[Brandon Holley]

When you are reflecting on a failed design it is important to not get caught up in incorrect interpretations of that design.

For instance, the fact that one team was using 8" mechanums successfully vs. your 6" mechanums is really irrelevant. I think you understand this already but I just want to drive the point home. The size of your wheels is irrelevant to other teams designs. Your wheels must fit with your design like everything else does.

It appears you are looking to use off the shelf components for much of the geartrain of your drive system. That is a great start, many teams find success using tried and true off the shelf components. It does limit some of your choices however. Make sure you are analyzing what kind of speeds and torques your drivetrain will put out when you design your system. Instead of thinking in terms of "I want 6" wheels, directly driven off of toughboxes", think in terms of "I want a DT that can move at XX feet/second". Pick a starting point and start to design towards that.

For instance, assume your target is 10 feet/second. You can start your feasibility analysis at a very broad level and begin to focus it down to what fits for your team. I suggest doing this:

-The first would be to forget what exists already and determine what theoretical gear/sprocket/belt ratios can get you to your speed. A quick rule of thumb is not exceeding a 4:1 reduction in a gear set (this is due to the torque increase over that reduction and the yielding of gear teeth in that application). Don't worry about part numbers or specifics just yet, just determine what combinations would work to get you there.
-Next, start to look at if using an off the shelf gearbox can help save you design and manufacturing resources (if this is important to your team, which it should be.) See if with that rule of thumb an off the shelf gearbox can get you to where you need to be. It's important to note that you need to constantly be evaluating your design criteria. In your findings you may discover that using an off the shelf gearbox you can get to 11 ft/s very easily. You have to ask yourself in that situation is 10 ft/s an absolute must have? What if you purchase/make different wheels? What if you add an intermediate reduction? There are so many variables you can play with, you just need to keep working towards what works best for your team.
An example: Our team is fortunate enough to have a great deal of manufacturing resources at our disposal. We’ve made many custom gearboxes and fully custom drive trains so we are comfortable doing so. However, one Achilles heel for us is wheels. We’ve made our own wheels many times and they were successful but they ate up a great deal of our resources. A lesson we have learned is to design around off the shelf wheels. We can tailor the gearbox and other components to make certain wheels work, so we use that as a design constraint to start nailing down the overall drive design.

I think you are off to a great start. Just thinking about your future designs is more than most teams do so you are doing a good job! If you have any other issues or questions feel free to ask them here. Just about everyone provides some form of constructive criticism so keep firing away!


-Brando
(sorry for the long winded response, I guess I'm just passionate about the design process)