Blade balencing

[Jason Wilkinson]

HI to all   Could someone  explain "COUPLE" as pertaining to blade balancing ?  I usually suspend the assembled rotor blades after finding the center , place a small spirit level on the back of each blade at the root and add weights to level them out. But COUPLE i don't understand   

[Flux]

This is really too complicated to explain simply without diagrams but I will try.

If you have a thin disc on a shaft with a heavy bit you can remove part of it diametrically opposite to balance it, if the shaft is on two knife edges it will stay in any position when balanced.

If you have 2 similar discs spaced some distance apart on a shaft and each disc is balanced the thing will still stay in any position. Now imagine you drill a big hole in each disc but diammetrically apart and at the same radius. Both discs are now out of balance but on your knife edge balancing method the thing will still stay in any position, the system is still statically balanced.

If you now rotate this assembly the heavy point of each disc will see a centrifugal force on it and one disc will pull one end of the shaft in one direction and the other end will be pulled the other way. If you have bearings at the shaft ends they will see forces trying to waggle them and this force will increase with speed. A pair of forces acting on the ends of a shaft in either direction constitute a couple.

You now have a system that is statically balanced but dynamically unbalanced. You can't detect this problem by any form of stationary balancing method on bearings or knife edges.

Obviously in this example you can visualise where you need to add a weight to each disc to cure this problem but if just presented with a non " thin" system you can't tell whether it is dynamically balanced or not. If it passes the static balance test and still vibrates when rotating it will have a couple.

There are machines that sense the forces acting on each end of the shaft and measure the angle they act at and from this information you can determine the position of balance weights but you won't have access to this for a normal turbine.

When on the balancing machine a perfectly balanced assembly will have no force on either end sensor. If it is purely statically unbalanced the forces on each end will be equal and in phase. If it only has a couple then the forces will be equal but in opposite phase. If you have static and dynamic unbalance the forces will not be equal and will probably be at some random angle.

I hope this makes sense. Something like a wind turbine blade is normally sufficiently near a single plane that the couples are too small to worry about but the same is not true of alternators where you have a pair of heavy discs separated by several inches.

[Jason Wilkinson]

Thanks  Flux  I'll have to read it over a few times . A thought came to me as regard balancing the magnet disk  to take them to a tyre shop and see if the attendant can put them on the balancing machine , but then again what if the "weight" needed to balance it is just where the magnet is ?   only way to know for sure is to do it
   Thanks     Jason

[tanner0441]

Hi

If your looking to balance the magnet rotors you can remove material from the heavy side as well as adding weight to the light. We used to drill holes in the outside edge of flywheels to balance them rather than adding weight to the other side. If you do need to add weight the drill and tap a hole then caulk lead into it, the thread is not important it is to prevent the lead being thrown out.

Brian.

[SparWeb]

Everybody uses wikipedia these days:   http://en.wikipedia.org/wiki/Couple_(mechanics)

Very broadly, the words "couple", "moment", and "torque" are synonyms.  They do relate to different constructions of things, but basically the concept is the same.

The "couple" implies that there are two forces, acting in opposite directions to each other, and separated by a distance on the object.  Each force will bend its respective arm with a "bending moment".  At the center of the arm is a "torque".  Again, Wiki provides us with a diagram (so I don't have to draw one, but if I did, it would look like this anyway).



In the lower diagram, the bar has two forces "C" which represent the couple, making it want to turn.