Shafts in Drivetrains

[Tottanka]

Iv'e recently been looking into many custom Drivetrains, and noticed that comparing to the classic KitBots, in many of them the wheel shaft is only mounter through one place, and only supported in one 'end' of the shaft, while in the kitbot it is fully supported on both ends.

This is a nice way to save weight and simplify a drive train - but my question is - are there any negative results of the shaft being not supported enough, getting bent by some torque on it, and perhaps resultins in some damage, or some wheel coming off...

So i ask the people who had this kind of Drivetrains - what were your results? Had anything of this sort ever happened to you? How do you prevent that from ever happening?

Thanks.

[AndreaV]

Cantilevered shafts!
You should do a search for it, I am sure many people go over it in great detail.
Last year we had .5" hardened steel cantilevered shafts for our drivetrain,they stuck out just under 3". After 2 regional, champs, and IRI the shafts were exactly the same as they were during assembly. Many teams use aluminum with no issue, it was just our first year trying it so we played it safe. We used bolts and circlips to hold the extremly sticky colsons on, no problems. the weight isn't too much less, aluminum shafts would have saved some weight. Yhe biggest advantage is space, the ease of wheel/drive train access.

Cantilevered wheels are a must have for West Coast Drive, I suggest you look into that too.

As long as the shaft is sufficiently supported, there's absolutely no problem with cantilevering the shafts. It's commonly found on a West Coast Drive, where the axles are live and well supported by a bearing block.

[Akash Rastogi]

Originally Posted by Andrew Lawrence

As long as the shaft is sufficiently supported, there's absolutely no problem with cantilevering the shafts. It's commonly found on a West Coast Drive, where the axles are live and well supported by a bearing block.

This is an overly simplified statement. Quite a few factors go into shafts for a WCD, including material used (I don't use 6061 to cantilever since friends reported bending of shafts on their bots), stress risers along the shaft, length of the shaft, load at the end of the shaft, etc... These factors are most important for the output shaft of the gearbox.

I haven't seen teams have wheels "fall off," but I have heard of the occasional shaft snapping near a retaining ring groove, and causing issues. You can retain wheels using snap rings, I prefer to tap the end of the shaft in order to drop in a large washer and bolt to hold the wheel on. The retaining rings at the ends of a shaft don't/shouldn't cause problems though.

I only have anecdotal evidence from other teams and from 3929's 2013 drive, but as far as I can tell, cantilevered shafts are A-okay with the proper setup. Check out any 254/968/973/1538/1323/987/60/696 bot as examples.

There's a good amount of discussion on this photo as well - http://www.chiefdelphi.com/media/photos/35912 but deals with a shaft used in a shifting PTO gearbox. I don't think single speed gearboxes have nearly as many issues to worry about.

Originally Posted by Akash Rastogi

This is an overly simplified statement. Quite a few factors go into shafts for a WCD, including material used (I don't use 6061 to cantilever since friends reported bending of shafts on their bots), stress risers along the shaft, length of the shaft, load at the end of the shaft, etc...

I should have gone into more detail - my apologies - Yes, as Akash said, you shouldn't have a problem with cantilevered shafts if you use the right grade material in the correct manner. For a live axle setup, 7075 aluminum in 1/2" hex should work perfectly. I've yet to have problems with it. If you're doing anything smaller or dead axle, I'd go with a steel alloy (I hear 4140 is good), though I find it better to stick with live axle and use 7075 al.

Also, as Akash mentioned, I also prefer to retain parts on the shaft by drilling and tapping the two ends of the shaft 10-32 by about 3/4" and putting a 5/8" 10-32 bolt with some washers on for size to keep everything on the shaft.

[Tungrus]

In our experience it's is the robot weight supported by the shaft that has more impact. Also the distance between the wheel and the bear shaft is supported on. Our team has successfully bent 1/2" keyed Aluminum shaft. For cantilever shaft steel is the probably the best choice.

[sanddrag]

We've bent 6061 and 2024 1/2" round shafts. Never a problem with 7075 1/2" round for wheel axles. Just don't cantilever it out too far. In 2012, we had 6" pneumatic tires and 7075 axles, and we jumped the robot over the center field bump (and speed bumps at school) to where the whole thing was a clear 8" in the air, and it was never a problem. Also, I would sometimes ride the robot, and I weigh twice as much as it does.

[Tottanka]

im planning on a 3/8'' dead shaft, for a 6x6 or 8x8 DT.
is that a problem in your opinion?

[Chris is me]

Originally Posted by Andrew Lawrence

I should have gone into more detail - my apologies - Yes, as Akash said, you shouldn't have a problem with cantilevered shafts if you use the right grade material in the correct manner. For a live axle setup, 7075 aluminum in 1/2" hex should work perfectly. I've yet to have problems with it.

You're still missing way too much detail to speak in general here. The point of interest is more than just the shaft sizing; how the load is applied and supported also matters. Essentially you want the cantilevered load as close to the support as possible and each of the support bearings a reasonable distance apart. In a drivetrain it's fairly easy to get those supports far enough apart, so unless you're doing something really strange like half-inch tubing, the main area of attention should be getting the load as close to the support as possible. Don't space out wheels or use excessive spacing between sprockets.

If you're doing anything smaller or dead axle, I'd go with a steel alloy (I hear 4140 is good), though I find it better to stick with live axle and use 7075 al.

You're saying that for an equal size shaft and load, you need steel for a dead axle but can use aluminum for a live axle? I don't follow; the bending loads on the shaft should be identical. If anything the lack of torsional loads on the axle would slightly reduce the stresses involved on a dead axle.

OP: If you're deviating from the tried and true 1/2" shafting west coast type setup, you should probably do some math to be sure. Very simple statics, just draw a free body diagram and look at the forces and moments at play. If you need help with this I can walk you through the math; I'm a bit busy at the moment though.

[Qbot2640]

Originally Posted by Tottanka

im planning on a 3/8'' dead shaft, for a 6x6 or 8x8 DT.
is that a problem in your opinion?

There are many variables to consider: total load, length of shaft from support to load, Modulus of Elasticity of the chosen material, etc. But the comparison of section modulus in 3/8" round to 1/2" round (and to 1/2" hex) is rather surprising.

3/8" Round has a section modulus of 85mm^3
1/2" Round has a section modulus of 202mm^3
and 1/2" Hex has a section modulus of 213mm^3

We have bent 1/2" steel round in a non-drivetrain robotics application, and we have bent 1/2" bolts used as dead axles supported on both sides of the wheel (estimate 5" spacing)...this comparison would make me very hesitant to use 3/8"

Sources:

http://67.237.145.72/PDFs/SECTIONPROPERTIES.pdf
http://www.engineersedge.com/calcula...re_case_15.htm

[TheFrozenSlink]

If you're using 1/2 in Hex shafts for a live axle, with wheels that have a hex hole for the shaft, what do you use to keep the wheels from coming off then, as you cant thread the shafts?

[iyermihir]

Originally Posted by TheFrozenSlink

If you're using 1/2 in Hex shafts for a live axle, with wheels that have a hex hole for the shaft, what do you use to keep the wheels from coming off then, as you cant thread the shafts?

From what i have seen, most people either tap the end of the shaft and use a screw with a washer, or use snap rings.

-Mihir Iyer

[Tottanka]

i was thinking in the lines of:
shafts:
http://www.vexrobotics.com/vexpro/ha...aft-stock.html (the tube)
or the usual AndyMark 3/8'' bolts used as dead shafts.
along with
http://www.vexrobotics.com/vexpro/ha...t-collars.html
those collars to put everything in place nice and well.

i was thinking 3/8m but it seems like i should rather go 1/2'' - which ,might have another problem as i wanted the base structure to be 20X20 mm profiles, and drilling a 1/2 hole in it will cause some problems.

[Chris is me]

Originally Posted by TheFrozenSlink

If you're using 1/2 in Hex shafts for a live axle, with wheels that have a hex hole for the shaft, what do you use to keep the wheels from coming off then, as you cant thread the shafts?

There's a few ways to do this. Hex shaft collars are the "zero-machining" way to do it, but they take up space and weigh more than other solutions. Drilling and tapping a hole in each end and using washers is another way to do it. Works just fine, though you want to Loctite those bolts and check them frequently. Snap rings work great on the ends of shafts, not so much between loads, provided you can cut grooves.

At one point we actually turned our hex stock round at one end and used a thread die to cut threads in the ends. We just popped in a nut and were good to go. I wouldn't recommend this as shaft collars, tapped end holes, etc are much simpler but if you must you can do it.