Chassis Connections

[Chris is me]

Originally Posted by Katie_UPS

So, I'm on a team that does a lot of welding. And by a lot, I mean that every robot since I've been on the team has been welded- the arms, the frames and -the title of the thread- the chassis. The other three robots that our team has made have been kit frames. So, to incite your responses:

How do you keep your 'bot -but more specifically, your chassis- together?

This isn't strictly true. http://www.chiefdelphi.com/media/photos/24740

Anyway, Shaker used a pretty simple - if non optimized - chassis with some 1/4" AL plate, standoffs, and 80/20 extrusion to connect the two wheel modules. Standoffs were made on a lathe and secured with 1/4-20 bolts (1/2 bolts for the combination standoffs and axles), and the 80-20 was fastened by tapping the extrusion's holes and bolting them onto the chassis. It weighed a fairly reasonable weight: I think a fully driving base with compressor and everything was 65-70 pounds, with no attempts at all to reduce weight. (We wanted a low CG and we were consistently underweight the entire year)



The chassis worked just fine all year, but it really isn't that well designed. Look at the cross section and you'll see it's a lot more C-shaped than we wanted. Prechargning a pneumatic kicker caused it to noticeably deflect. We also had the electronics mounted several inches higher than we needed to which took room away from a hanger. If we did it again I would take the 80-20 structure and mount it much lower on the chassis.

[Aren Siekmeier]

2007 we used kit chassis with LOTS of loose bolts (I wasn't around then, but many alumni have memories of the robot dropping things everywhere during matches and in the pits)

2008 was a little different. We used kit chassis rails that we bent around a bunch a bunch to make a cool looking but structurally weak octagonal frame with an indent in front for the trackball, and then bolted a bunch of 80/20 up off of that. The 80/20 was quite strong, and fortunately the chassis rail didn't have any problems.

2009 (yay for the basement!) we had kitbot chassis on the bottom and then two sheets of plywood up the sides. Everything inside the bot was then bolted to this plywood, with a nice plywood roof to top it off, making for a rather sturdy setup.

2010 we welded for the first time thanks to a local prototyping company who did it for free . It was definitely the strongest we had ever made. We used the chassis for wheel mounting convenience and then aluminum box tube for a superstructure around that. Initially it was all riveted together, and then we sent it in to be welded, with gusset plates at the corners.

Attached are pictures from the years I could find.

[JamesCH95]

Here is an FSAE car chassis that I welded together my senior year of college:



Frame weight of 58lbs with all brackets included, welded from 4130 steel. I could imagine a FIRST chassis made in this manor weighing less than 15lbs. Just food for thought for you welding-intensive teams.

[Andrew Schreiber]

Originally Posted by JamesCH95

Here is an FSAE car chassis that I welded together my senior year of college:


Frame weight of 58lbs with all brackets included, welded from 4130 steel. I could imagine a FIRST chassis made in this manor weighing less than 15lbs. Just food for thought for you welding-intensive teams.

If I recall correctly Kettering's Formula Zero team (hydrogen powered car) is using a similar chassis. Ours is a little smaller due to race restrictions though. Something tells me 1501 used welded 4130 in 2007 for their chassis.

397 has used the KOP frame the last couple years, we just bolt it together. There is something to be said about having a simple chassis together in one day. We have welding capabilities but only use them in situations that require it. No reason to over complicate things.

2337 used box tubing and some custom made blocks. These blocks were basically caps on the tube. Personally I feel this was heavy and labor intensive.

[DonRotolo]

Based on Billfred's experience and recommendation with 1/4" pop rivets, we've used them for 2 years with excellent results.

[AdamHeard]

Originally Posted by JamesCH95

Here is an FSAE car chassis that I welded together my senior year of college:



Frame weight of 58lbs with all brackets included, welded from 4130 steel. I could imagine a FIRST chassis made in this manor weighing less than 15lbs. Just food for thought for you welding-intensive teams.

15 pounds is a bit much for a drive chassis, we usually end up with a welded aluminum frame around 7 pounds.

[JamesCH95]

Originally Posted by AdamHeard

15 pounds is a bit much for a drive chassis, we usually end up with a welded aluminum frame around 7 pounds.

...which is less than 15lbs last time I checked

Do you have a picture of this 7lb aluminum frame?

[EricH]

A 7-lb Al chassis versus a 15-lb steel chassis--that sounds about right, steel being roughly 2-3x the weight of aluminum. (And stronger, so you can use thinner wall thicknesses, further reducing weight, such that for equivalent strength, 2x is closer.)

[AdamHeard]

Originally Posted by JamesCH95

...which is less than 15lbs last time I checked

Do you have a picture of this 7lb aluminum frame?

2009 robot, 6.9 lbs.

2010 robot, 7.2 lbs.

2010 preseason prototype, 7.0 lbs.

Weights are from memory, but I'm sure they're accurate within .5 if I go back and Check the CAD. Weights not including bellypan, which varied from 1.5-2.5 lbs each year iirc.

I didn't point out the weight difference to be harsh or brag, but to get people thinking. We don't do anything new or innovative with our frames (still ripping off Glenn Thoroughmen's fantastic designs) and we've never failed one.

[ttldomination]

A bit off topic, but is the 2010 preseason prototype a 254/968 style with cams/diamond bearing blocks?

- Sunny

[AdamHeard]

Originally Posted by ttldomination

A bit off topic, but is the 2010 preseason prototype a 254/968 style with cams/diamond bearing blocks?

- Sunny

All three posted bases are of that style, except the 09 didn't use cams.

[JamesCH95]

Originally Posted by AdamHeard

2009 robot, 6.9 lbs.

2010 robot, 7.2 lbs.

2010 preseason prototype, 7.0 lbs.

Weights are from memory, but I'm sure they're accurate within .5 if I go back and Check the CAD. Weights not including bellypan, which varied from 1.5-2.5 lbs each year iirc.

I didn't point out the weight difference to be harsh or brag, but to get people thinking. We don't do anything new or innovative with our frames (still ripping off Glenn Thoroughmen's fantastic designs) and we've never failed one.

They look good. If you've got access to the CAD file would you mind switching the material to 4130 steel and making the wall thickness either 0.035 or 0.045? If that's easy to do I would be curious about the weight.

[AdamHeard]

Originally Posted by JamesCH95

They look good. If you've got access to the CAD file would you mind switching the material to 4130 steel and making the wall thickness either 0.035 or 0.045? If that's easy to do I would be curious about the weight.

Prototype Frame for 2011.

Aluminum (6061-T6) 6.1 Lbs

4130 .045" wall 9.3 Lbs

4130 .035" wall 6.7 lbs

I'd like to point out that a substantial amount of the "weight saved" was in changing the .125" wall 6061 for our siderails to the .045/.035, which would be very impractical for us to do without a redesign of our bearing block system (which would just be more work for a net gain in weight). It's convenient to have that .125 wall. For reference, this is generally our only use of .125 wall on the robot.

I'd also like to point out that for our particular team, switching to a welded 4130 frame would take a significantly longer time to machine and then weld, along with costing us more (in the short term, as we don't stock steel or any filler rod, etc...).

Also, I hear it mentioned all the time that welded reduces the temper of metal. This is true, I do not disagree one bit. I just want to throw a counterpoint out for the inexperienced people reading who might otherwise assume "Welding aluminum is AWFUL! it turns to butter!" (I know I did a few years ago). The weakened material will be in your welded joints obviously, and with a good frame design these won't be loaded to failure. We've never broken a weld in a driveframe before.

I encourage everyone reading who is re-evaluating their frames for steel, to also do so for aluminum; I'm not saying one is better than the other, but if you analyze the steel and fail to do the same effort into aluminum, you aren't really proving anything. For some teams one will be better, and for others, the other; do the math, evaluate your resources, and make a decision.

[Brandon Holley]

Originally Posted by AdamHeard

I encourage everyone reading who is re-evaluating their frames for steel, to also do so for aluminum; I'm not saying one is better than the other, but if you analyze the steel and fail to do the same effort into aluminum, you aren't really proving anything. For some teams one will be better, and for others, the other; do the math, evaluate your resources, and make a decision.

I couldn't agree more with this statement.

Often when we're making engineering decisions, we end up choosing between 2 or 3 options and weigh the benefits. Sometimes when making these decisions we all have a tendency to declare one a "safe" bet or automatically declare it as valid, while trying to prove an alternative as feasible. While this is sometimes a good way to explore your options, its important to keep an open mind. Keep all of doors open, and gradually close them one at a time. Closing each door can be based on a variety of circumstances, just make sure you're choosing what works best for your particular situation.

-Brando

[JamesCH95]

Thanks for the response Adam.

I would add that although steel is harder to machine, the thinner wall thicknesses mean that there is less material to remove, so the machining/cutting times tend to balance out. Using abrasive tools might even expedite cutting time.

I agree that every option should be explored when a team is making a design decision. I remember when I thought "steel? that's too heavy" and dismissed it, oh how wrong I was.