Mechanical Auto-shifting

[asid61]

Originally Posted by Ether

Please explain what you mean by "just flywheels".

I don't mean it in the derogatory sense. I just asked if the flywheel shifting design was the only kind out there.
Flywheels are cool; they make CVTs pretty simple. I just want to avoid using them just yet due to space constraints.

[BBray_T1296]

How about a variable diameter pulley system? seems pretty simple, and could be done with a simple Vex motor. We have one in our drill press. Just a turn of a knob and it varies by a spread of ~3x.

Basically you can relate it to a dog gear and receiver, except they always touch (the teeth are long), and are two sides of a V pulley. By pushing/pulling the sides together on one side of the two pulleys (that the belt runs between), the running diameter of the belt is increased or decreased. The other pulley has a spring attempting to lightly compress it, maintaining tension in the belt. As the motorized pulley expands, it demands more belt length and thus reduces the diameter of the spring pulley.

In ASCII terms:

|\....../|......|\..../|.....|\B/|
|.\..../.|......|.\B/.|.....|./\.|
|..\B/..|......|../\..|.....|/..\|
|=====-->====-->====
|../B\..|......|..\/..|.....|\../|
|./....\.|......|./B\.|.....|.\/.|
|/......\|......|/....\|.....|/B\|

The problem with this is it can only shift on the fly. It is unrealistic to shift while not moving, as it would be very difficult to pinch the belt upwards, so, in other words, not an FRC robot. It is a cool idea though

[asid61]

Originally Posted by BBray_T1296

How about a variable diameter pulley system? seems pretty simple, and could be done with a simple Vex motor. We have one in our drill press. Just a turn of a knob and it varies by a spread of ~3x.

Basically you can relate it to a dog gear and receiver, except they always touch (the teeth are long), and are two sides of a V pulley. By pushing/pulling the sides together on one side of the two pulleys (that the belt runs between), the running diameter of the belt is increased or decreased. The other pulley has a spring attempting to lightly compress it, maintaining tension in the belt. As the motorized pulley expands, it demands more belt length and thus reduces the diameter of the spring pulley.

In ASCII terms:

|\....../|......|\..../|.....|\B/|
|.\..../.|......|.\B/.|.....|./\.|
|..\B/..|......|../\..|.....|/..\|
|=====-->====-->====
|../B\..|......|..\/..|.....|\../|
|./....\.|......|./B\.|.....|.\/.|
|/......\|......|/....\|.....|/B\|

The problem with this is it can only shift on the fly. It is unrealistic to shift while not moving, as it would be very difficult to pinch the belt upwards, so, in other words, not an FRC robot. It is a cool idea though

Yeah, I've seen those kinds of things before. THey used to use a similar system in cars IIRC.
The reason I am shying away from such things is simply because I don't trust our team to design it properly. We don't have any mentors who could feasably design one, and in any case the students would want to design it anyway. These v-belt systems are a little beyond our scope right now. We haven't even used timing belts before, although we plan to change that this summer.