The RHS steel beam was used for mounting test ribs on the bench, to simulate the rigid mounting conditions of an installed sound board. Tapered spacers were positioned between the sound board test strip and the larger mounting blocks on the RHS, to simulate the sound board seating conditions of Overs piano sound boards.First published 10 June 2004
Compression crowned ("CC") ribs, such as the control rib shown above, will have a smaller 'unstrung' crown radius at the feathered ends. CC rib sound board panels typically are grossly overloaded in compression, even before they are subjected to the downbearing force of the strings. It is not unusual for CC boards to exhibit significant signs of collapse within twenty years, and often much earlier (some overstressed sound boards can exhibit signs of collapse even before the instrument is sold from new). Overs Pianos is currently (2004) replacing a CC concert grand sound board (circ. 1962) from a 'leading' manufacturer, with reverse crown between the bridge and the belly rail in the second top string section (colloquially known as the killer octave).
An early-development-stage I-rib is shown under test below.
A test rib for rib no. 11 of the Overs 225 piano, under a 24 Kg downbearing test load
With reference to the images, please note;
- The 24 Kg down bearing load, on the test rib above, is the equivalent of the down bearing force experienced by rib 11 at 1.5 degrees of downbearing angle on the Overs 225 piano. For actual sound board loading the downbearing was set at around 1.3 degrees (strung) for rib 11.
- With the dial guage mounted on the RHS, deflection under load was measured at several points along the length of each rib. Various alternative rib profiles were tested and evaluated until a suitable set of rib profiles was determined.
- The test rib illustrated was too weak for use in a real soundboard (note that the bottom flange of this rib was made from King William Pine), although many production CC boards are overloaded to at least this degree. It is not at all uncommon to inspect new production pianos where there is some measurable downbearing throughout the instrument, yet the sound board panel is pushed almost flat.
- Typical sector-of-an-ellipse feathering allowed the I-rib to be overloaded at the ends. Straight feathering was found to be superior for the I-rib section ribs.