The secret revealed: Rotary engine Bob weight calculation.
 

After searching the entire internet for information on how to calculate the bob weights for balancing the rotating assembly, calling and emailing the manufacture of the balancing equipment I will be using and pm'ing multiple people, I have come up with zilch.

I did find that the bob weights are the sum of the weights of the rotor, all seals and the weight of the oil that would be in the rotor while it is running, but I haven't seen anyone reveal the weight they use to compensate for the oil. So before I lose faith in the rotary community I figured I'd give this thread a shot. So please help me out here.

How much weight is added to the bob weights to compensate for oil.

I've never heard of anyone refering to the counterbalance weights as "bob weights". Unless you're going to be building a full high performance race motor, balancing the rotating assy is not neccessary. Just use the correct factory counterbalance weights/ flywheel for your year rotors. If you want to balance the rotating assy for race application or just because you want to then I would recommend sending it out to a professional and don't advise you trying to do it on your own.

Here is some good information from Mazdatrix on rotor compression and weights, identifying flywheels and counterbalance weights. Hope this helps.

http://www.mazdatrix.com/faq.htm

I guessing you have looked at this Page of the Mazda Comp catalog
http://www.who-sells-it.com/cy/mazda.../page-148.html

I was able to witness and help on the balance of my assembly at CLR.
There are also SAE papers on it also, you will need to search the SAE site for them.
That is all I can help you with.

Thanks Jay, but the bob weights or dummy weights are weights you attach to the eccentric shaft to a simulate the weight of the rotors, seals and oil when using an engine balance.

Mazda original balancing for non renesis engines was only to within 50 grams, and I don't believe it took into account the weight of the oil in the rotors while the engine in running. The reason I believe this is I read that part of what makes the renesis engine rev higher is that it is now dynamically balanced to include the weight of the oil. This implies that previous rotary engines did not take this into account.

This will be an interesting discussion. Unfortunately I don't think any of the real pros will confirm or dispel any of the beliefs. I really wish Carlos Lopez would tell us what he does...

If anyone can find that SAE paper I'd appreciate a link...

I think the factory accounted for the oil in their bobweight calculations. Why wouldn't they? This should be described in an SAE paper somewhere...

"Dynamically Balanced Rotors: To further refine the superior balance of the twin-rotor configuration, Mazda shifted from the previous static balance setting and instead adopted dynamic balance calculated from the mass of oil entering the rotors. This improvement, together with the effect of the long span engine mount system (mentioned previously) realizes extremely low vibration during acceleration."

http://www.rotaryengineillustrated.com/renesis.php

So what's the calculation?

interesting

Still working on this guys, I'm getting close though. I'll post on it in the next few weeks.

My factory Pre-renensis cosmo engine has no problems revving to just over 10 000rpm and making 3-4X the power a renesis makes with factory balancing(or the lack of it).

wouldn't the weight of the oil be different at different temperatures? The front rotor has been known to run cooler than the rear, wouldn't that effect things to an extent?

reviving this thread

i'd have to say that most shops came to their own conclusion based off of testing various shaft configurations and the results, averaging out to a consistent rotor weight for each series.

of course that is all assuming that mazda's theory on running rotor weight was correct, but who is to say that neither is correct but both are "close enough". even with factory balancing the engines can easily handle over 10k revs, that is mainly because not much stress is applied to the stat gear bearings in these engines.

I really wish someone could answer the question or at least pm, I live in Greece and no one here has ever balanced rotors. It's not that they don't have the tools for it, but there just isn't any high demand for it. I'm currently building my new engine and I wanted to use a RX8 shaft and also using slightly differently weight rotors in my 13bre.

You can always send it to me and i help you with that ;)

Wankltrim, thanks but I'm not willing to spend about 350euros just for shipping

Mazda's basic formulas were published 40 years ago in the book Rotary Engine by Kenichi Yamamato, one of the leaders of the early rotary engine program.

It can be found in the section "engine vibration" on PDF page 88 of this document on Google Docs: https://docs.google.com/viewer?a=v&p...hl=en_US&pli=1

Yes, but "M" (mass of rotor) does not specify the weight of the oil. Did they consider it negligible?

From my interpretation, yes. This might have more of the answers you are looking for though: http://papers.sae.org/890325/ but you will have to pay $22 for it.

To me it would be pretty hard to account for the weight of the oil--how are you supposed to know the instantaneous mass of the oil inside the eshaft, and spraying on to the back of the rotor? It seems to me that only Mazda would have the instrumentation and proprietary models to account for this. They might be doing it now on the Renesis for example, but clearly they did fine in the past with considering it negligible.

In one of Mazda's papers there is brief discussion of how they balanced the R26B in the 787B race car. No mention was made of the mass of the oil. That doesn't necessarily mean that they didn't account for it though.

mazda either filled all the pockets with oil one by one, weighed the oil and accounted it's total volume or omitted it completely from the equation.

i would have to say that the oil is slung out of the rotor just as fast as it enters it, equating to no additional weight over initial rotor weight.

i have been contemplating this also as i am planning on putting together an e-shaft balancer for the 26B shaft.

Back from the past,

anyone found a recommendation on the amount of oil for particular engines? (20B, 13B, 12A)

Worst thread title ever.

That said, I would assume that the rotor is constantly full of oil, because of centrifugal force. Even though it's orbiting, it's still spinning at 2000rpm or whatever. I've heard people say that the front rotor starves for oil when accelerating and the rear one starves when braking, but given the rotational forces, I don't really see that happening.

What I think is wierd is that every documentation I was able to find says to take the weight of all the seals into account, including the apex seals. The people that do rotary engine balancing therefore also need to know what kind of apex seals your using, since the weight difference between steel, carbon or ceramic seals is pretty huge. But when you think of it, when the engine is running the apex seals ride against the rotor housing, and are not attached to the rotor, the rotor just moves the apex seals around, so I think that alone causes a big error. In fact, the seal springs kindoff push the rotor away from the rotor housing. What it boils down to is that a large part in engine balancing is probably based on experience, and using what has worked in the past. Which is what I did too with my bob weights, I found out what has worked for others, and then checked if it's accurate by weighing the rotor, the rotor bearing, all the seals, and I actually filled the rotor with oil and weighed how much oil fits in there. The 2 matched up within a few grams, so I'm confident that I'm on the right track.

^yes.

The apex seals and springs should be omitted, as they are not radially supported by the rotor. As for mass of oil, I cannot give an answer other than to mention that not much oil stays in the rotor at any given time, and that the assumption that centrifugal force keeps the rotor full is not correct. I will try to find a publicly available information source if I can.

Thank you for your input!
I can just agree what you say and I know that the figures differs on the type of the seals you use. What I would like to know is the amount of the oil inside the rotor itself to add it to my own calculations. I have measured the amount myself and I have numbers, however, I just would like to double confirm.
If you don't mind to share your figures (PM is welcome) the info will be much appreciated.

REgards,