Team 610 Preseason Sprocket-Hub Drive

FIRST Robot Showcase
1

http://i.imgur.com/5cKNwEb.jpg

Team 610, the Coyotes from Crescent School, has quite been busy during pre-season. We’ve been working on a new project which we like to call Sprocket-Hub Drive.

Last season we decided to use 6" x 1.5" Colson wheels with press-fit Vex Pro hex hubs. However, last season we noticed that the hex shaft we used were not very precise. The tolerances were too large and we would much rather use precision ground keyed shaft. In addition to this, we found that in our past sprocket based designs, the chain would eventually start to drag. We traced this issue back to the material of the sprocket and chain. In order to eliminate this issue, we’ve used steel velodrome sprockets and bike chain. However, the sprockets we decided to use do not have a conventional torque transfer method as they are designed to be fit onto a bike shaft. To solve both issues simultaneously we’ve designed custom aluminum press fit hubs that allow for the sprockets to be directly mounted to the wheels. By cutting these hubs using our Haas TM-1p CNC mill we were able to create a perfect fit with the Colson 5" x 1.25" wheels and repeat it very quickly.

However these were not the only improvements that we made. Taking inspiration from our 2013 drive system, we decided to opt for a similar structure only replacing the plate with C-channel. To connect these channels together we used a combination eighth inch aluminium plates on the top and bottom and two end blocks at each end. These end blocks allow for two roll pins to be inserted into each side to manage shearing stress. To hold the frame together, two threaded rods are run through through two inserts that lie at either end of a 1” by 2” tube. When taught, these rods will manage tension stress on the drive. Lastly, to manage impact force, a cross beam has been mounted between the middle of the two rectangular tubes.

The power distribution board has been mounted on the cross beam to allow for the easy connection. The belly pan has been made of 1/16th inch black fiberglass and allows for a light yet rigid construction. Vex Pro single speed double reduction gearboxes with the standard gearing for 9.52:1. As the drive does not have a chain reduction, we choose these gearboxes as they were easier and quicker to employ than a custom built one.

All of these factors together produce a new lighter take on the classic drop center chain drivetrain.

Feel free to ask any questions!

TLDR:

  • Velodrome sprockets and bike chain torque system.
  • Custom dead axel hubs for 5" x 1.25" Colson wheels.
  • Double C-channel construction held together by roll pins and threaded rods.
  • ⅛” Drop center.
  • Vex Pro single speed with the standard gearing for 9.52:1.
  • Light and versatile construction.

For CAD files of the assembly please send me a direct message.

Photos:
http://i.imgur.com/ODd1Ygd.jpg
http://i.imgur.com/r5B3MV4.jpg
http://i.imgur.com/C0FSXgk.jpg
http://i.imgur.com/oEXSQ7Q.jpg

2

Looks nice. Any reason you can’t make the system dead axle and put bearings in the hubs?

3

Very cool solution to using the not so easy to work with bike sprockets. When I was on 1310 in 2013 we also used bike chain. Our solution was to make a plate sprocket with the andymark hole pattern. The tooth profile was generated with sprocketeer and then we made the necessary changes to make them first friendly.

4

I like it. I’m glad you went with the Miche system instead of trying to thread for a standard cog with a reverse thread to lock it…which is what I thought while reading before I saw the pictures. Are you guys using 3/32 chain or 1/8?

5

Possibly they decided to make them all live axles since the direct driven center wheel would need to be a live axle anyway.

Edit: If I was designing something like this, I don’t think I could resist the urge to use dead tube axles on the ends, though.

6

In order to make the hub press fit, we had to make one side slightly smaller. We explored this option in CAD but found that the manufacturing would be easier if the bearings were fit into the plates. Furthermore, the center hub had to be live axle in order to power the drive.

7

We used 1/8 chain. Bike chain for 3/32 width sprockets has some slop built into it to handle angular misalignment with derailleurs. I figured the 1/8 chain is better suited to our application since it is designed for single speed straight chain runs.

The Miche sprockets are quite nice. The heat treated steel is probably bullet proof for FRC purposes. And the bore is so large that they are very light.