Hot-Air engines (stirling cycle and
atmospheric) produce very little power in model sizes. In fact, the
engines will not run at all if there are any binds in the moving
parts or a little too much friction anywhere. An operating engine is
not proof that most friction that can be eliminated actually has
been eliminated. It may simply mean that it is still just able to
run in spite of the friction. The point I want to make is that an
engine that runs with other than ball bearings at least on the
crankshaft would run much better if it had precision ball bearings.
Ball bearings come in many different types and quality. Cheap (read
crude) bearings which probably could just as well be made by chimps
who work for a banana a day may be better than sleeve bearings -
actually, maybe not! Due to the mass production of precision ball
bearings for the aero-space and computer industries, really superb
instrument ball bearings are extremely low cost items these days.
Any engine worth the time and effort invested to construct it surely
warrants high quality bearings.
Bearings generally are available in three styles: Sealed, Shielded
and Unshielded. Sealed bearings have some type of non-metallic
material (usually neoprene) that makes running contact with the
inner bearing race to prevent dirt, water and other foreign matter
out of the bearing. Do not use sealed bearings because the running
contact of the seal will probably cause more friction than a plain
sleeve bearing would. Shielded bearings have protectors usually of
metal that cover the side of the bearing and almost but not quite
touch the inner bearing race. These prevent anything larger than
dust particles from entering and are extremely free running. Some
applications may require shields on both sides of the bearing, but
most applications really only need a shield on one side because of
the way the bearing is mounted. Unshielded bearings should only be
used in totally enclosed spaces because they are completely
vulnerable to dirt otherwise.
Ball bearings also come in two other types: Plain and Flanged. Plain
bearings need a housing with a step in the bottom to prevent the
bearing from moving deeper into the cavity than desired. Flanged
bearings look like railroad car wheels. Mounting is very easy
because all that is needed is an appropriate hole in a block or
plate for the bearing and the flange prevents the bearing from going
deeper into the cavity.
Ball bearings in sizes suited to model engines should never be force
fit into their mounting cavities or onto the shaft. If the bearing
is too tight a fit on the shaft, polish the shaft down. A light hand
push fit is as tight as they should be and even a slip fit is
perfectly O-K. If there is worry that a shaft may turn inside the
inner race, a tiny amount of thread locker adhesive may be used - do
not allow any of the adhesive to get into the bearing! The outer
race of a slip fit bearing will not rotate in the cavity due to the
large area of contact and the speeds we are dealing with - unless
the bearing is defective to start with.
If you have a bearing that has been contaminated with dirt (the
bearing feels gritty when turned by hand) there is a good chance it
can be saved. Using a magnifier and a sharp tipped x-acto blade or a
sharp needle, the "C" clip can be removed from it's groove
and the shield removed. Spin the outer race by hand while the
bearing is submerged in CLEAN solvent and then blow the solvent out
of the bearing with compressed air. Try the bearing to see if it now
turns silky smooth, if not do the above steps until it does. If the
bearing still feels gritty after several times of this process then
the best use of it may be as salvage of the "C" clips and
shields.
Once, I attempted to clean some ball bearings with an ultrasonic
cleaner. They were a little gritty when I put them in - when I took
them out they were bound up solid. I'll have to try it again when I
get a bearing with dust in it and see what happens again! In the
meantime, think twice before you do it.
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