goldedge

Hi I have just had the rotating assembly balanced for a 12a race motor,

the excentric shaft and counterweights had been balanced previously,

the rear rotor gear was damaged so the rotor has been replaced with

a new rotor 40gms lighter than the existing front rotor.



The engine balancer has added 40gms weight to the front counterweight

and not touched the rotors or the rear counterweight at all leaving the

front rotor 40gms heavier than the rear rotor, does this sound right?



rotor weights front 4580gms rear 4540gms



Regards

Michael

Lynn E. Hanover

I know just enough about balancing to make a fool of myself, so here goes:



The rotors balance each other. Period. So they must weigh the same.



Simple.



The counter weights take out the rocking couple caused by the rotors not being in the same plane. This is the ends of the crank wanting to wiggle because of the rotor offset. Notice that the counterweights are not even close in weight to the rotors. They don't need to be.



Adding the weight to the counterweight will static balance the system, (at rest it will balance) but will not balance it dynamically. (spinning)



If you spin it up to 9,000 RPM a good machine should still show it as out of balance. The bobweights should be adjusted to carry the extra weight of the oil in the rotors. Otherwise the counterweights will appear to be too heavy.



A balancer with extensive rotary experience should be consulted.







Lynn E. Hanover

diabolical1

i think that's the bottomline.

goldedge

Thanks for the reply Lynn,

( and your many other excellent posts :-) )



I have spoken to the engine balancer and he tells me that the 40grams

difference between is within spec and that the added weight to the front

counterweight corrects for this accurately.



The engine will most likely not exceed 9000rpm more likely to be used

at 8500rpm max so this std of balance may be OK? (semi stock pro7 class).



The engine builder is reputed to balance most of the race rotary engines

in this area.



I have been told that the balancing of individual rotors can be a very long and

expensive process? by another balancing company.



I have also seen reference to Mazda allowing upto 50grams difference

between rotors as their acceptable standard ie 50gms between rotor grades

A-b B-C etc ( not sure as to the accuracy of this ).



Personally I would prefer no weight difference, how would you recommend

balancing a modified rotor to ensure it was balanced correctly after lightening

etc.



I am guessing that you would need to find and maintain the center of balance

of the rotor while ensuring that the weights at each rotor corner on both sides

was identical in at least 6 outer apex corner positions?



Does any one know how much an Rx8 rotor can be lightened

/ modified to lower compression slightly for Turbo use?



Regards

Michael McCarthy

89 Rag

Kiwi isn't helping you with this problem?

Lynn E. Hanover

Well,

a penny weighs about 3 grams. put 13 pennys in a sock and have a friend hit you in the head with it.



Feel like too much now?



It sure does.



I have balanced my rotors for years using home made balance beams. If you loose one rotor of a balanced set, you need to get a new rotor back to the weight of the good rotor lest you loose two rotors.



A piece of tubing, or a 2 X 4 rocking on a 10 penny nail on your bench is easy and fast to reproduce anywhere. Or two nails hammered through the boad to act as the low friction fulcrum. A tape measure and a pencil. No talent involved. A drill press and some time. Put a stop on the press to limit drilling to just the drill point depth. Practice on a junk rotor. It is easy. Blow off the chips each time.



drill a bit from about the same place on the three points on the rotor. Alway drill all three corners. If you need more off, turn it over and go again. The rotor stays balanced on its own axis so long as you dril all three points each time. If you build a nice beam from square tubing with two pointed bolts as a fulcrum you can see a paper clip added to one rotor. Its just too damn easy not to do it.



If you screw up and drill through the rotor, (I know you won't after all of that practice with the junk rotor) just TIG the hole shut and keep going. there you are identical rotor weights.



Engrave each rotor with some system of numbers or whatever to make them a set.



Lynn E. Hanover

goldedge

The weight at 40grams is also equivalent to 2.5 aa batterys

( I have the bump on my head to prove the effect), thanks Lynn :-).



Thanks for the info re using balance beam in the above post.



I can weight match a pair of rotors using a balance beam with a center pivot,

my concern was to make sure that the balancing is even and that the rotating (dynamic ) balance

is kept correct so that the forces in rotation were still centered in the same place on the excentric shaft.



Wouldn't it be advisable to remove an even amount on both sides of the rotor, that is from all six corners to

maintain the balance through the vertical center of the rotor? or is this not neccessary?



I am assuming that the weight distribution at the front and rear of the rotor should be kept even so that

the forces acting on the excentric shaft are spread evenly and the factory center of balance maintained?



Regards

Michael

goldedge

Thanks for the suggestion :-)



One of the problems with this type of question is that many engine

builders do not want to share information that may help the competition.



I have not approached them but will keep it in mind, I assume they have their

preferred balancing company.



Regards

Michael

goldedge

Helpfull balance info link.



http://www.mazdamotorsports.com/webapp/wcs...bject=balancing



More engine prep info.



http://www.mazdamotorsports.com/webapp/wcs...h?storeId=10001



Regards

Michael

Lynn E. Hanover

The ideal is always the objective.



However, you are say 40 grams out of balance. Even if you took all 40 grams off of one side of the heavy rotor it would be way better than leaving the 40 gram imbalance.



So, you drill in the three corners every time. That maintains the dynamic (spining on its own axis) balance of the rotor. Now it does move the actual center of mass away from the newly drilled side.

Butt, you are removing part of a gram from a 9 pound rotor. It is impossible to detect this change with any equipment that I know of.



So once you get close with your home made scale, you still drill all three corners even if it makes the heavy rotor into the lighter rotor. For example, you started 40 grams heavy. You get to within a half gram, but in the last drilling sequence you have drilled only two of the three holes to get dead even weights. Go ahead and drill the third corner in the last sequence. (and keep track of what corner that should be)



This rotor will now be lighter than its mate, by a microscopic amount, but it will still be in balance on its own axis.



Keep flipping the rotors from end to end on your scale so as to make sure you can still see the difference in weights. This to check the accuracy of your scale.





You are now doing it better than Mazda did it.



A 6 foot 2 X 4 with two 3" drywall screws through the center line works just fine. Keep the screw points on a piece of steel sheet stock for a near perfect zero friction bearing. Allow only an inch of travel total at the ends.



You can get fancy with screws to locate the rotors each time, or steel or aluminum tubing (rectangular) and so on. It must balance with nothing on it. The weight of a penny should move it profoundly. You can tape a penny near the center to bias the scale to zero, as the wood will absorb water more from one end than the other from day to day And you can see it change.







Lynn E. Hanover