# How to simply connect a plastic wheel to plastic body of a robot?

I have a robot that it's down part is something like this photo: http://www.sanatbazar.com/components/com_jshopping/files/img_products/full_Capture3.JPG . This chassis has 4motors and 4wheels are connected directly to motors, but my robot only has 2motors and 2 rear wheels are connected to motors directly. And the problem is about forward wheels! I am looking for a simple and cheap(but good) way to connecting them.

As I saw in some toy cars, they used a shaft through 2wheels and connecting them to shaft, but as I have electrical parts inside the body and need smoother movements, I prefer to use another way. I searched about bearings but don't know how to attach them to my chassis?

At the end I should mention my robot has made by 3D printed parts and I can change the down part's design to matching something like bearings.Also the chassis dimension is about 16*16*6 CM^3.

EDIT: These photos are my chassis, wheel and two 3D printed white parts that I wanted to use as connection, but I saw this mechanism is very unstable and wheels slipping:

I can change my chassis design and 3D print it again, but I am looking for a proper design for my purpose. I don't like to pass a shaft trough side holes, because I want to put my electronic board there!

Edit2: I bought a bearing with maintainer as you can see below:

But it's about 2times bigger and heavier than what I want! So the only other option do I have is using this simple bearings, but I don't know how to maintain them to the body:

• Your question is vague because you don't make it clear what your motor type and transmission type is and how you plan on connecting them. Also you want to connect the axle to the plastic body, not the wheel. Nov 4 '17 at 15:18
• @daemondave: Maybe it's because of my bad English. If you see the photo, that chassis has 4 motors(each wheel directly connected to a motor), but my chassis has 2 motors (only rear wheels connected directly to motors, and I have no problem with this part). My problem is about my free wheels. I don't know how to connect my free wheels to my body(chassis). this is my question.
– WDR
Nov 4 '17 at 19:13

The problem with asking for a specific answer without providing all the requirements (it must do this, it can't do that, etc.) is that any answer becomes a circular argument as to what might work or what won't.

There are many possible answers, there are many possible designs.

What you are asking is this:

"Go find me a rock."

Then someone offers up a rock without knowing your master plan.

"Not that rock, a better one."

When engineers (like myself) design, they start from a specific set of requirements to avoid wasteful tautologies.

Before you start playing with a 3D printer you should decide major things like where do the wheels go? Must they be in the body?

If they don't have to be in the body then you could use a technique like these Badger Wheels:

and mount them on the outside of the chassis but still keep a single front (?) axle.

Every design choice delivers you good (pro) and bad (con) consequences to your overall design. You can't keep all of them in mind when you are looking for a design one rock at a time.

Resubmit this question with a complete (exhaustive list) of requirements and someone might be able to give you the very best rock in one attempt.

Given the updated post, here is a rough idea of what you want:

Since you are mounting an axle that will carry 1/4 the load (vehicle mass) and the wall is plastic you need a method to reinforce the mounting location and at the same time not flex so the wheel drives straight. Short axles cause mechanical deformation if the mounting methods aren't strong enough because they are an unbalanced moment arm (see torque and inertial moment Wiki Torque) - the longer the moment arm the greater the torque.

$\tau = R \times F$

That is the reason, in general, that people normally use a single axle because it cancels out one side of the vehicle against the other in a lateral sense. Do you understand?

Reinforce the mounting area on both inside and outside the plastic wall, ideally join it to the structure. I can't guarantee this will solve your problem. The longer you can make that inner axle, the more torque it can dissipate.

Use metal for the axle! Not plastic!

• OK Dave, So I will go to the computer that has windows and my solidworks on it and try to upload more information by photos, less than a hour.
– WDR
Nov 5 '17 at 6:53
• I edited my post and put it's real photos
– WDR
Nov 5 '17 at 7:22
• Thanks Dave, I had the same idea and bought this bearing from internet( uupload.ir/files/cyqd_photo_2017-11-05_20-42-46.jpg ) but when I received it I saw it's about 2 times bigger what I want and also heavy for my robot. So the only other option I have is these bearings: sanatbazar.com/components/com_jshopping/files/img_products/… or sanatbazar.com/components/com_jshopping/files/img_products/… that have no maintainers!
– WDR
Nov 5 '17 at 17:24
• If you don't reinforce this you will pay for it down the road. Better to use stronger solutions early than try to refit electronics later on. Also, don't forget to upvote if this was useful. People need to know it worked for you. Nov 5 '17 at 17:44

I don't see the problem here. You just need to pass a metal shaft across the body on the underside. This shaft needs to fit the wheels but I suppose that part is sorted already. To fit the shaft to the body you could buy attachment points or make them yourself (3d print or adapt something, for instance cut a few short lengths from a thin copper pipe as used in a/c systems and attach these to the body using hot glue or even just ties, then pass the shaft through them.) Personally I prefer being inventive.. Hope that helps.