Quote:
Originally Posted by KrazyCarl92
Is there a compelling reason to use a live axle on that center wheel? Dead axles can better help strengthen the sheet metal frame if used properly. Common reasons to use live axles on a center wheel in an application similar to this is to direct drive the center wheel from the gearbox or to run chain for a wheel on the opposite side of a frame member from the wheel. Neither of these criteria are met, so using a live axle on the center axle in this particular design is not taking advantage of the benefits of using live axles.
If the corner wheels are on dead axles, why are there bearings in the model? The axles do not rotate in a dead axle setup, so the axles should be in some way constrained to the frame. Most commonly bolts are used as dead axles and tightened into the frame.
With the available motors from the past 2 seasons, it's difficult to justify an RS550 in a drive train application. CIMs and MiniCIMs have much more thermal mass and do not rely on the high speed of the motor to cool with an internal fan. This means that they are better suited for the constant use and sporadically near stall conditions that are seen in the drive train. Although RS550s have similar power to these other motors, they are better suited to most manipulator conditions where they are not subject to constant and heavy loads. With 6 CIMs and 4 MiniCIMs at your disposal, there are still many powerful motors left over even with 6 CIMs in your drive train. I'd expect at least the very least 4 CIMs and 2 MiniCIMs to be allowed by the rules, so even with all of those in the drive train you would still have a sizable selection of motors to use on manipulators.
EDIT: Why use the optional third stage of gearing on the ball shifter gearbox if the reduction could be achieved by sprocket and chain from your gearbox to your wheels? This could save weight with 1 less stage of gearing, less chain, and smaller sprockets with the same force on the chain.
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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).