Quote:
Originally Posted by matthewdenny
Ok, got it. Yes there is noticeably more friction with the wheels out from the turning axis, but we calculate that we still can turn 90deg with a fully weighted robot in <.1s, which was acceptable for us. The 2 wheel arrangement allowed us to lower the cg, and keep everything COTS, which was a primary design concern.
As far as the bearing goes, we aren't sure where to find the spec on the bearing load limits or what kind of shock forces we can expect. We do anticipate that most thrust forces would be held by a needle roller bearing between the bevel box upper gear and vex gearbox though. The side and cantelever loads are a bit of a mystery, and any insight you could share on calculating that would be appreciated.
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I fear your steering motors will be very unhappy.
You have a 48:1 ratio from the BB to the swerve axle. Let's say that each stage is 90% efficient, two stages, that is an effective ratio of 39:1. The BB has a stall torque at 12V of 0.8N-m so you've got 31N-m at the swerve axles.
BUT, your wheels look to be perhaps 4-5" on center, let's call it 110mm and be done, that means you're 31N-m of max steering torque can put about 280N as its dragging those grippy wheels over the carpet. That's about 60lbs. No problem you think to yourself. Even if the robot full weight in on one swerve module and the CoF of the wheels is 1.0, your BB motors can do the job. And you'd be right, they can turn the wheels, bearly. But they are not going to be happy about doing it for long periods of time.
Let's do the less drastic case: 120lbs robot, 1/4 on each wheel. That is a normal force per wheel of 15 lbs (67N). With a CoF of 1.0, it takes over 7N-m just to drag the tires over the carpet (which you have to do if you want to turn the wheels) 7N-m is about 1/4 of the stall torque you have available and will take 20A for each swerve module. Yes, the BB can to do this but again, not for long periods of time.
I am happy that you are trying this, I just want to put some things out the for you to consider.
Dr. Joe J.