Chiaphua Gear Box

[Cory]

Originally Posted by Matt Adams

nobody's going to convince me that it's better to have wheels cantilevered off to the side in FIRST.

No more $.02... I'm dropping a quarter.

Matt

Nobody said it was better. It might be appealing to some because of the lost weight on an outer rail, and bearings. It's just different, and not necessarily bad if you do it properly.

Cory

[NateBot16]

Last year we had wheels cantilevered off the side of our robot, very bad idea. During testing our pillow block broke (more likely to cheap parts) so we made our own. And in the Florida regional our bot lost a wheel. Luckly we were the only team still functioning at the time, so we had no trouble going up the ramp and winning the match. I still laugh when I think about how the camera zoomed in on our lone wheel surounded by boxes and panning over to our robot sitting on top of the ramp victorious. Sorry I got a bit off topic there.

In general we always try to keep a shaft supported at two ends. If a gear, sporket, what have you, comes loose during a match it doesn't fall off that way.

[Joe Johnson]

Matt,

I don't want to budge you, but I do want to urge you to open your mind a bit.

First, let me agree with you: All things being equal, simply support shafts are better than cantilevered ones, especially in cases where there are high loads (like wheels) and where the stakes are high if the shaft fails (like wheels).

BUT... ...engineering is always a case of tradeoffs and things are never equal. Specifically, there are advantages to cantilevered shafts. If you can deal with the problems associated with them, it can make good engineering sense to use them.

My larger point is this: Engineering is full of such issues and trade offs. While it is good to make people aware of the potential problems associated with this or that path, I think it is bad form to turn such decisions into religious debates where one side or the other is just wrong.

Over the years, I have used cantilevered shafts from time to time, even on wheels -- ZOUNDS! I might be a heretic...

...or maybe a good engineer.

Joe J.

[Matt Adams]

Originally Posted by Joe Johnson

My larger point is this: Engineering is full of such issues and trade offs. While it is good to make people aware of the potential problems associated with this or that path, I think it is bad form to turn such decisions into religious debates where one side or the other is just wrong.

Joe, (and everyone in general)-

As I'm sure many of you have figured out... I haven't been around these forums too long, I'm not a practicing engineer, and I have a BOATLOAD of opinions. This could typically make what would a rather... frighting sort of poster. Perhaps I am.

As a general heads up... a lot of time I'll close off my posts with some sort of little "twang". Perhaps this comes off a little more cocky than I mean it to be, but I've noticed that people (often the most qualified) tend to "bite" on those sorts of thing, which is the reason I do it. Though some people like to read posts where people state the obvious and mumble about how there's exceptions to every rule, I like to make sure that people DO list the exceptions, DO tell how things work in the real world, and DO explain the how's and whys.

To maybe make the case... I love to have a bold general opinions that others can shoot down with exceptions. Some of the best threads I've participated in so far were because I stuck my neck out and held up generalizations as truths, just to see what people would say.

And looking back, I definitely came across pretty arrogant on this thread, and I apologize if I came across rude to Veselin or Cory or anyone else who read it.

Let the great exchanges continue.

Matt

[EStokely]

Getting back to one of the original questions.

Bronze bushings. I have only used them on non-drive train parts. Especially for arm gear boxes and such. something that will see a Pi or less revolution in a matter of a few seconds. And then return to its starting point.
Its a personnal bias against bushings for high torque/high rev speeds. But they are all over copiers and printers. And these things can put a high load on bearings/bushings.

So there you have it. An unsupported opinion. But I don't see designing around bushings in drive train gear boxes anytime soon

So close we can almost touch it....And on the West coast its up before the Sun to make kick off :-)

[Matt Adams]

Just as a heads up... there were a lot of good thoughts in this thread about ball bearings vs. bushings.

http://www.chiefdelphi.com/forums/showthread.php?t=23023

Hope it helps,

Matt

[Andy Baker]

I agree with Matt that it is a good rule of thumb to always contain your power transmission device (sprocket, gear, etc.) with two bearings on either side of the device.

This is a very conservative way to design. For instance, FIRST may (or may not) give us some new motors to deal with. Typically, teams have difficulty getting info on these motors. Ususally Dr. Joe or Paul Copioli finds specs on the motors during week 2 and the rest of us benefit. But, in the mean time, we gotta get designing.

Since we will probably know little about this motor and none of us will have experience using it, I will urge my team to put an extra bearing to support the end of the output shaft unless there is obviously no need to do so.

Now, as for CIM motors, I have seen many good designs which did not support the end of the shaft. As I have used these motors over the past 2 years, I have supported the shaft, being the conservative designer I am. It worked, but the CIM motor is probably better off without a supported shaft. Since the bushing holding the 8mm output shaft is long enough to support heavy side loads, supporting the output shaft with a bearing just adds an efficiency loss by introducing a slight bind in the shaft.

If FIRST gives us CIM motors again (and I hope they do), I'll not support the output shaft.

However, if FIRST gives us the same Drill motors as last year, I stick with supporting the output shaft. These motors are designed for an application where side loads are equalized in the planetary gearbox.

Andy B.

[Andy Brockway]

We have not supported the CIM motor based on the 2002 version. That one had an integral gear and included a cluster gear. Since the original application of the motor was unsupported than we felt our application was solid.

One word of caution - we still place the gear as close to the motor as possible. The farther from the bearing the larger the moment force. This goes for cantileverd wheels also. Place a bearing as close to the wheel as possible.

As for as the gearbox output shaft, we generally run a double sprocket with the chains pulling in opposite directions helping to balance the load on the shaft. With the low life of our robots (10 to 20 hours) you would probably not see any problems with side loading.

BTW, we did not support the the end of the drill motor in our shifting gearbox this past fall and do not plan on it for this next year. One of those engineering trade-offs. We may need to replace the motors at some point but I believe that they should last through two regionals, we will see

[Rob Colatutto]

My high school team didn't support the CIM output shaft in 2003. Never seemed to make a difference since its less than 1.5 inches. We also had an output sprocket outside of the gearbox not supported on one side (on a 1/2 inch steel keyed shaft). This I would not suggest as much if your going to have a long length of chain before the first wheel. If you are still going to do it, I would recommend putting a collar on the outside of the sprocket as well as using its setscrew to hold it in place on the keyed shaft. We just relied on the set screw last year and towards the middle of the championship we needed to tighten the setscrew after each match. Never had any problems with the shaft bending after the gearbox ended, but if at all possible to have the output sprocket inside the gearbox plates I would suggest doing so.