Knex CV Joint

Welcome to my Instructable on how to build a CV joint out of k'nex!

A CV joint (continuous velocity joint) Is a device which allows a rotational force to be transferred through an angle, much like a U-joint (universal joint). The difference between the CV joint and U-joint is the fact that the CV joint provides a output equal to the input continuously, where as the output speed of the U-joint oscillates around the input speed which can create vibrations. There are quite a few different types of CV joints all of which have different methods of achieving constant velocity.

This video describes some of the differences between the U-joint and a CV joint (Specifically the Thompson CV joint. (Skip to 3:10 for CV vs. U joint)

I designed my CV joint to be used in a large k'nex car I am working on. I designed this CV joint to be low profile and capable of withstanding fairly high torque and it needed to be fairly balanced for high speeds. The design is based off of a standard CV joint.

The major disadvantage to my CV joint is the fact that it has quite a bit of play in it (I estimated around 40 degrees of play), so it would not be suitable for something that requires precise movements. The CV joint also is not capable of banding as far as the U-Joint. My CV joint has an effective angle up to 22 degrees from center, in comparison GWorks' K'nex Universal Joint has an effective angle of about 55 degrees. Another requirement of this CV joint is that both sides of the joint need to be completely supported in order to keep the joint together, else it will fall apart or break.

Here are the parts you will need to complete the CV joint:

3 grey clips

1 orange connector

2 blue spacer

1 head top (black slide on part)

4 white circle connectors

4 blue rods

2 red rods

The CV joint is constructed with two individual parts, the socket and the ball. we will start with the ball. Just build whats in the picture!

Now that the ball has been built lets start building the socket.

Slide two white connectors to the center of the rod and secure them together with the sticks.

Then slide the spacer on followed by the last white connector.

Now clip the four blue rods on the three white connectors as shown.

Then slide the whole assembly to the end of the rod so that the rod does not protrude into the socket.

Now slide a spacer on the rod followed by the white connector. This white connector prevents the blue rods from sliding back. When installing the CV joint, be sure to fill the rod with spacers for it to work properly.

Slide the ball into the socket.

And that's it! Thanks for checking out my instructable!

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

Update: I have just tested the CV joint and have determined that the output speed is fairly continuous up to 15 degrees. After 15 degrees oscillations occur.

Here Is a video of the CV joint in action!

The CV joint is on the top and I am comparing it to a Double Cardan Joint built with GWorks U-joint design. a Double Cardon Joint is basically two U-joints connected together so that the oscillations from the first joint are canceled out by the second joint. Thus creating a continuous output.

The Double Cardan Joint output is displayed by the white rod, and the CV joint output is displayed by the green rod.

In order to account for the play in the CV joint, I added a small load (a friction rubber band) to the CV joint output so we could see its true output under load.

The CV joint appears to be fairly continuous up to about 15 degrees. After 15 degrees the grey clip starts brushing the blue rods which creates erratic jumping.

Credit goes to NUCAP_KDM and his Double Cardan Joint Assembly for the testing fixture design.

This video shows GWorks U-joint against the Double Cardan Joint for comparison.

The video starts with a 20 degree bend. (20 degrees is where oscillations started to become noticeable.) As the angle increases the amplitude of the oscillations increases significantly.

So in conclusion, my CV join may not quite achieve the CV functionality any better than the U-Joint. It is hard to see the oscillations at small angles. However the benefits of my CV joint over the U-Joint include: Lower Profile, and higher torque capability.