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Do ultralight engines need a thrust bearing?

Most gas engine applications only have a radial load ( perpendicular to the shaft).

Aircraft, not only have a radial load but also an axial prop thrust load pushing/pulling the shaft out/into the engine depending on whether the prop is a tractor or pusher prop, which usually requires a different kind of bearing able to take a thrust load.

So...do ultralights need a thrust bearing?

If so...

I assume the bearing of the output shaft of a reduction drive would have to be a thrust bearing.

However, we see 1/2 VW engines being used in direct drive applications, and I don't think they change the final bearing into a thrust bearing.

The reason I'm asking is, that I'm thinking of using a $400 15hp 4 stroke industrial engine for an ultralight application, just as we see in Rutan's quickie(?), ultralights using the vanguard brigs and stratton engines, and M Columbine's new design ( designer of the Cri Cri). Yes they are heavy, but they are cheap, 4 stroke, a TBO of 2,000 hrs, and are rated for continuous duty.

I plan to run at 75% power, which is about 2,500 rpm so I really don't need a reduction drive.

If I need a thrust bearing, I could have a reduction drive with something with a 1.1 reduction. I could also have just a support thrust bearing bolted directly to the engine.

If I don't need a thrust bearing, then I would be off to the races.

I don't think the Lazair had a thrust bearing or a reduction drive when they used the non rotax engines.

So... do GA airplane engines need a thrust bearing, in the 15 to 30 hp range?

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Any aircraft engine that spins a prop to generate thrust will necessarily contain a thrust bearing to prevent the propeller thrust force from pulling the crankshaft out of the block. But since a bearing designed for mostly radial loads will also be capable of resisting a certain amount of thrust load, some automobile engines converted for aircraft use can get by without having a thrust bearing added to their crankshaft supports.

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    $\begingroup$ Most automobile conversions (and some ultralights) use a speed reducer, either gears or belts -- and the thrust bearing is on the reducer's output shaft; hence protecting the engine bearings from thrust loads. $\endgroup$ – Zeiss Ikon Sep 16 '19 at 17:29
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    $\begingroup$ yes yes. without it, the engine would quickly fail in an ugly way. $\endgroup$ – niels nielsen Sep 16 '19 at 23:47
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    $\begingroup$ Actually, the main bearings in most engines, anything from a vertical shaft lawnmower type up to a racing V8, are journals, not ball bearings -- and they don't take end loads at all well. In typical automotive engines, only the end bearings have any thrust load tolerance at all; the mains will have the crank rubbing on the block and cap if there's any thrust load. $\endgroup$ – Zeiss Ikon Sep 17 '19 at 11:05
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In automotive engines in general, the main bearings and front and rear end bearings are journal bearings, with near-zero end load tolerance (the end bearings have small flanges, but mainly for location tolerance). I think if you look at the 1/2 VW conversions, you'll find they're building an entirely new crankcase (as well as cutting the crankshaft and camshaft, assuming they don't make those new), and they do in fact incorporate thrust-tolerant bearings in the ends of the case -- different bearings from the ones Volkswagen originally used in their four-cylinder air cooled engines (designed in the early 1940s and barely updated until the 1970s), which were specified for the intended use, with near-zero thrust loads.

If they didn't, even with only 30 kW in the minimal versions, the engine would have a very short running life.

If you're buying the engine already converted, you probably don't need to worry about it as this work will already be done for you, but if you plan to do the conversion yourself, be sure you look in detail at existing conversions, including their running life before overhaul in combination with the bearing type.

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