Which bearing shields on model Diesel with ball bearings?

[rklopp]

I am building a pair of Nalon Viper 2.5-cc "Diesel" model airplane engines per Ron Chernich's plans purchased from MEN.org. The design calls for two ball bearings to support the crankshaft, one just in front of the crank disk and one just behind the prop driver. I bought shielded bearings. I am wondering whether I should leave all the shields in place, or pry off all but the front one, or all but the front and rear one, or what? If I leave the rear bearing open, the primary compression will be slightly reduced due to the exposed volume of the bearing. Does it matter? I am favoring prying out all but the front one. The crank has 0.001" radial clearance with the crankcase nose bore, so the leak path to the front bearing is fairly choked, but oil should find its way to the front bearing. If I leave all the shields in place, the bearings will be lubed by their native grease.

 

[Lawijt]

Just a open bearing inside the engine. Good cleaning them. The oil in the fuell will lubricate those.
The front bearing you take one with metal shields. Remove one shield. The other shield come in the outside.

Barry

 

[Hopper]

Primary compression will be the same with or without shields. Shields (ie the metal covers, not the neoprene seals on a sealed bearing) do not seal around the inside diameter, They are just a close fit, designed to keep dust out and grease in, mostly, sort of. Usual SOP for internal use is to remove the shields. This not only allows the fuel/oil mix to lube the bearing, but allows the mix to cool it too as it freely flows through.

 

[Tinkerer58]

Only problem you could encounter with metal shields if you take the internal shield off the oil may leak out the outer shield as it is not a sealed fit. It's OK for grease because it is thicker. In the older cars and gearboxes they used to use open bearings but these days they use sealed bearings as the bearings are filled with 30 to 35 % grease which is the recommended amount. I would personally use bearings with rubber shields which are non contact and indicated as UU this will reduce friction, I would not use DD or 2RS type as they are contact seals. The rubber seals are Nitrile and will withstand up to 100 deg. C as will the grease which as a standard is usually Alvania 2. I trust this helps.

 

[rklopp]

Thanks, guys. This is very helpful. I can't wait to run the engines.

 

[mu38&Bg#]

I have found that even contact seals do not retain grease nor keep the fuel out of bearings in RC engines. There must be a shaft seal present in order to prevent the crankcase pressure fluctuation from blowing through the bearings. Yet there are a few two stroke designs out there relying on bearing seals to seal the crankcase at the crankshaft. Shields don't do much. One reason people use seals or shields in model engines is to help prevent catastrophic damage if the cage was to fail while the engine was running by preventing the parts from getting into the crankcase.

 

[MCRIPPPer]

am i missing something here? if this a a 2 cycle or some form of 4 cycle that uses the crank case for pressure, then you need rubber sealed bearings or a seal on the shaft. or are you relying on the close fit of the crankshaft to keep pressure in? even rubber sealed bearings will leak where the shaft goes through the inner race!

 

[rklopp]

Apparently conventional practice is to have no shields except the one just behind the prop. It must work well, or it wouldn't be the conventional practice. In my engine there is only a 0.001-in radial clearance between the crankshaft and case in the area between the two ball bearings. That represents a high-resistance leak path, especially when filled with oil. It should behave much like a journal bearing, which I've used successfully many times on engines. The engine is a conventional two-cycle, with a rear rotary valve.

 

[mu38&Bg#]

This topic is often discussed on RC forums. Many an engine was supplied with open bearings and never had a problem. With a sealing feature in the engine design this will not be a problem.