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MMD > Archives > August 1998 > 1998.08.14 > 14Prev  Next

Bronze Bushings & Motor Lubricants
By John Rhodes

Some cheaper oils (perhaps "3-in-1" -- I don't know) are not highly
refined and contain some residual sulfur.  This can be hydrolized into
acid by moisture adsorbed on the bearing and shaft surfaces.

Automobile oils are carefully refined to eliminate the acids which
etch bearing materials, *but* one of the products of combustion of
gasoline (one of the not-so-low sulfur fuels) is hydrochlorous acid.
This acid etches metals, and is the reason internal-combustion engine
oils must be changed regularly.  The modern engine oils contain
buffering compounds which attempt to neutralize the acid, but they're
only partially effective.

Many oils also polymerize, especially in the presence of nickel
(a fine catalyst), producing the familiar gum.   Watch makers spent
a lot of time developing superb oils which resist not only gumming,
but also spreading.

One of the historic best watch lubricants was sperm oil from the
sperm whale.  Modern watch and clock synthetics are almost as good.
A horology catalog (or a watch-maker friend) is a good source.
I agree that Singer sewing machine oil is also excellent.

A porous bronze bushing (also called sintered bronze, because that
is the process by which it is made) is _always_ equipped with a felt
wick when placed into motor service.

Even the cheapest box ventilating fans have motors with sintered
bushings which are sized to an accuracy of about 100 millionths of
an inch.  They operate on shafts which have been centerless ground
to a similar tolerance, and the typical life approaches 20,0000 hours.
(That's a lot of music!)

Bernt Damm's comment about reaming reminds me that a common mistake in
machining sintered bronze is to use dull tools.  If a reamer, or other
cutting tool, is dull, then the metal being machined is smeared over
the pores, which effectively shuts off the lubricant source.  Rapid
failure results.  Sharp machine tools are a *must*.

One of the nicest -- and cheap -- sintered bushings I ever saw was
in a General Electric refrigerator: a fractional-horsepower shaded-pole
motor used as the air circulating fan.

These refrigerator fans will typically run 30 *years* at about a
50% duty cycle.  The vendor (whose name I forget) had machined helical
grooves in the shaft which served to _circulate_ the oil _through_ the
porous bronze bushing into a capacious wick (1/4" shaft, 1/2" bushing
length, about 2 cc of wick volume).

I examined one of the wicks after 10k hours of service; it was stained
with a dark deposit which was slowly being transported through the wick
-- but hadn't yet reached the  outlet/recirculation side.  I'm pretty
sure the stain was wear products -- most likely steel from the shaft.
It was a beautiful design!

John Rhodes

(Message sent Fri 14 Aug 1998, 05:56:17 GMT, from time zone GMT-0700.)

Key Words in Subject:  Bronze, Bushings, Lubricants, Motor

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