Steel on a FRC Robot

[Grim Tuesday]

I can't imagine making anything out of steel on our robot (besides some drivetrain components)

On our elevator this year, we even replaced a steel axle with an aluminium one to save weight!

[Joe Schornak]

Our robot has an all-steel welded frame, and it's worked quite nicely so far. Although steel is indeed heavier than aluminum, we make up for it by using thinner-walled material and very few bolts.

The trick with using steel is to take advantage of its greater strength. We're able to save weight by using many fewer supports than we would have with aluminum. Our frame is basically a flat rectangle, solidified by some diagonal corner brackets. It comes out a bit heavier than the aluminum equivalent, but it works nicely as a counterbalance for the elevator. The whole robot weighs 119.2 lbs, and is able to complete every element of Logomotion.

There are still many aluminum parts on our robot. Things that really need to be lightweight, like our gripper and minibot deployer, and things that aren't in high-strain positions are vastly aluminum. Our in-house machined components are also primarily aluminum, due to ease of work and speed. It is simply a matter of weighing pros and cons.

Steel's good to use in FRC, as long as you have the proper tools and skills at your disposal, and plan carefully. I suppose that magnetic minibots could be an issue... ours uses a fairly powerful neodymium magnet, and we haven't gotten it stuck to the frame yet. Yet...

[kgzak]

Last time we weighed to robot it was around 112 but we still had parts to put on. then we realized we had the battery on so we are still in a good position. still waiting on minibot deployment (less than 8 pounds) and we took off 22 pounds for our claw/lifting mechanism.

[Tristan Lall]

I'm not a fan of ordinary structural steel for FRC robots. The strength to weight ratio is awful, and the available sections are generally inconvenient. You see it occasionally, and you always wonder what set of design tradeoffs made that worthwhile for the team. (To be completely blunt, it's probably more like a lack of design that precipitated it.)

Alloy steels, particularly chromium-molybdenum steels like AISI 4140, are very useful, however. In the annealed condition, the strength to weight ratio is around the same as AA 6061 wrought aluminum. The difference is that you can weld annealed AISI 4140 with negligible loss of strength, while AA 6061 will drop dramatically in strength when welded. That makes welding a viable construction method for high-stress, high-strength parts.

The trouble is, because the steel weighs more, you have to use thinner material. This can be difficult to work with. Also, since 4140 typically comes in solid and tube sections (and doesn't work very well as sheet metal), you'll see it used in space frames, rather than stressed skins.

And of course, steel (of all kinds) is stiffer (in terms of elastic modulus) than aluminum, so when distortion is a major criterion, you may want to choose it.

But as for stainless steel, I've got to ask: why? (There are lots of grades of stainless, but in general, while it's probably a slightly better material than structural steel for an FRC structural application, it has a lot of other undesirable properties—for instance cost and machinability, under most circumstances—that make it a surprising choice.)

[Billfred]

We used a little bit of 1/8" thick flat steel on the end of our arm; originally we intended to tap its magnetic properties for a little extra holding force to keep our claw steady, but we decided to go with an active mechanism later down the road. Beyond that, it's pretty much fasteners.

[kgzak]

Originally Posted by Tristan Lall

for instance cost and machinability, under most circumstances—that make it a surprising choice.)

as I said for cost, it was donated (I am assuming they use stainless for what they do and said we could use some) and we didn't machine it all so we didn't have issues with that. With the load on it we need something stronger than aluminum. Stainless steel was what we had available so we used it.

[tutkows1]

Last year my teams robot was made complete out of angle iron steel, and this year our part of our arm is stainless steel.

[Zholl]

One of the axles on our claw mechanism is currently made of steel, mostly because the aluminum rod that was in there before kept bending under load. Otherwise, it's aluminum and a wooden board

[MrForbes]

Last year we made our chassis of MIG welded thinwall mild square steel tubing. It's strong, a bit heavy but not too bad, and it's relatively easy to build. Also it's locally available and not very expensive.

We also make the Typewriter Repairmen underwater ROV frames from steel strap, for similar reasons. A coat of spray paint keeps it from rusting.

If you do a good job designing your manipulator to be light, then a bit more weight in the chassis really doesn't hurt anything.

[Hawiian Cadder]

last year the drawback for our kicker was a pair of wheels with a little pins to drive each other, this needed to be made out of steel because we did FEA on the part and aluminum was not strong enough, an aluminum part on a high torque part must be made larger to offset the torque. steel can be made smaller and thus easier.

[s_forbes]

2840 made an appearance at the Duel in the Desert and was sporting a steel chassis, so I know of at least one steel framed robot this year.

Steel is a great building material... cheap, easy to work with (if you have a mig welder and a chopsaw). It ranks up there with wood and fiberglass in terms of my favorite robot building material.

[JamesCH95]

Originally Posted by Tristan Lall

I'm not a fan of ordinary structural steel for FRC robots. The strength to weight ratio is awful[...]

Alloy steels, particularly chromium-molybdenum steels like AISI 4140, are very useful, however. In the annealed condition, the strength to weight ratio is around the same as AA 6061 wrought aluminum. The difference is that you can weld annealed AISI 4140 with negligible loss of strength, while AA 6061 will drop dramatically in strength when welded. That makes welding a viable construction method for high-stress, high-strength parts.

The trouble is, because the steel weighs more, you have to use thinner material. This can be difficult to work with. Also, since 4140 typically comes in solid and tube sections (and doesn't work very well as sheet metal), you'll see it used in space frames, rather than stressed skins.

4130 steel is available in many tube cross sections with very thin wall thicknesses, down to 0.035in. If one was going to make structures out of steel that's what I'd recommend. I'd then advocate the use a weld-in nuts to fasten everything to the frame.

[flippy147852]

I know in the past 1918 has used steel bars as ballast to get weight where we want it

[SGS]

Our whole forklift/arm is made of steel, completed the arm weighed about 30 lbs, and ultimately caused our robot to weigh about 124. We had to disassemble and cut holes in everything, but our robot is VERY robust.

Last year, our whole frame was made of 1' square steel tubing , it was the only tunnel robot near the weight limit, but in the finals at Lone Star, it was an indestructible defensive robot and still quite fast.

Granted steel may not be the best material, but it is often a practical option and it is possible to create a decent if not relatively good robot with it.