Plate Materials for Swerve

Over the off-season we designed a swerve module for use in the 2020 season. The main plate that mounted to the robot frame was 5/16in thick aluminum. Looking at other teams designs I’ve seen a lot of alternate materials used for main plates. As I iterate on our design I’m looking into switching the material to save weight and make machining easier. I would be mounting the plate on top of the frame (made of 2x1 aluminum). I was thinking about either 3/8in thick polycarbonate or 3/8in thick nylon. What would be the performance and machining differences between the two? Will there be too much flex in the plate because it is mounted above the frame?

This is a picture of the plate design, the gray sketch lines show where the frame would be
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Just use thinner aluminum- we’ve never built a swerve but we use 5mm (0.2 inch) aluminum for all our gearboxes and it holds up just fine, i’ve seen other teams get away with 4 (roughly 1/6 inch) and 3 (1/8 inch) millimeter gearboxes. You can also pocket it if you really wanna save weight.

The thing is, the main plate on a swerve module receives a lot of bending load, as it has to basically carry the robot, where as with a tank drive gearbox it doesn’t have to take a lot of bending forces, therefore you would need a thicker plate

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Does it have to be 5/16"? Couldn’t you use 1/4"? Let the 6 or 8 bolts capture what I assume is a large bearing. Both PC and nylon will have some flex at 1/4" That could be bad. We used 1/4" G10. Our router can cut fairly fast in the G10. We use a end mill specifically designed for composites.

Having it thicker than a 1/4 is really helpful. That way the hole for the 1/4in thick bearing can have a lip on one end to prevent the bearing from pushing back out the top of the plate. Bolts would be less strong and it needs to hold 1/4 the weight of the robot. Some flex is actually beneficial because it adds some compliance that will keep all of the modules on the ground

Use a 6061-T6 and it should hold up. But I’d check after every comp for it’s strength and wear. We’ve never built a swerve module but we find that 6061-T6 (1/4 or thicker) is very hard to bend.

Maybe you could try a 7075-T651?

Or add another brace across the frame, so the plate does not have so much bending load on it? Then you could use thinner material, and it would also be stronger.

Yeah yeah 6061 is great at 1/4" and above for this use case, I was responding to Benjamin saying that 4mm could be strong enough, it might be, but I wouldn’t bet on it

Anything under .175 is destined to bend even in the slightest (I’m looking at you RSL light holder)

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That could be a good solution, but at that point I would just go for a thicker plate

Roughly 35 lbs static. Pretty much nothing for 5/16" Al.

The case that’ll wreck a weekend is if you drive off a platform, the wheel that hits first will see a near-full-weight shock load. For FIRST purposes, you can model that as 5x static, so assuming the full weight of 150lbs is behind it, apply FEA for 750lbs.

I modeled it as an 80mm bearing hold in a 6" plate. I fixed the bolt locations in the friction area around each fastener, and applied 750lbs to the lip on your 5/16" plate in 6061-T6. Maximum stress is ~8x10^7 N/m^2 at artificial islands created by the modeling method, and is still 3x below yield stress (~3x10^8).

Out of curiousity, I re-ran at 3/16" plate thickness, keeping a 1/16" lip for bearing retention, and found the maximum stress in simulation is just about equal to yield stress. Because those maximums are at artificially high stress areas created by the analysis method, I would expect a 3/16" plate to last the season easily.

Then I got really curious, and re-ran it with the 3/8" polycarbonate suggestion (full 1/8" bearing lip). This got some fun displacements… which might actually be to your benefit, the stresses still don’t actually exceed material strength and under “normal operation” you’ll have an easy time keeping all 4 wheels on the ground.
Look into how 2767 designs compliance into their module to get consistent wheel contact & resulting improved module controllability.

IF you are looking for anecdotal evidence, 192 has used 1/4" acrylic and 3/8" wood in their modules without field failures :open_mouth:

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Thank you so much for all of that info. It really helped.

The modeling of the plates above give a good image where stresses occur. But,they are just a simplified model. One thing to note is the number of fasteners in the model and they appear to have large washers under the bolt heads. In a real module there will probable another plate with stand offs and motors attached. Motors have mass. Stand offs and 2 plates increase stiffness an share the loads. However there is a point stress at the stand offs. The large bearing will have the steering pulley or gear inside it further adding complexity to the model. The forces applied when a module slams into a barrier like the ball coral this year are different then the forces modeled above. So while the model above can be useful, It does not truly represent the real world forces the module sees. To fully model the stress in a real world module are well, that’s complex. So how to make this decision. Fortunately there are many teams that have done destructive testing on real modules. 910 used .375" nylon plates with success. However they used small wheels and there modules are very compact compared to others. 2910 and Swerve Specialties used 5/16 AL very similar to yours and ours. They are thinned and relived. 33 killer bees also have designed a small module. They are different in that they are printing theirs out of carbon nylon. They have 2 plates and they sandwich the frame rail between both plates and they fully enclose the module between the plates. This full enclosure also adds stiffness to the whole assembly. Triple Helix also has a module with a single plate attachment. There are many other team also. Look at their module for real world examples. We have been redesigning ours too. We’ve looked at what other teams have done. We went with .25" plates and have the 6 bolts retaining the bearing. Many teams have AL wheel supports with a small point contact on the bearing. We are printing our wheel supports and have a very large contact with the bearing. 33 did this too. Our module plates sandwich the frame rails. This means we are high. Trade off. Some members wanted to print 1 or 2 plates out off carbon nylon. We can not reliably print them and print time is a killer. We can always use AL but have gone with the G10 over nylon. The flex is not acceptable for our module and we can rout it fast. Dust control is a issue we are addressing. We have a drive base that is in the hands of the programmers. The g10 looks to be good. We will beat it in the next 2 month and find out. Driving off hab 3 should be a good test. Also flying into the ball corral and other floor barriers. High school student excel at destructive testing. So what I’m saying is do your best to make a design choice. Then get a driving base and beat it. You could make 2 AL and 2 nylon. Then test and make a choice. It’s the end off Oct. still time to get it done. We should have been testing all of Oct but still do not have a moving base. Hopefully next week. Our 2020 season will hang on our design choices.

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