My gut tells me running threaded rod as a live axle on a drive is a bad idea, but I’m curious if any folks have hands-on experience with this or some words of wisdom about what the pitfalls may be.
It’s entirely possible to avoid using threaded rod through the bearings, but it’d simplify things a lot and make me a much happier person if it were an acceptable alternative.
Okay, so… go!
i think that if you lock the threaded rod to the id of the bearing somehow you’ll be fine and it would eliminate some interesting looking wear patterns
I can’t speak on live axles, but 1293 used threaded rod as a dead axle on a total of 16 different wheels over the past two seasons (two on Ockham, six on Chomp, and then double that for their practice robots). I can count the number of problems we had with it on zero fingers.
Team 1882 is thinking of threaded rod as well. We think we may fit threaded rod inside 1/2 od tubing to make a long bushing that the wheel rides on (dead axle). This will fit inside the 1/2 id bushing and lock the side rails with nylon lock nuts outside. We could modify this to be a live axle as well. This will be the first time we try this so it will be a wait and see at this point. Good luck with the 2007 season.
The following are some thoughts, where if I’m understanding you correctly, you’re asking about using a “live” threaded rod with a wheel fixed to the end, running through a roller bearing.
Assuming the roller bearing is running on the threaded rod itself…
As you may already know, roller bearings are designed to run on hardened “smooth” shafts. Being that the threaded rod is most likely not hardened, and obviously not smooth, I would suggest it be avoided, as there will most likely be bearing issues.
Regardless of whether or not the roller bearing is running on the threaded rod…
You’ll have a pretty high stress concentration at the “v” of the threads (since you’ll have a cantilevered load), so depending upon the diameter of the threaded rod, the distance from the bearing, and the load, you’ll possibly cause fatigue failure of the threaded rod (since it’s a live axle).
While it may work at first, it then becomes a question of how long. If it were me, and I had an alternative (which you stated you do), I would not risk my drive train.
Maybe I’m a bit conservative, but that’s my 0.02.
I suppose it makes little difference if the bearings are riding in the wheels or in the frame.
Coupling nuts are a small miracle, by the way. 
McMaster-Carr doesn’t carry studs of convenient length, so I was resigned to assigning some poor student to threading steel rod with a die, but if we can cheat a bit and use rod that’s threaded along its whole length, that’ll save me from the dishonor of assigning more kids to sweatshop duty.
If you used large enough diameter rod, and assuming you only need threads on one side of the rod, you could turn the rod down and fit it to a smaller bearing. 1/2" threaded rod could easily be modified to fit 3/8" bearings.
For you people that don’t have access to a lathe, mount the threaded rod in a drill press and then file away while it’s spinning.
Here’s a cross-section of the drive arrangement I’m considering.
The dark gray rod is the axle; potentially a steel, 3/8-16 threaded rod. It runs through two bearings, shown in white near the center, and has a cantilevered sprocket on one end and a wheel on the other. Threaded onto each end is a 1/2" hex coupling nut. The ends of the axle are drilled and tapped to accept left-handed screws which’ll hold plates against the wheel and sprocket to prevent them from backing off.
I was looking specifically at Grade B7 threaded rod available from McMaster-Carr – part no. 98750A068.
We’ve successfully used rod with jus threaded ends for axles. Although they did not transmit power. I believe we bought then at either Lowe’s or Home Depot. Good Luck!
Thread a bronze or steel bushing and this goes onto the rod to offer a smooth shaft for the ball bearings to ride on. All it requires is increasing the inside diameter of the bearings.
looks to me like you are actually planning on using ball bearings, not roller bearings?
As long as the inner races of the bearings are clamped tightly to the spacers, using the threaded rod and nuts to hold it all together, it should be ok…
Yep. I associate “ball bearings” with the balls alone and not the whole bearing, so I’ve always called them roller bearings. It’s fixed now.
OK, I work on cars a lot, they have both types, and there is a big difference between roller and ball bearings…so I’m kind of picky about the terminology.
If you can’t find the right length at McMaster, you might want to try calling your local Ace hardware stores. Numerous times they have had odd ball size/ threads. But it won’t be listed online, you will need to call local stores.
We did alot of our rollers for the ball system last year out of threaded rod. We didn’t want to, but we didn’t have a lathe, and it was taking too much time threading the ends of solid shaft. So we went with threaded rod. It was a bad idea. Most issues you wouldn’t see (we had nuts that tightened as time went on, binding the system), but we did cut into the bearings with the thread.
Good luck!
Madison, the two issues are both related to the threads:
The effect of the threads as stress concentrators on the rod. However, if the rod it thick enough, that is, not near it’s strength limits, there isn’t much to worry about…
The effect of the relatively small contact area between the rod and the bearing ID - that is, will the threads mush down from the force? You need to know the force (in PSI) on the thread tips from the bearing, and run that against the yield strength of the thread material (steel?) and if they’re on the same order of magnitude, it’s a problem. But, consider a 120 lb robot, maybe 10G force, and 0.2 SqIn of contact area (=6000 PSI) at 40,000 kPSI Yield, you’re safe. (The contact area is very conservative here - it is likely more, and thus even better). I also ignored that there are two bearings, each taking about half the load, etc.
(There is another effect, the threads and their reaction to torque, but the effect is small compared to the capacity of the rod, and so can be ignored)
So, back of the napkin, seems OK.
Don
Can you use 10 mm threaded rod and turn/file it down to 3/8" so that it’s a tight fit in the bearings? That would give you a larger minor diameter and more strength, as long as there is sufficient thread depth left to snug things up. Any possibility that you could use a socket shoulder bolt (stripper bolt) as an axle?
At any rate, I would be concerned about using such a small axle for a wheel that is cantilevered out.
that’s a good point…although from the drawing, it’s kind of difficult to figure out just what all the parts are, and how everything is supposed to be held together.
also it’s not very difficult to use a die to put threads on the end of a round steel rod, rather than using threaded rod. One could even buy a long bolt, which has threads only on the last inch, cut the head off, and thread the cut off end with a die.
Thanks for the replies, everyone.
Here’s an isometric sort of cross-section that may make it easier to discern what’s what.
http://img304.imageshack.us/img304/7078/annotateddriveisogh9.th.jpg](http://img304.imageshack.us/my.php?image=annotateddriveisogh9.jpg)
The alternative to threaded rod is to thread the ends of steel rod – and that’s why I was looking at what lengths and variety of studs were available from McMaster-Carr. This would allow for round shaft to ride in the bearings, while threaded shaft was exposed from either side.
I understand also that there’s some chance of the coupling nuts tightening against the bearing blocks. That will occur if it’s easier to tighten the nut on the input side than it is to spin the shaft – a situation most likely to occur at start and at stall. I don’t have a good idea about whether it’ll be easier to compress the coupling nut against the nylon spacer and bearing block than to overcome static friction, say, or what effect something like Loc-tite will have in binding the nut to the threads. Conceivably, I could pin the nut through the shaft to prevent it from moving at all, but the goal while designing this was that it’d require the absolute minimum amount of maching work possible. The left-handed bolts in each end of the shaft are to prevent the coupling nuts from backing off when the drive spins counter-clockwise.
