Strengths and Frailties of wood
Prunus serotina or Black Cherry –
Specific Gravity .50 (dry), MOR = 12, 300 lbf/in2
From a structural standpoint, wood is pretty amazing stuff. Under a microscope, you can see that wood is made up of many spindle shaped cells. These Cells are mostly filled with air (or water when it becomes humid or wet) but the walls of these cells are made up of long helical chains of cellulose. You can see Cellulose in a purified form as cellophane - Wrigley’s Spearmint gum used come in cellophane wrappers.
Once for ounce, wood’s tensile strength is
greater than steel. It is easy to
machine and takes a wide variety of adhesives.
Compared to man-made materials with similar strengths (Kevlar or exotic metal
alloys), it is inexpensive. That said,
there are number of foibles that a builder should be aware of.
Wood is non-isotropic,
that is its strength and other physical properties are not constant it all
three dimensions. It is fairly easy to
split a stick of wood along the grain with a hatchet. It takes a lot more energy to split wood across the grain. Because of this, it is important to orient
the grain in specific directions on
highly stressed components.
Wood is a natural material,
and defects like knots, splits and shakes can limit its strength. Wood is an organic material – remember it is
made up of these empty cells that will shrink and swell as the ambient humidity
changes. Almost all of this shrinking
an swelling action is across the grain.
Over time, wood will slowly deform. Take a look at an old roof beam or joist and
you will see a definite curve downward.
It will try to run away from the load.
A great book on harps is Armstrong’s The Irish and Highland Harps
(Preager, NY, published in 1969). Mr.
Armstrong describes a dozen or so ancient Celtic harps, depicting (in vivid
detail) their demise, as they slowly collapsed under the tension of their
strings, hypothesizing continually on successive repairmen’s attempts to forestall
the harp eventual structural demise.
The harp builder can take a number of steps to
mitigate the weaknesses of wood:
- Select
clear grained stock
- Ensure
that it is dry. A lot of
woodworkers use expensive moisture meters. I’m saving that money for a surface sander.
Instead I make do by patronizing suppliers that I trust or buying
green wood from a local sawyer and storing it for a few years.
- Consider
lamination for the neck and other parts under high stresses.
- Use
designs and joinery that allow for Expansion/contraction
- Look
on structural failures as a tuition – a chance to improve you design or
building skills
Picea sitchensis or Sitka Spruce -
Specific Gravity .42 (dry), MOR = 10, 200 lbf/in2
If you really want to get technically educated
on different woods and their mechanical properties, visit the U.S. Forest
Services Forest Service
Products Laboratory for reams of data on wood species and their
properties. Chapter 4 of the Wood
Handbook is a particularly useful reference on the mechanical properties and
testing procedures used for lumber.
Back to the Structural Analysis
of the Folk Harp