Designing FIRST robots is a very unique challenge. In some aspects, the robots are exposed to large, unexpected, and often damaging forces and loads. On the other hand, these machines need only to run for 100 hours, so teams have found all sorts of tricks that may seem very unconventional to typical design engineers that they can get away with and succeed in FRC. There are also many “normal” designs that are common among successful teams. This thread is a place to share any mechanical design tips, tricks, or methods that apply to FRC robot design.
Introducing my friend, a design engineer, to FIRST showed how unique some elements of robot design are. To this day, he cannot get over using an aluminum sprocket with steel chain.
Some examples that I’ve learned from CD are -aluminum gears, aluminum sprockets, 118 style chain in tube, 254’s zip tie encoder mount, roller bearing and teflon pad elevators, Aren Hill’s choo choo linkage, 67/254’s smooth bumper material, somebody’s method of using a hex sleeve as a removable coupling between gearbox and transmission, 254’s (I think?) method of freezing the main breaker with upside down cans of canned air so it doesn’t trip.
We were lucky enough to have Aren mentor our team when he was at Iowa State, and he had really effective design techniques that helped us build one heck of a bot. Some of the ideas I encourage my FTC team that I mentor to use include building with subsystems to allow subsystems to be built independent of each other and to be implemented into the robot as a whole (kinda like cooking where you make a sauce and a chicken, then combine them into one). The other idea was to use a standard building system (like what the Vex Versaframe or FTC Tetrix parts) to allow for building/attachments with minimal tools/shop.
Hex shaft is one thing that is impractical for normal engineering designs. It creates stress concentrations within the inner bearing race with 6 sharp corners, and it must be designed with a slip fit. That slip fit is slowing wearing over time, but it does fine for a few hundred hours of FRC time. It is a slick way to not deal with key locks.
Along those same lines, you can get away with slip fits and some loc tite in axles/bearings, where you would normally press fit details. That is what took me a while to understand, because you don’t need the thousands of hours of reliability.
This year was my first involved with any type of design, so I learned a bunch.
-When putting snap rings on 1/2" hex shaft, use a metric size. It’s easier to get the snap ring on.
-It’s really helpful to drill and tap a hole in a shaft you may want to remove in the future so you can put a bolt in it and pull it out
-Over center linkages are really cool too. We used one in our pneumatically loaded spring shooter. It let the pneumatic cylinder extend to load the mechanism, then retract without it firing. We needed this because the shooter fired faster than the cylinder got out of the way.
-Use surgical tubing as a shaft coupler for encoder mounts. This is an awesome way to deal with misaligned and weird sized shafts. You just slide it over the shaft and add a bunch of zipties.
-Don’t use non sealed bearings, especially in places where junk is likely to get into.
-Vex Pro’s chain and previous chain bought from McMaster have different master links. Vex Pro needs the “Heavy Duty” one.
As for the breaker freezing, how much do you really think it does? Is it considered unsafe and against the rules? I was hesitant to try it after I got in trouble for shorting the pressure switch to reload air tanks after a stopped match, but after a practice match with a cooled breaker, it reached normal temperature by the end. I don’t think the breaker has enough heat capacity to stay cold for that long.
It makes a huge difference. It’s not about staying cold as much as it is about not going into a match with it at an elevated temperature.
We’ve had far more issues with main breakers prior to cooling them religiously between elims matches. There are other things you can do too, like cooling down the piston area of your compressor post match and after you’ve charged your tanks before the next match and cooling your motors. Everything will operate more efficiently when it is at a lower temperature.
I don’t have the data in front of me right now, but when I ran some tests it was obvious how much performance gain there was in cooling the breaker. While cooling is significant by itself, cooling also has a much greater impact if you have back to back matches (ie Qualification/Finals) where the breaker doesn’t have time to return to room temperature. When running back to back matches with the breaker chilled between matches, I think we saw a 3x increase in trip time, although I’m not 100% on the actual number. Regardless, it gives a definite performance gain.
As far as legality, it resides in a gray area and until the rules/GDC/Q&A explicitly state otherwise, teams will continue to use this for it’s performance enhancing aspect
Please clarify your definition of “trip time.” I don’t think you’re intending to say that command packet round trip times issued from the driver station took 3 times longer than normal to (a performance loss), but I’m having trouble finding another interpretation of your response.
I sure he was talking about circuit breaker tripping… the main circuit breaker is delayed action (lost the correct phrase), which means it takes few seconds after the current exceeds the limit (in this case 120 Amps). As I understand the main breaker responds to heat generated in due to high current. So it would make sense to cool the breaker before the next match. We have not done it.
I wasn’t sure where to post this, but thought maybe a few teams would use it.
We are building an off-season drive train and were trying to find a way to drill aligned bearing holes through box extrusion easily. After trying a few standard hole saw sizes (1-1/8" & 1-1/16") that were either too large or not large enough, I found this item on Amazon: Link
After drilling a few holes tonight, we hit very close to 1.125" for our hole size. For 7 bucks I think this could be an inexpensive solution for a lot of teams out there that don’t have the money or resources to invest in expensive hole drilling options. Plus they ship via Prime!