Back to the pyramid model. I'm making a few more changes to the design. Initially I had the centre section lifting and dropping under the power of a Geneva drive. I wanted to add a little more to the mechanism so I'm now working on a design where the centre section lifts, turns a quarter turn then lowers back into place. I think I have most of this worked out. It is going to feature the tour toothed ratchet gear shown above left in a mechanism sometimes features in retractable ball point pens.
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A bi-directional ratchet to download, print out and make. As the rocker is rocked back and forth the two ratchets drive the wheel continuously in a single direction. Members can download the parts sheets for free, non-members can download the parts for a small fee.
Print out the two parts sheets onto thin card. (230 micron, 230 gsm) Dotted and dashed lines show where to score, solid black lines are cut lines and the grey areas show where to glue. Carefully score and cut out the holes in the parts before cutting out the pieces.
Assemble the two ratchet housings in the same way as each other. Note that they are mirror images of each other. Fold the main body of the piece in half and glue it together to make it double thickness. Fold the end of the long tab over and glue it together to make a double thickness ratchet pawl.
Carefully cut out the circle on the main piece with a sharp knife.
Cut along the two grey curves to make a semi-circular end to the ratchet housing.
Fold the two tabs at the top over and glue them down.
Fold the remaining tab over and glue it down. The ratchet pawl piece should be free to move up and down. Notice the colour of the square on the top of the housing, in this case, red.
Find the link point with the same colour square and glue it to the top of the housing using the two triangular grey areas for alignment. Notice that the dotted line on the link point is a valley fold.
Fold the gear in half and glue it down to make double thickness pieces. Notice that the four centre tabs are not glued down. When the glue is completely dry, carefully cut it out. Repeat this process with the other gear.
Fold round and glue together the square sectioned drive shaft. Glue one of the washers into place lining it up with the red line.
Thread a gear onto the drive shaft as shown. Make sure that the teeth are pointing in the same direction as the picture.
Find the same housing as the one in the picture. (remember that they are mirror images of each other) Thread the gear into place in the housing as shown above.
Make up the box and rocker stand as shown.
With the rocker stand to the left, thread the ratchet assemble through the hole on the box so that the housing is flush with the box.
Flip the box over. Thread the remaining housing onto the drive shaft. Thread the washer into place and glue it to the shaft lined up with the red line.
Fit the second gear lining it up with the blue line and making sure that the teeth are again pointing the right way.
Fold round and glue down the two long tabs on the rocker to make triangular sections as shown.
Fold round the end tabs and glue them down. Notice that they are valley folds.
Glue the two hinges onto the two grey areas.
Glue the hinges to the top of the rocker shaft.
Glue the linkage to join the ratchet housing to the rocker as shown above.
Repeat the process with the other linkage.
That's it. Once the glue is dry, rock the rocker back and forth and the wheel will turn. Use the Bi-Di-Rat as an interesting mechanism in its own right or as the starting point for your own design.
Have fun!
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All the parts cut out and ready to go. I spent the morning putting together a double ratchet model and taking photographs for a blog post. It was working nicely and I was playing with it whilst sipping an espresso. As I worked the mechanism I noticed that ratchet 2 and the secondary ratchet both clicked at the same time. I was quite pleased with the accuracy of my construction. Such timing and precision is rare indeed.
But wait. If they are both clicking at the same time, which one is doing what work? I twisted the model round and worked the mechanism back and forth looking at it from various angles. Hmmm.
If the drive wheel was slipping past ratchet 1, it was being driven by ratchet 2 and visa versa. So just what was the secondary ratchet for?
I bent it out of the way and the mechanism worked just as well. Yep. Completely unnecessary. I always like it when I can simplify a mechanism but it does mean that I'll have to remake the model without the secondary ratchet and re-photograph the assembly. I'll make a start now but it looks like it'll be tomorrow before the download is available. I hope you can wait :-)
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A mechanism for mechanism's sake. I have put together the final stage prototype of the bi-directional ratchet model - and uploaded a youtube video of the model in action. My apologies for the sound quality, I forgot to connect up the external microphone. Still, once I've completed the download file you will be able to make you own and listen to the glorious tick-tocking in real life!
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Almost done. I've made up a working prototype. I'm happy with the mechanism though the layout needs a little more work.
This version only has a ratchet on one side but I can see that it'll work once complete. I've changed the ratchet housing again. Back to double thickness card but this time wider to that the ratchet itself fits fully inside the housing. It seems to be more stable like that. I've reduced the diameter or the gear a little and reduced the number of teeth to seven.
To complete the model I'll add a ratchet to the other side and a second linkage. That's for tomorrow though, I'm off to experiment with html 5 animation. Fun!
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I've been thinking of a few different ways of laying out the parts on the double ratchet. I'm wanting to change the housing that holds the ratchet pawl from the double sided as in fig 1 below, to single sided assembly so that ratchet pawl is visible in all its glory.
I had originally used double thickness card on the side of the housing to add rigidity. With a single sided housing I needed it to be even more rigid so I decided to make it as a thin box, 2. Finally, I decided to simplify the top of the ratchet housing by making it flat. 3.
Here it is fitted together. I've changed to nine teeth on the ratchet. Because the handle that drives the ratchet will now work in both directions that means that four and half goes back and forth on the handle will equal one full turn of the drive wheel. I'm going to reshape the box to make it slightly lower and a little longer. I'll add a rocker on the end where my thumb is and link the whole lot together. Its getting close to being a working prototype now. It promises to be an interesting little mechanism!
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After experimenting with a couple of different ways of connecting together the two ratchet in the dual ratchet model, I've settled (at least for now) on a 180º bell crank.
I must admit that it is all starting to look rather complicated for what is essentially a naked mechanism. Still, it looks like fun! So here's the plan. A 180º bell crank is basically a bar with a hinge in the middle. I'll locate it at a distance from the two ratchets and join it to the ratchets with a couple of linkages. Here's the first of the problems I forsee. The linkage that joins the ratchets to the bell crank need to be hinged in two different axes. Both ends need to be able to flex both up and down and side to side.
First axis of swing, side to side. I've added a couple of hinges lined up and down to the top of the ratchet housing.
The linkage has a hinge at each end and a twist along its length. The twist is more for artist effect than engineering need, looks nice though and does make a simple yet strong linkage bar.
I glued the linkage with its hinges running perpendicular to the ratchet hinges, that way it gives a solid joint which is still flexible. It moves easily both up and down and side to side yet still moves the parts effectively .
The prototype linkage is w-a-y to long, at least double the length it needs to be. Also, I think I'll move the bearing hole in the box further along to the right. I hope to have a working prototype tomorrow though I do have to take #1 Son into town for a quick look round the shops. Fingers crossed.
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Ratchets are interesting mechanisms I'm sure you'll agree, I like the way the back and forth movement of a handle is converted into rotary motion. The problem is, though, that the drive wheel only turns as the handle goes one way. When it returns to the start the drive shafts sits there - stationary.
Would it be possible to make the drive shaft turn on both the forward and return stroke of the handle? Here's my first attempt to see if it is possible.
Two ratchet gears, both fixed on to one shaft. Two separate ratchets, each ratchet has a sprung pawl inside it which engages with the gear teeth in one direction only. As one ratchet moves forward, the other returns to its start position. By moving them back and forth like this the drive shaft should move almost continually. At the moment the two ratchets housings are unconnected. To complete the mechanism I need to connect them so that as one moves forward, the other moves back.
Here are three possible ways I could link the ratchets together
Rack: A small gear on the top of the housing connecting the two ratchets with a rack gear. As the left hand ratchet moves down the gear turns anti-clockwise driving the other ratchet upwards.
Link: The two ratchets are joined together by a bar that pivots on the top of the box.
Belt: A belt loop running between the two ends of the centre box connects the two ratchets.
Time to crack out the sketch book and the double espresso.
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A ratchet driven paper animation kit to download, print out and make. Rock the ratchet handle back and forth and the early bird tries in vain to pull the worm from the ground. Members can download this model for free. Non members can buy the file for a small fee.
Print the parts sheets onto thin card. (230 micron/230 gsm.) Score along the dotted/dashed lines and cut out the holes before cutting out the parts.
Fold up and glue together the top and the base as shown above.
Fold over the tabs on both side pieces and glue them down to create triangular tube sections.
Fold round and glue together the two triangular pins.
Slide the pins into place through the triangular holes in the top piece. It is not necessary to glue them down.
Glue on the two sides, the red line lines up with the edge of the top piece. Notice how the side has a notch which lines up with the pin.
Glue the end cap into place between the two sides.
Glue the body top to one of the bird sides starting with the curved tab. Line up the edge of the curved tab with the edge of the body and with the small dot at the top of the body.
Fold round and glue together the leg supports.
Glue the leg support to the body side lining up the edge of the grey area with the end of the leg. Make sure you use the longer leg support on the longer leg!
Glue on the other side again using the dot for alignment.
Glue down the remaining tabs on the body top.
Fit the body to the top through the holes in the top.
Use the grey areas to line up glued areas.
Glue together the first 20mm of the beaks.
Fold the worm in half and glue it down to make a double thickness card. Once the glue is dry, cut out the two worm pieces.
Cut out the marked hole in the bird beak.
Fold the cam in half and glue down to make double thickness card. Once the glue is dry carefully cut out the part. (Like the nails ?)
Repeat the process with the two gears.
Fold up and glue together the cam follower. Glue the cam follower end to the grey area.
Complete the cam follower as shown.
Glue the neck to the body. Note the the centre tabs are folded forward.
Glue the head to the centre neck tabs using the dots on the head for alignment.
Glue the cam follower to the end cap.
Glue the small tabs to the end of the worm.
Fold a small dog-leg into the top end of the worm body.
Thread the worm up through the hole in the top. Glue the tabs on the end of the cam follower, thread the other end through the hole in the beak.
Glue the worm head onto the tab. Make sure you don't get any glue on the beak.
Glue together the square section of the crank shaft. Thread the cam into place on the crank shaft. Make sure that you put the cam on the right way. Note in the picture above the cam lines up with the red line and the yellow line is to the right. Glue two washers to the green rings on the crank shaft with the tabs pointing inwards.
Wrap a small (20mm diameter) in a small offcut of card and glue it to the front on the cam follower.
Fit the cam shaft into the box. In the picture above the purple ring is towards the front.
Glue on sides to the base.
Glue the cover into place.
Fold the pawl over and glue it to make double thickness card.
Glue the bottom of the secondary pawl to the base...
...the glue the top to the box side.
Thread one of the gears to the yellow ring on the cam shaft aking sure that the pawl is in place as shown
Fold over, glue down, and cut out the hole in the pawl housing. Fold over the top to make a triangular section then fold over and glue down the pawl to make it double thickness.
Thread the pawl housing over the cam shaft. Fit it into place with the washer.
Assemble the stop as shown
Fit the second gear to the cam shaft. Check front and back to make sure that the teeth are lined up.
Glue the handle into place. Positioning of the stop is more art than science. Hold it roughly over the marked area without gluing it. Rock the handle back and forth. Move the stop so that both pawls click as the handle is rocked. Once you have found the best location, glue it into place.
Your Early Bird is now complete!
Hold the handle, rock it back and forth and the bird will pull the worm from the ground a little at a time before the worm pulls back and the cycle begins again.
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Okay, that's the parts done. I've opted for an open ratchet so that the curious user can see what is going on. Hopefully the you can see what is happening in the picture above. The housing, containing the pawl (with magnified picture, top right) rocks back and forth advancing the gear one tooth at a time as it does so. The two stops are there to constraint the movement. I'll be putting together a final test piece as I've made a couple of small changes to sizes then it'll be onwards with colour and final layout. Not long now!
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