[Andrew]

We have used 20 dp on our final stage of the gearbox for 2003, 2004.



In 2003, we used 5/8" shaft with a 3/16" key way and a .5" face width, and the key ways started to run. I.e. this was a marginal condition. We performed a lot of pushing with 2003's robot, so this may be about as far out as we would ever go with a FIRST design. The gear teeth showed no appreciable wear.

We had a six wheel design, so the robot's weight was distributed over six wheels and two gear boxes. The middle wheels were chain driven to the front wheels.

The worst wear in the key ways occured in the gear box which had the BDMs powering and the middle+front wheels. I.e. we had more torque available and we had more normal force on the ground contact side.

In 2004, we used 1/2" hex stock and a .4" face width. We had six driven wheels, each with its own motor, so that, nominally, each wheel had the same normal ground contact force. We saw no appreciable wear in either the hex shaft or in the hex 'hole' in the gear. We saw no appreciable wear in the gear teeth. This robot also did a lot of pushing (we tried to move the center platform in autonomy a couple of times); however, since we had added another set of motors into the drive system, we backed the gear ratio down a bit.

Nonetheless, since the force on the output gear is more strongly affected by the available force at the ground than by the gear ratio, the loads on the output gear and shaft would be similar. I still think 2003's robot saw more load on the BDM gears than in 2004. However, the difference in performance with using hex stock instead of round shaft+key ways was remarkable.

In the intermediate stage in the gear box, we use 24 dp in 2003 and 20 dp in 2004. The reason for the change to 20 dp in 2003 was that we decreased the number of teeth in the pinion gear. We used 3/8" hex stock for this stage and the ID for a 24 dp gear was so close to the hex size that we did not have enough wall thickness. In order to keep the design gear ratio, we bumped up the pitch from 24 to 20 and moved the center distances.

We used a face width of 0.2" for these gears and saw no appreciable wear in teeth, shaft (aluminum yet!), or gear 'hole.'

In our mini-gearboxes for the FP motors, we use 32 dp to match the motor's pinion gear. In our mini-gearboxes for the BDMs, we use 0.7 module gears to match the motor's pinion gear. Although we used metal gears in 2004, we would probably use plastic gears in this stage in the future, both to save on weight and cost.

However, we'll have to see what the design rules for 2005 are (i.e. can we buy from anywhere? do we have BDMs and FPs? Etc.)