Section 2 Rotating machines

2.1  Assessment criteria

The assessment criteria are given in Figure 7.2.1 Rotating machinery assessment .

2.2  Scope

Assessment of vibration severity for rotating machines above 10 kW.

2.3  Vibration severity

The severity of rotating machinery vibration is defined by the root mean square (r.m.s.) value of velocity for frequencies up to 1000 Hz.

2.4  Position and direction

Measurements are to be taken at or adjacent to each main bearing in two orthogonal radial directions and in the axial direction, all referred to the shaft axis. Care must be taken that the measurements do not include local resonance effects.

The assessment categories given in Figure 7.2.1 Rotating machinery assessment apply independently to each measuring direction.

2.5  Machine classes

Class I: Individual parts of machines, integrally connected with the complete machine in its normal operating condition. Production electric motors up to 15 kW are typical examples of machines in this category.

Class II: Medium sized machines (typically electrical motors with 15 to 75 kW output) without special foundations, rigidly mounted engines or machines (up to 300 kW) on special foundations.

Class III: Large prime movers and other large machines with rotating masses mounted on rigid and heavy foundations which are relatively stiff in the direction of vibration measurement.

Class IV: Large prime movers and other large machines with rotating masses mounted on foundations which are relatively soft in the direction of vibration measurement (for example turbo-generator sets, especially those with light weight sub-structures).

2.6  Rigid and flexible supports

The distinction between “rigid” and “flexible” supports is defined as follows:

• Rigid support: the natural frequency of the predominant vibration response mode is greater than the excitation frequency
• Flexible support: the natural frequency of the predominant vibration response mode is less than the excitation frequency.

Where the support stiffness cannot be classified the assessment should be based on the “rigid support” values.

2.7  Guidance on assessment categories