Mechanical Auto-shifting

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.

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

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

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

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

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.

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.

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:


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. :frowning: We haven’t even used timing belts before, although we plan to change that this summer.

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.

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 :slight_smile:

We build out of a snowmobile repair shop so we have a great deal of hands-on knowledge with this sort of technology hanging around the shop (pretty much all snowmobiles use CVTs of this type).

First off, the traditional design does not function in reverse as you depend to a certain degree on the torque of the belt to influence the movement of the secondary sheaves. It would require extensive reworks of the helix and ramp angles to get it to even function marginally in reverse. (I have a friend who built a gokart and tried mounting the secondary running backwards due to space constraints and we could never get it to shift properly).

Secondly, the losses with a belt based CVT are fairly significant for an FRC application I doubt you would want to incur them.

The comet system is designed for engines from 5 to 8hp. To use it on a low power application would require massive reworks of the springs and weights inside. Again we come back to the power losses once you did get it to work. Those comet belts are fairly beefy when you’re talking about CIM motors. Also the ratio I believe varies only from 1.6:1 through to 0.89:1 which isn’t too much of a spread.

Typically, the purpose of these types of CVTs are to keep an engine running within a very specific power band. Gasoline engines, especially the 2 stroke variety have a very narrow range of RPMs where they produce peak power. CVTs of this type are designed so that the weights and springs which control them are at equilibrium with each other when the engine is at a specific RPM. Any faster, they upshift- any slower, they downshift to balance the forces within the sheaves. The result is that the machine is always running as fast (Given WOT) as possible by keeping the engine in its optimal power band. With electric motors, you don’t exactly have the same goal since their powerbands are vastly different. You can trade amperage and voltage for torque whereas on a gasoline engine you do not have the same luxuries. It would take extensive testing to develop a purely mechanical system which optimizes the power characteristics of an FRC drivetrain. For this reason I would say an active system controlled by encoders, current sensors and programmed logic would be vastly superior and much easier to tune.

If you were to do away with the mechanical weights and springs in favor of a servo motor to control the sheave separation you would solve the reversing issue. If you were to downsize the whole thing and use something like a silicone O-ring as a belt running at high speed to reduce your losses, I think you would have a viable system.

All this being said though, I think you would enjoy much improved reliability, reduced complexity, weight and improved efficiency to just pick up some ball shifters. I don’t know that the benefits would outweigh the drawbacks. The again I have always thought it would be cool though to run all your motors at full speed and control the drivetrain through an infinitely variable transmission like a hydrostatic system :smiley:

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

DC brushed motor has no torque at full speed. I think you meant full voltage.

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.

If one were hellbent on an infinitely variable transmission, I would start with something like what is found in a snowblower:

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.

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