MODEL ENGINEERING TIPS > 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 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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