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.
cp in bc
(still grinding)
----- Original Message -----
From: "Gabriel Orgrease" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Saturday, February 09, 2008 2:47 AM
Subject: [BP] wood bearings
What woods are best used for bearings?
crosspost from hurdy-gurdy list
"Even if this one is not really linked to the hurdy-gurdy: A friend of
mine owns a water mill from baroque times and the bearings of the water
wheel (he´s actually replacing it) are traditionally made from apple
wood. Not really what I would have used but exactly what he found and
what he was told from several experts and museums here. But I don´t
think that it´s a good idea to transfer this to the hg - the humidity is
hopefully at least a little different "
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