power transmission NOT using gears/chains?

[Madison]

Originally posted by KenWittlief
The thing I like about this idea the most, you could do this without the oneway clutch (ratchet) with gears and/or chain drive , and to make it a two speed self shifting tranny, all you need to add is one moving part - the little spring loaded cog level thingy.

How could this work without the one way clutch? If there's no clutch to slip when the output spins faster than what the high-torque motor is capable of at free-speed, that motor will drag and be spinning faster than it's free-speed.

...or, because you're making it spin faster than it's "no-load" spin mean that there won't be any appreciable effect on on the output? That is, at its free-speed, the high-torque motors does no work, so, conceivably, it requires very little work to keep it spinning at that speed? Would spinning it faster than it's free-speed damage it?

[Jnadke]

Originally posted by M. Krass
How could this work without the one way clutch? If there's no clutch to slip when the output spins faster than what the high-torque motor is capable of at free-speed, that motor will drag and be spinning faster than it's free-speed.

I think he's talking about using a reversible clutch bearings, like the one's used in socket wrentches. There's no way this idea could be implemented without the use of a clutch bearing.

I've thought about using such a design before, but ruled it out because reversible clutch bearings are rather complex.


As for spinning a motor faster than it's free speed, Lenz's law prohibits this. If a motor is electrically connected, it will act as a generator, creating a resistive force to the object that is turning it.


Although, the speed controllers do technically have the ability to disconnect a motor electrically. You could implement the idea without a clutch bearing as long as the 2nd motor is off when you try to use the high-speed motor. The negative side of this is that you'd be adding to the overally inefficiency of the drive system because you're rotating more mass inside the motor. Also, the brushes and bearings add inefficiencies. Overall, your efficiency would probabaly drop by 20%. I'd only do this if the high speed motor is 2x the low speed motor, because of the inefficiencies.

[KenWittlief]

what I was trying to say is you can build a drive train with two motors driving one shaft (or wheel) that is not self shifting

and to turn it into a self shifting two speed system, all you have to add is one moving part - the spring loaded tab that locks into the gear in one direction only.

This is not complex - its extreemly simple. Get you hands on the back wheel from a ten speed bike and take the rear gear cluster off (all the bearings will fall out - use a junk wheel)

You dont want to do this without the one way clutch - spinning a motor past its top speed, loaded, connected or otherwise, is still a huge drag on the driving motor.

and besides, motors have a physical top speed - if you spin them any faster the centrifical force rips it apart.

[GregT]

Why so complecated? Its possible to make a efficient CVT using only gears, its not used in cars because it requires 2 torque sources.

greg

[Adam Y.]

How could this work without the one way clutch? If there's no clutch to slip when the output spins faster than what the high-torque motor is capable of at free-speed, that motor will drag and be spinning faster than it's free-speed.

Actually I do not think you would destroy the motor by spinning it faster than its free-speed. The speed-controller is another story.

[Jnadke]

Originally posted by GregT
Why so complecated? Its possible to make a efficient CVT using only gears, its not used in cars because it requires 2 torque sources.

There is no way (that I know of) to create a CVT using only gears.

A real CVT allows for infinitely variable gear ratios (or at least between two points). Why? If you look at a motor curve, typically maximum efficiency (ratio of input power to work performed) is at 75% of free speed (25% power). The theory behind the CVT is to keep the motor operating at this free speed and vary the gear ratio during the acceleration phase.

The only CVT's to date are the Belt CVT and the Toroidal CVT. There are also hydro-mechanical CVTs.

[ChrisH]

Originally posted by Jnadke
There is no way (that I know of) to create a CVT using only gears.

I'm not sure if it would be considered a true CVT or not. But you can use an epicyclic or planetary gear system to create a similar effect. As GregT mentioned, it does require two torque sources and the ratio is changed by changing their relative rotational speeds.

I have a little hand operated demo version our machinist built that shows the concept pretty well. We were going to use it in 2002 but it didn't fit the robot concept. This year we didn't feel we had the weight available so we haven't put it into practice. Maybe next year, now that we feel a little more confident building gearboxes.

The Thunder Chickens' Crazy Chicken Transmission of 2002 worked on this principle. I think that the Gila Monsters used it this year. There may be others out there that I don't know about, but FIRST teams have done this in the past and it has been effective.

I'm not sure how efficient it is. Paul Copioli posted a white paper a while back that indicated the power of the second motor is basically lost. I went over the equations and didn't see anything obviously wrong, but a different configuration may require the equations to be set up differently and so give a different result. I haven't gone back and checked this myself, yet. I may get around to it someday, but I have a different area of interest at the moment.

[Jnadke]

Originally posted by ChrisH
I'm not sure if it would be considered a true CVT or not. But you can use an epicyclic or planetary gear system to create a similar effect. As GregT mentioned, it does require two torque sources and the ratio is changed by changing their relative rotational speeds.

I don't think I'd consider that a CVT. Yes, I've studied it before, and even created my own design. It's merely a dual-input viariable speed gearbox.

As for efficiency, in high speed it is extremely inefficient, because you must provide power to the central motor in order to give a surface for the high speed motor to react from. But it does serve its purpose since we have a limited set of motors to work from. I suppose you could make it fairly efficient if there was only a 2:1 difference between the two motors.