610's Preseason Drive

[Nate Laverdure]

Originally Posted by Ty Tremblay

Why use threaded rod across the entire width of the robot instead of nuts and bolts?

This is clearer when you consider the 2013 design. There was no machined inner block** that could have accepted any shorter fasteners. In that design, you use the whole end face of the ladder bar as a bearing surface to take any bending loads.

The ladder bar is a preloaded spacer. Because the threaded rod is so long, the spring rate is very low, which makes it possible to maintain big preloads on the rod, and therefore big clamping forces on the assembly.

**The opposite approach is taken in the 221 SimpleTube chassis... they use machined inner blocks to connect the ladder bars to the chassis rails.

[Ty Tremblay]

Originally Posted by GeeTwo

That's what's holding the two sides of the robot together. They aren't using any gussets or other brackets that attach to both the cross-tubing and the wheel tubes*. If they used nut & bolt at each end, the cross tubing would slide right off of the milled piece linked from post #3.

Edit:
* except the belly pan.

If they used a nut and bolt on each end, yeah. But that wouldn't make much sense...

My question was why not modify the milled piece such that machine screws can be used.

Originally Posted by Nate Laverdure

This is clearer when you consider the 2013 design. There was no machined inner block** that could have accepted any shorter fasteners. In that design, you use the whole end face of the ladder bar as a bearing surface to take any bending loads.

The ladder bar is a preloaded spacer. Because the threaded rod is so long, the spring rate is very low, which makes it possible to maintain big preloads on the rod, and therefore big clamping forces on the assembly.

**The opposite approach is taken in the 221 SimpleTube chassis... they use machined inner blocks to connect the ladder bars to the chassis rails.

Thanks. I understand how it works.

I'm asking why that method was chosen mostly because there IS an inner block in this chassis that could have been designed to accept shorter fasteners. Granted, would have taken an additional setup or two in the mill.

[Brian_R]

Originally Posted by Ty Tremblay

I'm asking why that method was chosen mostly because there IS an inner block in this chassis that could have been designed to accept shorter fasteners. Granted, would have taken an additional setup or two in the mill.

The long fasteners allow the whole tube to be preloaded compressively. This should give some additional rigidity and resistance to bending, which I'd find pretty useful in a chassis member that might get some good impacts in defense situations.

Screwing into the end caps only would be much lighter, but you lose the preload. I wish we had thought of that in 2014...

[Nate Laverdure]

Originally Posted by Brian_R

The long fasteners allow the whole tube to be preloaded compressively. This should give some additional rigidity and resistance to bending.

No, that's not really what's going on. In fact, loading the ladder bar in compression will make it more sensitive to perpendicular loads! Here's a better explanation:

The compression forces, commonly called preload, determine to a degree how stiff the joint is because immense friction is created at the part interface thanks to those compression forces. The basic idea is that the preload force must be overcome before the structure will even begin thinking of maybe shifting, just a tiny bit. Hence, properly designed machine structures are predictably stiff in their operating regimes.

[Drivencrazy]

Originally Posted by Nate Laverdure

No, that's not really what's going on.

+1 for video use.
+1 for column buckling formulas

[Brian_R]

Originally Posted by Nate Laverdure

No, that's not really what's going on. In fact, loading the ladder bar in compression will make it more sensitive to perpendicular loads! Here's a better explanation:

Thanks, the youtube link was helpful.

I hadn't considered buckling, so I took a stab at the numbers. I estimate the buckling load thusly (ignoring the pocketing of the tube because I'm not able to do it with FEA at this time):

Fixed-fixed: n=4
I=0.034 in^4 (2x1x1/16)
E=1E7 psi (aluminum)
L=24 in

F=23000 lbs

Let's err on the side of maximizing preload. Even a very fancy grade 8 1/4-20 bolt is listed as having a clamping force of 2860lbs, so it's about an order of magnitude away from buckling under the preload alone.

Granted, this doesn't say much about sensitivity to side loads. It's been a few years since I've broken out the calculations for side loads to trigger buckling, so I'd be very interested if you can estimate that under these simplified parameters.

I'll happily concede buckling is a concern with a slender, compressively loaded member, but I don't think 610's preseason chassis is near that state yet. Again, this ignores the pocketing, which'll of course weaken things, but is a separate design choice.


For preload to reduce bending:

It seems sensible to me that preloading the tube axially, to ensure a compressive stress state, prevents a tensile component and thus reduces the risk of a bending failure. The shear stress increases, but again, I think it's within what the aluminum can handle. As it's statically indeterminate, I'm not sure how to crunch the numbers without setting up a simulation.

Finally, to look a bit further into the source you provided (Step 10):

The Effects of Preload on Spacers

We aim to exact a slightly different end to using preload on bolted spacers. It's not so much the tensile loading that is beneficial so much as the ability to change the type of loading on the spacer's walls from bending to tension and compression. The total increase in rigidity comes from two main sources:

1. The outside of the sleeve is put into compression. A bending load will tend to compress one side more while relieving the other side. If there was no existing compressive stress, then the material will deform more before the same levels of stress occur within it. The stronger the material, the more compressive stress can be added (the stronger the preload). This works until the bending causes the compressed side to rupture (buckle outwards), and the other side to irreversibly stretch (plastic deformation).

[Nate Laverdure]

Originally Posted by Brian_R

For preload to reduce bending: It seems sensible to me that preloading the tube axially, to ensure a compressive stress state, prevents a tensile component and thus reduces the risk of a bending failure.

This explanation is a good one. Thanks.

Originally Posted by Brian_R

Thanks, the youtube link was helpful.

Yeah, sorry.