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String bearing felt

  • 1.  String bearing felt

    Posted 07-04-2018 16:04
    What is the main purpose of String Bearing Felt? Most grands have it between the tuning pin and the bearing bar before the string slopes down to either the agraffe or the capo termination.

    In some pianos I find that this felt is so tight, or dirty, that the string won't smoothly slide over it when tuning. This causes jumpiness and makes tuning difficult. A drop of Protek at the string/felt contact has frequently reduced this friction enough to make tuning possible. But every once in a while I come across a piano where there appears to be little to no friction between this felt and the string. This makes tuning extremely difficult because now even the gentlest movement of the pin cause the string to change pitch uncontrollably. And with a pin that is tight this can make tuning, again, extremely difficult.

    Is the purpose of this felt to make the string quiet, add friction, or what? If too tight, again, a drop of Protek can work wonders. But what if it's not tight enough? What can be done?

    Geoff Sykes, RPT
    Los Angeles CA

  • 2.  RE: String bearing felt

    Posted 07-04-2018 16:48
    Edited by Jim Ialeggio 07-04-2018 16:50
    Coming from the speaking length through the agraffe, the string has to traverse whatever unique conditions the plate presents. The Speaking length termination is always lower than the tuning pin field. Sometimes there is a significant vertical offset between termination and pin field, and sometimes the vertical offset is minor. Either way, the bearing felt allows the string to follow the vertical offset created by the plate conditions, without having the string bear directly on the cast iron. There is also some quieting, but the amount of felt needed to quiet front segments is in reality quite small. Great expanses of felt are not needed to quiet a front segment.

    In, say, a Bechstein, or Kawai RX series( I think), the vertical offset between termination and pin field is very minor. This means the termination angle coming through the termination is also very small...sometimes well less than 10 deg. When the SL termination angle is that small, the friction coming through the termination is reduced...sometime too much. There is not enough friction in the termination alone. So in these cases, a long stretch of bearing felt is used to increase the rendering friction in the front segment from too low to reasonable.

    In a Steinway tenor, the vertical offset is greater than the Bechstein. SL termination angle is higher, usually a bit more 15 up to 22 deg or so...sometimes much more. In a 15 deg termination angle case, the friction coming through the SL termination is such that adequate friction is imparted into the front segment. However, since the string must also offset vertically after coming through the agraffe, the string bears on a sloped portion of plate on the way to the pins. This adds friction to the already adequate termination friction. When strings are new, the added friction from this felt bearing is not too cumbersome. But as string age and corrode, and the string takes a deformation at the termination, this added contact at the felt, in addition to the friction at the termination creates a rendering difficulty to be overcome in tuning.

    In the case of the Steinway, some eliminate the felt bearing, and allow a very long length of unsupported wire to come up to a brass counterbearing bar. This eliminates the felt friction, but adds a second metal friction point which may or may not be problematic all on its own. If the second friction point (counterbearing bar) has a wide radius contact, there may be a hard to read segment between agraffe and counterbearing.

    So each plate and each segment presents termination conditions that have to be played like chess, in order to get the friction in the easy to read zone...harder said than done...and age changes the conditions sometimes, but not always, depending.

    In the not-enough-friction scenario you describe, the termination angle is probably very low. In a restring, by adding an adequate amount of  felt covered maple between agraffe and pins, you can increase the termination angle, and add some more helpful friction.

    Its a big topic, as the plate conditions are highly variable. I spend a lot of time working this segment, as it has a serious impact on how easy or hard it it to tune the piano with stability.   Most of the time, however, this area is a complete throw-away, as everyone is itching to get to the stringing. Messing with the front bearings takes considerable brain time, particularly, if the original had too much friction up there.

    Jim Ialeggio
    Shirley, MA
    978 425-9026

  • 3.  RE: String bearing felt

    Posted 07-04-2018 18:47
    Jim --

    Thanks for this. I understand what you are saying. Great tutorial.

    The piano that motivated me to make this post is a 1975 Yamaha G3 that was rebuilt about a year ago. I'm not sure, but I'm guessing that the rebuild was recommended as a correction for distortion and overtone problems that the owner was having that voicing, string leveling, etc., were not correcting. With the rebuild those problems are still there but now it's also impossible to tune because of the low felt issue. I'm glad to learn that I was reasonably correct about why that felt is there. However, I doubt that the owner wants to go to the trouble of correcting the felt problem considering that the original problem was not corrected with the rebuild. He's ready to trade it in on a high end Clavinova.

    Geoff Sykes, RPT
    Los Angeles CA