wreckerx7

Hello!
I was hunting around online looking a different rotary overhaul and performance shops. Rotorsports Racing and Engine Balancing LTD in NZ caught my eye, in particular, their nicely detailed write-up on how they balance the moving parts of a rotary.
Having some experience with balancing piston engines, (admittedly somewhat different, but balance is balance) I found myself somewhat skeptical of their method.
Here's the link to their balancing info:
» Rotary Engine Balancing Rotorsport Racing & Engine Balancing Ltd (Previously Rotary Power)

I was surprised that bob weights are used for dynamic balancing, and especially surprised that the bob weights are weighted to duplicate the individual rotors they represent.
I'm no engineer, but it seems to me that if a given point is considered. (let's say one where an apex seal is in the 12 o'clock position, halfway through the intake cycle) there will be three different conditions of balance (one for each rotor corner) for each of the three shaft rotations required to swing a rotor around 360°, assuming the average rotor has unequally weighted 'corners' due to normal manufacturing variation.
Given there are three different conditions, it seems to me that fixing a bob weight on the eccentric shaft is only introducing significant error, as the bob weight does not mimic the actual motion of the rotor.
Further, I think that even if an elaborate apparatus could be devised to allow such a bob weight to mimic a rotor's motion, and an imbalance were noted, the rotor would have to have weight added or removed, not the eccentric shaft, due to its 1:3 rotation ratio-- the shaft cannot be changed to account for rotor imbalance.
It would seem that this alone makes the effort to weight the bob weights to duplicate the rotor balance condition completely unnecessary, as the bob weights are also unnecessary, and in fact, detrimental to dynamic balance.

Given the dynamic relationship between rotor and eccentric shaft, I can only see that the shaft must be balanced separately from the rotors and counterweights. The rotors must be statically AND dynamically balanced separately themselves, similar to how one can statically and dynamically balance a car's wheel.
After this, the total rotor weights can be duplicated by bob weights (there will be no difference in the number of washers on each bob weight screw as the rotors are already 'corner balanced' and no longer present three different conditions of balance) which are then installed onto their respective eccentric lobes. The counterweights are installed and the assembly's static balance is then checked with material removed from the counterweights as required to achieve stability in any position. (like statically balancing an aircraft propeller) This would indicate the counterweights now match the rotors in effective weight. The assembly as a whole is now statically balanced.
At this point, (I'm starting to run out of 'smarts' here) I believe with the bob weights removed, the shaft and counterweights should still remain statically balanced. Perhaps now a dynamic balance test can be performed with material removed from the counterweights to achieve dynamic balance in them, too. I'm not entirely sure of this point, but it 'feels' right. If yes, the whole assembly is now dynamically balanced.

I believe only in this way can the shaft, rotors and counterweights be balanced completely.
To me it seems that RR&EB might be operating their balancing procedure according to unbalanced theory!

Any opinions on this?

bumpstart

IMO.. no method is 100% right till it accounts for the weight of the oil inside the rotor .
bit concerned when they find one balance is 6.51 grams out.. and the other 6.50 grams out
.. yet they have spun the assembly with simulated bob weights instead of the rotors with some method of compensation for the oil weight inside .. i guess maybe 6.5 grams worth

i guess they use the simulation masses as a method of remaking you a cloned rotor should only one fail and need replacing .. so saving a costly full pull down and rebalance

wreckerx7

That's an interesting point! I bet if the oil areas within a rotor are symmetrical, AND it was known that these areas would always be full of oil, then I don't think oil would be a factor to individual rotor balance, static or dynamic.

However, when considering the balancing of counterweight mass, the weight of the oil within the rotors definitely does play a role!

The trick would then be to somehow measure/calculate the volume of oil in these areas and add its mass to the bob weight total. If the rotor internal oil areas are not symmetrical, corner to corner (to corner), that would certainly complicate things quite a bit more.

score +1 for bumpstart!

John Huijben

iTrader: (1)

Your probably overthinking it a bit. Rotors are balanced individually without accounting for oil (If they get balanced at all, not everyone even bothers). When dynamically balancing the rotating assembly the bare rotor weight is taken, then an X amount of mass is added for seals and springs and oil. Mazda motorsports published the amount of mass you should add for this in one of their tech articles.

If you ask me, balancing everything to 0.1 grams is complete bogus. The amount of oil in each rotor varies with different engine conditions, and what about carbon buildup, and the apex seal springs pushing against the rotor. OEM rotors can vary over 50 grams in weight, and they seem to do just fine without a rebalance.

Not saying balancing isn't important, it is! But I would just go with the tried and tested method.