For those who are skeptical about propellers - Team 2526

[Akash Rastogi]

Originally Posted by David Brinza

I wouldn't be surprised if FIRST issues guidelines regarding safety measures for propellers (just remember the guidelines for last year's catapults). FIRST is (and should be) risk adverse when it comes to safety.

As an inspector, I'm going to ask a lot of questions about the safety provisions and considerations in high-energy, potentially lethal features in robots.

What margin of safety do you have in the hazard containment? (Prove to me that you've got at least a factor of five in keeping all parts from penetrating your safety shields.)

How do you verify that there are no flaws in the propeller and attachment components? Do you have proof of inspection for cracks in the propeller, shaft, etc.? Do you have safety-wired fasteners and a fail-safe design (i.e., if a fastener fails, do you still have margin against catastrophic failure)?

Sound tough? This is nothing compared to the aftermath of a failure where someone is seriously injured or worse.

I was thinking, what if there was a way for teams to have a touch sensor near their fan mounts, like really really really close, so that if the propeller moves even like .125" from its original mounting position, the motors to the fans shut off when the limit switch or touch sensor is touched? Seems simple enough to me.

[Andy L]

Originally Posted by Akash Rastogi

I was thinking, what if there was a way for teams to have a touch sensor near their fan mounts, like really really really close, so that if the propeller moves even like .125" from its original mounting position, the motors to the fans shut off when the limit switch or touch sensor is touched? Seems simple enough to me.

I had the same thought, my afterthought though was teams that would take this precaution would probably have very good safety precautions already for any other situation.

[writchie]

Originally Posted by Akash Rastogi

I was thinking, what if there was a way for teams to have a touch sensor near their fan mounts, like really really really close, so that if the propeller moves even like .125" from its original mounting position, the motors to the fans shut off when the limit switch or touch sensor is touched? Seems simple enough to me.

A 12 inch prop rotating at 7500 RPM has a wing tip speed of about 120 meters per second. It stores considerable energy. Even if you could anticipate ALL of the possible fault scenarios and detect when they occur, removing the energy source is not going to change the mechanical energy much over the short term. It's too late to have any effect.

IMHO The only realistic measures are distance from the hazard (used in R/C flying) and containment and only the latter is available to us. For the containment scenario, I think that teams will have to demonstrate that their containment materials and configuration can easily contain a prop failure at max RPM.

There are tradeoffs. Using steel mesh wire, the open area drops dramatically as you increase wire size and reduce mesh spacing. Engineering a mechanism that is both effective and safe is a challenge and either one without the other is just not acceptable.