I Think I Did Good Today

Here's the board from Rigging class today. I'm in a university classroom and it has one of those dual, three panel counter-weighted chalk boards that are so much like the ones we don't have in Purnell. It's not completely paradise though, as often these boards act like they have helium in them and just start rising of their own accord. Still, it's nice to be able to do a whole lecture without erasing. (what with all the crap on the board in 33 I think I spend as much time erasing as writing)

So, my phone-photography isn't that good. I guess if I am going to do this I need to try to get the same angle on all the pics, lest we get vertigo looking at the combined image. In any case, I give you the blackboard from today's topic:

MA and Double Purchase Systems

The reason I think it went well is because for the first time I can recall, when I got to the end nobody looked confused. I suppose it is possible there was just a bunch of well hidden confusion, but my take was that people left understanding and able to do the homework. Much as I have tried, I haven't really felt that way before.

"How did it go?" you ask. Well I will tell you. It starts on the upper left looking at architectural configurations without available full arbor travel - places where double purchase systems make sense. Then we go to the top of the right board and draw up simple, kind of spot-line hemp systems showing 1:1, 1:2, and 2:1. Looking at these we get into travel ratios, pulling load, line tension, and structure load - defining all of these traits prior to really even mentioning mechanical advantage. Then we establish the notion of trading force for distance and M.A.

With everyone having made this trip we go into some particular weirdnesses about sheaves, changing direction, load-increase factors, and alternate drawing configurations. This lets me talk about how tying off to the load puts less strain on the structure than tying off to the rail. This is one of my favorite little rigging facts because there's this moment in the thinking when 50% of the load just seems to go poof and its gone. After traversing that material we do classic pulley systems, 2:1, 4:1, 3:1 and then offroad a little to 6:1.

And then with all of that established we go on to the Double Purchase Counterweight System, the 2:1 "against" lift line rigging and then the 2:1 hand line rigging giving the 1:1 handline to batten travel ratio with a 1:2 arbor to batten ratio. This is where just about everyone is usually glazed over. Today, less so.

Pretty cool.


(Also, just as a sideline, I think the idea of doing powerpoint slides for lectures consisting exclusively of photos taken of the chalkboard from a prior lecture would be too totally awesome. I think I will have to do that at least once.)