For those who are skeptical about propellers - Team 2526

[ChuckDickerson]

Originally Posted by Russ Beavis

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

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?

[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.

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.

[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)

[zrop]

Originally Posted by Akash Rastogi

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

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

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.

[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.*

[zrop]

Originally Posted by rocketperson44

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.*

Alrighty.. We'll take care of it. We really posted this here for more the propulsion design rather than safety concerns.

[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.

[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.

[engunneer]

Cool.

What are the pegs on your rollers? What's the diameter of them? If they are too small, you might have inspection problems.

[Woodworker88]

Nice work, looks like you put some thought into the design. Good use of model airplane props.

Are you worried that having the props mounted in a biased configuration (not dead-center on the robot) will tend to make the robot turn or move awkwardly? I assume you offset them to make room for a ball-handling system. Will the airflow pass through the ball system or will you reverse the rotation to reverse the direction of the robot?

On a separate note, have you checked the metal hardware on your collector to make sure it's not going to rip up the balls? Looks iffy to me.

[JVN]

All safety considerations aside...

Have you done any performance testing as compared with a standard kitbot drivetrain? How about one with some basic traction control?

I'm not overwhelmed by your performance videos, and I'm curious why your team made the design decision to go this direction. Was there testing or prototyping involved? All about theoretical calculations? None of the above?

If the decision was all about "cool factor" then... rock on.

However, if you believe this has higher performance than using those CIMs in the traditional "rubber meets the road" way, I'm curious why you believe so, and if you have supporting data. If you DO have supporting data, I'd absolutely LOVE to get a peek.

Enlighten a man who sometimes has difficulty understanding why others stray outside the box, when the box appears to be an optimized and elegant solution.

-John

[Andy L]

Originally Posted by JVN

"plastic meets the road"

fixed.

I'm curious on your reasoning also, the main reason I dismissed propellers was not only safety, but slowing and stopping. How well does it stop and change direction to reverse?

[zrop]

Originally Posted by Andy L

fixed.

I'm curious on your reasoning also, the main reason I dismissed propellers was not only safety, but slowing and stopping. How well does it stop and change direction to reverse?

Theoretically, we're supposed to be getting 30-some newtons of thrust from the current arrangement... and with the calculations (which i long forgot) it was supposed to be able to accelerate faster than bots that use standard wheel drive. However, I emphasize that this is not the final arrangement -- we should have standard triblade props delivered this monday with a pitch of 4, and furthermore, if we get more thrust out of that, we'll try to custom order triblabe or quadblade props with a pitch between 1 - 2, idealizing our max speed (at around 7-15 mph) while boosting rpms and increasing power. THEN, with the addition of a duct (ideally, increasing power by an addition 5%) we should be operating at pear conditions.

Also, the idea of hybridizing our bot has came up: i.e. powering the back two wheels for extra acceleration. Although that's a ton of power stress on the battery, the 3rd CIM for the powered wheels would only work during acceleration, so we should be alright. =)