Polycarbonate side plates for lighter gearboxes

[Joseph Smith]

Hello all,
My team this year machined some custom side plates out of quarter-inch aluminum to lighten up some Andymark toughboxes. We spaced them out with some simple aluminum spacers, and it all held together very well. I was wondering if it is plausible to make these plates out of polycarbonate, probably quarter-inch. Are there any advantages or disadvantages of using polycarbonate? Do you have any advice for working with it? Any help would be appreciated. Thanks!

[MichaelBick]

I don't know about polycarbonate. But 254/968 and some other teams have used delrin to significantly reduce weight.

[Kevin Sevcik]

It's interesting that you did that to lighten up some AM Toughboxes, since the TB back plate is polycarb (Lexan) and the front plate is 1/8" aluminum. And AM will sell you aluminum spacers as well.

At any rate, 1/4" Lexan will be about 10% lighter than 1/8" aluminum, for starters. If you're not direct driving out of the gearbox, 1/4" Lexan will be plenty strong enough. The main downside to Lexan is that it's prone to cracking. Not as much as acrylic, but much more so than aluminum. So you'll want to be careful about tightening things and subjecting it to heavy shock loads. There's a reason AM chose aluminum for the front plate of the TB, that's the plate that takes the most load from radial forces on an overhung shaft.

Since 1/4" Lexan only saves 10% over 1/8" aluminum, perhaps you should look elsewhere in the TB for weight savings? AM will sell you aluminum versions of the large gears that will save you 0.56 lbs together. In the past, I've tossed the large gears on a CNC and webbed them to save a similar amount of weight. And there's the aluminum hex output shaft that would save you another 0.1 lbs or so.

So what I'm basically saying is that if all you did to lighten up a TB is custom side plates and spacers, then you have some other low hanging fruit available.

[Dan_Karol]

you mean something like this?:




This year our team decided to integrate the gearbox into our chassis rail and use a piece of polycarb as the other side wall. It took a couple of tries for our students to get the hang of drilling the holes, but we never had any problems with the gearboxes.

[akoscielski3]

Originally Posted by Kevin Sevcik

It's interesting that you did that to lighten up some AM Toughboxes, since the TB back plate is polycarb (Lexan) and the front plate is 1/8" aluminum. And AM will sell you aluminum spacers as well.

At any rate, 1/4" Lexan will be about 10% lighter than 1/8" aluminum, for starters. If you're not direct driving out of the gearbox, 1/4" Lexan will be plenty strong enough. The main downside to Lexan is that it's prone to cracking. Not as much as acrylic, but much more so than aluminum. So you'll want to be careful about tightening things and subjecting it to heavy shock loads. There's a reason AM chose aluminum for the front plate of the TB, that's the plate that takes the most load from radial forces on an overhung shaft.

Since 1/4" Lexan only saves 10% over 1/8" aluminum, perhaps you should look elsewhere in the TB for weight savings? AM will sell you aluminum versions of the large gears that will save you 0.56 lbs together. In the past, I've tossed the large gears on a CNC and webbed them to save a similar amount of weight. And there's the aluminum hex output shaft that would save you another 0.1 lbs or so.

So what I'm basically saying is that if all you did to lighten up a TB is custom side plates and spacers, then you have some other low hanging fruit available.

I have no Idea what kind of Lexan you are using but the Real Lexan is Virtually Indestructible. And I mean the actual brand name "Lexan" not cheep stuff.

I believe you should be fine to change to the lexan, even though i am not really sure how much weight you will save.

[sdcantrell56]

We used delrin for a year in our drive transmissions and are now back on aluminum heavily pocketed for sideplates. Plastics are really too flexible for this application where you want all the efficiency you can get. I know 254/968 both swapped back to aluminum as well for the same reason

[Chris is me]

Originally Posted by akoscielski3

I have no Idea what kind of Lexan you are using but the Real Lexan is Virtually Indestructible.

It depends on thickness and application.

I tend to agree with Sean in terms of absolute efficiency. That said, I've never done a side by side test or anything like that. In my mind, if you're looking to cut weight from a gearbox, start with your gears.

[thefro526]

Originally Posted by Chris is me

It depends on thickness and application. That said, I've never seen 1/4 lexan crack in a gearbox application.

I tend to agree with Sean in terms of absolute efficiency. That said, I've never done a side by side test or anything like that. In my mind, if you're looking to cut weight from a gearbox, start with your gears.

Chris, not to argue with you, but the Polycarbonate (Lexan is a brand name) side-plates on the Toughbox have been known to crack. I've never seen one fail during a match, but I have seen plates with plenty of spider cracks after a season's worth of abuse. That being said, I'd stay far away from using Polycarb or any other plastic for the side plate in a drive transmission.

OP, if you're looking to save some weight from your gearboxes, it's worth playing with lighter gears first - Like Chris said above. West Coast Products and AndyMark sell drop in replacement AL gears (1/3rd the weight of a comparable steel gear) for many of the standard toughbox ratios.

[Hawiian Cadder]

Originally Posted by thefro526

Chris, not to argue with you, but the Polycarbonate (Lexan is a brand name) side-plates on the Toughbox have been known to crack. I've never seen one fail during a match, but I have seen plates with plenty of spider cracks after a season's worth of abuse. That being said, I'd stay far away from using Polycarb or any other plastic for the side plate in a drive transmission.

OP, if you're looking to save some weight from your gearboxes, it's worth playing with lighter gears first - Like Chris said above. West Coast Products and AndyMark sell drop in replacement AL gears (1/3rd the weight of a comparable steel gear) for many of the standard toughbox ratios.

you can also pocket a steel gear out to about that weight as well. Working on an off-season upgrade I found it was easy to remove 60% of the mass of a steel gear with no practical strength reduction. With a little more time on the mill one could probably get it down to 25% of its starting mass.

[Cory]

Originally Posted by sdcantrell56

We used delrin for a year in our drive transmissions and are now back on aluminum heavily pocketed for sideplates. Plastics are really too flexible for this application where you want all the efficiency you can get. I know 254/968 both swapped back to aluminum as well for the same reason

Moreover it's nearly the same weight when you pocket the aluminum one.

[Tristan Lall]

Add my vote to "too flexible to be worthwhile". However, I've been toying with the idea of using DragonPlate carbon fibre laminate. Basically, bond something very stiff to the large faces, and something light, dimensionally stable and durable on the inside.

[Kevin Sevcik]

Originally Posted by akoscielski3

I have no Idea what kind of Lexan you are using but the Real Lexan is Virtually Indestructible. And I mean the actual brand name "Lexan" not cheep stuff.

Originally Posted by Chris is me

It depends on thickness and application. That said, I've never seen 1/4 lexan crack in a gearbox application.

Perhaps I should take this up with AndyMark, then. All I know is that I have several Polycarb TB back plates that my students have cracked by overtightening the screws on the gearbox. I'll grant that they're small cracks, but they're cracks nonetheless. And Rule #1 of cracks is they never get smaller.

Also, one of the recommended methods of cutting (thin) Lexan is to score it and bend it to crack it along the score. Which works surprisingly well if you get a good sharp score.

Also, also, you might want to look into the phenomenon of solvent stress cracking in polycarb. Wiping down a (small) piece of polycarb with acetone is a fun and enlightening demonstration. But even barring that extreme reaction, the Google suggests that aromatics in your cutting fluid or residue on your tooling can be enough to significantly weaken Lexan around areas you've machined. Loctite or superglue are also really bad for Lexan.

Which is all to say that, yes, Lexan is tough stuff, but it's more prone to cracking than your standard 6061-T6 aluminum and more care needs to be taken when using it.

[Chris is me]

Thanks for correcting my misconceptions - I've removed the inaccuracy from my post. Now that I think of it, 2791's only assembled three or four regular Toughboxes, ever...

[pmangels17]

We usually use .125" aluminum for plates, not .250". seems to hold up fine. sometimes, we use 4"x2" tubes, or any size applicable. Even that is usually .125". The max we go is 3/16" for really high power stuff. We reserve 1/4" plates for the highest stress applications. We dont have a single piece of 1/4" on the bot this year. Drives are .125" plates, the rest are 2"x2"x1/8" tube with lots of excess milled out. Hope this helps. Good luck!

[Gray Adams]

We have a gearbox this year that is one side 1/4" delrin, other side 1/4" aluminum. We pocketed the aluminum plate out and it weighed almost the same as the delrin. It really doesn't seem to be any better not to use aluminum, and the aluminum looks a whole lot nicer after bead blasting and anodizing. In fact, the delrin was a little bit warpet because it was such a large piece (9"x 6" I believe). We only used delrin because we mounted Banebot 775s to it and that way we didn't have to worry about the cases shorting to the frame.