Vibration sources in rotating machines:
Vibrations Sources | Disturbing Frequency fe Hz |
---|---|
Primary out of balance | 1 x rpm x 0.0167 |
Secondary out of balance | 2 x rpm x 0.0167 |
Shaft misalignment | 2 x rpm x 0.0167 |
Bent Shaft | 1 & 2 x rpm x 0.0167 |
Gears(N=number of teeth) | N x rpm x 0.0167 |
Drive Belts (N=belt rpm) | N,2N,3N,4N x 0.0167 |
Aerodynamic or hydraulic forces | (N=blades on rotor) N x rpm x 0.0167 |
Electrical (N=synchronous frequency) | N x rpm x 0.0167 |
Typical support natural frequencies (fn) for vibration sources:
Structures | Natural Frequency fn Hz | Isolator Frequency fni Hz | Isolator type |
---|---|---|---|
Suspended concrete floor | 10-15 | 3-5 | Helical, Air Springs |
Ground Floor | 12-34 | 6-8 | Helical, Air Springs, Elastomeric |
Soft Clay | 12 | 6-8 | Helical, Air Springs, Elastomeric |
Medium Clay | 15 | 6-8 | Elastomeric Isolators |
Stiff Clay | 19 | 8-10 | Elastomeric Isolators |
Loose Fill | 19 | 8-10 | Elastomeric Isolators |
Very dense mixed grain sand | 24 | 10-12 | Elastomeric Isolators |
Limestone | 30 | 10-12 | Elastomeric Isolators |
Hard Sandstone | 34 | 10-12 | Elastomeric Isolators |
Significant problems occur when the disturbing frequency fe is near to or coincident with the natural frequency of the supporting structure (floor, foundation or subsoil).