Welcome to my Instructable on how to make a k'nex automatic CVT!

A CVT is a Continuously variable Transmission. A CVT is able to achieve an infinite number of gear ratios within its range while the input rpm stays relatively constant. This eliminates the need for shifting and can keep the motor/engine running at its optimal rpm.

This differential is a Dual Differential CVT (DD-CVT) which uses two differentials connected in parallel to achieve a continuously varying ratio. This transmission has an output to input ratio range of 0:1 to 1:1. If a 0:1 ratio is not desired a ratchet can be added to change the range to 1:5.9 to 1:1.

This CVT is not a perfect design. It can be quite inefficient at lower gears (more in "How it works") However the main benefit of this design is that it requires only one input, the motor. Pretty much all other transmissions and CVTs require 2 or more inputs to change ratios. (at least in my research)

My sources for this CVT concept:

K'nex CVT by Austron

K'nex CVT V2 by Austron

Lego CVT by Nico71

Lego CVT by zblj

Video demonstrating the DD-CVTs operation. Watch for the different speeds of the differentials.

Step 1: Why Dual Differentials?

How the dual differential design was derived:

This DD-CVT is virtually identical to an orbital CVT. An orbital CVT connects two orbital gear assemblies together in parallel to achieve its CVT function. (check out this orbital CVT by Austron) My definition of an orbital gear assembly: An orbital gear assembly is a differential which has different gear ratios to its outputs. By this definition, one could simply construct an orbital gear assembly by gearing up or down one of the outputs of a differential.

The first picture shows a differential with one of the outputs geared down. The second Picture shows my orbital gear assembly. The function of both assemblies are equivalent.

Step 2: How It Works

How it works:

Power is provided to the input which spins the differential. The differential then divides the power to the two axles of the differential. The power from the two axles travels through different gear ratios to the axles of the second differential. The power from the axles of the second differential are added together to spin the differential and send power to the output.

At high torque and while accelerating, the first differential directs power to the geared down side of the differential, slowing down the output speed and providing more torque.

As the torque decreases, power starts to be provided to the 1:1 ratio side of the differential until the majority of the power is provided to the 1:1 ratio side and the output is at maximum speed. The sum of the speeds from the first differential are added together in the second differential. This allows infinite ratios within its range to be achieved.

When an excessive amount of torque is applied to the point where the output stops, the transmission goes to an idling state. In this idling state power is still provided to the input which is spinning the first differential. Because it is under high torque, the power is sent to the geared down side of the differential. Now because the output is stopped by excessive torque, the second differential does not spin, instead the power goes through the differential and causes the 1:1 gears to backspin. When in Idle, torque is still applied to the output. This allows the transmission to idle. If this feature is unwanted, a ratchet mechanism can be added to prevent idling.

Note that this transmission does not convert torque. But is designed to provide a constant torque though an infinite array of speeds. Any extra torque acquired at low gear is dissipated through friction. This friction is necessary to achieve the higher gears for more speed. Because of the friction at lower gears, this transmission is most efficient at its highest gear ratio.

Step 3: The Parts List

This is the list of parts required to build this CVT:

2 - differentials. Build them here.

48 - blue 3D connectors.

10 - white connectors.

2 - red connectors.

9 - yellow connectors.

5 - orange connectors.

4 - grey connectors.

13 - brown clips.

42 - green rods.

36 - white rods.

29 - blue rods.

8 - yellow rods.

2 - red rods.

1 - grey rod.

32 - blue spacers.

1 - low friction, light blue or gold gear. Must be light blue or gold!

4 - blue gears. (can also be light blue or gold)

4 - red gears

Step 4: Starting the Structure

Build two structure panels.

Then add 4 blue rods to one panel.

Step 5: The Center Axle

Build the center axle in picture 1.

Mount the Center axle in the center of the structure.

Step 6: Assemble the Input Differential Axles

Place 4 white sticks on a white connector. Make six of these. (3 are for the next step.)

Build on the axles of the differential.

Then mount the differential as shown in pictures 4-5.

Step 7: Assemble the Output Differential Axles

Build on the axles of the differential.

Then mount the differential as shown in pictures 3-4.

Step 8: Input and Output Axles.

First Build the final support for the differentials and mount it on the structure.

Then build 2 I/O axles shown in picture 4 and mount them to both sides of the transmission as shown in the last picture.

Step 9: Complete the Structure

Complete the structure by connecting the second structure panel on top of everything.

Step 10: Ratchet Option

This is my ratchet solution. It uses an orange connector over a red gear, armed with a rubber band.

For those who do not want the transmission to idle, the ratchet will prevent idling. This provides greater torque output for the lower ratios. It can be quite noisy though.

Step 11: Tuning the CVT

If your transmission is unable to achieve its 1:1 ratio you need to add friction on the "step down gear train". Adding friction also increases the torque output before the transmission resorts to idling. (note: increasing friction will decrease efficiency. This, in turn, will require a more powerful engine/motor.) Lubricating the snap gears in the differentials can also help (I used WD-40) Though this might decrease the max output torque before idling.

One method of doing this is to link connectors to the axles at the back of the transmission (picture 1).
You can also use spacers to push the red connector into the red gear to create friction (picture 2). Spacers can also be added to the bottom red connector as well for more friction.
If you still need more friction to achieve the 1:1 ratio you could add a friction gear (picture 3). It is possible to add up to 3 friction gears around the red gear if needed.
If that still isn't enough power for you, then maybe this CVT isn't suitable for your application.

If you feel that too much strain is being put on the transmission, you could use two CVTs in parallel to divide the work load.

Step 12: Finished!

Congratulations, you've just completed the K'nex Dual Differential Continuously Variable Transmission!

btw, I think this transmission would be absolutely perfect for driving a large Ferris Wheel. The transmission would accelerate it slowly up to speed. and if the Ferris wheel gets bumped or stopped, the engine would go into idle and prevent the motor from "Clicking".

Please Note: Using the designated output as the input is also possible and yields similar results. However this method relies on backspin which means that idling is unpreventable.

I hope you enjoyed my Instructable and that you find the transmission useful.

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