Concept sketch for mounting a sprocket on a 6" x 2" COTS pneumatic wheel hub. Requires no permanent modification to the wheel hub and no fabricated components. Sprocket is modified by enlarging center hole and drilling 4 new bolt holes.
Are the only places to attach the sprocket are the 3 bolts on the wheel. and why would the sprocket need to be modified other that attaching to the 3 bolts?
I believe there are four bolts. In which case, it will not match up with the typical FIRST 6 bolt circle.
To the creator:
I’d be curious to know what size sprocket you needed to use?
It’s a 4-bolt pattern. You just can’t see the 4th one.
You could probably get away with not modifying the sprocket, but you’ll need longer standoffs. The bigger hole would be to allow the hub to get through the sprocket.
330 did something similar for sprocket attachment, except that the sprocket was attached using the AM/IFI hole pattern (6 holes) put in the adapter, which replaced the stock hub attachment.
Longer standoffs would also take a higher load (think cantilevered beam bending problem) which would make the hub / wheel / bolts more likely to fail depending on the materials used.
Unless they were properly designed. The “probably” reflects that one would hopefully do the proper design work.
They will always be more likely to fail (no “proper” design makes a longer cantilevered beam all other things equal less susceptible to bending and shear) but proper design will ensure the materials used won’t fail with a comfortable safety factor.
(engineering school education at work!)
The model shown here uses nuts and washers as standoffs. I built a prototype using standoffs between the wheel flange and sprocket that were made of 3/4" OD aluminum bar with a hole bored in the middle. They were about an inch long. It also used a nut between the hub flange and the wheel flanges, and the bolt heads are on the sprocket end of the assembly.
The sprocket shown here is about 27 teeth. The real thing would probably have 36 teeth and be driven by a single stage gearbox (AM CIMple). We would bore out the center hole to pilot the OD of the hub. That would center it and carry the radial chain loads. The four 5/16 bolts and standoffs would only carry the torsional loads.
Another thing - The axle is 3/4" aluminum with 3/8" shoulder bolts at each end. The shoulder bolts tightly connect the axles to the frame rails, which are basically parallel plates (inboard and outboard) with 3/8 holes, spaced and located as needed for ground clearance, quantity, etc. In order to control and adjust center wheel drop, we are thinking of putting the holes at each end of the mid-frame axles off center by about 1/8". By rotating the axle, we could have about 1/4 total adjustment range.
We hope to try 8 wheel drive (4 per side) tank drive, with the pneumatics in the center and skyway or plactions at the corners to improve turning and reduce bounce. We could fit this arrangement into either a long or wide axis.
Of course, the 2011 game could make this all a moot exercise…
I am not seeing/understanding the use of shoulder bolts at each end of the axle. Wouldn’t it be easier if the axle is essentially a 3/4" round standoff with holes in the end? Then you come in with a 3/8" SHCS from the outside of the frame rails to tighten them down to the standoff/axle. As it is drawn now, if your shoulder bolt bottoms out in the axle before it is tight against the side rail, the axle will be loose.
I love the idea of adding the sprockets without major fabrication/modification to the wheel/hub. Your idea is doable by many less equipped teams.
Also, is this drawn in SolidWorks? Where did you get the model for the wheel?
General question for everyone: The idea of a 6WD with drop center is to enable easier turning, but then aren’t you effectively only pushing with 4 wheels when shoving someone?
General answer: it doesn’t really matter how many wheels you’re pushing with, as long as you’re applying all of your power to the all of the wheels that are touching the ground.
For the 3/4" axle, the shoulder bolts on the axles could be replaced with bolts. The intent is for the shoulder bolt NOT to bottom out in the axle hole, but rather to mount the axle tightly to the frame. If we do traditional wheels with 1/2" axles on the corners, then a 3/8" shoulder bolt with 5/16-18 threads would be used. This would allow a common frame hole (3/8") and permit interchangeability of axle types.
One of our CAD mentors modeled the hub using AutoCAD. He may have gotten some of the data from the OEM (Wagner ?), but the detailed dimensions on the hub were reverse engineered from the one we bought. The Grainger catalog isn’t too specific. This is Grainger p/n 1ZPE1.
And depending on your drop, that might be all 6, with two only lightly skimming, or all 6 (think team 25), or 4.
Just out of curiosity, did you CAD that wheel or did you pull CAD file for the wheel from some website.
Just remember that the CAD wheel does not expand when inflated or loaded like the real wheel does. This means, put the sprocket 1/2" further out that you think you’ll need, or you’ll find yourself cutting new spacers because the chain rubs the tire.
I learned this the hard way.
Oh, and the larger OD your spacers have, the more torque they can transmit to the wheel hub before failing.