How do ball shifters and dog shifters work?

[tahmid0517]

My team's looking to use Vexpro's ball-shifting gearboxes next season if it's a game with a good amount of contact. While we don't really need to know how they work to use them, I was just curious and wondered how they work.

[Chris Endres]

Ball shifters are quite interesting. There is a primary drive chamber, which holds balls (usually three on each level) at different heights. Those balls, when forced outward, engage the gear which is at the same level as the balls. The balls are forced outward by the piston plunger, some use servos, but VEXPro uses .5" throw pneumatic piston. Anyways, the plunger is either engaging the top gear or bottom, when the plunger shifts, it allows the balls from the previously driven gear to recede into the main drive chamber. Then the second gear is engaged by the plunger forcing the balls outward into the gear's pockets for the balls. This system makes sure that the two gears are powered at the same time, and it is a bit more complicated than dog shifters (but not by much). A good way to really see how they work is by opening the shifters up and move the plunger up and down (just watch out because the balls will fall out without a gear holding them in).

Overall, the drive shaft is powered through three little balls (yeah, weird that all of your robot's motion is in the hands of a few little ball, but it definitely works!). The main drive chamber is then connected to an output shaft, in VEXPro's case, a .5" hex output.

I'm sure there is a better way to word and name everything, but this is how I know they work. You can also look at CAD designs of custom shifters as well as VEXPro's model, you can see the geometry of the drive chamber/shaft, plunger, balls, and gears.

[asid61]

Here is a video that explains it quite well.

[Chris Endres]

Here are some cross sections of the ball shifting system.

The first is VEXPro's shifter, and the second is my custom shifter for a summer swerve project.

[KrazyCarl92]

Others have already responded with explanations of how the ball shifting mechanism works, but for the sake of the OP I'll answer the question at a higher level.

The idea for both of these shifting mechanisms is to be able to couple and decouple the rotation of gears and their shaft. In both of the styles of shifting gearboxes sold by VexPro you have a first stage reduction of CIM pinions to a larger gear followed by 2 pairs of gears, 1 pair each for high and low gear.

At all times, the high gear gears and low gear gears are meshed. However, the gears that interact with the shifter mechanism (either the ball shifter or dog shifter) have bearings pressed into them to allow them to ride on the shaft but not necessarily have the same rate of rotation as the shaft. When the balls or dog are engaged, the torque applied to the gear will be transferred to the shaft and that particular gear and the shaft will rotate at the same rate. When the balls or dog are disengaged, the gear's rate of rotation is allowed to differ from that of the shaft and torque is not transmitted by that gear to the shaft.

In high gear, the smaller output gear is engaged with the shifting mechanism, its torque is transmitted through the shifting mechanism to the shaft, and the shaft rotates at the same rate as this smaller gear. The larger gear is disengaged and rotates at a slower rate than the shaft without transmitting torque to the shaft, as allowed by the bearing that is pressed into the gear.

In low gear, the larger output gear is engaged with the shifting mechanism, its torque is transmitted through the shifting mechanism to the shaft, and the shaft rotates at the same rate as this larger gear. The smaller gear is disengaged and rotates at a faster rate than the shaft without transmitting torque to the shaft, as allowed by the bearing that is pressed into the gear.

The shifting mechanism and its associated gears and shaft allow for the ability to switch between these two states.

[EricH]

Seeing as I haven't seen anybody answer this one yet...

A dog shifter has a slightly different setup. Two gears on the same shaft (one for high speed and one for low speed) have machined hubs that match the dog. The dog is able to slide a short distance between the gears, just enough to disengage one and engage the other (or vice-versa). I can't recall offhand exactly how it's set up to shift (I'd want to go read through the CAD and drawings again), but a pneumatic cylinder forces the dog into one gear or the other--servos are generally too weak for this particular application.


One other method of shifting--though it's generally used in custom transmissions--would be to simply slide the entire shaft over with the gears, but that involves some rather wide gears and some side impacts gear-gear--not exactly the most durable solution if you can find a better one.