Jerry E. Howell Model Project Plans & Kits
Outpost Enterprises, LTD - 695 Godfrey Road - Hollansburg - Ohio 45332 - USA
(World Wide Supplier of Quality Model Project
Plans & Kits Since 1992)
Hot Air Engine Bearings
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 bannana 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 forein 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.
© Copyright 1999 - 2007 Jerry
E. Howell - All rights reserved