Most shifting gearboxes have this basic structure (
this schematic might be helpful):
- The drive motors all drive a single gear on what many call the cluster shaft.
- The cluster shaft has two other "cluster" gears on it. All three of these gears turn together.
- The two cluster gears drive the two gears on the shifter shaft (in this case, they have a profile in their side to accomodate a dog, or, alternatively, a different profile for the ball shifter). These dog gears spin about the shifter/output shaft, but they are not mechanically linked to it. This is because they are spinning at two different speeds, as each dog/cluster pair has a different gear ratio. Instead, these dog gears have bearings in them to let them spin freely.
- A dog (or ball shifter equivalent) is mechanically linked to the shifter shaft by a hex profile on the shaft (like a hex hub sliding on a hex shaft). The shifter shaft is hollow, so a small rod coaxial to the shaft(usually attached to a pneumatic cylinder) is linked to the dog. By sliding the dog back and forth along the shaft, you can select which dog gear it engages, effectively controlling which cluster/dog ratio is engaged with the shifter/output shaft.
-sometimes, depending on how you plan to deliver power to your wheels, a third stage gear reduction is added after the shifter, but this isn't always the case.
This is the standard practice used on the AndyMark, VexPro, and WCP shifters, as well as many shifting gearboxes that various teams have designed and built themselves, but many adaptations and variations have been developed. Team 192 comes to mind as having some really gorgeous, non-conventional gearbox designs.