Re: Re: A question of physics


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Posted by Rick Denney on February 24, 2003 at 12:28:12:

In Reply to: Re: A question of physics posted by AW on February 24, 2003 at 11:57:10:

The point of roller and ball bearings is to provide a load-carrying ability not possible with the oil film on a journal bearing. With the extra moving parts, lubrication would provide viscous damping, and you'd have to run a ball bearing dry or with very light oil to maintain the speed of operation. That would make them noisy if they have enough tolerance not to be sticky. But ball bearings are not design to operation with that much tolerance, and they wear quickly if they carry much load with that tolerance. They are usually preloaded to make sure that all the balls are in complete contact at the microscopic level, which will induce some drag at the expense of load-carrying capacity.

And if they were sealed against dirt, the seals would provide further drag. If the rotating item has a large diameter compared to the shaft (as with, say, a bicycle wheel), this drag is negligible. This isn't the case with rotary valves.

Also ball and roller bearings are designed for continuous rotation, not the back-and-forth rotation of valves. That back-and-forth action quickly squeegies the lubricant away from where it needs to be. Journal bearings (which is what rotary valves use) is actually better for this sort of operation.

There's nothing in the world wrong with a well-lubricated journal bearing for this sort of application, but it depends on a film of lubrication. With that lubrication, wear is negligible. Without it, wear and drag will both be problems. One problem is that people use piston-valve oil on the bearings, which is too thin. A heavy household or light motor oil works better, but only on the bearings, of course. A light piston oil (if any oil is needed other than saliva) on the rotors themselves is what you need to provide the seal you mentioned. If the rotor bearings are where they should be, there should be no mechanical contact of the rotors against the housings.

Rick "who oils his rotary valve bearings often" Denney


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