Roller Chain Based Three Speed Transmission

[Madison]

Originally Posted by Joe Johnson

Is the forward red thing a spring loaded tensioner for the chain?

If so, I think you many need one on the back side also (for when the tension in the chain is on the front chain -- this would tend to pull the tensioner solid on the front side, leaving slack to cause issues elsewhere).

Yes, it is. Yes, it does. I didn't include the backmost tensioning mechanism because I was feeling too lazy to mirror the parts in this assembly. It's designed to accomodate a second tensioning mechanism, however.

If not, I am thinking perhaps the shifter will bind during shifting, when the chain is routed over 2 drive spockets. This is especially a problem since the chain will not like to sit over the two sprockets because the they are spinning at different speeds.

That is a concern I share. The easiest solution seems to be avoiding shifting while the chain is moving.

I am concerned about what keeps the two triangular carriers in line with eachother. Perhaps the carriers could be gears and a wide gear could keep them in phase by bridging both sides. Not a perfect solution, but I suppose it could work. Just a thought.

As good a thought as any, really. I hadn't considered that the carrier would twist under torque.

Thanks for the comments, Joe

[dlavery]

Originally Posted by M. Krass

That is a concern I share. The easiest solution seems to be avoiding shifting while the chain is moving.

Joe caught on to the same concern that I have, the potential for binding between the carrier gears/sprockets and the chain while shifting. Just based on an eyeball estimation when looking at your illustration, it looks like two sprockets will be in contact with the chain through at least 60 degrees of arc as you rotate the carrier. Given that the C-to-C distance between the sprockets is fixed by the carrier, the chord length between the interfaces of the sprockets and chains will remain constant. If the chain is not moving, then the sprockets must rotate as the carrier moves them through their arcs (and effectively moves them along the chain). Since the chord length is constant, they must rotate at the same rate. If I am interpreting the illustration correctly, the sprockets are fixed on their respective shafts, along with the associated gears on each shaft. Each shaft will attempt to rotate at the rate of the sprockets. But since the gears are also fixed on the shafts, and of different ratios, the shafts will need to rotate at 1/3, 3x or 9x relative to each other (based on the pair in contact with the chain). Since they can't do both, they will lock up and cause the carrier to bind. If the carrier were only moving through a very small arc (<3-4 degrees), you might be able to rely on slop in the system and available backlash to deal with this. But through 60+ degrees of arc, I think there is going to be a problem.

This is a very creative idea, and you are taking things in an innovative direction. But I can't see how the carrier can rotate without binding (whether the chain is moving or not). Am I missing something?

If you can figure out how to resolve this, I do have a suggestion regarding regulating the motion of the carrier. Rather than using a motor to rotate the carrier (and using up one of the available motors, one of your stated "disadvantages"), you could attach a very simple ratchet and pawl mechanism to the carrier, and have the carrier rotated by extension/retraction of a pneumatic piston that reacts against the "arms" of the carrier. It would limit the transmission to shifting in a 3:1/1:1/1:3 pattern, but based on the needs of the game or robot design, that might be OK.

Keep going with this - it is a cool idea!

-dave


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Vmax = 10 ft/sec

Yes, there are two paths you can go by
But in the long run
There's still time to change the road you're on.

[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