DoN. Nichols wrote:

I had agreed in another branch of this thread that it was not
for measuring electrical frequency -- but the mechanical frequencies
will be related to the electrical power frequency assuming an induction
motor instead of a series or parallel wound DC or universal motor.

It suddenly occurs to me that it's probably not for 50 - 80 Hz but 50 -
80 RPM. Isn't the arm too long, the weight to big, and the spring too
light to vibrate at 60 Hz?

This page says the steam engine on a Liberty ship was designed to turn
at 76 RPM. I suppose that was at full speed, so you would want a device
that could be tuned to lower RPMs to check vibrations when the ship was
cruising slower.

http://liberty-ship.com/html/topics/engine.html