A tone bar screw is a metal self-tapping screw that is inserted between a tone bar and the harp of a Rhodes piano. They are sometimes called grommet screws. Its function is to anchor the tone bar to the harp, so it remains in position for the hammer to strike the tine and generate a tone. There are two screws in each tone bar assembly.
Function:
The screws works alongside the tone bar grommet, and the tone bar spring. Therefore there are two of these per tone bar along with a washer that sits between the head of the screw and the grommet. The grommets helps remove the vibration of the tone bar transferring to the harp. The springs keeps the tone bar assembly under tension so it does not directly touch the harp. The function of both of these is to improve sustain.
These screws are important in the sound of a tone bar. Because they are metallic, they have a resonant property and will affect the sound of the note. Therefore they must be right density and mass in order to deliver the ideal sound. Naturally this means that every screw must be the same. However, because they are screwed into the harp, they also transfer some of the vibrations into the harp. Too much of this will cause tone problems and poor sustain, which is why the rubber grommets are seated between the screw and the tone bar. A tone bar screw has a Philips head. It has a portion of the length without a thread, and a portion at the end with a thread. Only a small amount of the screw length is screwed into the harp.
Screw Types:
There are two tone bar screws per tone bar, one in front of the other. These are typically called the voicing screw, and the escapement screw. The voicing screw is closest to the centre of the tone bar, and is the screw that is adjusted for voicing of the note. By adjusting the screw, the tine is raised or lowered relative to the pickup. This will change the amount of fundemental and harmonics frequencies in the note. This can vary based on tonal preference, but an even sound throughout the Rhodes is aimed for.
The escapement screw is further away from the centre, and its function is to anchor the tone bar assembly at the correct height so even escapement occurs throughout the piano. The correct height for the escapement screw is so the bottom of the tone bar is 3/8″ above the harp.
On some later Mark I and Mark II pianos, there is only one tone bar screw, grommet, and spring in the top octave or so. This is because the tone bars have poor sustain, and removing the escapement screw aids in improving this. In pianos with both screws all the way up to the top notes, if adding tone bar clips does not help in sustain then removing the escapement screw can improve this. Voicing is more difficult with only one screw, but because the highest notes have less variation in the voicing anyway, it usually is not too much of a problem.
Restoration:
When undertaking a Rhodes piano restoration it is common practice to replace the tone bar screws along with the grommets. This is because the screws can lose their resonant properties as they corrode, and the philips head itself can become stripped. The grommets crucially are also replaced as they become hard and brittle and allow more vibration to transfer to the harp, reducing sustain. Replacing the springs is optional, but can aid in an improvement of the sound. For the majority of Rhodes pianos, replacement of these will make a positive change to the instrument.
