Chiaphua Gear Box

[mzitz2k]

(1) If one were to use a Chiaphua motor for the gear box, would it be necessary to support the end of the output shaft (from the motor itself). I have seen several other designs here on CD that have Chips as part of their gear boxes and I haven't seen any gear box that supports the end of the shaft. I tore apart a Chip and noticed that there were two bearings (one on each end), hence a third bearing would break Woodie's old law - avoid odd numbers of bearings in a line.

(2) Also, on many of these designs that I spoke of earlier, I have seen output shafts that continue past the gear box's aluminum plates. I assume teams are connecting a sprocket to these shafts, but just outside the gear box. So, my second question is: Is it necessary to put a third bearing in line (outside the gear box) so that the sprocket doesn't cantilever the shaft? Again, if the answer is yes, it would be breaking Woodie's old law.

(3) Thirdly, have teams found bronze bearings or the usual bearings best for supporting the shafts in the gear boxes?

Thanks

[sanddrag]

If you go more than about an inch and a half out from something, you will need support on the end. As for supporting the Chia shaft, it depends on exactly what you will be doing with it. We have never tried bronze bushings but have had good results with standard bearings.

[Matt Adams]

Some quick thoughts on your concerns...

In general, it's a good idea to avoid more than two bearing on a single spinning shaft, and it's best to have a bearing on either side of your power transmissing device. (gear, sprocket, etc) Since the shaft is presumeably already in line, more than two ball bearings tends to just add friction... there are naturally some exceptions.

You're always going to want to support both ends of a shaft if there's going to be any sizeable force applied to it.. such as wheels. Don't ever have wheel's cantilevered off the edge of your robot! As for a motor, since its only function is to spin about its axis, it has no other forces acting on it besides the force trying to kick the gears apart. Since the length of output shaft on motors is so small, you can typically get away without supporting both ends, which is why in many of the designs you've seen they don't.

Hope this helps,

Matt

[Veselin Kolev]

Originally Posted by Matt Adams

You're always going to want to support both ends of a shaft if there's going to be any sizeable force applied to it.. such as wheels. Don't ever have wheel's cantilevered off the edge of your robot!

Matt

Well, you can have wheels hanging off the side of your robot, you just have to support them well. Like at Cal Games in California, team 254 had joined up with Kingman to build another bot over the summer, and it had 6 or 8 wheels, i forget, all of which were outside the body. Of course to prevent damage to them they had sheetmetal guards. But.. they had 2 bearings on each wheel axle, with a sprocket in between, and the wheel off to a side. And it worked, WELL! That bot was high quality, it was so fast it was seriously getting air off the ramp.

[Matt Adams]

Originally Posted by Veselin Kolev

Well, you can have wheels hanging off the side of your robot, you just have to support them well. Like at Cal Games in California, team 254 had joined up with Kingman to build another bot over the summer, and it had 6 or 8 wheels, i forget, all of which were outside the body.

And it worked, WELL! That bot was high quality, it was so fast it was seriously getting air off the ramp.

Obviously robots can do this, and in a situation where you have 6 or 8 wheels, some of the problems are reduced . It still doesn't make it a good practice. <edit> in my opinion </edit>

You need bigger shafts to hold the wheels, there's more stress on the ball bearings... lots of extra problems with no immediate benefit coming to mind. Whereas I was a bit more open to other opinions on using #25 chain... 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

[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.

[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.

[patrickrd]

Originally Posted by mzitz2k

(1) If one were to use a Chiaphua motor for the gear box, would it be necessary to support the end of the output shaft (from the motor itself). I have seen several other designs here on CD that have Chips as part of their gear boxes and I haven't seen any gear box that supports the end of the shaft. I tore apart a Chip and noticed that there were two bearings (one on each end), hence a third bearing would break Woodie's old law - avoid odd numbers of bearings in a line.

A third bearing would indeed break Woodie's law. In this application, however, I don't see the need for the bearing at all. The motor is applying a torque to output shaft of the motor. This torque is transferred essentially tangent to the gear to another gear. Thus, there is no non-axial load on the shaft, and a bearing would be redundant (it would not support any load).

Now, this is not 100% true, actually only about 75% true. Your typical spur gear has a 14.5º pressure angle. This means that the force is not actually transmitted tangent to the shaft but 14.5º off from the tangent direction. Thus, whatever force is being transferred (motor torque ÷ gear's pitch radius), you need to multiply by sin(14.5º), which yields the "radial force" (force perpendicular to the circumference of the shaft). The radial force on the gear must be supported by the shaft's bearings. Since the motor already has two bearings in it, it is designed to support a radial force at some distance. You can check the motor specs to see if your radial force at your given distance is within the motor's specifications.

The other issue is deflection. If your shaft is too thin and the force is too far away from the motor, you could have significant deflection. If it deflects too far, the gears will slip. If it deflects just a little, the gears will temporarily become misaligned, and the gears will wear down quicker. If it deflects insignificantly, then Bob's your uncle (there will be no problems).

Having worked with these motors (and seeing that the output shaft is not that long), my guess is that the deflection will not be significant at all. But if you really are concerned about deflection, go ahead and put the third bearing on. But it MUST be perfectly aligned, otherwise the third bearing is deflecting the shaft, and applying a constant torque. Even if it is just a couple thousands of an inch off center, that may be enough to bust one of the bearings in the motor. My recommendation is to not use the third bearing. But you can do the calculations and find out for sure.

Originally Posted by mzitz2k

(2) Also, on many of these designs that I spoke of earlier, I have seen output shafts that continue past the gear box's aluminum plates. I assume teams are connecting a sprocket to these shafts, but just outside the gear box. So, my second question is: Is it necessary to put a third bearing in line (outside the gear box) so that the sprocket doesn't cantilever the shaft? Again, if the answer is yes, it would be breaking Woodie's old law.

I think others have sufficiently answered this one.

Originally Posted by mzitz2k

(3) Thirdly, have teams found bronze bearings or the usual bearings best for supporting the shafts in the gear boxes?

I think you are speaking of bronze sleeves, which are not ball bearings, but rather a low friction surface that the shaft slides within. For gear boxes, I recommend steel ball bearings! These have the lowest friction (much lower than bronze sleeves), and the friction does not increase as a function of load. So even if the shaft has a lot of load, the ball bearings can handle it and still maintain close to 100% efficiency.

- Patrick

[Joe Johnson]

At 2 N-m stall torque and min. a radius of 4mm (a bit less if you carve the a gear into the 8mm diameter shaft like the ones that they put in the kit the first year the Chiaphua motors were in the kit), the tooth load would be 500N.

If we double this number to account for dynamic effects and the force from the pressure angle of the gear, we get a max load on the shaft of 1000N.

If we assume that the load is cantilevered 15mm from the bearing and we get a bending moment on the 8mm shaft of just under 15N-m.

Now, if we get out our old Beer & Johnston textbook, we see the following formulas (I am assuming a cantelivered beam, which is not strictly true, but when you arbitrarily put in factors of 2 for this or that reason, it is pretty hard to get picky at this point in the calculations ;-)

Max Deflection: -M L^2 / (3EI)

Max Stress: Mc/I

M = 15N-m (from above)
L = 15mm (from above)
E = 30,000 psi = 200,000 MPa (property of steel -- look it up in a ref. book)
I = (1/4)pi r^4 (also from Beer & Johnson -- I LOVE that book)
I = 2E-10 m
c = r = 4mm

Max Deflection: .03mm

Max Stress: 300 MPa

The deflection is practically non-existant. The stress is getting high for crummy steels, but that shaft should have a yeild stress of something like 400 MPa or so I am not terribly worried especially since this is about twice higher that we expect under stall conditions of the motor. Fatigue should not be a problem since the number of cycles at that relatively high stress should be pretty low (note that if we are running at 4000 RPM or so then the shaft must not be loaded very much at all -- it is hard to rack up tens of thousands of cycles at over the stall condition on a FIRST robot).

Bottom line: I do not think supporting the shaft is required.

Joe J.

P.S. Beer & Johnston is perhaps the most valuable textbook I have from my college days... ...actually, I foolishly sold it back to my college bookstore while I was an undergrad. Only later did I realize my error and buy another one.