There is nothing that eliminates a long shaft from the design of a
PSRU. Nonbelievers might be advised to consider ship propulsion; long
shafts, low cylinder counts, propellers operating in uneven flow, often
via a gearbox. Sound familiar?

The important issue is torsional stiffness of the shaft, not
length. A long shaft can be torsionally stiff or soft, depending on
diameter and material. The engineering process will tailor torsional
stiffness of the shaft (along with a number of other factors) to adjust
natural frequency.

The information you need is found in engineering texts, not on RAH.
The subject can be complicated, but there are no unknowns. You will
find most of the torsional vibration classics listed in the
bibliography of Taylor's "Internal Combustion....". Some texts, like
Wilson's "Practical Solution.." (the ultimate reference) will be
difficult to locate. Try a large university library. The best readily
available text (sort of the ultimate primer on all matters vibrational)
is JP DenHartog's "Mechanical Vibrations". You can buy it for less
than $15 at Amazon. Here is a short list:

CF Taylor, "The Internal-Combustion Engine in Theory and Practice",
1966 (vol. 1), 1968 (vol. 2), MIT Press

W Ker Wilson, "Practical Solution of Torsional Vibration Problems", 3rd
Ed, 5 Vols., 1956, 0412091100, Chapman & Hall

JP Den Hartog, "Mechanical Vibrations", 1956, 070163898, McGraw-Hill

My compliments to Mr. Christley, whose comment (re frequency) was a
sole beacon of accuracy.


Dan Horton