We have never used a BAG motor or a VersaPlanetary before, and therefore seeking guidance. We are considering using a BAG geared at 147.00:1. The JVN-Design Calc estimates that an adjusted Drivetrain of 1.27 ft/sec. We are estimating a 150 pound bot (hopefully we will be lighter), using singe 4 inch driven wheel to drive on the Generator Switch. The “Pushing” Current Draw is 36.25 Amps.
Under the rules this year it appears that we can run 2 BAG’s on one controller, thereby saving one PDP slot. The other Bag would be used on our ball intake. Our idea is that even though both motors would be spinning at the same time, we would not be running the intake and driving on the switch at the same time.
What potential problems would exist with this arrangement? We are trying to weigh the pro/cons completely before we spending the money.
Uhhhhh, are you thinking about driving your robot with a BAG motor. That is a horrible idea. You’re barely going to move and when you do it’s going to be really slow.
Looking at your robot reveal, it seems as though you guys are using a WCD and if you weren’t using versaplanterys, then you likely had custom gearboxes for your mechanisms. If you can continue to do custom gearboxes, go ahead. If you can’t, then use a versaplanetary.
I really hope using a BAG as a drivetrain motor is a joke.
We are talking about using it to help balance the “Coat hanger” We are wanting to go slow. We are trying to save PDP slots as we are running swerve this year.
Well you can use a versaplanetary but I’ve heard on here throughout that a versaplanetary geared more than 100:1 (especially with a 775pro) starts to break. We personally have never broken a versaplanetary with a 100:1 but YMMV. Also, where does it say you can run 2 BAGs off a single PDP slot?
while im not sure about the rules on PDP slots, I would warn against putting 120lbs on the outputshaft of a VP, they arent the greatest at taking loads like that. try to at least support the other end of the shaft or use a gear to drive the wheel.
Check out the load ratings guide. Vex says a BAG motor is good up to 280:1 reduction. A 775Pro can do 180:1.
The issue is with the torque in the gearbox. Since each motor can output a different amount of torque, you get different ratings for each motor. Personally, I wouldn’t want to push the limit for any of the ratings, as it’s not worth even a small risk of failure.
Many failures teams see are due to shock loads when direct driving from a VersaPlanetary. Those shocks (like running into a wall with an arm) can exceed the load even if you’re in the green on the guide. In many situations, it’s worth while to add an additional chain reduction to help absorb the shock load and reduce the reduction within the gearbox.
Yes, definitely don’t cantilever the output shaft–planetary gearboxes in general aren’t very fond of that kind of abuse, so make sure that you’re supporting both ends of the output shaft. With a whole robot on it, I would put a bearing on the output shaft flush up against the VP front surface and support it with a bracket on that side, too.
I think you can absolutely accomplish the task at hand with a BAG motor. However I recommend having your last stage of reduction external to the gearbox using chain or something. Take a look at how we did this during FIRST Capital Ri3D. The mechanism didn’t make it onto the robot but it worked really well for a prototype. We used a 775 but with the correct gearing a BAG motor would certainly work too. It should be in this video towards the end.
We appreciate the input. We were definitely planning on having the shaft support on both sides, so no cantilever. We will probably look at doing a final reduction via chain as suggested as well.
My experience. In 2017 we used a cim with 100:1 VP to lift our 100lb robot up the rope. In 2018 we used that same gearbox to drive our lift and pulled our 130lb robot (we did strip a ring gear during prototype but that was because of user error). In both cases they were cantilevered. By the end of 2018 the shaft was nearly round.
But that was 2 years ago and we didn’t know any better than to abuse a VP like that.
If you do this all in VP stages, put the 7:1’s nearer the motor and the 3:1 as the output stage. The smaller gear ratios are more robust for torque and shock loads than the larger ones. Also check the load capability of the shaft you’re using; the CIM and 3/8" output shafts aren’t as robust as the 1/2" ones.
In 2017, I assume you had a drum to wrap the rope around, and probably something similar in 2018… what size? That plays a big role in how much torque is transferred through the gearbox. If I recall correctly, we’ve successfully climbed in multiple years with a ~15:1 reduction off a CIM and a 1" drum to wrap the cord around, without any issues at all.