Unibody/Monocoque Construction

Does any team have any experience or knowledge concerning unibody construction? Could anybody compare the construction method with welding or other styles of chassis out there?

My team is researching different chassis construction methods and any input concerning the above mentioned technique is appreciated.

Sam N.

look up team 1501…

Team 1501 uses monocoque construction in their robots. Search CD media for some pictures of their robots. They always come out with a very sleak and professional looking robot. Sorry already said.

Team 33 has used monocoque construction in our last two robots. We use .063 aluminum and are able to make a very rigid frame for roughly 9 pounds. You can contact Jim Zondag for details.

Yea 1501 is all monocoque and if you need any help with learning ask us and you can go to are website www.huntingtonrobotics.org and people their and it gives more info on it to so hope you will go monocoque because its fun and easy to learn and it also comes out with one beautiful bot!!!:smiley: :smiley: :smiley:

Yep, 1501 would be a team to look at. There was a good section on them in “Behind the Design” which I just bought yesterday.

One of the students made up this page and they did a video. I dunno if that helps or not…


Video Link

No skin pictures of 2005 robot.

The page is pretty impressive and well done for a student.

Just curious, since then have you moved on to designing the parts with the sheet metal features of inventor, or do you still make a 2d sketch and then transfer to plywood? Or was the plywood just a medium to transfer the unfolded sheet metal pattern to sheet metal?

2005 and 2006 robot are all 2D 1:1 scale plots transferred to plywood, then sheet metal.

2007 robot was drawn in 3D for the first time. I can’t remember if they plotted and used the 3D drawings from inventor or not. One of the students was showing me the sheet metal tool in inventor (me personally I don’t have a clue how to use the software), and acutally I think it was a student from team 1720 that was showing me :wink:

2007 was different in a way that it was alot of tubing, the same tubing they use in the landing gear on the airplane. Chromoly tubing, then the wings are sheet metal with rib spars. So it was a bit different than the 2005 and 2006 robot but still based on airplane parts. :wink:

You guys are using Chromoly? wow… that stuff is heavy, I can’t really tell from some pics I just looked up, but what size tubing was it and was it worth it compared to a larger piece of aluminum tubing? Is it something worth doing again? I’m not criticizing, I’m just trying become more familiar with construction techniques

Don’t put off learning CAD… I did for two years and it was totally not worth it… It seemed intimidating to me, but I just took the tutorials in inventor and fiddled around; Now I’ve become pretty good at it. If I can do it as I high school student, you should take some time to learn it. Unless you’re not a M.E.; in which case, sorry for harassing you.

It’s not as heavy as you think. Our whole tube frame was somewhere around 36 lbs I think. I am not for sure on that. But I was very impressed. The biggest down fall of the tubing was the welding. We had one mentor and two students that could weld it, the process was a bit slower than 10 guys with rivets guns for sure, but I can’t complain, the end result was amazing to me.

I am not an M.E. I am a Controls Engineer. About the extend of my cad is AutoCad Electrical 2008 (<<<–use to be VIA, until AutoDesk bought them out) and I like Google Sketchup. I like Sketchup and have been learning the concept of 3-D drawing, maybe that will carry over to Inventor, if I ever have a need to use it. So far, drawings electrical prints in 3D some how doesn’t fit into my schedule so far :slight_smile:

Although I wonder if the electricans would freak out if I gave them a 3D wiring diagram??? hmmm…

I was the mentor that helped with the welding of the frame last year. Chromoly can be built very light if done properly. That’s why it’s commonly used in race car chassis, high stress points in air frames, etc… Given our design goal (killer lift bot), a fully monocoque frame was not appropriate last year. The tube frame could be designed much faster than it would have taken to do it fully monocoque. Aluminum tubing could have been used, but we didn’t have the capability to weld it in-house and I’m not sure it would have ended up much, if any, lighter keeping the same strength. The total weight of the tube structure was #25, I believe - and this included the 8 big rollers, 16 bearings and 16 pillow blocks for the main lift. In all but a few small tubes, the wall thickness was only 0.035". This kept the weight down and all the tube triangulation made the entire frame and lift very stiff. The main tubes of the center tower are 1" OD. All the other tubes are either 5/8" OD or 3/8" OD. At one time there was a video on youtube of our bot lifting a 300lb guy on one wing without any showing any flex. As the primary designer of the tube structure I would definately use chromoly again if apprpriate for the application.


that’s the kind of info I was looking for; Very good use of it. Could you post some better pics of the frame than what is in CD-media currently? you can only see the uppermost portion which doesn’t reveal much. Also, when you say 25 pounds, how much of the robot frame is that? Is the chromoly weldament the entire frame, or are there other pieces in there?

If you weren’t building that robot, but instead building a standard 6 wheel drive base with some sort of maniupulator that didn’t affect the base shape, would you prefer the monocoque method, or a welded chromoly frame?

I belive that is the frame without the two pneumatic cylinders and the two gas struts. I could be wrong, but I believe that was how we measured it, the frame straight up, nothing added.

Can’t tell you that, this was my first year and I’m still learning lots about monocoque design, and enjoying every minute of it:D

To Follow up on Woody’s note above:
Team #33 has used a Structural Sheet Metal chassis design on several of our robots. Not exactly a monocoque, but similar. Like elaborated above by other teams; this is the strongest and lightest way to make many elements of a Robot. Our structural Chassis module is under 9 lbs including axles, motor mounts and battery tray, and it is very strong and very rigid. It has easily survived over 13 FIRST events of other teams relentlessly pounding on us and I have yet to have a failure of any kind. This design is a mix of .063", .050" and .040" T6 6061 Aluminum. We use 5/32 Alum/Alum Rivets for most of the jointery any use 10-24 screws with Jnuts for the wheel enclosure in case we want to go in an service the chains. We plan to post the complete set of CAD models, shop drawings and Assembly instructions to our team website later this summer (www.killerbees33.com)

Wow, that is beautiful… It’s hard to imagine that whole frame weighs less than my 12lb battlebot.

Are the wheels sheet Al as well? How much do they weigh?

The entire frame was chromoly and consists of two main pieces - a main “tower” and a “trolley” that rolls up and down the tower. These two pieces together are about 25lbs with the rollers and bearings. The lifting platforms (aka - wings) used monocoque construction (mostly .040" 6061 T6). These were made just like aircraft wings - skin over two main spars made from sandwiched aluminum. The actual wing pivots are made out of 0.1" thick chromoly plate - these plates ARE heavy, but very strong. The first set of wings were 17lbs each, I believe. I think the lightened wings were about 15lbs each (you may have seen us frantically finishing these in our pit in Atlanta). Other than these main parts, of course we had our control panel (monocoque box), monocoque battery box and pnuematics panel, two outriggers also made of chromoly (2-3lb each, not included in the 25lb), transmissions, wheels, etc… I’ll have to let one of the other team members post some good pictures of the frame.

If we were doing a more standard base, we would have used monocoque. We used entirely monocoque the two previous years - frame and manipulator/shooter. It goes together much quicker and is plenty strong if you have a “normal” shape. To accomplish our design goals last year, we needed a large part of our robot to move 12" and it needed to support large internal forces (upwards of 1000lbs) from the wings and it needed to have a big hole through the middle of it. This could have been done with monocoque given enough analysis and design time, but it just wasn’t practical to do in 6 weeks. The tube structure was much easier to design and is more durable than monocoque would be in this application. The chromoly tubing is tougher than mild steel, so thinner wall tubing tubing can be used most places which keeps the weight down.

Monocoque is ideal for distributed loads - like the areodynamic loading of an aircraft wing. The rolling trolley in this robot concentrates very large forces on just a few points (the rollers). Using monocoque, these points would require some fairly thick material along the roller tracks to distribute the point loads. We may have saved a few pounds using monocoque for the lift bot, but it would have taken a lot more design effort and for the same weight, the tube structure is a lot stiffer.


Thank you all for sharing your design tips and techniques–there is a lot of great information here, showing off some quality work.

For the past 4 years we have used chromoly steel (.035 thick) tubing for our robot frame. It is slightly heavier than aluminum and is 3 times as strong.

04: http://www.cooneyrobotics.com/photos/2006/100_0277.JPG
05: http://www.cooneyrobotics.com/photos/2004/2004-7.jpg
06: http://www.chiefdelphi.com/media/photos/23825
07: http://www.chiefdelphi.com/media/photos/28616

Where we buy it:http://www.aircraftspruce.com/catalog/mepages/4130square.php
(Correct me if I’m wrong Ricky)

The frame you see for the 07 bot weighs around 12.5lbs