oldschoolbaby

I have built loads of two stroke bike engines in my life and they all use needle bearings on the crank. These engines rev in excess of 13grand and the bearings are tiny. some go down to 40 mm in OD. I dont know the amount of load of a 2 storke compared to a 12A or 13B, but Ive seen and heard of 250cc v twin engines that develop 80 hp plus at 18k rpm.

I have been thinking of this for a very long time but never got to testing.

Keeping in mind, in a two stroke you dont have any oil pres at all! Oil is mixed with gas and this is the only lube the crank gets(apart from the main bearings that sit in gearbox oil)

I had a Suzuki RGV 250 with 15k on the clock and the only thing that failed was the cylinder liners. This engine saw 15grand every day.

I have a few dead 12A rotors, e-shafts and s-gears. all of these will need machining as they spun bearings.

If anybody can get part numbers on bearings I will gladly have these machined and build a test engine.

One thought though, are either the e-shaft or s-gears hard enough to use as a bearing race. If so, it will allow a larger needle witch inturn might increase life?
Saying this, on a 2 stroke the conrod is steel and this is used as the bearing race, aswel is the crank journal (witch I believe to be a lot harder than the con rod though).

RETed

This would be really neat if we're talking about ideal conditions...

Now, when I'm talking about "ideal", I'm talking about a perfect...circular...motion.

The big problem is that the rotary engine does not do this.
Therefore, you get assymmetrical loads...which needle (hell, any roller / BB) bearings do NOT like.
There are diagrams out there that show eccentric shaft (and therefore bearing) loads.
I'd really question how long the bearings are going to last under such conditions.

Do we have an engineer qualified enough to answer those questions?


-Ted

oldschoolbaby

But i`ve seen worn out con rods and they always wear on the top?

Black91n/a

Just because it's not perfect circular motion isn't that bad, so what if it has asymmetrical loads? In "normal" uses there's almost always some sort of asymmetrica load. Centrifugal pumps load up one side due to the thrust loads from the outflow, horizontal things are loaded from gravity and so on.

oldschoolbaby

I was thinking about this just now. I remember a very old suzuki 150cc 2 stroke air cooled engine that was used in a go-cart. This engine vibrated a lot because of the bigger clearences on an air-cooled motor. But stil, it had a rollar bearing crank in the same way the new ones do and it ran fine. keeping in mind that the bearing still coped with these vibrations.

BLUE TII

iTrader: (9)

So, Rotomax rotaries are base on Mazda 13B dimensions but use air cooled rotors instead of oil cooled (brings intake charge through the rotor before going into ports.)

Do they use needle bearings lubed by premix?

I know the Aixro XR50 (294cc 50hp wankel) uses a needle bearing rotor.

RETed

This is the bearing load info taken from the "Rotary Engine" book by Kenichi Yamamoto.


-Ted

rotarygod

iTrader: (1)

We get the same types of asymmetrical loads on the bearings of our mud pumps. We run full rollers not only on the crank as mains but also in the connecting rods. There's no problem with it at all and our pumps get up over 2000 hp. I am one of the engineers there (and there's only 2 of us) so I guess I am qualified to answer the question. It's the same principle. The fact that it's a rotary vs a piston is irrelevant. Bearing loads do not need to be symmetrical. Babit style bearings are FAR less efficient. Bearings are the first things that typicaly fail in a pump but this is usually after continuous use for up to 25 years. A babit style bearing can not live this long in the same location. That's the average we aim for when determining bearing loads although bearing quality may shorten that from time to time. That's pretty damn good. The question isn't whether rollers are superior or not. There's no question there. They are all things being equal. The question is can we get one to fit in the small confines of the space available to have a high enough safet factor? Maybe is the best I can answer until I actually do more homework. I can get them custom built too along with an FEA stress analysis for verification but that wouldn't be cheap. The FEA would as I could run it myself. A custom made bearing isn't cheap.

oldschoolbaby

Rotarygod, seeing that you have loads of experience with bearings, do you think the e-shaft and s-gears are hard enough to be used as a raceway?

Rotary-Celica

One other thing to consider is what bearing speed can they handle? Since the bearing speed is rather high in a rotory.

rotarygod

iTrader: (1)

I don't see a problem with the e-shaft or stationary gears being hard enough. You can always get them hardened to a different spec if that's a concern. However typically when we harden our eccentric shafts on our pumps (they can weigh up to 20,000 lbs!!!), we do it pre-machining of the eccentric lobes. We try to hold a +- .0015 tolerance over the entire machined surfaces and heat treating them after they are machined would alter this. Our tolerances are this tight because we use rollers. We could get away with less if we used babits.

Bearing speed is important to think about. Jsut because we can physically find a bearing that fits where we need it to doesn't mean it will do what we need it to. Bearing stresses rise exponentially with an rpm increase. There comes a certain point where just a little more speed is a ton more stress. We may find a bearing that fits fine but won't live long above a low rpm limit. Let's say 5000 rpm. It may be fine below this but not above. That wouldn't be too useful. Max bearing speeds that I have seen that may be functional are rated at up to 11,000 rpm. The rpm is fine but the next question is whether or not the stresses at the rpm limits we want exceed those of the bearing. I prefer to have a safet margin in there and I also prefer to calculate it based on continuous duty. Even though we won't be at redline constantly, it would be nice to have a bearing that could do it. There's alot to think about and then you need to calculate it all out.

I don't have the time to figure this all out. I guess I could sit down one night and create a virtual rotary engine in Inventor. I'd have to get the cross section of the rotors correct though but cutting one apart and miking it. That isn't hard. I've got spares. Then after a virtual is created complete with material specs applied to it all, I can use Finite Element Analysis to look into it more. This would take awhile though and I really don't have the time. If I used the FEA software in Inventor it would only tell me stresses in the e-shaft. For bearing stress in the bearings I'd need to use ANSYS but I only have that at work.

It's possible to do but time consuming. I have the ability. Just not the time. The easiest thing to do is to pick the toughest bearing you can find that will fit where you need it to and try it. Pray it works.

I'll try to get some pictures of a couple of our mudpump eccentrics and bearings here so you can get an idea of how really large these things are.

TonyD89

iTrader: (10)

The main idea of this thread, roller bearings for performance, has been really interesting. I hope I'm not becoming obsessive but I find myself thinking about the idea a lot. As other people have stated, I've seen countless small piston engines (most two cycle) that use them at high RPM and no sump or oil pressure.

Here's a thought I want to throw out there.

If one was able to make a full roller rotary, two piece crank with a center bearing, couldn't you also decrease the clearances of the rotor. I'm not just talking side clearance. I got flamed once for suggesting that the rotor face to trochoid was about 1mm. What is it then? Does anyone know? But maybe you could use .25 mm for a face clearance and instead of .010" at the tips for side clearance you could bring it in to maybe .005"? Mainly because it would now be locked in and not flexing. This would really help high RPM operation since the seals become less effective at high RPM.

Of coarse this brings up the fact you would have to make your own rotor. Even so, the thought about this roller bearing thing led me to think you could really tighten it all up. I believe the limiting factor on face clearance would be the clocking of the rotor by the side gear when changing from front-lash to back-lash. You have to have clearance to run. This would prevent an even more precision fit.

rotarygod

iTrader: (1)

I found the relevant needle rollers from Timken. Here's a link to their catalog along with descriptions, dimensions, and load ratings:

http://www.timken.com/products/beari...20Bearings.pdf

dj55b

iTrader: (2)

this might be a far strech, but why not use a planetary roller system like in the rotrex supercharger. Some radical modifications will need to be done to adapt it in there but might work

ronbros3

I think one of the main probs with the single peace shaft is it flexs to much , can you imagine how all those taper joints and splines would be under stress flex ,they would loosen up and cause other probs. I think just better material in a 1 pc shaft would help. note: yrs back porsche tried some roller brg engines they caused more trouble than all the expense involved. Ron

rotarygod

iTrader: (1)

If a roller bearing is a problem, the real issue is that it was a poorly chosen bearing for the job. It's as simple as that. As I said, I'm not sure how easy it would be to integrate one in properly. This assumes that one can be found that works good and I'm not sure one can that will fit within the small space that we have.