Discover more from An Engineering Self-Study
An Inventor's Quest for the NHL Pt. 11
Similar Results with a More Positive Outlook
You rejoin me climbing back up from rock bottom. I ended last year thinking about giving up on this project, because it seemed like my invention might put goalies at a higher risk for concussions.
A small part of me was ready to quit; ready to return to the warm embrace of a steady paycheck and to a boss I could silently blame for everything. But taking a week off to recharge and receiving your emails of encouragement and advice helped me to back off the ledge. To everyone who wrote to me, thank you!
Truthfully, I was probably overreacting. “I think that possibly my design didn’t work exactly like I expected. Oh the horror! There’s no hope! I couldn’t possibly try to change the material or do anything at all to fix the issue.”
But I’m getting ahead of myself. First, I needed to be absolutely certain if I even had a problem.
According to the Standard Specification for Head and Face Protective Equipment for Ice Hockey Goaltenders (ASTM F1587 − 12a), I didn’t. The only criteria for the cage is that when it’s hit with a puck at 80 mph, it shouldn’t break or deflect into the goalie’s face. I think I’m probably very close to that with my current design. The “shock-absorbing capacity” is only tested on the actual helmet part, by dropping it and measuring the acceleration.
So should I even bother with the shock-absorbing capacity of my cage? The official standard doesn’t care, so why should I?
The simple answer is that I don’t want to give people concussions. But you’d think if that was a concern, the standard would specify a test for it. I guess I’m still thinking through things as I write this.
In any case, I wanted to see where my Kevlar cage stood compared to a metal cage, so I started doing a bunch of testing. In the interest of telling a coherent story, I’ll skip past my confused bumbles and focus on the test that seemed to make the most sense. But please remember, despite how effortless and straightforward I make this all look, confused bumbles are integral parts of the engineering process.
Here’s where my tests ended up.
I had a puck on a string hung from my hockey stick that I could swing into the hanging mask. The mask was hung with four strings to prevent it from twisting or shifting and weighed down with 12 pounds of assorted lentils to simulate the weight of a human head.
The idea was to measure the acceleration a goalie’s head would experience when taking a shot to the cage. Head acceleration seems to be the best marker for concussions.
I measured the acceleration experienced by the mask/head with this fantastic app called Physics Toolbox on my iPhone, which I shoved in between the bags of lentils.
After exporting the data, this is what a plot of the impacts looks like. After 43 impacts on my Kevlar cage and 43 impacts on the metal cage, the results seem pretty conclusive. The average acceleration for my cage is 1.949 g while it’s 1.599 g for the metal cage.
So I think that proves that my cage is worse for head injuries right now. If you see anything that I’m overlooking, please let me know. One thing giving me pause is that the app can only record 100 samples per second, so the acceleration peaks are a little lumpy. I have plans to (a) get more fidelity there and (b) to calculate the head injury criterion, which “includes the effects of head acceleration and the duration of the acceleration”. If my design has a higher but shorter peak, it could still be safer.
Of course, I couldn’t leave things there. I had to try to dig a little deeper. This time, I went back to elastic and inelastic collisions. Here’s a nice little refresher if you need it, but basically the difference is that in an elastic collision, kinetic energy is conserved. In an inelastic collision, some of that energy is converted to sound, heat, etc. Visually the more elastic a collision, the more things bounce apart. The more inelastic a collision, the more they stick together.
If you remember, that was sort of what brought me down this path. The bounciness of my cage seemed like a bad thing. I wanted to take a second look at that, but in a more controlled way.
So I placed the helmet against a wall, taping it in place, and wedged the hockey stick into a ledge in the garage, a puck suspended from its blade. Again, I swung the puck into the helmet and expected that the amount of bounce back would tell me about how much energy was being absorbed by the mask/helmet.
I included the puck hitting the wall for a baseline – to see how energy would be absorbed by the puck itself. It looks pretty obvious to me that my cage is basically absorbing zero energy somehow. I don’t know how else to interpret what I’m seeing. Again, if you see something different please do let me know!
So where do I go from here? First, I’m going to try to redo the head acceleration experiment with more fidelity so I can be sure I’m not fooling myself. Then, if I’m still seeing the same things and if nobody manages to convincingly explain away everything I’ve seen, I’ll get to work on trying to make my cage absorb more energy.
Thanks for reading and thanks again to everyone who offered advice and encouragement.