Threaded rod through ball bearings
 

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!

Re: Threaded rod through roller bearings
 

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

Re: Threaded rod through roller bearings
 

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.

Re: Threaded rod through roller bearings
 

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.

Re: Threaded rod through roller bearings
 

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.

Re: Threaded rod through roller bearings
 

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.

Re: Threaded rod through roller bearings
 

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.

Re: Threaded rod through roller 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.

Re: Threaded rod through roller bearings
 

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.

Re: Threaded rod through roller bearings
 

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!

Re: Threaded rod through roller bearings
 

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.

Re: Threaded rod through roller 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....

Re: Threaded rod through roller bearings
 

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.

Re: Threaded rod through ball bearings
 

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.

Re: Threaded rod through roller bearings
 

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.

Re: Threaded rod through ball bearings
 

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!

Re: Threaded rod through ball bearings
 

Madison, the two issues are both related to the threads:

1. 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..

2. 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

Re: Threaded rod through ball bearings
 

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.

Re: Threaded rod through ball bearings
 

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.

Re: Threaded rod through ball bearings
 

Thanks for the replies, everyone.

Here's an isometric sort of cross-section that may make it easier to discern what's what.


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.

Re: Threaded rod through ball bearings
 

I know you're probably trying to avoid machining it, but if you filled the bearing portion of the threads with a metal-filled epoxy and lathed it down to the correct diameter, you'd do away with the majority of the stress concentration problems, and ensure yourself a precise fit in the bearing. And if you were using mild steel (not hardened) threaded rod, you could even fill it with weld filler, and lathe that down (you'd probably want to clamp it when depositing the weld filler, to avoid distorting the rod, though).

However, I'd tend to agree with the use of a shoulder bolt and a locked nut (threadlocker or a good-quality locknut). McMaster sells several Ø3/8" shoulder bolts in appropriately long sizes, and Flexloc locknuts (which tend to be very secure).

Re: Threaded rod through ball bearings
 

What will transmit the torque and keep everything from slipping?

Re: Threaded rod through ball bearings
 

ahhhhh.....

It appears you are planning on transmitting the torque for driving the wheel thru the threaded rod, being drivin by the hex shaped coupling nut on one side and driving the wheel with the hexed shape coupling nut on the other end.

Frankly, it doesn't look like a very durable design....I would try to come up with a different design, but then I have an old South Bend lathe sitting in my garage. :)

If you can put metal spacers between the bearings and between the bearings and coupling nuts, so that you can actually tighten the coupling nuts against each other solidly, then it might work better. Having a long nut like that will probably let loctite work well enough to keep it from turning, although it may work so well that you won't be able to diassemble it for service. A pin thru the nut and rod might work, but would weaken the threaded rod somewhat.

Also the frame tubing could use a metal spacer over the bolt that goes in the center, to keep from crushing the frame when you tighten the bolts as needed to keep the custom bearing blocks from moving around.

Sure would be nice to find some hex shaped tubing, eh?

Re: Threaded rod through ball bearings
 

The previous revision used shoulder bolts as axles and used the bolt head as a sort of hex shaft. It required a number of custom cut parts that I tried to eliminate with this version.

It seems like it'd be simpler for us to thread rod on our own than to mess with filler, even. It's probably not very much work, all things considered, and it'd save a lot of headaches.

I'm not sure if I understand what you mean by slipping. If, as I mention above, we thread our own rod, we can stop the coupling nut from tightening itself down against the bearing blocks by ending the threads before they get inside the bearing. The coupling nut will turn as far as it can on the thread before driving the shaft -- and likewise, on the other side, that coupling will turn the wheel after it can no longer move on the thread.

That's the idea, anyway.

Re: Threaded rod through ball bearings
 

just curious, have you calculated the maximum torque on the rod? low grade 3/8" hardware usually breaks at about 40 ft lbs or so.

Re: Threaded rod through ball bearings
 

It looks like you are going to depend on the nuts being very tight, rather than having keyways or a hex shaft or pins to keep the wheel and the sprocket from loosening and spinning with respect to each other.

Re: Threaded rod through ball bearings
 

It's geared for about 57 ft*lbf of torque at stall.

Also, there'll be a tensioning mechanism that occupies the space inside the tubing, enveloping both the bolts and the shaft. This'll

I suppose that's one way of describing it. I'm relying on the coupling nuts' inability to move along the thread -- that is, to tighten or loosen -- to drive the shaft. I'd just as soon use hex shaft as this ridiculous scheme if I could decipher some way of sticking a hexagon into a small round hole. :) It assumes that it's easier to drive the robot than it is to strip the threads -- which may not be at all true.

This is an (over?)extension of a method that was used to drive FIRST robots years ago. The 2003 KoP included an adapter that threaded onto the Bosch drill output and was secured in place by a left-handed screw. The adapter accepted a 5/8" dia. keyed shaft on the other end, so maybe that's my clue that what I'm trying to do here is a bit stupid.

I'm a terrible machinist and usually the person responsible for explaining to students and mentors alike what processes they need to follow to make the parts I design. If I don't know how to make something, nobody does; so I often stick to what I'm most comfortable with.

I appreciate everyone's help a lot. I'm a bit dense sometimes and don't always think things through until long after I've invested myself in making them work. :)

Re: Threaded rod through ball bearings
 

Is there any way to upgrade the design to a 1/2" or 5/8" shaft? Something with enough meat in it to allow pinning or a shaft key or just more torque in general? I can remember using an adapter off the old drill transmission to couple a shaft. However, that was before the final stage of gear reduction, and hence subject to a lot less torque.

Re: Threaded rod through ball bearings
 

We don't have broaches or an arbor press, so I try to avoid keyed shafts whenever I can. If it comes to that, we'll just use dead axles again in 2007 as we've done for the last several years.

Edited to add: I've been poking around the 'net for a few hours trying to see what I could find about calculating the torque required to strip a nut or bolt of its threads. I've come across some things suggesting good practice -- that the bolt shaft should fail before the thread strips -- but nothing explicitly offering an answer to the question. I understand that the answer is a function of the nut and bolt material and the shear area. The shear area is, in turn, a function of the number of threads engaged between the nut and bolt. Beyond that, I think that my brain is fried and I'm missing a connection somewhere that'd let me equate the output torque of my gearbox to the force required for the coupling nut to strip the threads on the shaft.

Re: Threaded rod through ball bearings
 

Keyed shaft material is readily available. You may need to turn and thread the ends, or use shaft collars to accomplish what you want. We don't have machining capabilities, so we use keyed shaft material and collars. We do tend to use a live axle supported at both ends, but once used a cantilevered design without problems, though frame flex had to be prevented.

Re: Threaded rod through ball bearings
 

Nice isometric view. There's a big stress concentrator where the shaft leaves the outer bearing and before it enters the coupling nut. The shaft will fatigue and break there.

Epoxy or most other fillers will not increase the yield strength of the shaft in bending, nor remove the stress concentrators due to thread geometry, since their yield strengths are orders of magnitude below that of steel.

I can't give you the forumlae, but the number of threads engaged is the key to calculating the force to strip threads - in other words, a function of cross-section of material engaged by the nut. If you think of the thread ridge as a straight ridge instead of a spiral, it'll make the calculations easier while not being too inaccurate. A 3/8" coupling nut with more than about 10-12 threads engaged will have a good chance of breaking the shaft before the threads strip. Hope that's what you're looking for.

Seems to me that a hex shaft, turned down to a rod for the bearings and outer wheel, then threaded on the outer portion, would be plenty strong and not difficult. Do you have a lathe? Can it cut threads? Good learning experience...

Don

Don

Re: Threaded rod through ball bearings
 

I was thinking about lathes....I have half a dozen friends who have lathes....they really aren't that hard to find!

Re: Threaded rod through ball bearings
 

Though the epoxy is an order of magnitude weaker than the steel (estimated 3 500 psi tensile yield), it ought to significantly reduce the effects of fatigue on the stress concentration. (Fatigue being a major reason reason why parts fail at stress concentrations.) It provides an alternate load path for small but repeated stresses to be transferred within the shaft. The effective net stress in the areas of stress concentration ought to be much lower, assuming that the fatigue-induced stresses do not cause the epoxy to yield, or the joint to split. I'm not sure if you're running it enough to warrant protection against fatigue, though—the back of the envelope says that you shouldn't see enough iterations of stresses anywhere near the fatigue strength for the few hours that a typical FIRST robot runs.

When it yields, then yes, all bets are off. But shouldn't the shaft be designed not to yield due to bending loads, under normal operation? If it isn't operating in the elastic region (of the steel), it's probably a bad design to begin with, because you can expect it to need replacing, once it deforms. And realistically, for a 130 lb robot, is it likely to see enough stress in the shaft to plastically deform it? Assuming all the robot's weight is on one wheel, and the stress concentration factor is 2 (that ought to be conservative), you're looking at around 3 600 psi stress in the shaft, which would just barely cause the epoxy to fail, if it were withstanding the load on its own. With robots potentially bouncing and falling off of things, maybe this isn't enough—I'm not sure about the strain rate sensitivity. But when you figure that the shaft is still mostly steel, that the epoxy would mostly be in compression (where it's at least twice as strong), and that the thread crests would be bearing some of the compressive load inside the bearing, it looks like it ought to work.

By way of rough analogy, consider that epoxy-coated bolts don't lose their epoxy coating when they are pretensioned. Further, they don't lose their corrosion resistance under these circumstances. That implies that the epoxy coating remains intact and unbroken, even under tensile stress. While I realize that the epoxy is not fulfilling a structural role, and that the stress isn't repeated, it does suggest that a high-strength epoxy won't merely crumble and fail when the bolt is loaded.

Also, in mild steel, the yield strength (tensile/shear/fatigue) of weld material should be of the same order of magnitude as the unhardened base metal (and in many cases, nearly identical). If the bolt is unhardened, it's very likely that this will make it a fair bit stronger.

On another note, looking closer at the design (like clicking on the ImageShack picture to make it bigger...), I realized that you weren't using Trantorque- or Fairloc-like couplings, but rather a standard AndyMark hex hub. That would make it a little trickier to attach the wheel and sprocket to a shoulder bolt. Maybe there's another way to simplify this, though: try a hex-bore bearing and use hex shaft throughout.

Re: Threaded rod through ball bearings
 

If you do not have a lathe or milling machine, you might try checking with a local community college that has machining courses. Our local college was kind enough to turn down a bunch of sprockets for us when we had a weight problem.

Re: Threaded rod through ball bearings
 

We have one each of a mill and lathe, but they're always a bottleneck when it comes to getting robots together -- hence my desire to avoid using a lot of parts that need to be extensively machined. The milling machine allows us to put external keyways into shaft, but it's more difficult to key things like sprocket hubs, so I try to avoid using them whenever I can get away with it.

I did some digging last night and it turns out that all of the adapters and spacers and bolts needed to avoid using a keyed, steel sprocket make the weight saved almost negligible, so I've been reworking the design to use 5/8" keyed shaft and keyed sprockets. Since the wheel is comprised of three aluminum plates stacked together, I can cut plasma cut a key into the hub of each.

This means abandoning the use of threaded rod and coupling nuts altogether at the cost of redesigning the gearbox a bit. It seems right now that the simplest solution that'll allow me to have a hex output to the cantilevered center wheel and a hex shaft inside the gearbox is a bushing with a 1/2" hex opening and a .625" outer diameter. I can cut two or three thin pieces of metal or plastic to make the bushing since I can't broach a hex hole the traditional way.

Re: Threaded rod through ball bearings
 

They make hex bore bearings? Cool.

The epoxy on bolt threads (when mated) is under compression, but this shaft has no such forces between the bearing and the outer shaft coupler. Material under compression will have better tensile survival since the absolute force never gets far into the other size of zero. I would expect the joint to split, but that might not be reasonable on my part.

However, to go any further, I'd have to do some experiments. I do agree, completely, that if the design exceeds (or even approaches) the edge of the elastic region, it's not a good design.

But, Madison seems to be moving towards a 5/8" shaft, so I guess it's moot.

Don

Re: Threaded rod through ball bearings
 

as used in conveyor systems, apparently...

http://www.google.com/search?hl=en&q...=Google+Search

Re: Threaded rod through ball bearings
 

They're sold by McMaster-Carr as 'Conveyor Bearings' and are mentioned in the Timken catalog on a page about farm equipment. I didn't know they existed until Tristan mentioned them.

It appears there are very few varieties commonly available, however, and unfortunately, 1/2" is not among them.

Re: Threaded rod through ball bearings
 

These guys say that they can make 1/2" bores by special order. I don't know what the cost premium or lead time on those would be, however.