For those who are skeptical about propellers - Team 2526

[zrop]

Originally Posted by synth3tk

Pit area? Although safety should always be first, sometimes mistakes are made.

How about we put safety whistles behind the props so everyone can hear when they're on and uh.. think?

[Akash Rastogi]

Originally Posted by zrop

Who is dumb enough to stick their fingers in there?? Plus the props are far enough inside the cage that even if you can get your fingers in, they wouldn't touch the props.

PLUS who's running around the field trying to catch a robot by sticking their fingers into the cage with spinning blades???

Who's dumb enough to accidentally turn on a robot when someone is working on the blades???

Who's dumb enough to move the wheels when someone is fixing a sprocket on the drive train?????


I can make lots of question marks too. You can get as defensive as you want about your robot design itself, but when it comes to the safety of others don't take suggestions lightly.

If you're not in it for the safety and well being of your members and others in the pit, that's fine by me.

[zrop]

Originally Posted by Greg Peshek

Well, our blade/s pull too much power to gear it up any. We start tripping a 40A breaker at a 1:2 ratio. The size of our prop combined with the really high pitch, and the fact that the blade is cut to give a lot of thrust in one direction.. gave us a pretty good acceleration to around the speed you're describing at 3/4 ratio.

Of course we're going for different things. You have them for turning and maneuvering plus a good amount of power. And we're going for sheer power, mainly for getting across in a straight line fast, pinning, and then dumping.

Tell you what, you make it to Atlanta for Championships, we'll have an Overdrive style race.

-Greg

You might want to reduce your pitch then. High pitches are made for high speeds, therefore you're being really ineffective and ideally you'd have about 1-2 pitch prop from max speeds of about 7-15 mph. Otherwise, even with our pitch of 3.75, we hit ideal speeds at about 30 mph.

And yes, prop race at Atlanta ;D

[EricH]

Originally Posted by zrop

The propellers are perfectly safe as long as they don't exceed their maximum rpm, the rate at which we are spinning them is no where near their maximum rpm (the maximum is about 13k rpm). Having them spontaneously exploding is not a huge concern to us, having balls getting in the way of the props is a much bigger concern. (oh and its 12.25 in in diameter btw ;D)

12.25 inches????

Let's run the numbers. That's 374.15 feet per second if something breaks! If a prop breaks, it's going 7, yes 7, FRC fields per second.

As for perfectly safe if they don't exceed maximum RPM, that's all well and good, but R/C aircraft don't hit things. That's the facts of it--if they hit anything, they crash (or just crashed) and the prop is broken anyway. These props are going to be jostled through shock loads when they get hit. This could potentially weaken them. If they're weak, and they get one hit too many, somebody is going to get hit, hard. I haven't run the numbers on KE for, say, 1/3 of a prop, but that's far more than I want to have hitting anybody!

As for who's going to poke hands in, I can think of a few things--little kids, poles from a tipped trailer, unsuspecting students... I am pretty sure I could get my hand in enough to contact at least one prop.

Bottom line, don't worry about keeping balls out, worry about keeping the props in. If you do that (better than the cage shown will), the balls will take care of themselves.

Oh, and I just remembered--if you reverse those blades while they're going full bore, that's a pretty hefty acceleration force you put them through. This will contribute to propeller weakening. They aren't designed to run in reverse.

[XaulZan11]

Yes, the cage does make it more safe, but why isn't it on during testing? In the 2nd video, I think the person bending down gets a little too close; one slip and people will be watching the video for different reasons...

Creative design, but make sure it's safe

I believe the KE for 1/3 of the prop will be over 2000 joules so yeah, it's not something I want to be in front of when it comes flying off.

[zrop]

Originally Posted by EricH

12.25 inches????

Let's run the numbers. That's 374.15 feet per second if something breaks! If a prop breaks, it's going 7, yes 7, FRC fields per second.

As for perfectly safe if they don't exceed maximum RPM, that's all well and good, but R/C aircraft don't hit things. That's the facts of it--if they hit anything, they crash (or just crashed) and the prop is broken anyway. These props are going to be jostled through shock loads when they get hit. This could potentially weaken them. If they're weak, and they get one hit too many, somebody is going to get hit, hard. I haven't run the numbers on KE for, say, 1/3 of a prop, but that's far more than I want to have hitting anybody!

As for who's going to poke hands in, I can think of a few things--little kids, poles from a tipped trailer, unsuspecting students... I am pretty sure I could get my hand in enough to contact at least one prop.

Bottom line, don't worry about keeping balls out, worry about keeping the props in. If you do that (better than the cage shown will), the balls will take care of themselves.

Oh, and I just remembered--if you reverse those blades while they're going full bore, that's a pretty hefty acceleration force you put them through. This will contribute to propeller weakening. They aren't designed to run in reverse.

How about this: We'll put extra thick mesh on the sides of the safety cage because any flying props would.. exit there. But to keep proper airflow, the current cage in the front and back should suffice.

[Akash Rastogi]

Originally Posted by tanmaker

I believe the KE for 1/3 of the prop will be over 2000 joules so yeah, it's not something I want to be in front of when it comes flying off.

Yup. It is by my account also. (Emphasis mine)

[EricH]

Originally Posted by zrop

How about this: We'll put extra thick mesh on the sides of the safety cage because any flying props would.. exit there. But to keep proper airflow, the current cage in the front and back should suffice.

Thick mesh, or a sheet, would probably work. Even if it doesn't fully stop the prop, it'll slow it down to the point where it isn't nearly as great a danger.

[zrop]

Originally Posted by XaulZan11

Yes, the cage does make it more safe, but why isn't it on during testing? In the 2nd video, I think the person bending down gets a little too close; one slip and people will be watching the video for different reasons...

Creative design, but make sure it's safe

Haha. It just looks like I got close, but really, i grabbed it about a foot in front of the props. And as for the cage, we didn't have the mounting brackets completed at the time of testing... and we were anxious.

[zrop]

Originally Posted by Akash Rastogi

Yup. It is by my account also. (Emphasis mine)

Don't worry. It's double nylon nutted on.

[TotalChaos]

Originally Posted by zrop

Who is dumb enough to stick their fingers in there??

I have to agree with Akash and Eric. I wouldn't just assume that common sense will prevent all accidents. Why do you think they put "HOT" warning labels on coffee cups? Or warnings on electrical things, such as hair dryers, not to submerge in water? ... Sometimes things can happen, and there's no reason not to play it safe...

Other than that, very cool propulsion system! I was one of those skeptical about using fans, but it turns out I was wrong and they work great after all!

Good luck at the competition!

- Austin

[synth3tk]

Originally Posted by zrop

How about we put safety whistles behind the props so everyone can hear when they're on and uh.. think?

Even if I wasn't safety captain for my team two years in a row, I would still like to think that safe practices are of the utmost importance in every FIRST member's mind during the season. While this isn't as deep as thousands of people dying, you still need to take some caution in the real world, and all the FRC is doing is preparing you to make safety second nature.

Afterall, we don't want this:
http://www.chiefdelphi.com/forums/sh...ad.php?t=65978

or this:
http://www.chiefdelphi.com/forums/sh...ad.php?t=59436

to sound like something that might happen as a result of your team's machine. I take safety seriously, and hope that everyone else does too. If not, you need to sit back, forget drilling another hole, and think about that.

[Joe G.]

I would still have finger-proof mesh in the front and back, and possibly even wire the props so that they cannot be turned on with the mesh removed. Never count on "people won't be stupid" as a saftey feature. Take Murphy's law one step further, "Everything that can and cannot go wrong will go wrong."

*From someone who got his finger caught in Vex chain one time too many.*

Remember we are taking into account 1/3 of a propeller. For all of the smashing that could be happening, those props will no doubt be subjected to forces they are not designed for. These abnormal forces will weaken the propeller, causing it to ultimately fail.