Action design notes, Odell opus 653
Above is a picture of the backfall action (with offset rollers) of Odell opus 653, taken about a year ago when the organ was being installed. I came across this photo this morning and posted it on my own Facebook page today. It sparked some inquiry about the action design, so here is a more detailed explanation:
The original action for the organ was built in the 1870s, but had been removed and disposed of by the Kimball organ company in 1928 when they electrified the organ and connected it to their opus 6567. Hence we were free to start from scratch.
We naturally wanted a modern key action that would be light and responsive. To achieve that, we decided to deviate from the traditional 19th century action system. The main difference between the new vs. the historical is the absence of a common pivot axle and a mounting rail for the levers. Axles have a tendency to introduce friction and can rob motion when the holes inevitably wear. Here instead, the ends of our levers are 'registered' by two guides: one for the pallet pull down wires and the other for the sticker rods coming up through the tails of the keyboard. The levers pivot at their centers on a section of angle steel that is mounted and "floats" with the chassis.
With this chest layout (one very common to 19th century organs), the basses are split diatonically: the bottom 16 notes are arranged — 8 each — on the respective ends of the chest (C, D, E, F#, G#, A#, C, D on the left, and C#, D#, F, G, A, B, C#, D# on the right). The 8 roller arms you see transmit the action from the bass end of the keyboard over to the backfall levers at the C# side of the chest. From E17 onward, the chest layout is chromatic.
Just like with the original system, the backfall levers vary in length to translate the keyboard scale to the chest scale. These lengths are plotted by drawing them out at full size. We take that data and use it to generate a schedule for fabricating the levers. Note that while no two levers are the same length, they always pivot at the center, maintaining the range of movement so it is equal at both ends. The keyboard levers have a ratio of 3:2 and the backfalls are 1:1. Hence a normal key depth of 9mm translates to 6mm of movement at the pallet connection point. The keystack also has a 30 note pedal coupler activated via a hitch down pedal.