03-11-2014 11:44

jwfoss

Re: pic: 3CIM Single stage shifter

This is some slick packaging. Nice work!
Any chance you've got a view from the rear of the gearbox?

03-11-2014 12:37

billbo911

Re: pic: 3CIM Single stage shifter

I like the packaging a lot. That said, a 1.48 spread seems a bit tight. Unless you plan on using tiny wheels, it looks like you will need a third stage of reduction some where in the drive train.
Do you have plans for additional reduction?

03-11-2014 12:49

Lil' Lavery

Re: pic: 3CIM Single stage shifter

Based on the small amount of reduction and the relatively short output shaft, I'm going to assume you're planning on more reduction via sprocket/pulley after this gearbox.

03-11-2014 13:48

jman4747

Re: pic: 3CIM Single stage shifter

That's a positive on the extra reduction. Probably 12t - 26t sprocket to some 4in wheels.

03-11-2014 17:11

Deke

Re: pic: 3CIM Single stage shifter

That's pretty slick, I like it!

Question, what's retaining the gears on the cim shaft?

03-11-2014 17:56

jman4747

Re: pic: 3CIM Single stage shifter

It's not reflected in the photo but I was considering a few different ways. #1 right know would be to press plastic thrust washers into a holes concentric to each motor shaft to keep the gears on the Hex/8mm adapter. Also adding some sort of adhesive to the Hex/8mm adapter. Most of everything else I thought of required making my own hex adapter instead of just buying one.

03-11-2014 18:47

BBray_T1296

Re: pic: 3CIM Single stage shifter

Any reason you chose to put the higher reduction on the outside (further from the CIM)?
Your low gear would be operating at higher torques and there would be more deflecting force attempting to separate the gears.

This is probably not even a real issue but with a cantilevered shaft it is usually better to keep the net force closer to the bushing.

03-11-2014 19:26

jman4747

Re: pic: 3CIM Single stage shifter

Your right, my only real reason was: 1 if I wanted to replace a motor I could in this configuration pull it straight out (assuming I enlarged the hole the cim shaft goes through). If the smaller cim gear was closer to the motor the 60t gear and 24t gear would block each other. Granted that is dependent on exactly how the cim gears are attached to the shaft and how easily they come off. 2 I figured it would be okay with this short of a shaft. It seems almost arbitrary and if/when we use it we'll probably try both? Maybe one is significantly better.

03-11-2014 22:36

Chris is me

Re: pic: 3CIM Single stage shifter

It looks like there's *barely* any spread between the two ratios. How different are the output ratios?

I've spent considerable time trying to make a first-stage shifter like this work, and ultimately I gave up when I realized that I couldn't package a spread of two good ratios together well enough to justify the complexity.

03-11-2014 23:16

billbo911

Re: pic: 3CIM Single stage shifter

It would be quite simple to replicate spread of the standard VexPro Ball Shifter of 2.27.
All you would need to do is use the 8mm to .500in Hex adapter from Andy Mark on the CIMs. Then mount a 24 tooth .500in Hex bore (27-2704) and 40 tooth .500in Hex bore (217-2708) from VEXpro on the CIMS. Then use the standard gear set (60 and 44 tooth) on the shifter/output section.

04-11-2014 00:24

asid61

Re: pic: 3CIM Single stage shifter

+1. Also, the 8mm to 0.5in hex adapter fits two gears on it perfectly, so you can place two gears on the cims with no problem.
Seeing as you will need another reduction stage anyway, I don't think it can hurt to get a bit mroe spread. The main reason why I avoid single stage shifters like this one is mainly because it's slightly heavier with the extra outside-gearbox reduction that's needed. On the other hand, it's easier to replace a broken gearbox.

04-11-2014 00:55

R.C.

Re: pic: 3CIM Single stage shifter

I would personally opt for the VEXpro adapter:

http://www.vexrobotics.com/vexpro/ha.../217-3255.html

A little cheaper and a little longer.

04-11-2014 16:12

billbo911

Re: pic: 3CIM Single stage shifter

Agreed!
I didn't even know it existed. Maybe I should have actually looked.

04-11-2014 20:57

Nuttyman54

Re: pic: 3CIM Single stage shifter

The problem is that the reductions you get from those stages are to small to realistically do a direct chain reduction, so you'd need to add an additional gear stage...at which point you're back at something akin to the available COTS options. You're trading reduction for spread

The stages the OP has modeled are 18:60 and 24:54, which are 3.33 and 2.25 reductions, respectively.

The alternative gearing you propose to get the 2.27 spread (24:60 and 40:44) only give 2.5 and 1.1 reductions, respectively.

You could maybe do a 34:44 reduction instead of the 24:54 reduction, for a 2.57 spread (almost the same as an AndyMark Supershifter), but that still ends up awfully fast (20fps range) with a 12:36 chain reduction.

05-11-2014 11:30

BBray_T1296

Re: pic: 3CIM Single stage shifter

Where has this been all my life?

05-11-2014 13:08

billbo911

Re: pic: 3CIM Single stage shifter

No doubt about that, it is NOT an ideal way to go. There needs to be a more reduction at the first stage to make this a viable solution. I was just trying to get the spread a bit greater.
Honestly, after using the original ball shifter for two years now, we would like to see a bit more spread than 2.27.
2.57 is probably closer to where we want to be, although even that may not be absolutely ideal either.
Ahhhh compromise, the bane of all things Engineering!

05-11-2014 16:44

jman4747

Re: pic: 3CIM Single stage shifter

If you are willing to use 9-11 tooth sprockets from McMaster and Vex 28-33t plate sprockets you can get 2.92 with standard Vex Gears.

Fast: 30t CIM, 44t Ball Shift

Slow: 14t CIM, 60t Ball Shift

Final: 10t Sprocket, 30t Sprocket