If you have a little swelling (seasonal / environmental0 of a support of a wooden shaft, even if it is perfectly aligned, you will start to get a 'braking' effect on the shaft, and since the wood will NOT swell evenly, there will then be one or two points of contact on the shaft, and thus a harmonic or sinusoidal rhythm to the braking (the nature of the wood is to not be adhesive, but that does not mean it doesn't create friction. If you were to hook up a gurdy under these circumstances to a high horsepower, low rpm motor, you would probably find that you would get a reasonably constant sound from it, but because the human arm does not provide consistent pressure, rather it adjusts to small changes in resistance, you actually assist in causing the wah-wah in the drones (the sound is actually there in all the strings, but it is easier to hear in the lower ones).
interestingly enough, you can also cure this sinusoidal vibration rhythm by putting in a precision bearing, perfectly aligned, in the same place. Either no drag (bigger hole), or very even and uniform drag (precision bushing). This is not only common to gurdy mechanics, but it is a VERY common situation in mechanics, especially motor and drive system mechanics. If you have a shaft that can support it's own self then just support it at each end - if you need more support, then be EXTREMELY accurate in placing 0 tolerance supports only where you need them. I learned such things from the chief mechanic of a specialty steel rolling plant. It is amazing to see weakly supported shafts a foot in diameter go through this vibration. Chris *********** REPLY SEPARATOR *********** On 3/15/2007 at 11:14 AM Seth Hamon wrote: I was looking at the building of the luteback hurdy gurdy on http://www.edmonds.de/body6.html . What is the reasoning behind drilling the hole in the first rib for the shaft 4mm larger than the o.d. of the shaft? I know he said it relieved the wah wah sound problem he was having.. It just seems as though this would take away some of the structural integrity of the shaft system.. Is this a common practice for the construction of a hurdy gurdy. It does appear that he didn't use any kind of bearings for the shaft. It seems to be set directly into the wood. Cheers, Seth
