High Tensile Sprocket Bolts

[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]

Originally Posted by Tom Line

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]

Originally Posted by DonRotolo

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]

Originally Posted by DonRotolo

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]

Originally Posted by Trent B

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]

Originally Posted by 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.

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]

Originally Posted by Tom Line

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]

Originally Posted by Brandon Holley

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]

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?

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.