pic: Artemis 3083 West Coast

Hello everyone,
This is a part of team’s 3083 pre-season mechanics, design and CAD project.
We designed two drive bases for possible use in the upcoming season. This is the first of the two and it is a “standard” WCD. We are still working on the second design, but we expect to publish it soon.
I want to emphasize that this was designed and modeled by our student team members, with only little help by our mentors.
We are eager to receive your feedback and improve our design.

More photos: https://picasaweb.google.com/artemis3083.hofcarmel/Artemis3083WestCoast


Modeled using Solidworks

Drive train:
This is a “West Coast Drive”, using 6 cantilevered wheels and directly driving the middle one.
Wheels: 4" AM Performance Wheels, Roughtop tread (1" thick)
Center Drop: 4mm (0.16")
Wheel to wheel distance – 400mm (15 3/4"), 7mm chain tensioning distance
Transmission: ASA-25 size chain, 22T sprockets
Tensioning is done by moving the wheel and bearing block sideways, up to 7mm.
A screw on the side of the frame is used to assist in applying the force and controlling the amount of tension. Then the bearing block is locked in place by the tightening the plates attached to it.

11.9 kg (26.2 lbs.) - Total weight
6.8 kg (15.0 lbs.) - Without motors
4.9 kg (10.8 lbs.) - Without gearboxes
2.0 kg (4.4 lbs.) - Weight of frame

Custom gearbox based over the Toughbox
Gear ratio: 5.4 : 1
Two transmission stages: 14:45, 24:40
2 CIMs per gearbox
Calculated maximum speed: 4.3 m/s (14.0 ft/s) (15 km/h)

Weight: 2kg
Built from Magnesium alloy profiles, donated to us by our sponsors, Alubin.
The frame is all welded together.
Bond profiles: 20mm x 50mm, 2.5mm thick
Support profiles: 20mm x 20mm, 2.5mm thick

All I can say that someone is on the right track!

It looks like the chain would interfere on the cross beans in your design?
Also for the bumper mounts I would look at 973’s design, almost the same thing but a bit simpler.
And for mounting the gearbox, I would recommend using the KOP hex beams to brace them, or put in another set of bolts to hold it to the chassis.

But overall a great start, can’t wait to see more.

What made you choose magnesium?

It’s about 40% less dense than aluminum, but it’s yield strength is nearly 4 times lower than 6061 and it’s ultimate strength is 60% lower.

Magnesium? Which alloy?

Also, do you take precautions against fire when machining? Magnesium shavings—at least in pure form (I’m not sure about wrought alloys)—are flammable. Most shops wouldn’t have a class D fire extinguisher lying around.

Thank you all for your replies!

Thank you very much, those are some important tips. Here is a quick update:

I looked at team 973’s bumper mounts but couldn’t find major differences. How would you improve the design?

We are using magnesium profiles as the main frame component for most of our robot for three years and it does a great work every time. When comparing it to our practice robots, that are usually made from aluminum, the magnesium parts seem to stand in impacts (that you can except for an FRC robot) just as well.

I am not sure which of the alloys we use, but I’ll make sure to find out. I do know, however, that this is the same alloy that is used to make the frame for Alubin’s bicycles, in case that this gives you some useful information about it.
Magnesium alloy is much more stable than pure magnesium, and is similar to aluminum when machining. However it does require different treatment in some cases.
Thank you for your concern ::safety::

It seems your design means that to switch bumpers one would have to undo 8 or more bolts. Having had a similar system our first robot, I can say it wasn’t fun and we promptly changed it to something much faster.

Also, just wondering, how does one go about welding magnesium alloy? Same process as alum or steel?

With four individual bumpers (not a one-piece bumper), you need at least 8 fasteners for a solid connection. We had 10 this year, plus flanges that hooked into slots in the bumper supports.

The better solution is to either not have four individual bumpers, or use fitted bumper covers. We did the second this year, and it worked very well.

Consider moving your corner brackets on top of the frame, and pop-riveting them in. This will make sure your frame is all aligned before it is sent to your welder, and make it easier to weld.

Actually, our bumpers mounts are designed so it can be used with no screws at all, or with 8 M4 screws. In order to lock the bumpers in place we insert a screw or a pin through the bumper mount and the frame, so it is fast to insert and prevent the bumper from moving.
We also use a bumper with changeable color as seen in this video by team 1937: http://www.youtube.com/watch?v=7PVkTPe7XIc , so we don’t have to disassemble the bumper in order to get ready for a match.

We use gas tungsten arc welding for the magnesium profiles, so it is similar to aluminum welding, although it does require some special equipment and experience.

This is a good idea, I think we will do that.

Thanks you all

Just a quick follow-up on the suggestion about the corner braces assisting with the welding…

As the mentor who handles most of the welding for team 1730, I would NOT recommend riveting the corner brackets into place before welding the frame. Doing so would create 2 “three-wall” pockets at each corner of the frame that are very difficult to weld (especially without actually welding the corner gussets in with the 2 frame members)!

I’m sure every welder out there has their own preferences but my suggestion on “helping out” the welder on a frame like this would be to make 2 “jig bar spacers” that are the exact length as the space you want between the left and right frame rails. Your 4 perimeter frame members can then be clamped together with 2 bar clamps in each direction (using your jig bars near each end of the side rails) to ensure you are parallel left to right. Then, with everything clamped together, you can verify you’re square and weld.

Other Hints:

If possible, keep all your clamps on the same side of the frame, this allows getting a big framing square on the other side without obstruction (and allows easier corner-to-corner measurements as well.

Depending on your eventual frame design, these “jig bar spacers” can eventually become inner cross-members of the frame itself. Once your perimeter frame rails are welded, reposition the jig bars to where you’d like the cross-members and weld them into place as well.

If you want to use corner braces in addition to the welding (we actually haven’t on Team Driven’s frames) you should consider NOT making them triangles as you’re probably better off without the 90 degree corner on the gusset where it would interfere with the face-welded joint of the 2 frame members.

Shay did you considered using 8 wheels? we did last year with the same 4" wheels and it worked really good. you can also look at 2010 robot and see the same idea of 8 wheels the different is the size.
the main though was to do the 2 middle wheels in each side a little bit lower. this gives you easier turns & in case of big bumpers (2010) with the right size of wheels its will be helpful. the problem of making this idea with 3 wheels is the robot is unstable… can I ask why did you go with 6 wheels & 2 gearbox?
and I guess that the 4" comes to give a lower gravity center?
to see the robots I talked about you can go on our site under History and choose the year :slight_smile:

Mott, thank you very much for the great tips. I will make sure to work together with our welders when finalizing the design.

Shir, I remember your drive train and it did worked very well.
Our center wheels are indeed lowered.
A 6WD is not necessarily less stable than an 8WD. This is because a design that places the robot’s center of gravity away from the middle will result in much less swinging over the center wheel. A robot with eight wheels might do better in those terms, but we find the loss of weight and improved simplicity of a 6WD more important to us.
Our decision to use 4" wheels is based on a few reasons; one of them is to allow us to have a lower CoG.

2011 was good but our telescope and gripper was way heavier then planed and all though we used 4" wheels the CoG was too high. this is why it was swinging over… but also from my team’s experience 6WD doesn’t do the work as well as 8WD and its working differently - the 6WD is all the time on the middle wheels & either of the front or back wheels, the 8WD is all the time on the 2 middle wheels just while needed the other 4 wheels come to help.
anyhow see you at the Kick-off in a week!! :smiley:
and here starts the end of our life :wink: