Lu,
Not all of the N per rev. systems are dynamic. On the 206B the isolation mount reduces the lateral 2 per rev generated by the 2 bladed m/r system. The isolation mount is nothing more than a rubber shock absorber.
As the Sultan pointed out 2 bladed rotors have a 2 per rev and 3 bladed 3 per etc. I don't know if the reason is due to the traveling wave or not but this same phenomena can be measured in ground based fan systems. This is also known as the "Blade Passage Frequency"
In addition to the BOB weights as you called them the (Bell calls then Pendulum Absorbers), the 222 employs a Nodel Beam System to reduce the vertical 2 per rev's and a nose Frahm damper to reduce the lateral 2 per rev's. All three are dynamic but only one is mounted on the rotor. Which one reduces the traveling wave?
In addition to the SPA's (Simple Pendulum Absorber) you mentioned on the Bell 412 they also employ a nose Frahm dampner for lateral 4 per rev. reduction. The 412's you saw without SPA's were probably Agusta made as they have some customers that would rather live with the payload gain than having a smoother ride and the associated payload loss when the SPA's are installed. Weights haven't been added to m/r. The 407 doesn't have SPA's but does have the Frahm damper installed on top of the rotorhead.
Sikorsky uses the Bifilar on the 76 to reduce the 3 and 5 per rev's and a mass absorber similiar to the Frahm to reduce cabin 4 per rev's on later models. On the S76A model they use the VTA (Vertical Trim Amp.)to reduce the vertical 4 per rev. This unit mounted in the nose uses a combination of hydraulics and electronics to perform the magic feat.
The Aerospatial (ECF) AS350/355 employ the spring mass on top of the head to reduce lateral 3 per rev's and then hammers under the pilot's seats to reduce the vertical 3 per rev. The hammers are just a weight mass mounted on a flat spring and tuned to reduce the verical N per rev. of the m/r.
I don't know if I agree with your statement regarding loss of lift as the blade passes over the longitudinal axis. If this happens as you say then on a two bladed rotor one would think you would feel a 1 per rev. and on a 4 bladed rotor you would also experience a 1 per rev. In flight a 1 per rev. that is not a result of mass imbalance is usually caused by the unequal lift of one m/r blade.
Based on your theory of the traveling wave, what would cause a 2 per rev. imbalance on a Bell 412?
With respect to your 2 per rev in engine to transmission driveshaft, most any driveshaft that is misaligned will show a 2 per rev of shaft speed. I doubt the 2 per rev caused the grease loss but more likely was caused by burping the grease from the seals due to misalignment.
Chuckolamofola