High Tensile Sprocket Bolts

[AdamHeard]

Originally Posted by Chris is me

If the plastic was failing, a longer spacer would distribute the load across more of the plastic, but in this case the bolt is failing and that's a stress concentration problem... if I'm anywhere near approaching an understanding of what people are talking about.

The longer plastic spacer creates more issues, it's all about beams in bending here.

[Chris is me]

Originally Posted by AdamHeard

The longer plastic spacer creates more issues, it's all about beams in bending here.

Oh, that makes perfect sense then. The spacer would increase the load on the bolt, causing the failure when it otherwise wouldn't be present because the bolt would be sufficiently close to the material supporting it. Got it.

I think your suggestion of shear pins sounds best for this problem.

[Rob Stehlik]

This has been a very interesting discussion. I know bolts are quite weak in shear, but I figured 6 of them on a 4" wheel would be plenty strong. Considering the holes in the wheel and sprocket are about 0.206", my suggestion would be to replace three of the #10 bolts with 5mm roll pins. Something like this:
http://www.mcmaster.com/#roll-pins/=eqsixn
Roll pins are inherently oversized, so the actual diameter of these would be .212" - .220". They should fit nicely in the wheel and sprocket, and take all of the shear loads, allowing the remaining three bolts to simply hold everything together.

[Spirator]

Rob, I'm a little concerned about the roll pins deforming during compression. Do you have any experience to share about roll pins?

[Rob Stehlik]

I'm not entirely sure what you mean by compression. Do you mean that the roll pins will get squeezed to a slightly smaller diameter when exposed to side loads? This could happen, but even so, I don't think it would be a problem. Roll pins are made out of spring steel, which is very strong, and is design to spring back. Mcmaster carr lists the double shear strength of the M5 pins to be 3943 lbs. In this application, the pins are in single shear, so the strength would be half of that, 1971 lbs. Check this page for a diagram of single vs double shear:
http://www.roymech.co.uk/Useful_Tabl...ted_Joint.html
With three of these pins in each wheel, you should be pretty safe. Considering the pins are a drop in solution without changing anything else, I'd say it's worth a try.

[DonRotolo]

Originally Posted by Trent B

I guess where my confusion lies is if you have a physically longer spacer (eg one that goes into the holes on the wheel as well) would that offset some of the shear force and thus make the bolt more resistant to bending than it would be with a short spacer?

I think you mean a spacer that actually enters the holes in the wheel or sprocket? In that case, the larger surface contacting the spacer ends would help by distributing forces over a larger area. My previous comments assumed the spacer was on the surface of the wheel & sprocket. But a larger diameter spacer would help even more.

Originally Posted by Rob Stehlik

I know bolts are quite weak in shear, but I figured 6 of them on a 4" wheel would be plenty strong.

Well, yes, they are. But that's not the only force here. The real issue is the cyclic shear these fasteners are subjected to. If the robot only went forward (= no cyclic load), and never saw any collisions (= no shock load), the fasteners would last a long time. But shock loads, reversing loads, and reversing shock loads all take their toll, especially if the fasteners are even slightly loose.

Pins would be better, because they don't have stress-concentrating threads, but if they're even a little loose - and in plastic, everything is loose - they'll eventually cause a failure too, but more likely by damaging the plastic and not by failing like the screws. That can be mitigated by sleeves to help distribute the load across a larger area of plastic.

(The compressive strength of plastic is a few thousand Pounds per Square Inch (PSI). That of metal is tens of thousands of PSI. If you put a 500 pound shock load on a 1/4" inch square (0.0625 sq in)of plastic, that's 8000 PSI.)

[Trent B]

Originally Posted by DonRotolo

I think you mean a spacer that actually enters the holes in the wheel or sprocket? In that case, the larger surface contacting the spacer ends would help by distributing forces over a larger area. My previous comments assumed the spacer was on the surface of the wheel & sprocket. But a larger diameter spacer would help even more.

Yeah a spacer actually entering the holes on the wheel (like plaction wheels for example have those holes that nuts can slide into), what if you mounted the sprocket on that side for instance. I am guessing in this case there would be a certain point where larger radius would help more than the spacer in the holes on the wheel but not the radius immediately larger?

And I was trying to get at what Don was saying, cyclic loading will cause failures, this can be impacts, if the load on the bolts changes significantly in normal rotation etc.

Here is an S-N curve


If you notice based on the load the number of cycles til failure varies, this is the amplitude of the stress in the cyclic cycles, if steel is kept below a given stress it should never fail, otherwise it should fail fairly regularly after a predictable number of cycles (as it appears to be for you guys) so you need to find a way to make the stress on the bolt less be it spacers, less threads, thicker bolts, etc.

[Teched3]

Originally Posted by Gdeaver

This is our plactraction wheel set up for dead axle. May help.

Very nice CAD drawing! Just to make a few design suggestions that we use in our wheel design:

We use a flanged bearing in the sprocket rather than the wheel hub to gain a little wheelbase on the dead axle. Spacer bushings outboard of the flanged bearing will provide chain clearance and retain the bearing in the sprocket.

Rather than nuts or standoffs on the hub to space the sprocket, we make a spacer ring of aluminum or Lexan that centers on the wheel hub similar to what was provided in the kit with the Skyway wheels. We make a thin ring of aluminum or Lexan for the other side of the wheel to act as a washer under the bolt head.

We re-drill the sprocket mounting holes with tapped 10-32 or 8-32 holes, and then use jamb nylon nuts to give a "double nutted" assembly.

We have used only (3) 8-32 sckt hd bolts in this wheel assembly with no failures. It's also prudent to check these bolts in the pit to ensure they remain tight.

The bottom line is that I suspect that bolts/nuts are loosening, and with the continual reversing that occurs between the sprocket and the wheels, shear and tensile loads are applied to the bolts. Just my nickels worth.