3-motor transmissions

Based on a topic brought up in the "Why not shift" thread (http://bit.ly/UgKdrg)

With the new Vex 3-motor, single-speed gearbox, something I've been looking into is why certain teams have or have not used a 3 motor transmission, and the logistics of such.

Here are some of my main concerns:

1) Have you used a 3 motor transmission in the past before?
If no, why not?
If yes, why did you want one? Do you think it was worth the extra work? Did you have any problems? Was it a shifting transmission or a single speed?

2) What kind of motor would you use in the 3rd motor slot? I've heard a lot about teams ditching their 3rd motors due to burning them out too often. How is this caused, and what precautions can be taken to prevent it?

3) What kind of wheels did you use with the 3-motor transmission? Did they work into a strategy used with the 3-motor transmission? Had you had a 2 motor transmission would you have used different wheels? When in the design process did you decide to use a 3-motor transmission?

4) Last but not least, what other advice would you have for a person looking into using a 3-motor transmission?


Thanks!

[ttldomination]

Originally Posted by SuperNerd256

1) Have you used a 3 motor transmission in the past before?
If no, why not?

I'll add this one because, in short, this is the only experience I've had.

The short answer to this simply the fact that there wasn't a resource available that provided us with this capabilities.

One of my mentors always believed in using everything that gives us power in the kit. If we didn't have a PDV from AM, I'd pick up one of the VEX gearboxes and cross my fingers that we'll get a chance to slap six motors onto the DT.

- Sunny G.

[sanddrag]

3 motor shifting in 2005 with 696. Didn't really need nor using. Not a smart decision.

3 motor shifting in 2008 Overdrive with 968. Worked fantastic, but we ended up changing the robot design, and borrowing two motors off the drive. Robot accelerated noticeably slower without them. Geared for around 21 fps top speed that year.

3 motor single speed in 2012 with 696. Worked great. Geared for pretty good speed, gave extra torque to get over the bump, push robots up bridge, etc. Never a problem.

[dcarr]

Originally Posted by sanddrag

3 motor single speed in 2012 with 696. Worked great. Geared for pretty good speed, gave extra torque to get over the bump, push robots up bridge, etc. Never a problem.

Your 2012 bot certainly looked good moving around on the field. What third motor did you use?

Edit: nevermind http://www.team696.org/history/index.php?qhr

[Billfred]

2815 (and 1398) used a six-motor shifter in 2010; our design had the motors available, so why not?

We used two CIMs and an F-P with the AM Planetary on each side. No issues with either. AM Plactions were the wheels, with roughtop.

In hindsight, it was overkill for our one-zone strategy...but it also didn't harm things either.

[Nuttyman54]

The reason why not is that it tends to be overkill in a lot of cases, and the motors that add a lot as a 3rd motor (BB775's, 550's and FP's) tend to be your best motors to use elsewhere. The DT motors tend to be abused a lot too, and that can have a detrimental effect on air cooled motors like the banebots and Fischer-Prices if they're not designed in right.

There are two main reasons you would want to do it:

1) You want to be traction limited but have a high top speed (>13 ft/s) with decent acceleration and not require a shifting gearbox

2) You have a shifting gearbox and want a really high top speed without an acceleration penalty (>17 ft/s).

Both of those speeds are rough rules of thumb based on a 150lb robot (inc. batteries and bumpers) running wedgetop tread. That's just how the math tends to work out. The main advantage you get from the extra motor is the acceleration and torque increase at higher speeds. It really has no effect on the low speed performance, provided you're still traction limited*.

Personally, I don't see many games where it's necessary, again going back to the motors that are used. They need to be fairly high power to have any noticeable improvement (remember, 2 CIMS is a pretty beastly drivetrain already). I would argue that for the vast majority of teams, it's more worthwhile to go 6 motor driveline to get rid of shifting than to get that extra jump from 17 ft/s to 20 ft/s, but each team has to make that call. If the majority of your robot mechanisms are going to be pneumatic, then it's less of a sacrifice.

Don't forget to think about your robot from an entire system standpoint. While it might be great to zip around the field at 20 ft/s, if your arm take 3 seconds to score an object because all you had left were window motors, you might actually be taking longer to score than if you'd left your robot at say 15 ft/s, but take only 0.5 second to score.

254 has used 3-motor transmissions several times, although very few have been successful. I recall their 2005 (and maybe 2006?) drivetrain had FP's as the 3rd motor, and they burned several of them out. In 2012 they intended to have three motors (BB775's IIRC), but ended up reallocating them to other robot functions, and changed their gearing to a lower top speed to account for this.

*I would never, EVER recommend designing a drivetrain that isn't traction limited in it's lowest (or only) gear. That's a great way to burn out motors and blow breakers when you run into robots and walls. 3 motors will NOT make you stronger in a pushing match.

[roystur44]

A 3 motor 3 position shifting transmission could be used for a number of things. The third motor could be shifted to a winch or roller pulley for example(power take off) and still have your main drive working. But if you needed the extra turbo power the transmission could shift the third motor to the drive train gear. Couple that with a electrically actuated disc brake and a over current protection sensor and we have something pretty darn cool.

I hope this years game is a racing game with full size robots climbing poles at end game aka minibot style

[CalTran]

Originally Posted by roystur44

The third motor could be shifted to a winch or roller pulley for example(power take off) and still have your main drive working.

Is there anywhere one can find some in depth information about how an FRC power take off would work? I know there are teams who have used them, IE 1114 in 2010, but I've never been able to find a paper or an explanation or a CAD of how they work.

[sanddrag]

For a PTO, look at 254/968 in 2010. It was basically another shifting setup. Another piston and another dog and dog gear.

The first PTO I saw in FIRST was on Team 60 in 2004. Amazing mechanism they had.

[CalTran]

Originally Posted by sanddrag

For a PTO, look at 254/968 in 2010. It was basically another shifting setup. Another piston and another dog and dog gear.

The first PTO I saw in FIRST was on Team 60 in 2004. Amazing mechanism they had.

Funny, I'm wearing a Cheesy Poof sweatshirt right now...Would a PTO take more customization of a gearbox, as I imagine you would need to shift to a different axle instead rather than the axle the wheels are on?

[AllenGregoryIV]

I was just looking at 973's 2010 robot on FRC-designs.com last night and they have a power take off system.

http://frc-designs.com/CAD2010.html#frc973

[CalTran]

I'll check the CAD tomorrow and open a new thread if need be to stop hijacking this one

[roystur44]

Originally Posted by sanddrag

For a PTO, look at 254/968 in 2010. It was basically another shifting setup. Another piston and another dog and dog gear.

Not 100% sure but I believe they used a 3 position piston for the main drive shaft. They had 2 speeds and neutral. They shifted to neutral and activated the second piston to engage a gear on to the main drive shaft. All the power was routed to the second drive shaft via the engaged gear.

Boy was that cool to see.

[sanddrag]

Originally Posted by roystur44

Boy was that cool to see.

Even more fun to help design But most of the credit on that lies with others.

[AllenGregoryIV]

Anyone know what 3 position piston they used in these setups?