Gear ratios for launcher

[CalTran]

We've been running 2 BaneBot 550s with 20:1's and they work beautifully. We may just be able to pull off our master strategy after all

[JamesCH95]

Originally Posted by swwrobotics

Has anyone found that magic gear ratio that works best for the launcher?

Our team tried doing a direct drive with the CIM motor but the ball did not go very far. I remember reading that a 2.7 to 1 gear ratio works best but I can't be sure. Any ideas?

Originally Posted by CalTran

We've been running 2 BaneBot 550s with 20:1's and they work beautifully. We may just be able to pull off our master strategy after all

The gear ratio depends heavily on the diameter of your end effector, assuming you're doing a roller/wheel shooter. Direct-driving with a CIM to a 1" roller is very different from an 8" wheel.

OP, care to provide any details? My team's experiments show that you should find the exit speed you need for your range, assume the ball will go 60%-70% of your theoretical tangential free speed, and gear accordingly.

[CalTran]

Originally Posted by JamesCH95

The gear ratio depends heavily on the diameter of your end effector, assuming you're doing a roller/wheel shooter.

Our team is using 6in or 8in wheels for our shooter, I can't remember which one. (I'm not actually on the shooting team)

[swwrobotics]

Originally Posted by JamesCH95

The gear ratio depends heavily on the diameter of your end effector, assuming you're doing a roller/wheel shooter. Direct-driving with a CIM to a 1" roller is very different from an 8" wheel.

OP, care to provide any details? My team's experiments show that you should find the exit speed you need for your range, assume the ball will go 60%-70% of your theoretical tangential free speed, and gear accordingly.

We are doing a one-wheel design that uses the new six inch wheels that came in the kit of parts and a flap on the top to adjust the angle the ball comes out of.

We were just worried that a high gear ratio wouldn't have enough force to launch several balls and it would be hard to get back up to speed.

[MrForbes]

The new kit wheels tend to tear up the balls, according to a couple of reports from teams who tried them. We're using a single 8" kit wheel from a year or two ago, driven by last year's FP motor with it's 19 tooth pinion, which drives an older FP plastic first stage gear from the FP gearbox. The ratio is 19:72, I believe. This gives us a pretty good range....

http://www.youtube.com/watch?v=81vOT2KY2ig

But the gear ratio you want will depend on a lot of factors, as others have mentioned.

[artdutra04]

Originally Posted by swwrobotics

We are doing a one-wheel design that uses the new six inch wheels that came in the kit of parts and a flap on the top to adjust the angle the ball comes out of.

We were just worried that a high gear ratio wouldn't have enough force to launch several balls and it would be hard to get back up to speed.

How much compression were you putting on the ball?

Other than the fact that the tread pattern on the 2012 KOP wheels chews up the foam basketballs, any single-wheeled shooter with six inch wheels (when directly driven from a CIM) should easily be able to shoot the balls a more than adequate distance.

Simply changing the gear ratio won't solve this problem; the real problem is probably related to ball compression, shooter hood design, and/or misalignment/binding/friction in the moving parts of the shooter.


FYI: The 2.7:1 gear ratio is what is inside the CIMulator from Banebots. There will likely be a fair amount of teams using this gearbox this year, as this gearbox makes a 775 or 500-series motor "emulate" the output characteristics of a CIM motor. This allows teams without a lot of machining resources to keep their CIM motors in their drive train and use "emulated" CIM motors elsewhere on their robot.

[swwrobotics]

Originally Posted by artdutra04

How much compression were you putting on the ball?

Other than the fact that the tread pattern on the 2012 KOP wheels chews up the foam basketballs, any single-wheeled shooter with six inch wheels (when directly driven from a CIM) should easily be able to shoot the balls a more than adequate distance.

Simply changing the gear ratio won't solve this problem; the real problem is probably related to ball compression, shooter hood design, and/or misalignment/binding/friction in the moving parts of the shooter.


FYI: The 2.7:1 gear ratio is what is inside the CIMulator from Banebots. There will likely be a fair amount of teams using this gearbox this year, as this gearbox makes a 775 or 500-series motor "emulate" the output characteristics of a CIM motor. This allows teams without a lot of machining resources to keep their CIM motors in their drive train and use "emulated" CIM motors elsewhere on their robot.

Our mentor said the we should have as little amount of contact with the ball as possible to pervent too much deformation and loss of accuracy. So you're right, that definitely could be the problem. But just out of curiosity, would it be worth more to have a high gear ratio or a larger wheel diameter?

[JamesCH95]

Originally Posted by swwrobotics

Our mentor said the we should have as little amount of contact with the ball as possible to pervent too much deformation and loss of accuracy. So you're right, that definitely could be the problem. But just out of curiosity, would it be worth more to have a high gear ratio or a larger wheel diameter?

It's true that you should have as little contact with the ball as possible, but you still need some minimum amount of contact, otherwise you won't impart much energy to it. Many teams seem to have found that 1-2" of pinch on the ball will work well.

With a 6" wheel you could be looking for 2,000-5,000rpm axle speed, depending on what range you want. the actual ratio will depend on what motor(s) you're using.

[Ether]

Has anybody posted test data anywhere that includes wheel rpm, wheel diameter, squeeze, launch angle, and distance?

[edit] and wheel moment of inertia, if available? (yeah, I know: dream on) [/edit]

I've been keeping an eye out, but haven't seen anything yet. It would be most useful as a sanity check for some of the calculations.

[Aren Siekmeier]

Originally Posted by Ether

Has anybody posted test data anywhere that includes wheel rpm, wheel diameter, squeeze, launch angle, and distance?

I've been keeping an eye out, but haven't seen anything yet. It would be most useful as a sanity check for some of the calculations.

We took a decent amount of data with ball compression, I'll see if I can't dig that up and compile for public posting.

We're gearing for a top speed at the wheel edges of about 80 ft/s (54 mph!), since this is about double the speed of 40 ft/s that worked well for basically any shot in the near half of the field in our testing. So for a 4" wheel that's a 5:1 off of FPs (0673) and we have nearly the equivelant of 4 of them (2 FPs, 2 550s), so we're making up for lost torque (see here for more of the reasoning behind that). We're using the same 8" plaction wheels we tested with because that tread seemed to work better than others we tried briefly. I pushed for wheels smaller than the ridiculous 8 inches, but nobody ever tested anything smaller (we have screwed up ways of setting priorities, working on fixing that...) and people were worried about differences in traction with smaller wheels, we stuck with 8". The reasoning for smaller wheels was mostly weight, with a marginal benefit in how the frame could be layed out (more freedom), as well as just not having ridiculously huge wheels. But no one listens to me anyway... In any case, with our 8" wheels we have 10:1 total reduction off of 2 FPs and 2 550s in the works.

[Garrett.d.w]

Two weeks ago I made a pitcher using two rollers made out of 2.5" PVC covered in wedge top tread. This was powered by two 775s on banebot 46-1 gearboxes with no chain reduction. The compression of the balls was about 2 inches.

I would be comfortable shooting from half court with this thing. It is also good for rapid fire, the 775s have so much torque that the rollers will spin at a steady speed no matter what.

[wireties]

FIRST Team 1296 is trying 2 FP 801 motors at 6:1 and 2 AM motors at 4.64:1 driving 2 8" wheels (both under the ball) - have not got it running yet, a proof of concept with a CIM at 3:2 worked nicely compressing the ball 1-1/2"

HTH

[klmx30302]

Team 166 is direct driving 2 axels with 2 8" wheels each. Each axel has a cimulator with 2 rs-550 motors each. We have not clocked its top rpm yet but the wheels spin quite fast as you can see in this video: http://www.youtube.com/watch?v=YtiK87pE_9U
We don't know the range of the final shooter yet, but the prototype could shoot 46-50 feet, the prototype was direct drive cims instead of cimulators.

[pfreivald]

We're making very consistent shots from key range with AM motors and planetary gearboxes, though the spin-up time between shots is too long. We're working on that problem now!

[DampRobot]

Originally Posted by pfreivald

We're making very consistent shots from key range with AM motors and planetary gearboxes, though the spin-up time between shots is too long. We're working on that problem now!

My team is using a similar set up. Were getting about 16ft using belts driven by 1:1 sprockets off a AndyMark gearbox (the one for the Fisher Price). Accuracy is OK, but we've really been underwhelmed by its performance.