Min Thickness for Countersunk Bolts

[JesseK]

In attempting to improve last years drive train, I've run into a slight snag with multiple possible solutions. One is to use countersunk bolts in 4 locations to alleviate the issue. It's the most elegant solution so far, yet brings up a question.

The two pieces of metal to be attached are 1x1x1/16" box tubing and 1x2x1/8" box tubing. The countersink will happen in the 1x2x1/8" box tubing. Is this too thin? Is there a table of bolt diameters and/or minimum thicknesses for angles of countersunk bolts? There will be some sheer stresses on the bolts/boxes as well. Google hasn't been terribly helpful.

CAD to come on the overall drive train, soon. Thanks!

[AdamHeard]

We have used countersunk #10s in .125 without issue many times.

[Jeffy]

If it gives you a reference:
I have used #6 in .0625 with lots of issues.

[AdamHeard]

Do you really need countersunk? can you bore a large clearance and just bolt through one wall instead?

Could you use a larger diameter (like .5") insert the length of the tubing, and countersink that? That'd give a stronger connection anyway.

Originally Posted by Jeffy

If it gives you a reference:
I have used #6 in .0625 with lots of issues.

The had on a #6 is way thicker than .0625, I'd expect issues as well (unless you're not countersinking flush).

It looks like you have to go all the way down to a #2-56 to get close, and that's a .064" tall head still.

[MrForbes]

Are you going to run a screw through a piece of tubing, in a such a way that it could collapse the tubing? or will there be a spacer in it? or are you going to drill a larger hole through one side for access to the screw head, and have the screw only go thru one side of the tube?

I'd need more info before being able to answer the question, or suggest other ways of building it.

[Aren Siekmeier]

http://www.smithfast.com/msflathead.html. For both 82º and 100º screws it lists all the relevant dimensions of the countersunk head. It lists 82º #10 screws as between .098 and .116. 100º is even smaller, so you should be good (although just barely).

I really like finding websites like that that give every imaginable chain, fastener, or whatever dimensions you would ever need.

[JesseK]

Thanks for the responses! The CAD will clear some things up, but the idea is to attach the 2 box tubings along their 1" faces. The bolt will not go through the entire thing; instead only the two touching faces will be bolted -- like what Jim described in his last sentence. The bolt itself wouldn't be longer than 1/2".

The countersink is for chain clearance so the chain (or belt) doesn't rub against the bolt head if it jerks while slightly loose. The sprocket & chain will be housed within the 2x1. Other solutions are low-head machine bolts and/or simply putting teflon over the bolt head. Or I could put the chain elsewhere, but the new location is part of the potential improvement.

[AdamHeard]

Originally Posted by JesseK

Thanks for the responses! The CAD will clear some things up, but the idea is to attach the 2 box tubings along their 1" faces. The bolt will not go through the entire thing; instead only the two touching faces will be bolted -- like what Jim described in his last sentence. The bolt itself wouldn't be longer than 1/2".

The countersink is for chain clearance so the chain (or belt) doesn't rub against the bolt head if it jerks while slightly loose. The sprocket & chain will be housed within the 2x1. Other solutions are low-head machine bolts and/or simply putting teflon over the bolt head. Or I could put the chain elsewhere, but the new location is part of the potential improvement.

Is this a west coast style drivetrain? What are the advantages of moving the chain inside?

We (and many others) have run nearly identical chain placement tucked inside (as in adjacent, inside the driveframe but not the tubing) the 2x1 for seasons without any failure. If you look at 60, 254, 968, 1538 (and now 1323) and us, the number of drivetrains using that configuration including practice bots is probably above 60 among those teams. Thousands of hours of runtime as well.

I'm not saying it's a bad design without even looking at it, I just don't see what potential advantage internal chain has aside from being "protected". The cost of putting chain internal seem huge to me as well in terms of assembly and risk.

[MrForbes]

You might just want to use button head screws, they don't protrude much and are rounded, and not likely to cause problems for chains or belts, and no countersinking necessary.

Or just weld the two tubes together....or rivet them....or something like that. It's fun figuring these things out with just a few clues

[JesseK]

Originally Posted by AdamHeard

Is this a west coast style drivetrain? What are the advantages of moving the chain inside?

It's not like WCD, though I have thoroughly studied WCD and tried to deduce many of the design decisions along the way. Mainly the wheels aren't cantilevered and there's no need to add additional supports for bumpers. I know our welder would balk at a 973/254 side bumper rail, yet a couple of our mentors want more support than what 2793 and a few others had this year. The axles are live axles, which is why the 2x1 is needed. The big thing we lack is knowledge about automated CNC'ing*. While I understand we don't *need* CNC to do WCD-esque drives, I do have other mentors to convince (and they're a stubborn bunch). There are some other things that are 'wanted', like direct drive & a multi-level frame, yet we also have to live within our limitations. I think I've come up with a concept that can do those few things without increasing complexity of the drive train.

The sprockets are housed inside the box tubing blocks more for the fact that we won't have to purchase a hex broach and learn its process than anything. The 2x1 tubing will be in 4" segments, so the whole chain isn't inside the tubing. I still wonder about how much of a hassle it will be during assembly -- particularly with belts (I want to leave the option open for next year). We want live hex axles so there's a naturally-sturdy structure between the wheel blocks if we lathe the ends down for round bearings. We want live axles because we don't want slight imperfections in a bearing-wheel combination to throw off the chain on impact like it's done every year (e.g. Plaction wheel + pressed bearing was discovered this year). Hex axles will also allow us to use 4" AM performance wheels without an additional hub, which is a decent money saver when we're talking about 6 wheels. I may even try to find a sponsor with a 3D printer so we can get a custom wheel diameter and naturally-purple coloring (maybe).

We're going down to minimalistic mechanical mentor support for the Fall from what I can tell (work/school issues) so mechanical knowledge will be scarce. Additionally, we only have 1 mechanical FRC student returning this year. Sticking with what we know, at least for this year, will be key for us to expand upon our 2011 successes.

Jim, I'll look into the button head screws to see what's available. If we use 16T sprockets, there will be about 0.15" of clearance for a bolt head before a chain touches it.

*The year we tried auto-CNC at our knowledge level was disastrous (2009 ... record of 6-18-2 because we could barely move). We do have a no-slop, ~0.01" precision manual mill now, so I'm stepping up the game somewhat with direct drive.

[roystur44]

You might try a .125-187 Dia. stainless steel pop rivet. They are also available in counter sunk heads. A shoulder bolt could work if you don't mine the chain rubbing across the head.

High bond double back tape works wonders.

[AdamHeard]

Originally Posted by JesseK

It's not like WCD, though I have thoroughly studied WCD and tried to deduce many of the design decisions along the way. Mainly the wheels aren't cantilevered and there's no need to add additional supports for bumpers. I know our welder would balk at a 973/254 side bumper rail, yet a couple of our mentors want more support than what 2793 and a few others had this year. The axles are live axles, which is why the 2x1 is needed. The big thing we lack is knowledge about automated CNC'ing*. While I understand we don't *need* CNC to do WCD-esque drives, I do have other mentors to convince (and they're a stubborn bunch). There are some other things that are 'wanted', like direct drive & a multi-level frame, yet we also have to live within our limitations. I think I've come up with a concept that can do those few things without increasing complexity of the drive train.

Would they balk at our drivetrain with no welds (at all, anywhere)? or our .125 plate bumper supports that could be made by hand in 30 minutes? (seen pretty well here)

It's easier than you think, and sounds like it's easier/less parts than what you're planning.

Hex broaches are pretty cheap too for what they do.

[Chris is me]

Running chain in tube can be done, but I would recommend a bigger tube (3x1 at the minimum) so that you have room to do maintenance if it does fail. 2x1 would have been incredibly cramped for us, but 3x1 gave us some breathing room.

[AdamHeard]

Originally Posted by Chris is me

Running chain in tube can be done, but I would recommend a bigger tube (3x1 at the minimum) so that you have room to do maintenance if it does fail. 2x1 would have been incredibly cramped for us, but 3x1 gave us some breathing room.

Not trying to offend, but same question as before (and to Jesse as well). Why? What is the advantage?

[Chris is me]

Originally Posted by AdamHeard

Not trying to offend, but same question as before (and to Jesse as well). Why? What is the advantage?

We used belts in ours, so I think some advantages would be lost on chain, but the main reason is to protect the belts from damage and to keep them out of the way of the electronics. Since timing belts don't need any real adjustment once you get them "just right" (properly tensioned and aligned), ease of maintenance is not much of an issue. Even so, we can get a belt out in 7 minutes, enough to do a belt change between matches.

25 chain tends to stretch, and I would be concerned about that and sprocket wear. Especially because you cannot see the chain and thus have little indication how badly it needs to be retensioned.