Lycoming or Continental Counterweight Detuning

Close-up of Lycoming counterweight showing plates (the part with 3 holes in it) that hold the pins in place. Pin diameter determines the pendulum length and thus the frequency.

"Counterweights" are used on 6 cylinder Continental and Lycoming engines.

Your engine produces power in descrete combustion pulses that twist the crankshaft. Between pulses the crankshaft springs back. If one were to freeze crankshaft rotation so just the vibratory forces were left you would see that the back of the crankshaft rotates in the opposite direction as the propeller! If these pulses are at the same frequency as the natural frequency of the crankshaft then they have the capability of breaking the crankshaft at the fillet radius. In one test of a popular 6 cylinder aircraft engine, up to 20 degrees of crankshaft twist was measured!

Bifilar Pendulums (centrifugal pendulum vibration absorbers) aka "counterweights" pictured above absorb crankshaft  torsional energy produced by the power stroke and eliminates or greatly reduces torsional twisting of the crankshaft.

There is a limit as to how much energy pendulums can absorb before they stop functioning. The graph below (I apologize for the poor quality) shows what happens when they are fed too much energy - Detune is the popular description although I prefer the word "jump".

Bifilar Pendulum Jump Curve

During normal operation the Pendulum(s) operate between line A and B. Within the limits of power the pendulum maintains crankshaft torsional amplitude (twisting) to near zero. The pendulum swings back and forth producing an opposing force to the twisting force thereby cancelling it out. Pretty neat trick.

What happens if we increase the energy beyond C? Energy is increased by increasing horsepower at the resonant  frequency of the crankshaft (lets say resonance occurs at 2,200 rpm in our example). The pendulum "jumps" over to line D and the crankshaft torsional amplitude now is free to twist and untwist putting great stress on the crankshaft. The pendulums are in a state called "detuning". Not only is crankshaft stress much greater but the pendulum itself has stopped swinging on the pins and is instead rattling against its restraint. This can result in circlip and retainer plate failure that releases the pendulum. This is what happened to Cape Air/Nyannis Air Service Inc. in their Cessna 402 (reference Teledyne Continental Motors Service Instruction SSI07-5). In fact it has been found that a "jumped" pendulum can amplify vibration amplitudes.^1.

Once the pendulum jumps (detunes) great stress and destruction is occurring inside the engine without any outward indication to the pilot. But what happens if the pilot reduces the power or changes the rpm? The pendulums stay detuned! Once they jump to D they can only be restored to proper operation A-B by reducing the power to close-to idle.

Wear to pendulum bushings reduces the jump point (A-B). Normal bushing wear (and abnormal fretting wear) inside the engine shifts the pendulum's frequency resulting in not only a lower jump point but a slanting of the A-B line thus allowing more torsional forces. One reason why I don't recommend running pendulum equipped engines past engine TBO. 

Other failures possibly attributable to counterweight detuning:

• Impulse coupling attachment rivet failure

• Crankshaft cracking

• Propeller cracking

• Left magneto oil seal failure on some Lycoming engines

• Magneto drive shaft breakage

• Magneto distributor gear teeth failure

• Oil pump gear failure

What this means to the operator:

In the case of the Cessna 402 with TSIO-520-VB engines, don't operate at 2100 RPM and 27" Manifold Pressure (SSI07-5). 

• Operate within the engine manufacturer's operational envelope.  If I design a device and include a manual of operation, it is "safe" if operated within those boundaries. I have communicated clearly what those boundaries are.

• Ignore well-intentioned advise to operate these engines outside of the manufacturer's power/rpm recommendations.

• Pendulum bushing wear limits the safe operational service life of the engine.
• Modifications to increase engine power output put the pendulums closer to the jump point.
• Modifications to increase engine power output reduce allowable pendulum bushing wear limit.

Notes:
1. http://www.egr.msu.edu/dvrl/pubs/Nester-etal_HI04.pdf

2. Textron Lycoming Mandatory Service Bulletin No 245D

“Rapid opening or closing of the throttle can cause counterweight detuning…To avoid detuning during simulated engine failure, use the mixture control to shut off the engine and leave the throttle in normal open position until the engine has slowed down because of lack of fuel. Then, close the throttle to an idle condition. The throttle being open allows the cylinder to fill with air, maintaining the normal compression forces which are sufficient to cushion the deceleration of the engine. Another result of rapid throttle movement is severe strain on the supercharger gears and associated gears because of the inertia force of the high speed impeller.” 
Further reading:

Reduction of Periodic Torsional Vibration using Centrifugal Pendulum Vibration Absorbers

Article on propeller interaction