Clarinet: metal rotation analysis

On the bell of a clarinet is a metal band that loops around the wooden bell. One winter (and only one winter), the metal piece on my clarinet was able to be freely rotated around the bell. My friend said that this happened to her the previous year as well (I had not checked my clarinet before). At all other times it has never rotated that I checked. My friend first pointed this interesting ability out to me the first winter I was playing clarinet in my church choir. I assumed that it had to do with the cold temperature outside and much warmed temperature in the church causing some expansion of the metal ring and that it would happen every winter. Since that year, the metal band on my clarinet has never again been able to rotate.

I own a Buffett E11 clarinet. The wood is African Blackwood and the metal is most likely German silver (1). To discover how large the temperature change would have to be to cause the metal band to rotate, I tried to find the thermal expansion coefficients of both the wood and the metal and compare them. The coefficient of thermal expansion for German silver is 18.4*10-6 /K (3) and for wood is 2 to 6 *10^-6 /K (2). Therefore, the thermal expansion for the metal band would be between 3 and 9 times as much as the wood. Now, with this, it would be easy to see how the metal band would expand more than the wood and be able to rotate. In fact, if this was the only factor, it would be easy to see how the band could fall off the wood entirely.

There’s a catch. There are at least two complications to this over-simplified calculation. One, the coefficient of thermal expansion for the wood provided is for the direction parallel to the grain. The coefficient perpendicular to the grain can be 5 to 10 times this value (2). Considering that the metal band is around the wood at the edge where the expansion perpendicular to the grain would be more applicable, the metal may no longer expand significantly more than the wood. Secondly, the thermal expansion of wood is insignificant compared to the expansion due to changes in moisture (2).

Some more information about how my clarinet playing practices have changed over the years is needed to continue. Since joining the church choir I have adopted the practice of one of my fellow clarinet player – blowing gently into my instrument to warm it up. This helps to tune the clarinet faster. I only picked up this practice after my first winter playing with the group. The change from not blowing in my clarinet to blowing in my clarinet to warm it up might be thought to increase the moisture. As I started this practice after that winter of bell metal rotation, it might explain why the rotation has never occurred again. However, as the bell is at the bottom of the clarinet, this seems doubtful as not much moisture reaches it.

So, the rotation only occurred in winter. And it only occurred one winter for me and two for my friend. My hypothesis based on the evidence is that the rotation is a result of two variables: temperature and moisture. The temperature gradient between the outside and the church in the winter probably causes the metal and wood to contract then expand at different rates. Or the colder temperature in the church in the winter caused the relative sizes to be different than in the summer. As for moisture, it’s possible that the two winters that the metal piece could rotate were dryer than the winters since then.

Overall, I do not have enough data points or good enough tests to figure out the correct reason. If I were able to (i.e. if I didn’t care about damaging my clarinet or if I had the money to buy a bell to test on), I would devise a test to figure out the reason. Preferably, I would measure both the temperature and humidity of the building and the outside. Of course, it will be difficult to determine what caused the rotation if it never happens again. That remains to be seen.

(1) http://www.madehow.com/Volume-3/Clarinet.html

(2) http://www.fpl.fs.fed.us/documnts/pdf1989/wegne89a.pdf

(3) http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

P.S. some people are really crazy about their instruments. If you want to read about whether or not you should oil your wood instruments you can check out some blogs on that (http://forums.bobdunsire.com/forums/archive/index.php/t-109298.html).