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.