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Unread 30-01-2003, 14:10
kmcclary's Avatar
kmcclary kmcclary is offline
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Re: Tranny help

Quote:
Originally posted by Thunder360
Anyone have a good design for a 3 speed thats Pneumaticly shifted, I can only manage a 2 speed in the given space. And I'm also having a down shifting problem its a lil rough, but thats expected from helical gears right? This is all for this years Cim's I have the pneumatic set up for a 3 speed but I'm coming up short on space. So any help would be cool!
MANY teams are finding the trouble and complexity of a shifting gearbox AT ALL may not be worth the time and MTBF reliability risk. It'a amazing at how much you can get done with a ONE speed robot. I'm not sure that you WANT to do THREE speeds, ESPECIALLY if it is NOT running by NOW. The KISS principle should rule at this point in the game, or you may not have ANY drive.

However, If you're REALLY set on doing that, consider combining a TWO speed gearbox with a "wheel switcher". That gives you three or even FOUR speeds.

A "wheel switcher" is any assembly where you drive two wheels off of the same shaft at different tangent speeds (via different sized wheels and/or different fixed gearing), but only ONE of them touches the ground at any one time. It switches speeds simply by using a piston to drop/raise one of the wheels to "take over", or rotates a triangular assembly where the three vertices are: Motor/gearbox, Wheel 1, Wheel 2. Just be sure you use a FAST, HARD throw piston for it so the time where both wheels touch the ground is tiny. It doesn't have to move FAR, just switch tires QUICKLY.

Here's a simple wheel switcher from last year's CDF Gallery by Team 401, which uses a DIFFERENT motor:
http://www.chiefdelphi.com/forums/pi...&quiet=verbose

The big advantages of wheel switchers versus gear switchers lie in that the FLOOR is the "switching part" NOT a mechanically complicated gearbox (which could be torn up by improper switching) so you have a SIMPLE FIXED transmission, and you inherently get "switch on the fly" so you don't have to stop the machine or otherwise unload a gearbox to change speeds.

BTW, I haven't seen it done yet, but given enough SPACE there's no reason why you couldn't make a THREE tire switcher with a pair of cylinders for the three speeds, and run a FIXED transmission. A motor gearbox runs a center, fixed tire. Two fixed length bars, both pivoting concentric to the fixed tire, run off to either side. Attach a wheel to the far end of each, driven off of it's own chain set from the fixed one. A vertical piston drives each end wheel down. Select which one is to be on the floor at any one time.

I don't know if this'll come through or not, but the side view of one end of the robot would look like this (ignore the ASCII Art spacing "dots"):

....Piston....................motor............... .......Piston2
.......V.....................gearbox.............. .........V
WHEEL_B===chain===WHEEL_A===chain===WHEEL_C

The three wheels are running at three different speeds. Wheel A is on the ground when B and C are raised. Push down on either end wheel assembly (ONE ONLY) to jack up the robot onto that wheel. Note the piston's upper end will have to pivot L-R. With the pivot being concentric to Wheel A, the fixed length chain and pair of sprockets for each side wheel each individually "orbit" Wheel A.

[edit]
Note to save linear space you COULD put both the B and C wheels on the SAME side, adjacent to each other. The mounting mechanics might get trickier though to provide solid torque transfer to the frame. BTW... The orbit bar has to be tough, AND kept short, to prevent over torque at the pivot point.


My REAL advice though at this point is to get a ONE speed robot running. ANYTHING running at this point is a good thing. You should NOT still be in a design phase now for your transport chassis, but finalizing your payload package and practicing.

If you ARE intent on additional speeds NOW, try a simple wheel switcher. It can be added to any existing gearbox without much rework.
[/edit]

- Keith
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Last edited by kmcclary : 30-01-2003 at 14:43.
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