Chain-In-Tube Design

Over winter break I was playing around with a chain-in-tube drivetrain design. It is based on the SimpleTube chassis system created by 221 Robotics Systems.

The design uses 221’s custom ANSI-25 17 tooth sprockets made specially for their SimpleTube chassis to route the drive chain through the inside of the tube rather than having it outside the tube. In addition, I also designed the frame in a hexagonal configuration to improve defensive capabilities against other robots.

This version incorporates both a chain in tube and hexagonal frame and is made using COTS and modified VexPro parts. It provides several advantages:

  • The chain is protected, meaning that there is very little risk of small game pieces getting caught in the chain.
  • The chain is also constrained inside the tube, which prevents it from slipping off the sprocket.
  • The hexagonal sides provide a defensive edge by preventing the robot from getting T-boned from a side collision.
  • Without the chain taking up space inside the frame, there is more space for electronics and other things inside the frame.

However, there are also a few disadvantages:

  • Maintenance of the chain when it is inside the tube is much more difficult. This is somewhat offset by the fact that maintenance should rarely be needed given that the chain is implemented correctly. However, if something DOES go wrong it is very difficult to fix the chain when it is in such a confined space.
  • Assembly is also more difficult than a typical drivetrain, requiring the chains, shafts, and bearings to be put on in a certain order in order to deal with the constraints of the tube.
  • Some of the traction is sacrificed because of the omni wheels. In a normal 6 wheel tank drive, the middle wheels are dropped lower than the the rest of the wheels to ensure smooth turning. This works by dividing the wheel base into 2 very short wheel bases that have much smoother turning than the whole thing (short wheel base = better turning). This drop design is not possible in this case due to the constraints of the tube. Therefore, omni wheels at the ends are necessary for smooth turning, even if it sacrifices traction. However, according to Arpan the loss of traction is not very large so this may not be too much of a problem.

Based on discussions with my fellow mechanical team members, we have decided that we will need to wait for the game to be revealed to decide if this design can be used or not. This is mainly due the risks associated with it – the CIT concept has never been tested in real life by our team and as such we don’t have a clear idea of whether our team can properly implement this. However, if such a design provides major advantages in the game it may be worth the risk. Chain-in-tube became popular last year due to the fact that robots needed a lot of inside space to fit totes and trash cans inside the frame. If such a thing arises in this year’s game, it may be worth it to try our luck with CIT.

CAD models can be found here.

– David F