and was the first headmaster of the PCS Centers of Advanced Learning in Nampa and Boise, Idaho. Part of my program included a huge Lego (Engineering) Lab. Lego Techniks were a major part of my collection. We were the FIRST to control our 4WD Lego vehicles, and mechanical/electrical/electronic creations with a computer. We were also the FIRST to invent the Lego Smart Brick System because one of my students won first place in the NSTA/Duracell competition for the all yellow LEGO CAMLU (Computer Activated Lego Unit) that had a hard-wired on-board brain that followed a simple survival logic. One of the other PCS students went to MIT and worked in the AI lab with Dr. Rodney Brooks on the Lego Smart Brick. We were also the first to show (see my Lego PCS Differential Pumpkin on YouTube) Lego Corporation that their differential must have 4 and not 3 bevel gears internally for the Spider assembly. You can read the exchange between Lego and myself with the video clip. I am asking that you send me high-quality photos of the three picks that you have here showing three aspects of your differential, compact, system. I, with your permission, want to include it in our Robotics assembly and understanding curriculum. It you could attach them, and any other mechanical wonders that you have created, I request that you share them with the rest of the world through PCS Edventures. I have NEVER received $$$ for any of my work with ‘my’ company. At one time they paid me $500 per month to live on, right! I live for the ‘ah-ha LIght-Bulb Moment’ on the faces of my students…that is my payment, that is my currency of exchange. I am saying that if you are looking for me to pay for my requests I am sorry to say that I am asking that you donate your ideas in the belief that you are sharing a part of yourself that very few people are able to create. You are gifted beyond my wildest imagination and I hope that you see the wisdom in what I am telling you. Please share yourself and your Lego Technik creations/ideas/photos/descriptions/etc. with the rest of us, especially the thirsty youths of the world. PCS Edventures are in over 7,000 locations in all 50 states and 22 foreign countries and we have a huge home school group that use us online for a major portion of their home school educational program. Please respond and share yourself, and thanks for your past contributions. Due to a operation, gone horribly wrong, five years ago, I have been forced to live in a nursing home in Boise, Idaho. It has been an educational experience. I have become healthy enough to have accepted an invitation to visit with the Royal Family in Saudi Arabia. We have several centers there (Pakistan, as well) and I am excited about going. I must get back into circulation and I think that the Good Lord has kept me alive to do just this…visit our centers around the world as the ‘Founding Father’ though I am only 63.

  29. March 12th, 2009 at 02:54 | #29

    Good! So I’ll leave it for a later reference.

  30. Sariel
    March 11th, 2009 at 18:19 | #30

    I think some delay would be a good idea, because – as I said – this is going to be used in practice soon. The Hellbender may be ready in some two or three weeks, and the differential lock will be well visible there, and shown in detail, including a video.

  31. March 11th, 2009 at 18:14 | #31

    Just great and amazing as usual from yours!

    Wonder if should repost your developments a TBs, or if you may fell it somehow inconvenient ???
    Maybe a few months delay post, to recall the attention to it, could be the most fair approach.
    Nevertheless my available time for posting, is actually to less… 🙁

    Continue the excellent job!


  32. Sariel
    March 11th, 2009 at 15:51 | #32

    It’s going to be used in the rear axle of my Hellbender model. I have an entire axle assembled already, with the lock engaged pneumatically (there will be autovalve attached to it later).

  33. March 11th, 2009 at 15:46 | #33

    Very nice! Are you going to build a 4WD based on that? How many interlocking differentials? 🙂

  1. May 20th, 2012 at 10:55 | #1