MRO Magazine

Vibration Basics

To understand how vibration manifests itself, consider an electric motor.

To understand how vibration manifests itself, consider an electric motor.

Rotation: The motor and shaft rotate around the axis of the shaft, which is supported by a bearing at each end. One key consideration in analyzing vibration is the direction of the vibrating force.

In our electric motor, vibration can occur as a force applied in a radial direction (outward from the shaft) or in an axial direction (parallel to the shaft).

An imbalance in the motor, for instance, would most likely cause a radial vibration as the ‘heavy spot’ in the motor rotation, creating a centrifugal force that tugs the motor outward as the shaft rotates through 360 degrees.


A shaft misalignment could cause vibration in an axial direction (back and forth along the shaft axis), due to misalignment in a shaft coupling device.

Amplitude: Another key factor in vibration is amplitude, or how much force or severity the vibration has. The farther out of balance our motor is, the greater its amplitude of vibration.

Other factors, such as speed of rotation, can also affect vibration amplitude. As rotation rate goes up, the imbalance force increases significantly.

Frequency: Frequency refers to the oscillation rate of vibration, or how rapidly the machine tends to move back and forth under the force of the condition or conditions causing the vibration.

Frequency is commonly expressed in cycles per minute or Hertz (cpm or Hz). One Hz equals one cycle per second or 60 cycles per minute. As the machine operates, it could be vibrating in multiple directions (radially and axially), with several rates of amplitude and frequency.

Imbalance vibration, axial vibration, vibration from deteriorating roller bearings and more all could combine to create a complex vibration spectrum.