Using a Ratchet to Drive a Key

Tue, 27 Aug 2013

I'm working on a project for client that has a decorative clockwork key on its side. I'd really like key to turn as the model operates. The character itself will be jumping up and down so I'm trying to harness this motion to turn the key. If I can get the mechanism to work well it would apply well to a clockwork robot model like the one in the sketch below which I can then make into a model for

The striped tab at the back is the drive that turns the key.  This would be connected to the box.

Inside the body I have this small toothed wheel. Twenty teeth, eighteen degrees apart.

To match up with the toothed wheel I've made a kind of toothed belt. There is a base strip onto which I have attached a series of overlapping squares glued on like the tiles on a roof.

The wheel will now only turn anti-clockwise, try turning it the other way and the teeth catch on the square tiles.

I've fixed a set of tile teeth in the back of a box so that they just touch the toothed wheel. Friction of the wheel against the tiles is only minimal but the wheel is effectively stopped from rotating backwards.

With the box closed a second tiled strip, this time with the teeth facing the other way, is threaded into a groove in the box. This strip is free to move up and down. 

I've finished off the box with a clockwork key slotted onto the axle.

As the strip is pushed into the box the key turns, as they strip is withdrawn the key remains stationary. The result is a slowly unwinding clockwork key. I'm pretty sure that this will work for the client's project and will also make the basis for a nice clockwork robot model for the website. 
Win / Win!

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Comments (5)

  • Shelley Noble August 27, 2013 at 10:33 pm

    Stupendous! And a perfect

    Stupendous! And a perfect example of what is so special about, engineering brilliance.


    But my favorite line in this post is "a private client" ??!! You mean your genius brain pan is available for hire?! That's astounding.

    Mwah ha hah! – RI

    • Smelter August 29, 2013 at 7:35 am

      Oh yes, Rob is a “Gun for

      Oh yes, Rob is a "Gun for Hire".
      Check out the past comments and postings on the forum for sightings of Rob "out in the wild".
  • Simon68 August 28, 2013 at 12:34 am

    I Just loved this post. Very

    I Just loved this post. Very inspiring! I was looking for a solution to this problem:
    I need to make a big disc (R ~ 22 cm) moving at a relatively low angular speed (6 revolutions per
    minute) when powered by a small electric motor (900 revolutions per minute).
    The idea is to reproduce an old toy I had as a child (in the 80 's), called "Champion Pilot":
    one has to drive a little car over a circular track. The car is stuck on the track by a small magnet.

    The obvious solution for a big gearing reduction is a worm gear. A worm gear will reduce speed by a factor of 1:number of teeth on the gear. You need a 900/6 which is 150/1 so a worm gear and a 150 tooth gear will do the job. – RI

    • Simon68 August 28, 2013 at 11:49 pm

      Hi Rob.
      Yes, in fact I

      Hi Rob.
      Yes, in fact I thought about using a worm gear. By the way, I found your fantastic site after searching the web for a worm gear project. The problem is, It't to difficult to me to design the 'big' gear. The solution you put in this project "toothed belt" is more easy (at least to me) to do. The original toy (which is seen on the photo) uses some kind of this "toothed belt" made of plastic, that has hundreds of tiny teeth. And the motor is tieted
      to a rubber cylinder that has no teeth.

      Ps1: By the way, talking about your robot character and the mechanism you design, can't wait to see it done: I'm  robot maniac 😉

      Ps2: This post also inspired me to build a paper version of another toy: a japonese tin toy robot called "Space Explorer" by the Yonezawa company. There is a YouTube video if you are ineterested:

  • cool022883 August 28, 2013 at 10:22 pm

    that is great it has like the

    that is great it has like the effect of an old wind up toy

    .are you going to attach the ratchet to a crank so the crank moves it up and down and it attaches to a handle or just leave it as a a pull tab mechanism?

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