In the 2005 & 2006 FVC seasons, we felt that our competition robots needed both speed and torque. Our compromise designs emphasized torque, resulting in a speed disadvantage in competition.  Our team decided that we needed a gear shifter to avoid the speed/torque compromise.

Over the summer of 2006, Team Unlimited produced several prototype chassis to test our Vex gear shift concept. We used the off-season to produce a two speed transmission that was ready and reliable enough for the rigors of competition.  The team researched many different design of FVC, FRC and non-robotic transmissions. The team designed, built, and tested successive designs, until the final shifter was finished.

The final gear shifter settled upon by Team Unlimited is a simple throwing arm design, shifting two sides at once with one motor. Before the design of the actual shifter is discussed, some of the flaws the team found from the building and testing of their previous shifters should be discussed. The first flaw the team found was in the use of servos to shift. The shifters, although seeming almost perfectly designed for a use of a servo, proved unreliable due to insufficient torque.  Gearing to increase torque resulted in insufficient throw travel.  To fix this problem, the team went with a 3:1 ratio of the throw arm, with a motor instead of a servo. The second major problem encountered was the design of the throw arm. At first, the team used a 5x1 piece of Vex bar stock, but found a problem with arc traveled by the throw arm. The team needed to allow the robot to complete the arc for shifting. The easiest way to solve this was to substitute a 5x1 slotted section cut from a slotted angle bar piece, which allowed free travel throughout the arc.

The actual final shifter is fairly simple, only a few moving assemblies and one motor.

Gear shifter - entire assembly

From above

Gear shifter - entire assembly

From above - opposite end

Gear shifter - closeup

Tight on shift motor and gearing

Gear shifter - closeup

Tight on geartrain

Gear shifter - closeup

Medium on shift motor & gearing

An animation of the shifting process will be posted soon.

The key to reliable shifting in this design is in the gears that actually do the shifting. The gears, as seen here, have beveling on them, which makes for simple, reliable and accurate shifting. The beveling works well because it forms a triangle point on the surface of the gears, making the gears more prone to mesh from side to side instead of hitting a square surface and making the gears bend out of the way. This beveling also allows for shifting on the fly, while stationary, and while in the middle of turning.

The job of beveling the gears is best suited for a lathe. For those of us who don’t have a lathe or access to one, a simple power drill can be used to bevel the gears. To bevel the gears, take a small (about 2-3” long) axle, and attach it to a power drill. Next, take a file and place it into a vise to hold it steady. Turn on the power drill and press the gear against the file with medium pressure at a 45° angle. This will produce the beveling. Continue to file away the plastic until the beveled edge touches the rim of the gear, or when the beveling passes the halfway point on the gear. Once the gears are beveled, you will most likely have remains of plastic shards in the teeth, so run a small fine file through the teeth until they are free of debris. For our design, we needed to bevel two 60 tooth gears and two 36 tooth gears.

The next innovation on our robot’s shifter was the “block gear.” This is very simple, just three 60-tooth gears placed together with screws. Just cut the spacing off the side of the gears off and screw the gears together with two one inch screws. This “block gear” allows the removal of the intermediate idle state in the shifting. When the gears move side to side, without the block gear, the robot enters a intermediate state between gears, allowing it to be easily pushed by other robots at that moment.

Our final design lets us choose between a 5-3 and a 3-5 gear ratio, letting us vary the torque/speed ratios at will, allowing what we feel was a substantial advantage in competition.  And best of all - it is all FVC legal!

More photos from our earlier experiments are linked here.

Last modified: 12/05/14     All Rights Reserved, © 2013 by Team Unlimited [FTC 0001]