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]

Originally Posted by Spirator

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]

Originally Posted by 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?

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]

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.