Vibration for mechanics

A vibrating object has three properties:

Speed. How fast is the object traveling,
Distance. How far is it traveling (displacement), and,
Acceleration. What acceleration forces does it have (G force).

How fast (IPS) or inches-per-second is the standard vibration measurement as it is rpm independent. A 2 IPS vibration is 2 IPS at 100 rpm or 10,000 rpm. This makes comparison easier.

1. IPS is how often an object oscillates or moves back-and-forth from the neutral position to its upper and lower extreme limits.

2. Displacement measures the change in location. An earthquake can have a low IPS but a large amount of displacement.

3. The forces of acceleration or G forces often define the loading from the vibration. I recall one aircraft accident where the G forces from vibration from the airframe was of such intensity that it kept the pilot's eyeballs from focusing on the instrument panel!

Pendulum absorbers "counterweights" are used on aircraft piston engines to absorb torsional (twisting) vibrations caused by the firing of the cylinders. Shown here are two sixth order absorbers.

Typically six cylinder Lycoming and Continental engines use two sixth orders absorbers on the rear of the crankshaft. In the center there may be a 4th, 5th or 6th order.

Although "counterweight" is the street term it is entirely incorrect and adds confusion as to their function. They are bifler pendulum absorbers. By bifler we mean hung like a pendulum from two points. The pendulum swinging is tuned to the frequency they need to absorb. Their mass then swings in opposite to the vibration thereby absorbing it.

Order is the number of vibrations per revolution. A 6th order vibration is one that vibrates 6 times per each revolution.

An imbalanced rotating shaft has a 1st order vibration as the imbalance crosses the same point once each revolution.

Each cylinder in a 4 cylinder 4 stroke engine produces a combustion event once each 4 revolutions. Each cylinder produces a 4th order vibration.

A 6 cylinder 4 stroke engine produces a 6th order vibration.

Vibrations can be dampen or absorbed

Dampeners consume the energy of vibration and turn this energy into heat. Common vibration dampeners are rubber mounts and shock mounts.

Absorbers use energy. Helmets and modern cars absorb impact energy by crushing and changing shape. Pendulum absorbers use the vibration energy to accelerate a heavy mass. This acceleration is tuned to be equal and opposite to the vibration - thereby reducing vibration.