Classifying Woods as Stable and Unstable
DonStephan
| Posted in Wood and Materials on
Re-reading part of Understanding Wood today, specifically a few pages on radial and tangential rates of expansion/contraction. One paragraph mentioned radial movement, tangential movement, and a ratio of the two. The next paragraph identified several woods generally considered stable, and several generally considered unstable, but didn’t explain if this classification was based on one of those numbers, two of them, or all three. Other woods in the chart had one or two similar numbers. Can anyone satisfy my curiosity?
Replies
Don, you need to look for the cross grain ratio between tangential shrinkage and radial shrinkage.
A stable wood is American mahogany with small tangential shrinkage of 5.1% and a similarly small radial shrinkage factor of 3.7% as measured from 30% MC to 0% MC (green to oven dry.) The tangential to radial (T/R) shrinkage ratio is also low at 1.4 found by the sum:
Tangential Shrinkage Factor / Radial Shrinkage Factor = Tangential to Radial ratio. The sum therefore is, 5.1 / 3.7 = 1.4.
This is the important bit-- generally speaking timbers that have a T/R ratio close to 1.1 tend to be more stable. The tangential and radial shrinkage factors might both be large or both be small, but it’s the ratio between the shrinkage factors that gives the best clue to stability.
In other words, if shrinkage is large in one direction (tangentially) but small in the other (radially) there is a large differential shrinkage pattern, meaning uneven transverse sectional shrinkage as viewed from the end of a board, and this leads to greater distortion, ie, instability. If the amount of transverse shrinkage is very similar in both directions the shrinkage is even and much less prone to distortion and is therefore described as stable. Slainte.
I can't comment on the article you mention but there are three factors in wood movement, radial, tangetal and longitudinal. There isn't a way to know if these factors are calculated individually or volumetrically unless they say. Most likely, all three are factored in. Engineering stability calculations can be quite specific, complex and based on more than just the cooefficient of expansion due to moisture absorbtion. Since each board is an individual, calculations are only approximations.
Also, some woods like elm, sweetgum, hackberry, blackgum, and sycamore have spiral or interlocked grain where the fibers in the wood orient on a slight angle to the vertical. In some years, the angle reverses. This spiral grain sets up stress when the wood dries, making these species very prone to twist and warp.
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