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Direction Sensitive Gearbox

January 5th, 2009 Leave a comment Go to comments

Compact 2-speed gearbox, where gears are changed when the direction of motor’s rotation changes.

This gearbox utilizes a working principle of a ratchet with gears. Only one end of the ratchet is attached, the other one is loose. There are gears on both ends of the ratchet, and it causes it to tilt accordingly to the direction of rotation of the axle it is attached to. To create a pressing force, that helps the ‘loose’ ratchet gear to engage receiver gears, an axle pin with friction is used. That friction consumes some of the motor’s torque, but creates an effect of the ratchet being strongly pressed to whatever obstacle it comes in contact with – in this case, the receiver gears.

The design has mupliple advantages: is extremely compact, pretty robust and needs no external gear changing mechanism. There is one, quite crucial disadvantage: with default gears setup, it can only drive iutput in one direction. So, it is possible to have e.g. vehicle that runs at two different speeds, but only forwards. Obviously, the output can have its direction of rotation changed by a separate mechanism, but that is very troublesome. On the other hand, it is possible to use even numbers of gears for both outputs of the receiver gears, thus obtaining forwards and reverse gear with different gear ratios. Both these options have a limited range of application, however the default setup may be useful for compressors, where the direction of rotation doesn’t matter, but the speed does.

As of the end of 2008, this gearbox design was used in one of my constructions only, in a SandRail, a small buggy able to drive only forwards, where its ability to handle high torque was tested. The performance of the gearbox was satisfactory, and the main conclusion was that it works better when braced in bricks that when braced in liftarms.

An ldr (MLCad format) model of the gearbox is available here.


1.png dscn6575.jpg dscn6576.jpg dscn6577.jpg dscn6578.jpg dscn6579.jpg dscn6580.jpg dscn6581.jpg dscn6582.jpg gearbox2.png


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  1. Sariel
    December 5th, 2011 at 07:42 | #1

    That only affects output ratio, so it doesn’t really matter for the functioning of the gearbox itself.

  2. qwertyuiop
    December 5th, 2011 at 07:02 | #2

    in the main photo, the 12 tooth gear is the driver and the 20 tooth gear is the follower, but they are switched around in the video. was this intentional?

  3. Sariel
    February 17th, 2011 at 19:50 | #3

    Oh, I know perfectly well how you feel 🙂

  4. RichUk
    February 17th, 2011 at 19:44 | #4

    Thanks for the input and prompt reply I’ll give it a try soon, I’ve built both designs individually so I’ll couple them and see how it goes, I need a few more parts before I can do it though. Oh how my credit card hates me 🙂 @Sariel

  5. Sariel
    February 17th, 2011 at 14:21 | #5

    I think it’s worth a try, but I have some doubts about how reliable would it be all together.

  6. RichUk
    February 17th, 2011 at 14:07 | #6

    Hi Sariel, I have been interested in building a small vehicle, do you think it would be possible to couple this design with your 2 speed automatic gearbox to enable a vehicle to have 2 forward gears but also be able to deal with the resistance of an incline? Thanks, Rich

  7. Sariel
    November 17th, 2010 at 00:11 | #7

    Yes, that’s correct.

  8. November 17th, 2010 at 00:09 | #8

    You could replace those three t8 gears with two t16 ones, couldn’t You? Then You’d have for example forwards in normal speed and backwards in reduced speed which would work great for small cars.

  9. mientos
    October 26th, 2010 at 23:13 | #9

    Bardzo ciekawy projekt. Kosztem wstecznego można zrobić dobrą przekładnię np. do uphillu.

  10. RjbsNXT
    September 26th, 2009 at 16:07 | #10

    Omg just what i need for the PTO on my big build !!!! Thanks !

  11. kama12811
    August 14th, 2009 at 18:21 | #11

    ten pomysł jest bardzo oryginalny i pomysłowy(heh)bardzo dobrze nadaje się do podstaw żurawia samojezdnego:D

  12. Sariel
    May 1st, 2009 at 20:30 | #12

    @Daniel Purrington
    Check my direction sensitive gearbox. There is a solution that disengages motor once it stops.

  13. Daniel Purrington
    May 1st, 2009 at 20:27 | #13

    @Daniel Purrington
    I can take picture and show you if you like to.

  14. Daniel Purrington
    May 1st, 2009 at 20:25 | #14

    I’ve been trying to make 2 different prototype sensitive gearbox. Do you have any idea how to make it automatic release itself when motor stop rotate? I used rubber band and used 5 studless instead of 3 studless transfer. Also I added clutch to put pressure to transfer on each sides of 2 gears. For Example: Car steering that returns to itself in neutral.

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