Roller Chain Based Three Speed Transmission

[crazy_ratchet]

looking at that ilustration seems quite simple. it free spining small diameter sprockets are installed every 120 degrees, 60 degrees out of sink with the drive sprockets, then apon rotation, no 2 drive sprockets are going to touch the chain, eliminating the possibility of lockup.
the way that i think that on the fly shifting could be acomplished is if:

• free spining small diameter sprockets are installed every 120 degrees, 60 degrees out of sink with the drive sprockets
• the distance from center of input shaft to the point of contact of the sprockets with the chain is the same for all 3 speeds/dear ratios
• the angle the chain makes around the transmission assemly would be increased to 140 deg (current design seems to propose something in the range of 90 deg)
• transmission is placed closer to the middle of each side of the robot
• tensioners installed on the free spinning sprockets to take up slack, ortensioners installed on both sides of the transmission along the chain

any comments on my proposal greatly apreciated, sorry if i veered too far away from simplicity in design as the goals.

[Mr. Ivey]

The part you are refering to as a tensioner, is actually called a derailleur in bike terms. It's the part that picks up the slack in a bike chain. But we have passed that part. If you were to put a derailleur bot in front and in back of the power gear, and have them change orientation (the front and back derailleur move the chain up and out of the way from the gear that is being enguaged). One thing about a bike derailleur is that it has a bar under the gear, it's used to keep the chain from getting off of the derailleur. If you were to put this same bar, or a small cog to the under part of your tensioner, you would be able to lift the chain in order to allow the cogs to rotate and not bind. Just a though, it seemed easy enough to me to do this without possibably messing something up while shifting.
Ivey

[Joe Johnson]

Originally Posted by Mr. Ivey

The part you are refering to as a tensioner, is actually called a derailleur in bike terms.

Deraillers do much more than simply take up the slack. I prefer the term tensioner in this case.

[Joe Johnson]

Okay, now that I see that M. intended front and back tensioners, I am ready to pull the rabbit out of the hat...

How about a GENEVA MECHANISM as the shifting mechanism?




Shown above is a 6 lobe version, we would use a 3 lobe variety.

IT'S BRILLIANT!

Hear me out...

The indexed lobe would be the carrier. The shaft with the pin would be driven by a motor, with a switch to stop it at a particular location, specifically, when the drive pin is as far way from the indexed lobe as possible. Looking at the picture above, the pin would be at 3 O'clock.

Here is where the rabbit comes out of the hat: We use the rotation of the pin to disengage and re-engage the chain! Brilliant! Just Brilliant!

Allow me to explain...

We have the "lost motion" of the pin from 3:00 to 11:00 (if the shaft with the pin is rotating CCW) or the motion from 3:00 to 7:00 (if the shaft with the pin is rotating CW) to lift the chain in order to disengage it from the drive sprockets entirely.

Of course this will pull the chain tensioners tight, but that is the reason M. put them there is it not, so that I could do this engineering magic trick? ;-)

After we disengage the chain, we shift (from 7:00 to 11:00 or vice versa), then we lower the chain and engage the chain (from 11:00 to 3:00 CW or from 7:00 to 3:00).

At 3 O'clock we stop the shift motors.

TA DA!

As you can see, I LOVE THIS IDEA. Everything comes together, the shifter is lock when we are not shifting, the chain doesn't bind, we could have one motor drive both sides of the robot (4 Geneva mechanisms total, 2 per transmission), the shifter moves the carriers when there is no load on the sprockets, etc. The list of good things about the design goes on and on...

It is very clever, if I may say so. I think this would be really cool to see in action.

Now, having fallen in love with my idea, I have to tell you all, I really hope that nobody actually goes down this path for their FIRST robot.

Why? Because I don't feel that a 3 shifter is worth the bother in terms of time and weight and engineering effort.

Part of my reason for writing this message is as a cautionary tale to everyone building FIRST robots in 6 weeks.

Fall in love with your designs. Loving your ideas with all your heart is an important part of the design process. Love your clever designs and be an advocate for them.

But then... ...use your brains to decide what is needed and do what your head tells you, not what your heart wants.

6 weeks goes by SO fast.

Good luck to us all.

Joe J.

[Mr. Ivey]

Joe, of course a derailler is much more than a tensioner, it also changes the cog that the chain is on. But to keep it simple for teams without a big budget, and make less shop work to do, make a tensioner that is capable of moving the chain out of the way of the changing cogs.
Ivey