High Tensile Sprocket Bolts

[daniel_dsouza]

Originally Posted by thefro526

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.]

Originally Posted by 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.

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.

[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]

Originally Posted by Gdeaver

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

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]

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.

[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.