Team 1511 Prototype Drivebase CAD

[BrendanB]

Originally Posted by Vespasian

One thing I forgot to add is that the outer four plates (the big ones) are made of steel. This was a design decision that we wanted to try out. Though I think you are correct in saying that we will want gusseting.

We are also planning on having bumpers that are entirely connected (one piece), and that are reinforced with aluminum. I don't know if anyone else has had good results with bumpers that also serve to reinforce their structure, or if that is just not a good idea.

Could you explain more about why you are trying out steel? I am all for trying new stuff in the offseason that you wouldn't normally in the build season but this is one of those items I think most of us would say if you are having to try steel you are doing something wrong. Steel isn't the easiest material to work with compared to the ease of aluminum.

I would highly recommend you give this chain spacing calculator a try to properly space your wheels. We used it this past year on 35 chain and after a full season the chains have stretched slightly but they are still in great shape to run. Its been really nice not having to worry about chain stretching out over a season and eliminate some components from the robot. http://www.chiefdelphi.com/forums/sh...8&postcount=10

Also am I looking at the model right that you are using two shaft collars to keep your axles in place? I would recommend using slip rings or tap each end of the axle with a 1/4-20 thread. Shaft collars will come loose very easily and if you don't check them after every match it will come off eventually. Your drivebase is the most important part of the robot that will see the most run time. If you make it as reliable as possible and eliminate as many failure points as you can your pit crew will spend little to no time touching it in a season.

If you keep the gearboxes mounted as they are now you might want to consider running dead axles instead of live axles as you are now. Round bearings are cheaper to buy in comparison to 1/2in hex. Since you aren't directly driving your center wheel there isn't much of a need to go with live axles.

Another thought to consider is how to make each wheel/axle assembly as similar as possible to reduce the number of unique parts and make repairs a lot easier. This past year we made every wheel/axle assembly the same on both of or robots including color coded spacers so all parts were interchangeable between wheels and robots. Of the few times we did have to adjust our drivebase it was an pretty pain free. You can see a picture of one of our drive modules assembled for reference.

[KrazyCarl92]

Originally Posted by Vespasian

These are a lot of very good points, and much of it comes down to my inexperience. We may switch the live axle in the center to dead, since there is really no logical reason for it to be live (other than it is in CAD). I'm not sure it will hurt anything, other than adding a bit of complexity...?

About the bearings on the dead axles, those were originally live as well, and I didn't think to take the bearings out. The shafts are constrained using collars, is there a problem with doing this? It would add some weight, but for the purposes of this project, is this particularly detrimental?

The motors being used (other than two CIMs on each side) are RS-775's, not 550's. They are another sort of experimental idea, in case we don't have 6 CIM's to allocate to the drivebase next year. Larry (our team leader) brought this idea up, and we are at least going to try out the CIM-ile gearbox from VexPro with a 775 in it. If we don't like the results, then we will switch it to a CIM or Mini-CIM.

The third stage reduction was originally put in to achieve the proper ratio without needing a sprocket reduction. We have generally had a 1:1 ratio between our output shaft and wheels, and tried to stick with that. But it turns out the specific reduction we needed from VexPro was out of stock until at least September, so we bought a transmission with a larger reduction and compensated with sprockets. If that makes any sense.

I wish I could find the JVN gearing spreadsheet we used, but it is only on our school network at the moment. When I get in there on Monday I will upload it to Drive to show everyone. Our high gear theoretical speed came out to be around 17 ft/sec, which is a lot faster than our normal drivebase design (12 theoretical, 13.5 actual).

A dead axle in this application itself to the use of round bearings, which are more reliable than hex bearings. The dead axle can also help provide strengthening support between the rails of your chassis.

Echoing Brendan's comment, shaft collars are not ideal for this application. The amount of vibration experienced in the drive train will make the collars loosen even more quickly, and you don't want to lose your shaft retention in your drive train part way through a match. I'm no expert in dead axle shaft retention in sheet metal drives, but to my knowledge your options include retaining rings/snap rings (I believe this is what Brendan called slip rings, this is a misnomer as slip rings are an electrical component), drill and tapping each end of the shaft, or simply making your dead axles bolts like in the Kit Bot.

My personal preferences for shaft retention in FRC are to in general use snap rings on the outside of shafts, spacers in the middle, and selective use of shaft collars and shoulders where reliability and maintenance are not negatively impacted but manufacturing and assembly can be made simpler.

[SoftwareBug2.0]

Originally Posted by Vespasian

We are also planning on having bumpers that are entirely connected (one piece), and that are reinforced with aluminum. I don't know if anyone else has had good results with bumpers that also serve to reinforce their structure, or if that is just not a good idea.

I think you would be in good company with using bumpers for structural purposes. I can't find a picture of it, but 1114's 2013 bumpers had some interesting use of aluminum angle which I beleive was for neither weght distribution or aesthetic purposes.