High Tensile Sprocket Bolts

[Spirator]

Hello, in our rookie season, Breakaway, our robot managed to shear the drive sprocket off the centre wheel. We were using the standard Andymark #10-32 bolts on an 8" Plaction wheel and it was being driven off an Andymark Toughbox with 2 CIM motors.

We're concerned because we want to make sure that a 6WD Plaction set up isn't going to have a repeat and we concerned to the point of buying hi-tensile bolts as replacements.

Out of your own experience and knowledge, what do you think would cause such a break? The bolts holding the sprocket onto the wheel broke, if I recall correctly.

[thefro526]

We've bent quite a few #10's in the same spot over the years. What I've found is that in a perfect world, the sprockets should be bolted directly to the wheel, not spaced out at all. I've seen quite a few teams run into issues when spacing beyond about 1/4" or so.

Did you space your sprockets away from the wheel at all?

On the subject of bolts, we've usually been able to get away with common alloy steel bolts from McMaster Similar to part number 91251A353. They're made from a steel that's arguably the same as Grade 8 and hold up really well. In 2008, we used fully threaded Grade 5 10-32's from home depot that sheared between the threads at one of our off seasons - so now we try to stay away from fully threaded bolts.

[Chris is me]

You'll want graded bolts.

[Spirator]

I'd imagine that having a partial thread would be stronger than a full thread. I don't think the sprocket was spaced off the wheel at all.

Could it have been due to poor alignment or loose connections in other places or is it due to the amount of torque we were placing on the centre wheel?

[DonRotolo]

our robot managed to shear the drive sprocket off the centre wheel.
You need greater shear strength, not tensile strength.

It is possible that the fasteners were not as tight as they should have been. Loose fasteners in this application puts a lot of shear force on the fastener. Make sure they are torqued properly and use self-locking fasteners.

The goal is to use the clamping force of the fastener to create enough friction between the joined pieces to that the fastener is only in tension and not in shear.

Increasing the hardware size can help, but you can even shear 1/4-20 bolts if they're loose.

[thefro526]

I'd imagine that having a partial thread would be stronger than a full thread. I don't think the sprocket was spaced off the wheel at all.

Could it have been due to poor alignment or loose connections in other places or is it due to the amount of torque we were placing on the centre wheel?

Yes, a partial thread should stronger than a full thread in Shear (Depends on a few other factors, but usually it's always stronger). If the sprocket wasn't spaced off of the wheel at all, then you might be running into some other issues.

If the bolts holding the sprockets to the wheel weren't tight, it could cause some issues, especially if the sprocket was allowed to move under load. Poor Alignment could also cause the bolts to see more load then they should, but how much would depend on how bad everything is.

[kramarczyk]

Most bolted joints use the tension on the bolt to clamp the pieces together. Friction then prevents movement of the joint. Fasteners that are not tightened adequately will fail prematurely as the joint is allowed to move. This puts the bolt in shear where it will fail a ~0.6-0.5 of it's tensile strength. In the case of a sprocket on a wheel, not only does the loose joint have half it's strength, but the sprocket can get a small running start before contacting the bolts. This increases the load being generated.

Socket Head Cap Screws, SHCS, are actually stronger than grade 8 bolts of the same diameter. Grade 8 bolts have a Ultimate Tensile Strength (UTS) of 150 ksi* and proof strength of 120 ksi per SAE J429. SHCS have a UTS of 180 ksi and proof strength of 140 ksi per ASTM A574. This makes SHCS 20% stronger based upon the UTS. SHCS are also harder than grade 8 fasteners.

You can find this information and a lot more about bolted joints in the Unbrako Engineering Guide at http://www.unbrako.com/docs/engguide.pdf. The figures quoted above are from page 71 of this guide.

*1 ksi = 1000 psi

[daniel_dsouza]

If the bolts holding the sprockets to the wheel weren't tight, it could cause some issues, especially if the sprocket was allowed to move under load.

I can attest to that. We were using the 2011 kit bot for a demonstration at a school, but found that the robot was not driving correctly (complaining loudly when turning). Guess what we found when we checked our wheels (which had no spacing between the sprockets and wheels). Every single bolt was loose (resulting in alignment issues which caused the chain to make noise), and the treads had been destroyed.

[AdamHeard]

I've seen a lot of teams bolt on sprockets with nothing aligning the sprocket to the shaft (except for the bolts). If there is something (bearing works great), aligning it to the shaft it will greatly reduce the likelihood of the bolts failing.

[sdcantrell56]

We've gone so far as using 3 10-32's per wheel and never broken or bent a bolt. I'd look into the set up and take into consideration what Adam said. Use something else like a hub to handle the forces.

[R.C.]

I've seen a lot of teams bolt on sprockets with nothing aligning the sprocket to the shaft (except for the bolts). If there is something (bearing works great), aligning it to the shaft it will greatly reduce the likelihood of the bolts failing.

Agreed,

This helps quite a bit, we used to wonder why we would shear bolts as well... Until we figured out that a bearing or some sort of a bushing works wonders!

-RC

[Gdeaver]

This is our plactraction wheel set up for dead axle. May help.
http://wiki.team1640.com/images/thumb/0/05/Plaction_Detail_annotated.jpg/450px-Plaction_Detail_annotated.jpg

[Trent B]

As Don said you want bolts with a higher shear force rather than tensile force, and partially threaded is better than fully threaded. From a partially threaded standpoint you want as few threads between the bolt head and the nut as possible (eg get as few as possible to have a tight fit) because each thread acts as a crack and the stress from shear force will concentrate their increasing the odds of failure.

Additionally make sure the change in stress as the wheel rotates is at the minimum for each bolt (eg center the sprocket well, evenly tightened bolts etc.) Fatigue or failure as a result of changing stress amplitude through a cycled application is one of the leading causes of failure in parts.

[AdamHeard]

This is our plactraction wheel set up for dead axle. May help.
http://wiki.team1640.com/images/thumb/0/05/Plaction_Detail_annotated.jpg/450px-Plaction_Detail_annotated.jpg

Seems like 91920A280 would be a better choice for those standoffs. Keeps the same functionality, but replaces the fastener that holds the sprocket on with a bolt rather than a nut; It would be much more convenient, also a good deal cheaper.

[Trent B]

Adam did you mean sprocket instead of wheel on?

Also I agree that the other option may be a better choice, as the sharp angle at the base of those standoffs is another stress concentration area that would prove to be a weak spot and prone to failure.

[Tom Line]

Fun times. Our 2010 soccer bot was 8 wheel drive. The AM sprockets were small enough that we had to use spacers for the chain to clear the wheel hub.

We have sheared off close to 50 bolts. Grade 8, partial thread. We know they are tight, because we were tightening them so MUCH that we started bending the sprockets and compressing the plastic of the plaction wheel.

Simply put, the combination of AM sprockets, with AM spacers, on AM plaction wheels, can't hold up over time. The 10-32 bolts will sheer every time if you're putting serious loading on them. This is on a 4 inch wheel with 24(ish) teeth on the sprocket. 2 cims per side, run through tough boxes. A 3:2 sprocket ratio from the tough box to wheels.

AM could address this by putting locking indentations on both the plaction and the sprocket, and then putting bumps on the spacer so that when the three are clamped together, the bumps/buttons take the shear.

Bolts stink in shear. Tension is their game.

This year, we went with bigger sprockets so that we could get rid of the AM spacers. We never sheared a single bolt. Lesson learned.

[Chris is me]

This year, we went with bigger sprockets so that we could get rid of the AM spacers. We never sheared a single bolt. Lesson learned.

We just used it as an excuse to switch to live axle. Works well so far.

[AdamHeard]

Spacing off the sprocket will increase the likelihood of breaking bolts. We've never sheared 3 bolt patterns of the same diameter in situations with higher loading, but the sprocket was aligned to the shaft and there was no gap between the items being fastened.

For teams that keep failing the full 6 bolt pattern, have you tried replacing 2-4 bolts with shear pins?

A while ago I noticed the holes in Am's sprockets are .206", which is a bit big for a #10. This slop (and not accounting for it by aligning the sprocket on the shaft) is further weakening the interface.

[Trent B]

Inherently by using spaces you are cantilevering the bolts which extends the amount of shear stress it is feeling. I would go with adams earlier part suggestion of a hex shaped nut that can screw onto the bolt in fit in the placation wheel holes, thus the bolts will be rigidly held relative to the wheel hub and have extra support (via the nut) for the period they extend beyond the surface of the plaction wheel.

Shearing is greatly related to the amplitude of stress, if you can see concentric semi circles that all seem to be surrounding one point on an edge of the sheared face and expanding as they cross the face it was likely fatigue failure.

Steel bolts, if below their fatigue limit should never fail due to fatigue, if you notice a fatigue pattern (you can find pictures online) you need to reduce the load you are putting on the bolts through some means.

[DonRotolo]

If you are using spacers to offset the sprocket, the larger the outside diameter of the spacer the more it will resist bending (and thus shear on the fastener) forces. The hex spacer is not better if it is the same effective diameter as a round spacer. The 'ideal' (for strength) would be a complete ring with holes drilled for the fasteners.

Adding some kind of surface feature as Tom Line mentioned would further put the stresses onto components that can handle it (i.e., not fasteners) because, as Tom also mentioned, #10 hardware is far better in tension than shear.

[Joe Ross]

The 'ideal' (for strength) would be a complete ring with holes drilled for the fasteners.

Like this? http://www.andymark.com/product-p/am-0207.htm

[Trent B]

The hex spacer is not better if it is the same effective diameter as a round spacer.

Wouldn't the hex holes in the plaction wheel help absorb some of the shear force over a circular spacer that was barely unable to fit in the hex holes?

[Spirator]

Thanks for the advice guys! We're thinking of ordering some SHCS from AM pre season for our protobot. Hopefully they won't shear and if they do, we'll have an odd 40 lying around as replacements. ;)

[DonRotolo]

Wouldn't the hex holes in the plaction wheel help absorb some of the shear force over a circular spacer that was barely unable to fit in the hex holes?
Not really. Technically yes if the tips of the hex extended further from the centerline of the fastener than a round spacer (which wouldn't extend to the 'points'), but that would be very little help.

It has to do with lever arms and resistance to bending. A very thin spacer would be able to more easily move/tilt due to the torque of the wheel (that is, in the shear direction). A wide spacer (or the neat device Joe Ross linked to - thanks Joe!) wouldn't tilt as much (or at all).

Shear over a wide area is bending. Think of a bolt sticking out an inch, force on the end of that bolt will bend the bolt. Apply that same force at 1/16" from the supporting structure and it won't bend as much. That's why someone said that adding spacers to put the sprocket further out from the wheel makes things worse.

[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?

[George C]

If you're having that much trouble with 10-32 bolts shearing off, why not just drill out the holes and use a #12-24 NC or 1/4-28 NF? McMaster Carr doesn't seem to carry #12-28 NF but other suppliers might.

[Tom Line]

If you're having that much trouble with 10-32 bolts shearing off, why not just drill out the holes and use a #12-24 NC or 1/4-28 NF? McMaster Carr doesn't seem to carry #12-28 NF but other suppliers might.

I can't speak for other teams, but we don't drill out the plactions and sprockets because we reuse them each year. We don't have a tremendously huge budget, so we'd rather spend the $25 a year replacing the fasteners rather than the $150-$200 buying new plaction wheels and sprockets to meet the letter of the FIRST rule.

[Brandon Holley]

I can't speak for other teams, but we don't drill out the plactions and sprockets because we reuse them each year. We don't have a tremendously huge budget, so we'd rather spend the $25 a year replacing the fasteners rather than the $150-$200 buying new plaction wheels and sprockets to meet the letter of the FIRST rule.

Tom-

Can you just clarify what setup you had on your plactions. Was it a full combination of AM plactions, AM (plastic) spacers, and AM sprockets?

Was there generally a period of time before you would shear the bolts? For example, would you replace the bolts, get ~20 matches out of them, and then end up having to replace those bolts again?


I'm just hypothesizing about the nature of plastic parts in these situations. I'm wondering if the plastic parts creep slightly and therefore gradually lose the pre-load of the bolt. The parts may still appear tightly attached together, but losing some of that pre-load would then amplify any shear stress on the bolt and eventually cause a failure.

Just a thought I had.

-Brando

[Tom Line]

Tom-

Can you just clarify what setup you had on your plactions. Was it a full combination of AM plactions, AM (plastic) spacers, and AM sprockets?

Was there generally a period of time before you would shear the bolts? For example, would you replace the bolts, get ~20 matches out of them, and then end up having to replace those bolts again?


I'm just hypothesizing about the nature of plastic parts in these situations. I'm wondering if the plastic parts creep slightly and therefore gradually lose the pre-load of the bolt. The parts may still appear tightly attached together, but losing some of that pre-load would then amplify any shear stress on the bolt and eventually cause a failure.

Just a thought I had.

-Brando

We started seeing bolts shear about midway through our second district. We played through the semis of the first district, so that was 20-24 matches, + about half of the next competition - 10 or so. So call it 32 matches.

What we routinely see is that the locknut shears off, then the bolt head works its way out until it starts hitting the frame. It's always the nut side of the bolt that shears (probably due to the smaller minor diameter where the bolt is threaded).

The components are mostly Andy Mark. The plaction and sprocket is AM, the spacer is a copy of their design made out of ABS, so it's solid rather than having the pockets theirs has.

After the first failures, we put new hardware in and over-tightened them - the chain actually started skipping teeth because we bent the sprockets they were so tight. After that we taught the kids to tighten them but stop if they saw any deflection - essentially as 'tight' as they could be without causing damage.

We ran the same gear ratio in '11 but with no spacers and never had to replace one bit of the hardware. Perhaps the plastic to plastic interface is too slippery and allows relative motion.

[Chris is me]

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?

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.

[AdamHeard]

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]

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_Tables/Screws/Bolted_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]

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.
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 :ahh: 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]

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
http://www.efunda.com/formulae/solid_mechanics/fatigue/images/fatigue_SN_01.gif

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]

This is our plactraction wheel set up for dead axle. May help.
http://wiki.team1640.com/images/thumb/0/05/Plaction_Detail_annotated.jpg/450px-Plaction_Detail_annotated.jpg

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. :) :)