A friend of mine in British Columbia, a player of historical
instruments, posted this to another list in regard to my query as to the
wood used for bearings on grist mills... it was something that came up
in conversation here about selection of apple wood for bearings... I
think it interesting to the idea of a relatively small hurdy gurdy
compared to the sound of a fairly large grist mill, both of which
operate with turning mechanism to produce sounds, as per his last line
as to when the grist mill is in tune.
When I asked him for permission to cross post he asked me to convey, "
...that humans were much smaller then and that the physics of the ear
canal means that they could not hear the low pitches we can today."
Cross post as follows:
"Like almost everything else, what woods are best for bearings depends
entirely on who what where when and why. Ebony and lignum vitae were
and are commonly used in industrial applications for water immersed
bearings. It is not so much their hardness as their natural oily
qualities (preventing rot) that make them useful. Lots of vertical
shaft turbines from the 1800s have lower seat bearings and adjustable
(and replaceable) side-travel blocks of those woods, and top bearings
too if the turbine was totally immersed.
However, the 1876 pack horse era grist mill I restored in Keremeos, BC
still had some of the original bearings for the water wheel, line shafts
and machines. The water wheel, fitted with iron gudgeons and their 4"
shafts used Babbitted bearing blocks, but the first counter shaft had a
3" gudgeon shaft riding in a wooden bearing block made of Douglas Fir.
Neither was supposed to be wet, simply oiled. The water wheel (10 rpm)
weighed a couple of tons while the first counter shaft only had to spin
at 20 rpm with less than 50 lbs of weight.on each of its bearings.
Inside the mill, succeeding counter shafts and the main line shaft
rotated at progressively higher speeds but with either wood (Fir) or
Babbitted iron blocks depending on the gravity loads or side thrusts to
be accommodated. The fastest was a 700 rpm counter shaft and idler
pulley that used fir bearing blocks for a 2" steel shaft. Caked oil on
the wood showed the attentive care given to this one, but the shaft
still fits the hole quite nicely even after its dozen years of historic
use. I would love to get an analysis of the oil residue to see what
they used there in the 1880s.
The reason I mention all this is to illustrate that in that pack horse
lifestyle setting, efficiency was critical. It was two hundred miles
over the mountains to the next town and a five month sailing voyage to
Britain for spare parts, so you had to make efficient choices about what
to carry in and what to make on the spot out of local materials.
(Actually I exaggerate a little. By then it was only three months by
ship to Britain and two days to San Francisco by fast steamer - but
still at least three weeks on the trail to get to the ship. The
Hudson's Bay Company commonly sent two pack trains a year into the
region with supplies. There were even some wooden shafts, carved to
hexagonal shape with 4" diam. cylindrical ends for the bearings. They
were set into wooden bearing blocks. Speeds about 60 RPM with light
vertical loads.
A wood technologist here who performed a detailed analysis of every
major item in the mill, commented that he was surprised to find Western
Red Cedar used in many applications there, but that the wood structure
in those parts was much more dense than usual. He mistakenly said that
the wood had perhaps become compressed due to the loads, but was wrong
about than, not really understanding how the machinery worked. But he
also said that the unusually dense cell structure might also have been
caused if the wood had been exposed to fire. As a living history
restoration woodworker, that make a lot more sense. The original
builder of that mill performed some amazingly sophisticated work, and I
would not be surprised if he either selected wood for those parts from
timbers that had experienced forest fire or perhaps intestinally
hardened the wood by fire to give it the quality he needed for those
particular parts. Heck, we still do that with fence posts where they
are going to touch the earth.
The important thing about axles and bearings is to protect the precious
bits with expendable and replaceable mating parts. Shafts are usually
a lot more important and costly than the bearings, so the bearings will
always be softer or less wear resistant. Hence Babbitt sleeves in iron
blocks too. With slow speeds and low weights, even fir works just fine
as long as you keep it oiled so it doesn't catch fire from friction.
It is all about those three factors of friction, pressure, grip and
speed. In our modern age of alternating current and high speed tools,
it is easy to forget how slow and quiet things were in those good old
days. The only ear protection needed might be ear muffs against the
cold. The grist mill when operating was/is like a little chamber music
concert of delicate sounds and clicks. When in tune, it sings to you
about its pleasure of smooth operation, allowing you to hear all the
diverse voices in harmony; but it might just as well groan with dismay
at feeling friction."
--
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