Balancing a Pneumatic wheel

[Cal578]

Originally Posted by ksafin

My apologies, I meant centrifugal.

I don't know how this works against us though.

Considering our weights are on the very outer edge of the plastic hub, pushing them outward away from rotation keeps them more solidly in place.

Ok, centripetal vs. centrifugal is an easy mistake. If your weights are on the outer edge, that still sounds scary. Do you mean that they are on the inside of the outer rim? In other words, if you imagine a smiley face painted on the non-adhesive side of the weight, would it be facing towards the axle? That would be okay. If it's facing away from the axle, that's very scary; if it's facing parallel to the axis of rotation, that's a little scary.

Maybe you could post a photo?

Of course, if your team is convinced that it's safe, then you only need to convince the inspectors, not me.

[Al Skierkiewicz]

OK, time to add a few words of sense to this discussion.
1. Adhesives, particularly double sticky tape are not fasteners. We can argue industrial adhesives but so far I have not seen anyone discuss these yet.
2. The tread of the pneumatic tires you are discussing cannot take the force of the wheel speeds that will be generally encountered. I believe the operative phrase is "destructive disintegration". Have you noticed how hard it is to find any speed data on these tires?
3. The imbalance and vibration is telling you something, you are not listening.
4. Just because you haven't witnessed parts leaving the flywheel yet, doesn't mean it won't happen.
5. Most of you are using motors that free speed above 15,000 RPM. Coupled to a 3:1 transmission, what numbers do you get? Direct drive on a 775 is over 7,000.
I think you guys need to rethink your designs to be sure. Not pretty sure, not 'oh it will never happen' sure, really sure. Please remember that inspectors will be looking at your robot with this in mind...
R08
ROBOT parts shall not be made from hazardous materials, be unsafe, cause an unsafe condition, or interfere with the operation of other ROBOTS.
I am sure that if your design should cause any flying debris, an inspector will be asked to either disable your mechanism until such time as you can prove it won't hurt anyone or anything, or whatever it takes to prevent a re-occurrence. Now is the time to be really sure, not during competition.

[FrankJ]

For the tire wheel weights, mounted on the inside of a wheel: They are being used exactly as the MFR intended in a less severe than typical application. In this case the supplied double sided sticky tape is tested & sold for this application. Of course they need to be properly applied to a clean surface.

(squirrel alert) Industrial adhesives: Our shooter board & bumper backing is made of thin wood strips glued together with industrial adhesive. Fiber reinforced composite matrix to my materials professor. Plywood to my Home Depot salesman.

We are using the spin box with standard .500 keyed hubs and mounting hardware. The problem that we have found is the plastic hubs are out of round. No level of balancing would cure this issue, we tried. The id bore is to large for the hub and the thru bolt holes are to sloppy. This hub was not intended to spin at 5K rpm. We are now attempting to turn a new set of hubs out of HDPE and a drive shaft from 6061. We are hoping we can tru the hub and shaft. Our current setup runs and shoots OK but the vibration causes issues with the camera. As we progress I will post pics and CAD files if anyone is interested.

[Ether]

Originally Posted by Andrew Lawrence

No. Such. Thing.

In Newtonian mechanics, the term centrifugal force is used to refer to one of two distinct concepts: an inertial force (also called a "fictitious" force) observed in a non-inertial reference frame, and a reaction force corresponding to a centripetal force¹.

Centripetal force is the force exerted by the wheel rim on the balancing weight, pushing the balancing weight toward the center of the wheel. It's what makes the balancing weight following the circular path.

Centrifugal force is the reaction force exerted by the balancing weight on the wheel rim in response to the centripetal force. It is a real force.

So Ksafin's only mistake was that he should have said "centripetal force" instead of "centrifugal force", because he was referring to a force acting on the weight.

¹wikipedia

[FrankJ]

Our AndyMark hubs are round. Since they appear to be injection molded parts, I would expect them to be consistent. We are mounting them to a 5/8 dia Andymark hub with #10 screws. We made a 1-1/8D x 5/8D insert for the other side of the hub to keep it true to the shaft. If you are using 1/2 shafting, a 1/2 bearing would work there as well.

[Shu]

Originally Posted by Al Skierkiewicz

OK, time to add a few words of sense to this discussion.
1. Adhesives, particularly double sticky tape are not fasteners. We can argue industrial adhesives but so far I have not seen anyone discuss these yet.
2. The tread of the pneumatic tires you are discussing cannot take the force of the wheel speeds that will be generally encountered. I believe the operative phrase is "destructive disintegration". Have you noticed how hard it is to find any speed data on these tires?
3. The imbalance and vibration is telling you something, you are not listening.
4. Just because you haven't witnessed parts leaving the flywheel yet, doesn't mean it won't happen.
5. Most of you are using motors that free speed above 15,000 RPM. Coupled to a 3:1 transmission, what numbers do you get? Direct drive on a 775 is over 7,000.
I think you guys need to rethink your designs to be sure. Not pretty sure, not 'oh it will never happen' sure, really sure. Please remember that inspectors will be looking at your robot with this in mind...
R08
ROBOT parts shall not be made from hazardous materials, be unsafe, cause an unsafe condition, or interfere with the operation of other ROBOTS.
I am sure that if your design should cause any flying debris, an inspector will be asked to either disable your mechanism until such time as you can prove it won't hurt anyone or anything, or whatever it takes to prevent a re-occurrence. Now is the time to be really sure, not during competition.

Hi Al,

There are many teams using these pneumatic tires on their shooter but I am getting the impression from your various posts that you are deeming them unsafe to use in this application. My concern is that you as a inspector or someone else will rule them illegal during inspection based on what you specified. This will in turn render a robot's shooter useless until another "more safe" wheel is used.

Yes, these tires were not specifically designed to be used under these circumstances but I have yet to hear of one coming apart. I agree that great care needs to be taken in how they are mounted and balanced. Also, it is highly recommended some sort of shield be used to prevent any flying debris should something come loose.

If a team takes these steps, do you see them capable of being used in a safe manner?

[sandiegodan]

Originally Posted by FrankJ

For the tire wheel weights, mounted on the inside of a wheel: They are being used exactly as the MFR intended in a less severe than typical application. In this case the supplied double sided sticky tape is tested & sold for this application. Of course they need to be properly applied to a clean surface.

How are you evaluating "less severe?" These weights are sold for auto & motorcycle tire balancing. An automotive tire will rarely see over 1000rpm. Small trailer tires may be as much as double this but no where near 3k-5k rpm. I'm surprised no one is discussing the Dynabeads mentioned in an earlier post. Much better balance solution for this application. They claim to deliver precise balance and they are contained. I have not used them but I've seen a number of credible reviews on them from the motorcycle community.

This still doesn't allow teams to use components beyond their rated capacities. Even with the best balance solution, if the wheel/tire falls apart, they are all dangerous. If the vendor description of the item has a 5000 rpm recommended limit, that should be considered the maximum safe operation condition.

Regards,

[Racer26]

Originally Posted by sandiegodan

How are you evaluating "less severe?" These weights are sold for auto & motorcycle tire balancing. An automotive tire will rarely see over 1000rpm. Small trailer tires may be as much as double this but no where near 3k-5k rpm. I'm surprised no one is discussing the Dynabeads mentioned in an earlier post. Much better balance solution for this application. They claim to deliver precise balance and they are contained. I have not used them but I've seen a number of credible reviews on them from the motorcycle community.

This still doesn't allow teams to use components beyond their rated capacities. Even with the best balance solution, if the wheel/tire falls apart, they are all dangerous. If the vendor description of the item has a 5000 rpm recommended limit, that should be considered the maximum safe operation condition.

Regards,

I don't really think its fair to say that its the maximum safe operating condition. Manufacturers often leave a very large margin for error. Its also problematic for components such as the AM pneumatic wheel, which doesn't spec a 'recommended maximum rpm'.

The AM pneumatic wheels were DEFINITELY not designed for operation at, or anywhere close to 5000rpm. Neither are the banebots wheels many have used in their shooters. Nor ANY wheel I've seen in a wheeled shooter, though I would consider MOST shooters currently mounted on robots in their bags 'reasonably safe'. If they haven't blown themselves apart in testing, they likely won't during competition. Could it happen? Sure, there's wear and other things that aren't considered in my extreme oversimplification. I think robots should be designed to contain a failing shooter wheel, but restricting operation of robot parts to manufacturer provided values/use cases is just problematic.

[FrankJ]

Originally Posted by sandiegodan

How are you evaluating "less severe?" These weights are sold for auto & motorcycle tire balancing. An automotive tire will rarely see over 1000rpm.
Regards,

Centrifugal force (or centripetal depending on your point view) Depends on tangential velocity not RPM. Motorcycles & cars see easily 50 m/s which is more than our wheel, at least, is spinning.

All the wheels I have seen use on the Frisbee shooters are being used "off label" from what the manufacturer intended. Liability laws being what they are, I doubt you will get buy in from them if you asked.

So it is incumbent on us to make sure our robots are safe. This very much includes making sure they are no loose bits on the wheel that can fly off and hurt some one. Also that you are not spinning the wheel at a speed that will make it come apart.
Also checking the shooter wheel should be part of everybody's pit check list before every match.

[Ether]

Originally Posted by FrankJ

Centrifugal force (or centripetal depending on your point view) Depends on tangential velocity not RPM.

It depends on both.

F = mv²/r .... or .... F=mrω²

Centripetal acceleration:

20" at 120mph = 11,330 m/s² ....... 8" at 5000 rpm = 27,854 m/s²

see attached Motorcycle vs Shooter Wheel centripetal acceleration calculator

[Richard Wallace]

Originally Posted by FrankJ

Motorcycles & cars see easily 50 m/s which is more than our wheel, at least, is spinning.

50 m/s is about 110 mph. Motorcycle and car wheels that operate at such speeds are not made of polycarbonate and grey rubber.

[FrankJ]

I will grant you a 110% I will not put an Andymark pneumatic wheel on my motorcycle and try to go 110 MPH. I don't think my poor Volvo would go that fast either. The analogy breaks down pretty rapidly.

Here is a article on plastic car wheels. Doesn't really transfer to robot Frisbee shooters

[Al Skierkiewicz]

Shu,
I am not saying that inspectors will make any decisions on the use of a particular tire. We cannot tell it's use during the inspection process. I am discussing the use of certain of the mentioned methods of attempting to correct vibration and balance. Again there is no way for inspectors to determine during the process if your design is safe or not.
What I am trying to point out is that certain assumptions for high speed rotating objects may lead to unsafe conditions during play on the field. I don't want to see any team disabled by or for an unsafe condition and I certainly do not want to see anybody injured. From my calculations, many shooter designs exceed by more than 5 times the rotational speed of car tires. Even devices designed for road tires may not retain their design characteristics at your rotational speeds. Now is the time to be sure. For reference I suggest you look to videos of drag racing vehicles in the burnout box prior to a race. Be sure to take a close look at the tire profile during the burnout process.

[Racer26]

Also, anybody that has spun up one of the AM pneumatic wheels 1:1 off a CIM with no air in the tire will tell you that the rubber of the tire deforms rather alot. It quite visibly pancakes at that speed with no air pressure to help it hold shape.