Mechanical Auto-shifting

[asid61]

After seeing 971's friction clutch this year, I was wondering if any teams have made a mechanical auto-shifter in the past.
I'm not talking about code to shift when the current spikes or something like that, something more like a friction clutch that would automatically shift back into higher gear once load lets up. 971 definitely had a good design, but they were still using a pneumatic cylinder in place of something like a spring.

[IKE]

Historically these were accomplished using flyweight mechanisms that engage at s particular speed.

[Jared]

I know some lawnmowers have centrifugal clutches so that when you start them and the engine isn't going quickly there's no load.

http://en.wikipedia.org/wiki/Centrifugal_clutch

[Ether]

Has any FRC team ever tried using a comet torque converter in their drivetrain?

[asid61]

So just flywheels then? I remember that one team had a toroidal CVT a long time ago, but not recently.

[IKE]

I have thought that there might be some neat potential to use some window motors and a differential with some powerful motors to do a neat IVT style drive.

[Ether]

Originally Posted by asid61

So just flywheels then?

Please explain what you mean by "just flywheels".

[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.

[Brian C]

Originally Posted by asid61

So just flywheels then? I remember that one team had a toroidal CVT a long time ago, but not recently.

In 2002 team 190 from WPI had a CVT using a toroidal design. I believe they were made from cast iron.

[JohnFogarty]

I thought I heard from someone that 190 was using a CVT again this year.

Originally Posted by Ether

I think they're bi-directional. Next time I disassemble the comet on the go-kart to clean and lube it I'll take a closer look.

By the way, I suppose you get this a lot, but every time I see your name I get the 60's deja-vu (even though it's spelled differently :-)

That would be awesome to know since I don't have access to one myself at the moment.

Yeahh, I do get that a lot. It's still funny to me that the name had nothing to do with him, it was shear coincidence.

[Francis-134]

Originally Posted by JohnFogarty

I thought I heard from someone that 190 was using a CVT again this year.



That would be awesome to know since I don't have access to one myself at the moment.

Yeahh, I do get that a lot. It's still funny to me that the name had nothing to do with him, it was shear coincidence.

Sorry to disappoint, but we are not using a CVT, just a standard 1-speed gearbox to a six wheel drive.

[fox46]

If one were hellbent on an infinitely variable transmission, I would start with something like what is found in a snowblower: http://www.youtube.com/watch?v=hZD5toJyjyQ

Basically, the engine drives a round plate and small rubber wheel is driven by said plate. The shift linkage moves the wheel to different points on the round plate. As the circumference of the small wheel's contact patch changes you end up with different gear ratios.

It would be quite simple to build something like this- use a banebot wheel on a section of hex shaft and a mechanism to slide it back and forth depending on the desired gear ratio. You'd have to run it at fairly high speed though to minimize the slippage.

[JohnFogarty]

Originally Posted by Ether

Has any FRC team ever tried using a comet torque converter in their drivetrain?

A few of us mentors of 4901 were talking about making an FRC version of that.
I personally don't know if they could go in reverse.