Need Advice! - Shooter is not performing as expected

[AndrewSears]

We started with a direct drive from a single 2.5 CIM to a 8" Plaction wheel. We did not get the distance we wanted so we made a gear box to increase the RPM of the shooting wheel since torque is not needed. Right now our gear ratio is a 1:6 increase in RPM using two 2.5" CIM motors. With the CIM motors putting out 4000 RPM we expected 24,000 RPM at the shooting wheel. This does take about 2-3 seconds to reach max RPM but it does not appear that we are getting 24,000 RPM out of the wheel. Today we are connecting the encoder to measure our wheel RPM to verify. We seem to have good contact between the shooter wheel and back wall. The Frisbee is sandwiched between 2 UHMW sheets with about 3/16" clearance.

We have a slight flex (about 3/16") causing some vibration but have a solution to stiffen that.

Everything is held in with flanged bearings and we have a mechanical take-up. When we roll the shooter wheel with the power off the wheel rolls very smooth. We do not notice that the belts and chain are slipping.

We tested different settings to the motor controls and found that from the 0-1 scale, 60% power to the motor control visibly gives us the highest output RPM in the shooter wheel.

Our thoughts are that one or all of these are the problem.
- The CIM's are bad.
- The CIM's are not able to handle the load.
- We are loosing too much energy with the slight flexing.
- We have something wrong with programming and not getting the power needed for MAX RPM.

Any thoughts are welcomed and I will share the CAD model with you if this will help you analyze our problem.

Thanks,
Andy

andrew.sears@autodesk.com

[MrForbes]

We have chain drive, and have been seeing the same phenomenon. Loosening the nut on the axle that holds the wheel gave us a lot more RPM.....as did loosening the chain tension when it was too tight.

It boggles the mind.

[AndrewSears]

We were thinking the same thing. We found a spring chain tensioner last night and are going to try that after adding 2 links to the chain. We have a floating pin (1/2" bolt) in the take-up that allows slight motion and everything is captured with flanged bearings. We have double nut on the end of the bolt so we would not put a thrust load on the bearings. This sprocket is running entirely on the bearing inner race....

Now that I think of it, I wonder if this being allowed to float slightly in the bearings could be the problem.. ?? Since everything is sandwiched between flanged bearings, maybe we need to tighten this up a little so we lock everything to the inner race of the bearing.

[Dad1279]

I'm hope someone will check my math, but if you were to achieve 24,000 rpm on an 8" wheel, you would have a surface speed of 545 miles/hour......

Perhaps we are not going for the same distances, but we have found a direct drive on a 5" wheel satisfactory.

[Kevin Sevcik]

I think you're probably spinning your wheel too fast, at the least. If you ever managed to hit your 24000 RPM, the tangential velocity on your wheel would be 571 mph. With an expected maximum frisbee velocity of 285 mph.

You're never going to get a frisbee going that fast. So your wheel will always be slipping and you'll always be applying sliding friction to the frisbee. Sliding friction is only dependent on the compression force, not the speed of the wheel, so spinning your wheel faster isn't gaining you anything.

So back to the actual problem. You haven't mentioned whether this is a linear shooter or a 90 degree wrap shooter. Also, what's the distance between your wall and the rim of your wheel? I think most people are finding something around 10 1/2" to be a good distance there.

If you're running a 10 1/2" compression, then your frisbee is going to bow up a good bit in the middle. I think it'd end up squashed between your two sheets of UHMW if you only have a 3/16" gap. That'd add drag and lower your shooting distances as well.

[Justin Shelley]

Use a pneumatic tire

[Willyspu]

Make sure there is sufficient friction between your wheels, the surface the Frisbee rotates against and the Frisbee.

Hard wheels may not produce the friction needed to spin the Frisbee, we riveted our hard wheels with a traction material and that helped a lot (that's why the pneumatic wheels work well).

The Frisbee needs both force and spin to fly properly.

Jim Russell
Mentor 2144

[budzilla]

There may be other factors at work with your setup such as frictional losses (we had those as well), but we have experienced less distance with more speed based on a none contact tachometer. I think that it is just like a funny car breaking the tires loose on launch. There's a point where adding more power is just making smoke and slowing the launch, be it car or Frisbee. We were peeling rubber off the tire. The pneumatic tire offers a MUCH larger contact patch on the Frisbee because of the compliance and therefore helped us a lot. BTW, 6440 rpm on the pneumatic was too much, but now we can PID the shooter, YAY!

Best of luck to you!

[Ether]

Originally Posted by AndrewSears

8" Plaction wheel ... 24,000 RPM at the shooting wheel.

Please seek out an adult mechanical mentor to review this. It is unsafe, and unnecessary.

[smclean1969]

Any time there is a transfer of energy (e.g., from CIM to gear; gear to chain) there's going to be some energy lost, so this is going to be part of the issue. There's always the theoretical speeds or power levels that motors should be able to produce but as soon as they're put under load you'll be at some lower level. Going with a 6:1 acceleration is frankly unnecessary and very dangerous. We've been able to successfully shoot the full length of the field with a 2:1 acceleration.

I strongly recommend relooking at the approach to make it safer for the machine and the people around it.

[fsracer]

Watch that rpm. Most wheels are not rated to spin that fast and a catastrophic failure may result. (and serious injury if someone is near it.)
We enclosed the wheel so if it blows, it is contained.

We did our shooter with the short sim direct drive, 90 degree, with a 6 in wheel with traction tread riveted on, A bearing support for the wheel weight and the opposite side, also traction tread on the reaction surface. We set it up for 1/4" compression of the disc.

Our auton position at the back of the tower requires 11 volts to hit the 3 point target.(24 degree angle) If we crank it up to full, we get a 48 foot shot from the floor (20 degree shooter angle) that hits the wall with a bang at 11 feet high. may be good for the 2 point goal from the feeder station if can get some blocking from an alliance partner.
Good luck all, and watch the safety!!!

Mr. S. mentor team 3137

[AndrewSears]

Thanks for the replies. The information is very helpful.

I understand your concern and I am one of the adult mechanical mentors on the team. Every other mentor and the school are confident with our safety procedures, design and the thought process. We have safety in mind and we would not do anything that would put a student in danger. The students started with a direct drive and were not happy with the performance. They set goals and I do what I can to help them achieve those goals. We are being safe and this is completely necessary for their learning experience. What we are going to bring to competition will be safe, effective and will work the way they want.

FYI - We are decreasing this ratio and will keep you posted.

[protoserge]

Originally Posted by AndrewSears

Thanks for the replies. The information is very helpful.

I understand your concern and I am one of the adult mechanical mentors on the team. Every other mentor and the school are confident with our safety procedures, design and the thought process. We have safety in mind and we would not do anything that would put a student in danger. The students started with a direct drive and were not happy with the performance. They set goals and I do what I can to help them achieve those goals. We are being safe and this is completely necessary for their learning experience. What we are going to bring to competition will be safe, effective and will work the way they want.

FYI - We are decreasing this ratio and will keep you posted.

No. You are clearly unsafe in this case.


Please take this suggestion with the highest regard of importance. You cannot spin that assembly at that speed. There are COUNTLESS warnings on this forum for this type of rotating assembly.

[Gregor]

Originally Posted by AndrewSears

Thanks for the replies. The information is very helpful.

I understand your concern and I am one of the adult mechanical mentors on the team. Every other mentor and the school are confident with our safety procedures, design and the thought process. We have safety in mind and we would not do anything that would put a student in danger. The students started with a direct drive and were not happy with the performance. They set goals and I do what I can to help them achieve those goals. We are being safe and this is completely necessary for their learning experience. What we are going to bring to competition will be safe, effective and will work the way they want.

FYI - We are decreasing this ratio and will keep you posted.

Have fun.

[amesmich]

24,000 is the @ the speed of a router, and faster than a CD spins in a Drive. Mythbusters has proof of them shattering a CD at those speeds. A CD is far more balanced that any of COTS wheels you can buy.

As stated those speeds do little since the inertia of a still Frisbee is too great to be over come in such a short distance by anything spinning 24,000. There will always be slip. The only way you could take advantage of something that fast is is you had multiple stages to accelerate the Frisbee to some speed before getting to a 24,000 wheel.