rotarygod

This may sound like a weird question so hear me out. What is a bearing in an engine anyways? We don't use ball bearings or roller bearings. I admit there are some very small uses for these in specialty engine applications but for the most part in an engine we don't have them so we'll just leave it at that and go from there assuming we don't have those. The bearings in an engine are really nothing more than steel cylindrical inserts that are pressed into the rotors and stationary gears. These sleeves have a groove in them (some bearings don't) for the oil film to travel in. A bearing never touches metal on metal which is why bearings don't ever break in. The thin film of oil is what keeps them apart and what keeps everything happy. From time to time a bearing may come lose and "spin". This causes all sorts of issues but on a rotary that ususally means you are replacing the component that this happened to as well as the eccentric shaft plus whatever else it decided to take out with it.

Why do we need these inserts? Why can't we just machine a tighter tolerance into the rotors and the stationary gears so they work as the bearing themselves? Remember it's really the oil that does the work and not the bearing. Nothing should be touching. This would really simplify things. First off since bearings are never touching, they shouldn't ever wear out. When you press a bearing in, it can distort in shape a little bit and get out of round. That's bad news. We can easily avoid this is we just don't have one. If the stationary gear and rotor are machined good, we shouldn't ever have an out of round, out of true issue. If something fails we still have to replace everything anyways so longevity isn't an issue. If anything this should be more reliable because we can't spin a bearing. This is simpler, has less room for error, still does the same job, and has no drawbacks. Why couldn't this be done? The reason I bring this up is because this is actually how NASCAR engines work. No "bearings". Machining the tolerances into everything is no more difficult than what they do now. They still have tolerances they machine to on the rotors and stationary gears when they are machined now. They are just larger to allow a metal sleeve bearing to be installed. It would be so much easier to just adjust this machining spec to get rid of the bearing.

It all makes sense to me. What do you think?

lshu

I don't know anything about bearings, but from what I gather on this forum, if rotarygod believes we don't need bearings, then we don't need bearings.

zoom44

this article from 2000 claims the Nascar engine in question costs about 30k

http://66.102.7.104/search?q=cache:W...ient=firefox-a

zoom44

this article which is newer http://66.102.7.104/search?q=cache:I...ient=firefox-a

claims 60k to 80 k and possibly higher

rotarygod

What's the cost of a NASCAR engine have to do with it? I'm just talking about making a simple machining adjustment at the factory when they make these things and then not installing a bearing. That's it. If anything that should save Mazda money as they don't need to buy bearings and install them. The whole point of making the NASCAR connection is that the idea is sound. Cost is irrelevant in this case.

ps1726

The softer metal (babbit) used in bearings helps preventing dry start damage until enough
oil pressure is available to float the eccentric or crankshaft off the main journals, or housings in the case of rotaries, it is a very short time of metal to metal contact but in time it adds to the engine wear, remenber oil seeps out from the bearings when tthe engine stops.

rotarygod

I've got used bearings out of an old rotary I pulled apart that still has the opaque coating all around it that it came with originally. As I said, as long as the oil does it's job nothing touches. Most of the oil should seep out but not all unless the engine hasn't been turned over in a very long time. If there had ever been any metal on metal contact with my bearings this coating would be gone. I can remove it with a kleenex so any contact with metal would definitely remove it. No signs of it.

r0tor

iTrader: (1)

- tolerances that tight would cost big big $

- the babbit (aka lead) is softer then the eccentric shaft so if there are ever any issues the babbit of the bearing will wear before the e-shaft and you have a cheaper repair bill. It also has a better co-efficient of friction if there does happen to be metal to metal contact - which means it is also easier on startups where it takes a split second to get oil flow, or if you get a sudden vibration and some shaft rub (i don't know, like bucking the car).


...also roller bearings are not used because they are extremely unreliable and have lots of vibration issues when compared to a well running sleeve bearing.

rotarygod

I've never seen an eccentric shaft that actually made contact with a bearing not need to be replaced. It messes everything up.

Why would it cost anymore? If you are using a CNC, you program the tolerances in and it cuts it. It's a change in a value on a machine.

BRealistic

If you did not have separate bearings, what happens when you get too low oil viscosity or flow for the pressures being applied? Does the E-shaft just fuse with the block after it cools down? Does the harder metal flake off when severely overheated (since the harder metal doesn't give as easily, it will create more friction and more heat) instead of smoothly wear. How is that cheaper than spun bearings?

I would be more impressed with a perfect apex seal and spring combo that can withstand 11k rpm without bumping after passing over the lobes and lasts 200k miles.

bgreene

The machining adjustments to tighten up the tolerances could add significant cost to the whole process of production for multiple reasons:

1. To meet tighter tolerances, the final machining is done with many many very minor "cuts", which means much more machine time per part (and more machinist time per part.

2. Tighter tolerances mean more sringent inspections, and more parts getting reworked or rejected. This increases labor by inspectors and machinists, as well as meaning more stock material gets used to make the same number of parts.

3. Slower production of individual parts means more machines, machinists, factory space and energy consumption to produce the same number of units.

All this adds up to more cost, and a more expensive car (and more expensive spares). It's probably unlikely that a mass produced engine would see costs of $30k+ due to economies of scale, but it's easily possible that such a production method could add $5k or more to the cost of a vehicle (balancing the smaller number of production wankels vs the greater number of moving parts in ottos, the increase could be comparable for all cars).

We do super tight tolerances on things in aerospace all the time (+- 0.010 inch is frequently considered a fairly sloppy tolerance, with extreme slop being +- 0.030 in in most cases), and combined with exotic materials and small production runs, most "shoestring" cost numbers (with some notable exceptions, like Rutan) in this industry would choke an automotive bean-counter

lurch519

i dont know about in the rotary, but in a normal piston engine, lets say you seize or spin a bearing. what would be cheaper, machining the components, and installing a slightly thicker bearing to allow for the machining, or just replacing the components?

TeamRX8

iTrader: (25)

NASCAR engines aren't designed to last 100,000+ miles

therm8

you have to machine tolerances to fit the bearing just right, so machining tolerances for no bearings shouldn't be any harder. like cutting a perfect 1" circle isn't any different in cutting a perfect .5" circle, either way it has to be perfect.

rotarygod

When the engine is running, if the bearings actually touch from the oil failing to do it's job, the engine will also fail. If your oil does break down or get too thin and the bearings touch, you are screwed. Bearings do not touch. There is oil in between them. You can't have them touch and still be usable. At least not while the engine is running. Bearings do not wear. They can't if they don't touch.

I will concede that it probably is possible to get the bearings to touch at startup if you have a person who doesn't take care of their engine and runs on an oil that is breaking down. But the most damage is at startup. You can't have a bearing issue while it's running and not hurt the motor bad. So if we have them for mostly neglected use as a safety feature, that I can concede to. I don't buy any information of bearings wearing or touching when in normal running operation though as that doesn't happen. So basically a bearing is used because if there is any damage, it is at startup and a street engine will be started far more per miles run than a race engine will per miles run. I can live with that. This would be a great idea for race rotaries though as it is one less thing that can fail. As I said, if a bearing fails during operation and by fails I mean spins or even touches, the engine is dead anyways and needs a rebuild. Probably with usable parts as they don't fail and still remain rebuildable. At least rotaries don't.

rotarygod

Just thought of another reason why they can get away with it. I'm not sure as this is more speculation but it's plausible. Does NASCAR run a dry sump oiling system? If they do, oil stays up in the engine and bearings. They'd never go dry and wouldn't suffer the issues of a pan based system and it's oil draining back into the pan somewhat drying everything out. This could be a contributing reason it is possible.

yiksing

Do metals break down from extreme heat without touching each other.....hmmm.....

rotarygod

Not the low level heat in an engine.

Ole Spiff

What about heat expansion issues? There must be a good reason why they still use bearings, aside from being easier to accommodate the various tolerances of mass production components and assembly. Magnetic bearings are ideal; no contact, no friction...it's easy to get to 100k rpms because oil isn't involved at all; the shaft is suspended in a magnetic field. Wouldn't that be a great rotary engine? 0-100,000 rpm on a tach would make every ricer around green with envy hehehe.

TeamRX8

iTrader: (25)

Yes, they run a very extensive dry sump system. In general they don't suck air if that's what you mean by going dry, but everything still falls down into a pan where it's scavenged back to the sump tank, except for when the engine is shut down. I don't understand what you mean by "drying everything out"?

max5roadster

Good question... Why are they even called bearings? In reality, they are oiled bushings, not bearings. I believe in order to go without bearings that everything attached and spinning with it would need to be in perfect balance to maintain an even oil film during rotation. Further, the surface of most bearings, er bushings are actually soft and porous to a degree-allowing it to absorb/trap oil. Hard metals would not likely have this characteristic. just a thought.

Ericok

It's probably a doable concept (going without plain bearings) but someone would have to spend the money to develop it for mass production. There'd be the issue of ensuring the warranty could be met. Since existing plain bearings have a long history, are proven, and cheap, what would be the benefit to the typical consumer (Bragging rights to: My engine has no bearings?). The reason ball or roller bearing aren't used is that you would have to use a (pressed together) built-up crankshaft - so the races could be installed onto the shaft. Plain bearings allow a one piece shaft. Honda used ball bearings on its air-cooled formula one engine a while back using a built up crank. Didn't need pressure lubrication (like a plain bearing).

crossbow

Bearing wear does occur. You'll see it in UOA's (Used oil Analysis) and when checking bearing clearances in an old vs new engine.

bgreene

For matching the final fit, it's probably easier to grind down a "stock" bearing to match the "as-is" finished engine part size, as this allows the factory to tolerate more variation (0.020 in vs. 0.001 in, say) in the machining of the harder materials. Also, the bearing can better accomidate any minor (0.005 in or so) out of roundness in the surface of the shaft (although these should be exceedingly rare if that part of the shaft is finished on a lathe....)

bgreene

The machining and balance would have to be so perfect on a shaft made to spin at 100kRPM just to ensure that nutation wouldn't rip up the whole works, that the clearance and tolerance issues that bearings deal with would probably no longer be a concern.

Then there's the issue of flex due to local centrifugal force on an e-shaft (or crankshaft, camshaft, etc) since the CG of the cross section at many points is off the rotation axis (with the net of the whole shaft balancing). The e-shaft would probably have to be made from some exotic alloys, possibly even unobatanium...

It would make a fascinating concept engine (if you could figure out how to cool a wankel running at that RPM), but is probably not possible for production anytime in our lifetimes.