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Welcome to my Instructable on how to build a fully automatic transmission out of k'nex!

When I first built this transmission I was not planning on posting it because it wasn't very functional. I built this transmission as a "proof of concept" build to prove that a differential could be used to automatically shift gears based on the amount of torque applied to the output.

I originally got the idea from Steffan Whitt's Automatic Transmission. I thought that his design was over complicated and I thought that I could simplify the design and make a more effective load baring transmission if the centrifugal clutch was eliminated and the differential was used as the output and the gears "shifted" in between the engine and the differential. This works because the differential "feels" all of the torque even when in low gear. If the gears shifted after the differential, the differential would "feel less torque when it shifts into low gear and it would then try to shift back to high gear.


My original design was almost three times as long and was not very reliable. I decided to post instructions for this transmission because, over the past week, I have refined and redesigned the transmission multiple times and finally found a compact design which is much more reliable and functional than my original.

The transmission has two gear ratios: 1:1 and 1:5.9. The transmission is able to shift gears regardless of the direction the motor is spinning. When the differential "feels" torque on the output, it rotates the outer gear and pulls the transmission to a lower gear. When torque is reduced, the rubber band pulls the transmission back into high gear.

Here is a video of it in action:

Step 1: Parts List

These are the parts you will need to build the transmission:

1 - High Torque Differential with grey rods for axles. (use black rods if you have them)

a small amount of wire

2 - Rubber bands

2 - grey rods (use black rods if possible)

10 - yellow rods

41 - blue rods

34 - white rods

25 - green rods

8 - black clips

11 - brown clips

4 - blue clips

2 - orange tubing clips

3 - grey clips

88 - blue spacers (some may be substituted for grey spacers)

1 - hinge

7 - orange connectors

3 - purple 3D connectors

11 - yellow connector

5 - white connectors

49 - blue 3D connectors

2 - red gears

1 - yellow gear

4 - blue gears

2 - red connectors

Step 2: Base Structure

This is the base structure which will start off the build!

Step 3: Build the Output

This is the output axle which runs through a differential.

Build a High Torque Differential with grey rods as axles (use black reinforced rods if possible.)

Next, add the components on the axle of the differential and attach wire to the yellow gear on the axle. Pictures 2 and 3 show how it should be attached. The wire provides about the same space as 1/2 a spacer and is necessary for proper spacing between gears.

After the axle has been built, mount the differential to the base structure.

Step 4: Build the Shifting Axle

This axle is able to slide back and forth to shift gears. I have built special "sliding blocks" to allow the rod to slide back and forth while preventing the gears and components on the rod from sliding out of position.

This step is probably the most complicated part of the transmission. It is important that it is built correctly or the transmission will not function as intended.

Step 5: Build the Input

The Input is pretty simple to build. This is where the power source is attached to drive the transmission.

Step 6: Build the Automatic Shifting Arm

This is the mechanism which pulls the shifting rod back to shift gears.

Step 7: Build the Top Panel

Build and attach the top panel to the transmission structure.

Step 8: Arming the Shift Axle

Use rubber bands to arm the transmission. The rubber bands pull the transmission into high gear. Adding more rubber bands will increase the amount of torque the transmission must experience before it shifts down to low gear.

Congratulations, you've just completed building the fully automatic transmission!


Thanks for viewing my instructable! Let me know what you think of the transmission and be sure to check out my other instructables!

If you post an instructable which uses something from one of my instructables, let me know, I'll post a link to your ible in my applicable ible.

great instructable, clear instructions and pics. I made one and I am having trouble making it work. if someone could help that would be great.
<p>Could you describe what isnt working? Is it not shifting? The motor must be strong enough to overcome the elastics in order to shift. (also, if the elastics are too strong the shifting arm may break)</p>
it was adding more stress to the motor rather than helping to re leave the stress also i was using 7inch bands doubled up. the motor was the green box type. I think what happened was the .motors I have were not strong enough. this is an amazing peice but I think I built one that does more what I need. thanks.
I was also not using snap gears for the diff I think that may have caused some of it too.
<p>Cool! Glad you could get it working :)</p>
thanks, great instrucatable
<p>Nice! Works pretty well, no?</p>
It works quite well! At least it does on the bench. I have not used it in a practical application though.
Very clever, as usual :)
<p>Thanks :) </p><p>Were you able to see the video? When I logged on today I saw the video wasn't showing. I just updated the link to the video in the introduction.</p>
now it seems to be working:)

About This Instructable

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Bio: I have an Associates Degree in Electrical Engineering Technology and I am currently working as a manufacturing Engineer at Whelen Engineering Co. I have many ... More »
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