pic: Cobra box

Gearbox just for fun. Using a worm gear system. It is 200:1 but can be higher depending on the sprocket sizes. Dont know if it will ever be made but is great for high torque applications.

All of the gears, shafts, and bearings are all from Mcmastercarr. Trying to get off and away from AndyMark as much as possible and to stick with only Mcmaster.

You’re better off staying away from bent 1/4" Al. Might as well have two pieces bolted to form the L to mount. Just countersink the bottom screws. If you stick with the bent 1/4" just make sure you have the proper bend allowance in there. Tolerances can be thrown off a bit of bearing holes and such because of the tolerance of the bend, but I guess depending on your shop this could be negligible. Just a thought though.

… Why?

Yeah that was another option that I was thinking of, but we have two great companies that make our parts and should be able to handle those no problem. As far as getting away from AndyMark is concerned, Mcmaster has always given us great customer service and we have never had any delays from them. AndyMark has not, and Mcmaster has a much larger supply than AndyMark, this way we don’t have to order from multiple places. This is just the team’s experience with AndyMark and we decide to purchase from the place that is the best for the customer.

Fair enough. My experience with AM has been very different than yours it seems.

It might be cheaper and lighter to do the worm gear earlier in the reduction, so that it can be smaller. That will also reduce the thrust loads you’ll have to deal with in your gearbox. Do you have a thrust bearing on the shaft with the worm?

AndyMark doesn’t ship as fast as mcmaster for the same price, but who does? Mcmaster’s speciality is fast and good service for a price premium.

Andymark has been good to us, we’ve had a few defects and small issues over years, but they’ve always fixed any issues we’ve ever experienced. I love the variety of 20dp gears that are already good to go, saves a lot of time.

Excellent point Adam. I will look into it, the main reason I did it this way was for simplicity if the motor ever needs to be replaced it would be an easy swap out. I also had the idea of putting a system in place where one could simply disengage the worm gear and allow the arm to be moved manually. Really this was just for fun, and a possible solution to some problems from last season. The anti back driving effect of the worm gear is a huge advantage and allows a mechanical solution to a problem instead of adding in code to achieve the same effect. The bearings on the worm shaft will be thrust bearings however I did not have any thrust bearings in my workspace so I used just standard until I could CAD them sometime this week (only had larger ones from off of an old swerve module). I also did not place any shaft collars on the external parts of the box. Yeah I am a huge fan of Mcmaster, they are huge compared to AndyMark, and I feel like it allows a wider range of materials to do some crazy stuff with. We have had a lot of issues with AndyMark (probably because we are just unlucky). Last season we ordered some transmissions and all of the output shafts were way outside of the tolerances and none of the hubs fit well we had to grind down and press them on. When we bought another set for the real bot everything was fine. Thankfully we got the order in around week 2 and avoided the massive snow storm. Mcmaster is also a company that the students may use even when they are out in the industry so it is a good way to get them to understand how these sites work. This box is in no way attributed to the Bird Brains. We have always been a student designed team with mentors answering some questions. If they want to use this box or anything that I have designed I will usually do a lesson or talk on the design and encourage that they work on their own version, and then help answer their questions. I really hope though that we can get this fabricated to see how it works and functions (don’t want them using something that is useless).

Worm gears by themselves don’t necessarily have any anti back-driving properties, they are more efficient in one direction than the other depending on what gears you use and how well the tolerances come out, what grease you use ect. It may not stop back-driving as much as you might think, it would be good to prototype and collect some data. Although a 200/1 reduction with a CIM as the input wont be back-driven very easily anyway.

The often better way than using worm gears.

However, sometimes you really just need that worm gear or anti back-drive pins (like on a winch for hanging), but often you can get away with “passive assistance.”

Just wanted to make sure this option was considered. Best of luck, it looks pretty awesome.

I can attest that passive assistance works. My team’s 2011 arm had two extension springs attached to a “sissy bar” above the lower arm joint… two BB 550 attached to 1:123 planetaries were overkill, as one of the two motors was would move the arm… the two at full power would rais the arm in a second (0 to 10 ft), so in practice the the motors see at most (via PID) 70% power at any given moment.

on a side note, sissy bars also make good tilt camera mounts…

This is what we usually do with all of our arm systems. This is a great way to get a desired effect without having to change a gearbox. However designing an arm correctly with a transmission that can hadle the arm without assistance, is the best option. For logomotion having the ability to back up grab a tube and lift your arm all the way back over to the other side to score was a really big time advantage that I noticied 973 perform over and over again. They were able to hang a lot of tubes very quickly due to this advantage. Maybe Adam can say something about this?

It’s possible to counterforce even if the arm crosses vertical. It’s just slightly harder. But still possible.

Good design includes counterforce. Just saying.

I know many teams who could hang as fast as 973. Watch IRI.

I know that is is possible to add a counter weight system, however I just try to keep KISS in mind. The only point I was making was that it is better to design something right. By knowing that even if you have no counter weight your transmission should still be able to work your arm. I have no say in what the kids want to do, if they want to have a counter weight that is fine it is my job to make sure they know what to do, and to answer their questions. This was just a fun side project based off of criteria that I set.

Our robot did use the feature well, but teams like 1503, 233 and especially 987 used it better. We made our whole robot symmetrical at first (with rear floor pickup), we should have just allowed the arm to rotate over. It can save time if used right. We complicated the design of things a lot to allow that symmetry, and it wasn’t necessary.

As for scoring, we were good at champs, but just ever so slightly out of that top tier. If the best in our division were doing 8-9, we were probably doing 6-7. IRI we had a new driver and that average tanked unfortunately.

While taking another look at this, I thought of something else. You may want to consider putting the plates that support the worm on the “outside” and bolting into the side plates. This way the thrust loads from the gearbox (which will often be shock forces, not light constant ones) will pull in-line with the bolts used to bolt the plate on.


Wow a lot of great ideas I will add those in this week. Thanks everyone for the input.

This rendering was done in ProE 5/ Creo, but I am also working on a Solidworks duplicate at my college.

Which do you find easier to use? I was trained in Solidworks and find it very easy… but I am just curious.

Hmm good question. I was first trained in ProE and then self taught myself SolidWorks and Autodesk Inventor. I would say for making individual parts SolidWorks takes the cake, the ribbon structure and layout is very very simple to both understand and use. However when you get into more complicated assemblies, ProE is better. ProE does a better job at keeping individual parts organized in a complex assembly. SolidWorks puts mates under its own tab and you have to find which one you want to modify, ProE automatically keeps mates together by part. SolidWorks outlines most lines (you can turn this off) and makes the part look very nice and easy to see. ProE is a little hard to stare at sometimes, but you can get used to it. ProE creates far better renderings than SolidWorks in my opinion.