Vex differential

Ok the discussion has been talked about in the advanced gear thread. But lets get more specific.

The defferential is an open differential.

Open Differential
When one tire slips 100% of the torque goes to the slipping wheel.

Limited-Slip Differential (LSD)
When one tire slips a series of clutches and springs transfers 30 to 60% of the torque to the non slipping wheel. ( commonly found on cars )

Locking Differential
When one tire slips a series of clutches springs and gears transfers 100% of the torque to BOTH wheels. ( typically found on 4x4’s ) This also does not allow any wheels to freewheel while turning. ( Live axel ).

There are many advantages and disadvantages of these types of systems. What I would like to do is open up a discussion on how one could modify an open differential system to be one of the other two types.

There are times when the bots will traverse uneven ground and since there is no suspension as of yet for vex that means zero chassis flex. unless you forgot to tighten your bolts up, or have a weak frame. What are you doing to overcome the issues that arise when your wheels slip?

Now There is a link in the advanced gear topic that shows how someone modified a lego open differential. Well lego does not handle torque like vex does. so the lsd that idea demostrates will not work. For those of you that have not seen it it’s the left and right side connected to a common shaft with strong rubberbands. which allows for slippage. This is a great idea and works well for low torque operations.

I have built a system that when activated will become automatic locking. It’s very crude needs refining and better design work. Each output shaft uses a chain to tuen an auxulary shaft. one of the shafts is movable, as in able to slide in and out. It has 2 springs on it to keep it in open position. and when needed a servo could slide the shaft and push it into the other shaft. ( will post a pic of it in the morning ) Though it is not powered yet it is very effective under hand power. ( tangent… got another idea already to help lock the two shafts use a clutch… )

Eventually after it becomes more refined I would like to see a program that reads 2 optical shaft encoders on the aux shafts. and when one is no longer turning it will auto engage the locking mechanism for a durration of say… 5 seconds that would be enough time to get across the problem area i feel. if not it would engage again when it “sensed” that only 1 wheel was turning.

Anyway, Lets brainstorm. toss around ideas on what you think that will work. There is no wrong questions. If there is something you don’t understand ask. I’m betting that the Vex community can make this into a more valuable asset than what it already is.

Good luck.

Here is the pic

Sigh it will not allow me to edit my post this is odd.

edit: missing information

(A) Right hand side aux shaft, this will move back and forth to engage shaft b. spring is used to return it to an open state.

(B) Left hand aux shaft.

(C) Right hand drive wheel shaft.

(D) Input shaft from motor.

(E) Left hand drive wheel shaft.

(F) Locking bar with standoffs to be used as “fork”

(G) Stationary locking bar.

Ok I think I understand how this works but maybe you could show us a picture with the right hand aux. shaft pushed in.

Have you considered using two locking bars perpendicular to each other so that when the shaft is moved over it will engage sooner?

I spot a loose screw to the top left of the differential in the photo.

First off about that screw… i was rushing and well forgot to remove it after i realised that it would be in the way. i will see about engaging the aux shaft tomorrow afternoon and getting you a picture.

P.S. I bet your really good at the “spotting the difference” photos in the newspapers huh? :slight_smile:

Sometimes if you look hard enough. I was just looking at the photo think about ways to make it better and noticed the screw.

that’s not a good thing neccesarily because then if one wheel is in the air it will get all of the torque and the bot may not be able to move

Thats exactly why useing a differential is nto always a good idea. There are limited slip differentials that try counter this problem but they aren’t entirly effective all the time. Google them to find out more.

If you search the Vex Forum you can find a simple example of how to use Vex parts to convert the differential in the kit into one that has limited slip. There is a picture that goes with the post.

Searching (i.e. doing research) is a good habit to form…


I just looked up online, and this guy was saying that open differencials don’t work all the time, because of different traction.

For example, if one of your wheels is on ice, and the other one is on dirt or pavement, the torque on the ice wheel wouldn’t transfer to the differencial, and the whole thing would jam.

please correct me if i’m wrong.

Ok I was talking about the 3 Main types of differentials.


Limited Slip


Differentials are GREAT idea’s.

What this Post was meant to be about was the fact that Vex labs Advanced gear kit uses an OPEN differential. Which is a great device. It lowers the amount of friction on the tires by allowing a wheel to free wheel.

The true discussion was mearly a way to take this Open diff, and give it additional properties. Every application that incorporates a diff will not use the same type.

I enjoy the properties of an open diff. You get less parts to break, You have less moving surfaces which is less friction. and it’s compatable with a major part of enviromentmental variables.

But When I also enjoy the benifits of having a Locking differential. When I need the transfer power from the wheels that slip to the wheels that grip. I need it to be there not as a precentage.

I’m gonna go out on a limb but i’ll say that more than half of the professional 4x4 clubs of america use some form of a locking differential for rock climbing and mud bogging.

Differentials are a great thing to have. if not every 4 or more wheeled vehicle in the world would only drive with 1 wheel or there would be 2 engins/motor.

Or they would wind up replacing tires more frequently .

TR - If your question is aimed at me; I said to search this Forum, I did not say to “google” the topic of differentials.

ooooooh sorry.

For example, some persistent searching might turn up this thread