Vibrations & Balance by John Schwaner

Table of Contents

1. Warning and application
2. What causes vibrations?
3. What is balancing?
4. Vibration Orders
5. Orders defined
6. IPS defined
7. Dynamic and Static balance defined
8. Degrees of freedom defined
9. Crankshaft balance
10. Articles of vibration and balance
11. Book and article references

Cracked cylinder barrel caught before the cylinder blew off of the crankcase. Another good reason to carefully inspect all oil leaks!

An engine that exhibits unusual vibrations may be failing. If your engine has an unusual or out-of-character vibration, have a mechanic inspect the engine before flight. Only after the basic health of the engine has been established should you proceed to problems with balance. Something as simple as malfunctioning hydraulic lifters can cause engine vibration. 

• Unbalanced rotating parts
• Unbalanced reciprocating parts
• Combustion cyclic variations
• Cyclic variations in the inertial force of reciprocating parts
• Variations in torque and torque reaction

   

Vibrations require a restoring force. This is provided by Hook's law where strain creates a stress. Vibrations do not continue forever because there is a certain amount of internal dampening called "hysteresis". Hysteresis represents the error in "Hook's law".
During normal operation, the chief cause of vibrations is fluctuating engine torque -more so on four cylinder engines than six cylinder engines since the torque impulses are further apart on four cylinder engines. The turning force (torque) imposed on the crankshaft occurs on each power stroke and causes uneven torque loading because internal combustion engines are pulsating flow devices.
Torque vibrations are transmitted to the engine frame primarily from the cylinder walls, and, to a lesser extent, from the crankshaft. Torque is made up of two forces: inertia and gas pressure.

Balancing refers to the balancing of inertia forces acting upon engine parts. For balance to be achieved, the vector sum of the inertia forces must be zero. This means that for every force there must be an equal and opposite force (Newton's Third Law, Conservation of Momentum).

6th order minor component
Less than 1% inertia. Less than 1/2 of 1% connecting rod couple.
6th order major component
Gas pressure (torque)
6th order related orders
3,6,9,12
6th order cylinder influence
none
6th order note Counterweights (pendulum absorbers) absorb 6th order torque. Vibration measurements may vary depending on manifold pressure and engine rpm. Especially with high manifold pressure at resonant rpm (approximately 2400).
Bifilar Pendulum Absorbers (common name Counterweights) in Lycoming engine

Velocity is arrived at by simply integrating the signal that an accelerometer produces. If you integrate a second time, you get displacement.

It has been found through experience that velocity is a better unit of measurement to use if you want the numbers to track with how severe a vibration is independent of RPM. In other words, a 1 IPS vibration is pretty severe no matter if the object is turning at 200 RPM or 200,000 RPM. This is not true for units of acceleration or displacement.

Now, for harmonic oscillation, the acceleration is just omega times the velocity; for, say 2400 rpm (fundamental frequency of 40 Hz), your vibration of 1 IPS corresponds to an acceleration of 20.9 ft per sec^2 or about 0.7 Gs. That does seem a fairly healthy vibration for the front end of an engine with propeller.

It takes on average 65 grams to correct for a 1 IPS vibration. But this value varies greatly from ship to ship. From a low of 20 to a high of 130.

Rotating parts such as crankshafts can be dynamically or statically balanced. Dynamic balance, or balance due to the action of inertia forces, modifies the distribution of mass so that the center of mass is about the principal axis of rotation. At this location the resultant inertia forces of the rotating system is zero. Dynamic balance is performed by spinning the shaft. Since an unbalanced inertial force increases as the square of the speed, spinning the shaft magnifies the amount of imbalance to a measurable level.
Static balance is the balance of forces due to the action of gravity. Static balance involves weight matching and balance beam matching of components. If the center of gravity of a crankshaft does not lie on the mechanical axis, then the crankshaft will turn until the center of gravity is directly beneath the mechanical axis.
The difference between static and dynamic balance is that dynamic balance not only balances inertia forces but also centrifugal couples. Both static and dynamic balance only affects first order vibrations.

Consider an object floating in space. It can roll clockwise or counterclockwise; it can pitch up or down; it can yaw to the right or to the left; in all it is free to move in six directions. This is termed six degrees of freedom. Your airplane has six degrees of freedom. Your engine has another degree of freedom and that is crankshaft position. In other words the crank can be at top dead center, bottom dead center or any position in between.

Even though a crankshaft has external balance, it may not have internal balance. In theory, crankshafts can be perfectly balanced if we assume they are absolutely rigid; but crankshafts are not absolutely rigid; if they were we wouldn't have torsional vibrations and wouldn't need pendulum absorbers (counterweights) and center main bearings. Since the balancing mass is not exactly opposite each crankshaft rod journal, the centrifugal force on the throw creates a load on the bearings even when the crankshaft as a whole is in complete balance. Locally, each crank is unbalanced. The effect is seen in bearing wear where center main bearings suffer more wear than the outward bearings. Anytime the crankshaft loads the bearings it imparts a force on the crankcase causing vibrations to the engine structure.
 

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