It made a very loud crunchy sound. couldn’t tell if that was the plywood spitting, the robot crying or both lol. It was rather spectacular and also terrifying at the same time. We can laugh about it now but at the time it was a full on scramble to pick up the plywood.
Pneumatics are intrinsically sketchy. If you ever have the opportunity to do so safely, try taking two identical tanks (or cylinders) and pressurizing them to failure, one with air, and the other with water. When hydraulic vessels fail, they tend to do so fairly calmly, with a leak forming and thus depressurizing the vessel. Air tanks fail in a far more bomb-like fashion; since air is compressible, you can squeeze way more energy in there.
I’ll happily play around with high voltages, radiation, and lasers, because things like that are pretty easygoing and predictable. It’s fast-draining energy reservoirs, like gas bottles and big capacitors, that scare me.
I think the piston made it past it’s stop and made contact with the end cap. On most cylinders these days the end cap isn’t made to take an impact and the stops are less than adequate for what was described here. Usually whatever clevis or attachment was on the rod would make contact at the other end first, but this is a fun failure mode that is rare to see.
As to other statements on compressed air and safety, metal air storage and such is typically safer because rather than exploding they tend to peel open like a banana whereas plastic tanks have the easier ability to fail explosively. Not saying to take it lightly, but we are always happy to have the added weight in exchange for a little peace of mind.