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Unread 03-08-2014, 22:26
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Re: Team 1511 Prototype Drivebase CAD

Quote:
Originally Posted by Vespasian View Post
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.
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