Capstans, Inertia

[Bill Spurlock, RPT]

Thanks to David and Bill B. for your responses to my query on wippen springs,
etc.

>From Bill B.: > But there's more to fast and deep repetition than simply balance
weight and
inertia.>< Ken thinks that high checking is necessary, but one of the things
that
the NH Chapter's Junior Science Project hopes to find out (among others) is
whether the tail and check ever do contact during the deep and fast, or for
that matter the jack button and spoon, and the front rail punching and the
key.>

For what it's worth, I did a little experimenting a couple of years ago with a
Yamaha action model. I drove it with a cam on a variable speed shaft pushing the
key to about 90% dip. I was testing longevity of knuckle lubricants, but the
interesting thing I discovered was the relationship between checking height and
potential repetition speed. With proper tail shape & length, and correct
backcheck height, it was possible to make this action check at 5/16" from the
string. And when adjusted that way, the action would repeat in excess of 20
times/second when driven by this "playing machine". However, when forced to this
speed, the backcheck wire would soon bend, increasing checking distance. The
repetion would begin to skip, so I would lower the speed until it would repeat
reliably again. Then after a few minutes it would begin to check lower yet,
repetion would become unreliable, and I'd have to lower the speed some more.
When adjusted to check at around 3/4" from the string, the action would repeat
reliably at around 12 times/second, and would maintain this speed for an hour or
so until gradually the backcheck wear caused checking distance to become
greater. There was a direct correlation between checking height and maximum
possible repetion speed.

This was of course an artificial situation, with perfectly uniform repetition
frequency and depth of key travel. Also, the repetition speeds were probably
beyond what anyone would need to achieve by hand. So, I don't claim that it
duplicates human playing conditions. However, this experiment clearly showed
that the backcheck is functioning during fast repetition. A simple illustration
of this is just to bend one backcheck out of the way and try fast drum repetion
on an action. Notice from the blur of the shank that it oscillates between the
strike position and the rest rail, and that many repetitions skip. Now readjust
the backcheck and try again. Now the shank will be seen to always stay well
above the rest rail and repetition is more reliable.

Bill Spurlock