Introduction to Vibration

What is Vibration?

• Mechanical vibration is the dynamic motion of machine components.

• Vibration measurement is the measurement of this mechanical vibration relative to a known reference.

How does Vibration Start?

• Energy must be put into the system through an applied force, either internal or external.

• The force may be instantaneous, an impulse, or continuous. In machines this energy is diverted from the process. This reduces the machine’s efficiency and may also result in damage to the machine’s components.

What Characteristics are measured?

• Amplitude
• Frequency
• Phase Angle

1. Absolute Phase Angle
2. Relative Phase Angle

 

What Affects These Characteristics?

• Mass
• Spring Stiffness
• Damping

AMPLITUDE

Peak-to-peak refers to the total amount of vibration.

Zero-to-peak refers to the total amount of vibration from the maximum height of either the positive or negative peak to the zero voltage axes.

Root mean square (RMS) is a function of the signal conditioning performed in the monitor or diagnostic instrument and not the output of the transducer.

Frequency

• Frequency is defined as the repetition rate of a periodic vibration within a unit of time.
• The frequency of vibration (cycles per minute) is most often expressed in multiples of rotative speed of the machine.
• Basic frequency measurements can be performed with an oscilloscope and Keyphasor signal.

Phase

PHASE ANGLE

The phase angle is defined as the number of degrees from the Keyphasor pulse to the first positive peak of vibration.

Form

• Vibration form is the raw waveform displayed on an oscilloscope and can be separated into two categories:

1. Time base presentation
2. Orbit presentation

VIBRATION FORM

ORBIT- is the output of two transducers at 90° angles to one another (XY plane) in the X-Y mode of the oscilloscope. The orbit is the representation of the shaft centerline movement.

Measurement References

The three basic frames of reference for vibration measurements on rotating machines are:

• Rotor motion relative to the bearing (Shaft relative Vibration).

• Casing motion relative to a fixed reference (Casing Absolute Vibration).

• Rotor motion relative to a fixed reference (Shaft Absolute Vibration).

Position Measurements

• Axial Position

1. Thrust Position
2. Rotor Position

• Radial Position

• Differential Expansion

• Case Expansion

• Eccentricity

AXIAL POSITION

Thrust Position

• Measurement of the position of the thrust collar in the thrust bearing assembly and how much bearing material may be used up.
• The probes are mounted on the thrust bearing observing the movement of the rotor position within the thrust bearing.

Thrust Position Normal/Counter

• Normal - direction the rotor would normally be forced to move due to the designed operation of the machine.

Steam turbine typically Away from high pressure (HP) inlet towards the LP inlet.

• Counter - direction of thrust opposite the direction the rotor is normally designed to move.

Indicates the machine is experiencing an abnormal operation 

 

RADIAL POSITION

Proximity Probes are used in the X-Y configuration to measure radial vibration, the dc signal from

the transducer can be used to indicate the radial position of the rotor within the bearing

DIFFERENTIAL EXPANSION

CASE EXPANSION

ECCENTRICITY