Rotating Assembly Balancing (Rotor Weight?)
 

Hi everybody

My first question here and also my first Rotary Engine build.
I want to do this the right way (for me) from the beginning. I know it be a bit overkill at some points.
The way to build is also important to me not only the finished engine.

This 13b Engine I’m working on is for a race car.

It takes me some time. I plan to do any step of my build by myself.
Next step is the balancing of the rotating assembly.
I have created a Balancing Machine (with Dynex Impulse kit) for this Job. It works very good on the lightened Rotors.
So, I go forward and put the E-Shaft with the other rotating Parts on the pedestals of my balancer.
https://cimg5.ibsrv.net/gimg/www.rx7...d35a696b26.jpg
https://cimg6.ibsrv.net/gimg/www.rx7...9471c4f9f1.jpg


But now I’m struggling to find out how much the bob-weight for the Rotors should be.
I tried to find any available information for calculating the needed weight. Unfortunately, I not found clear information on that.
I count the different weight of the Seals and Springs without the Apex Seals and Springs.
https://cimg6.ibsrv.net/gimg/www.rx7...ac6f8795b8.png
Kenichi Yamamoto wrote in his Book from 1981, there will stay only a small amount of oil in the Rotors.
How much oil is it?

Are there some other variables like friction I must add to the calculation?

I do some test-balancing on a S4 NA assembly. The only complete OEM set of parts that lay around. My plan was to find out roughly what Mazda was thinking on choosing the counter weight and flywheel. I add to my bob-weight more and more shims to bring the assembly in balance. The result was really not what I expected. Finally, my bob-weight is slightly over 5200g. I was thinking it should be between 4950g to 5000g.
What did Mazda calculate?

Maybe my bob-weight design is not ideal?

I understand that maybe some Pro-Engine-Builders would not share his own calculation-path. Are there eventually more technical Information’s around like Mazda/SAE technical Papers on that topic? Maybe I haven’t found them till now.

Someone please gives me an advice for that. I know it would be possible to send out my parts for balancing them. That is not what I want.

Thanks in advance for help me
Regard Reto

The 1997 Mazda Competition Catalog indicates for a 13B: a rotor weight of 4920 grams with a maximum rotor unbalance of 50 grams, a dummy weight of 5474 grams, dummy weight offset from E-Shaft centerline toward gear side of 2.70 mm and an rpm of 720.

link to said catalog http://foxed.ca/rx7manual/manuals/97CatComp.pdf

Thank you for that information’s, guys.

Luckily I found this page few days ago. I maybe overread this chapter last time I go over the Tech Tip pages.



Do someone know what the base are for the Competition Rotors?

13b looks pretty heavily and the 20b's impressive light. But for both Rotors are a additional weight of 554g added to the Rotor weight (without bearing).



New calculation for me:

4150g Rotor with Bearing

- 254g Bearing

+ 554g Additional Weight

= 4450g Dummy Weight



Maybe the webbing inside my S5 Turbo Rotors is a bit different. I think I can use this 554g. Oil volume should not make a big different.

Can someone confirm that?

I thought 20B would be the same as 1989-2002 rotors. 1986-1988 rotors are heavier and 1974-1985 rotors are heavier still.

not understanding what you did above, this should be your dummy weight calculation:


which would be after all lightening/machining/fitment procedures are completed.

I wouldn’t get hung up on the weights listed there. What you have is all that matters. It will obviously vary between a high rpm max lightened with ceramic seals for NA vs thicker high boost turbo with steel seals and such. I suspect that’s the difference on the 20B listing.
.

Apex seals should not count for balancing, I don't think. They move with the rotor but they don't hang off of it as it rotates.

The largest question I would have would be how much oil is inside the rotor with the engine running. My gut feeling is that centrifugal force keeps the rotor full, but I am not a balancing expert. Apparently this factor is where people keep their secrets. Either way, the amount of oil may change when the rotor is lightened, if you remove a significant amount of iron from the internal webbing of the rotor. It's like the opposite of drilling a hole in a weight to add a slug of tungsten, you are adding the weight of the tungsten but you are also removing iron, so the actual weight change is only the difference in density from iron to tungsten for that bit of volume.

I wonder if you could approach the bob weight question from the other direction. Could you send an assembly to an experienced builder for balancing, and then use your balanced assembly to find the correct bob weights needed for your balancing machine?

Probably I understand the "Tech Tips" wrong.

I understood the “Note by the Dummy Weight” that they add the Bearing-weight with the Seals and Springs to the Rotor weight.

So it would be more logical to me to add so much weight (554gr) to the Rotor weight itself.

Seals and Springs are not that much heavy.

That's what I thought about the Apex-Seals. They are only riding on the Rotors.

Yes, this is also an option.

Unfortunately the shipment to overseas would be expensive.

I don't think there are good shops with experience on balancing in Europa, especially in Switzerland.

there are two 20B engines. the 20B that came in the Cosmo, which used the 1989-1991 turbo rotors
and the 20B that was available over the counter as a race engine, which used unique rotors/bearings. we could say, its AKA, 13G