Re: For those who are skeptical about propellers - Team 2526
 

Just a reminder of how important safety is:
http://chiefdelphi.com/forums/showth...540#post815540

Re: For those who are skeptical about propellers - Team 2526
 

To any teams that will be using propellers on their robot, be prepared for serious scrutiny from your robot inspectors. I'm still developing the inspector training materials for this particular design element and I'm leaning towards placing a lot of the burden of safety verification on the teams.

Be prepared to defend your design. I would strongly encourage you to bring a "standard finger" to demonstrate general protection.

I would strongly encourage you to calculate the amount of energy stored in your propeller(s) at max speed and determine whether that energy is sufficient to punch through your enclosure. For example, how much energy is required to "rip" a steel wire of diameter X? Maybe you can find such destructive limits on online materials databases.

Bring plenty of documentation to support your design.

Good Luck and BE SAFE!
Russ Beavis
Chief Inspector

Re: For those who are skeptical about propellers - Team 2526
 

Russ,

Thank you very much for addressing the safety issue of propellers here on CD as a warning. I have two questions though:

1) Will the "inspector training materials for this particular design element" be officially released on the FIRST website so that teams will know exactly what they need to be prepared for?

2) Will the "standard finger" be defined? Something like a 3/4"D x 3"L wooden dowel or something? If left to the teams to define I would expect as many different definitions of a "standard finger" as there will be propeller designs. Actually it seems like this should be something that the inspectors provide at inspection rather than the teams if it is to become a "standard" similar to the sizing box or scale.

I ask because the safety considerations for propellers should be taken seriously but the only defined requirements that teams have access to at the current time is the Inspection Checklist. Maybe a section could be added to the checklist covering minimum propeller safety requirements?

Re: For those who are skeptical about propellers - Team 2526
 

Chuck,
The standard finger that Russ speaks of is a test fixture used by UL to test fan guards. As I understand it, the device takes into account the distance from the guard to the blade and the size of the openings in the guard as it would pertain to a finger intrusion. As this is a new concept in robot design for First, and has potential safety issues, I am sure that several different disciplines are involved in setting a standard for inspections.
Finger intrusion is only one of those concerns. As have been discussed above, noise, flying debris, robot damge, etc. are all concerns as well.

Re: For those who are skeptical about propellers - Team 2526
 

Dude. Those props are still awesome.

Re: For those who are skeptical about propellers - Team 2526
 

I look forward to seeing all documentation and substantiation that will support the safety guidelines that Russ will be issuing to the inspectors.

Re: For those who are skeptical about propellers - Team 2526
 

For those who are curious about how serious fan blade containment is the aerospace industry, check out this certification test performed by Rolls-Royce.

Think of all of the analysis that went into the design of that engine to ensure survival from such a catastrophic failure. The FAA and other aerospace agencies require proof that the design is robust, hence the analysis is backed by this very dramatic (and expensive!) test.

I'd be interested in seeing what came out of the the exhaust of that engine. I bet it wasn't pretty...

BTW, I didn't post this to discourage propeller advocates for robots. I just want them to see what kinds of challenges engineers sometimes encounter.

Re: For those who are skeptical about propellers - Team 2526
 

You're right. Years of effort goes into these fan designs in the hope that you only have to run this test once.

Some of the coolest testing we do is this test and the bird strike test, which basically involves shooting ducks at an engine running at full power. As was recently proven in NY, it's a good thing we do it. A friend of mine in the test department designed the equipment to do our most recent bird strike test.

As an MC and announcer...I hope the safety restrictions are very tight on prop-driven robots. Otherwise, I'll be announcing from outside.

Re: For those who are skeptical about propellers - Team 2526
 

As a scorekeeper about three feet from the edge of the arena, who has the added disadvantage of sitting down most of the time and is therefore less mobile, I couldn't agree more.

Re: For those who are skeptical about propellers - Team 2526
 

I agree that that needs some serious protection. Don't underestimate the energy in large diameter spinning objects.

Perhaps you could make a ducted design that turns with the blades. Then you would get more performance, while reducing the chance of pieces coming out of the fan space.

Re: For those who are skeptical about propellers - Team 2526
 

Check out the protection requirements in Team Update 11.

Re: For those who are skeptical about propellers - Team 2526
 

Only problem is that the update 11 doesn't have any specific requirements. It gives an example of a safe setup. It does not state any need for this particular setup. I personally won't feel that this is adequately covered until they specify max blade size, max rpm, and mandatory specific protective equipment. Any less and it will be too ambiguous.

Re: For those who are skeptical about propellers - Team 2526
 

They "expect that propellers...will be protected in a comparable way." That's not exactly just an example...

I'm not sure that they want to specify those items. They've already spec'd the bumpers to death, so it's either "Do we REALLY want to go farther?" or "Why stop there?". Judging by team reactions to the bumpers, I'd guess the former. I could be wrong.

If they do specify one thing, I would say they should do specific minimum protective equipment, possibly with relation to the size of the prop and the speed.

Re: For those who are skeptical about propellers - Team 2526
 

First off, Max blade size has little to do with the safety of a prop. A 32" prop can be made safe while a 10" prop could be very unsafe.

Max RPM is a function of prop size and construction. some props are perfectly happy at 20,000 RPM whil others are dangerous at 5,000 RPM. To set an arbitrary 'MAX RPM' may actually encourage poor design and unsafe robots.

As far as mandatory specific protective equipment goes, what may hold a 32" 5,000 RPM prop may not hold a 11" 20,000 RPM prop. Instead of blanket protective equipment (which may or may not work in all cases), require the teams to prove that the setup is safe with proper engineering numbers. Show that the cage will completely contain a catastrophic failure of the prop.

Re: For those who are skeptical about propellers - Team 2526
 

When it says comparable. It says that it is in respect to the danger of the blades. In reality, it really isn't saying much.

Also, I know it is the blade size that makes it dangerous. I also know it isn't the rpm that makes it dangerous. Lets compare to electricity. The current alone won't kill you and voltage alone won't kill you. It has to be together. However, there are standards for both. If nothing else, they could set up a simple formula that would allow for various rpm/diameter possibilities while still restricting the real danger.