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).
