Library / Driveline and transmission / DRV-17
Driveline and transmission
Order excitation frequency
Frequency of the kth order of a rotating shaft: k times its rotation frequency.
- Ref
- DRV-17
- Rev
- v1
- Field
- Driveline and transmission
- Result
- forder [Hz]
Orders are multiples of a shaft’s rotation frequency n/60. The kth order has frequency f = k n/60, with n the actual speed of the source shaft in rpm. A four-cylinder four-stroke engine fires twice per revolution, so its dominant vibration is the 2nd order; gear mesh, imbalance and misalignment appear at other fixed orders. Because an order moves in proportion to speed, it can cross a structural resonance during a run-up, which is what a Campbell diagram shows.
Frequency of the kth order of a rotating shaft: k times its rotation frequency. Engine firing, gear meshing and imbalance all show up at fixed orders of a shaft speed.
Calculator
- Rotation frequency, n/6025 Hz
- Speed at which this order reaches 100 Hz1,500 rpm
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- The 4th order at 1,500 rpm is at 100 Hz. A structural resonance at that frequency is excited as the engine passes through this speed; a 100 Hz resonance would be excited by this order at 1,500 rpm.
- Each extra order adds one rotation frequency: at 1,500 rpm the 1st, 2nd, 3rd and 4th orders sit at 25, 50, 75 and 100 Hz.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (Hz) | Local step | Change in result |
|---|---|---|---|---|---|---|
| k Order number | 1 to 20 | 25 | 500 | 475 | +10% | +10 % |
| n Shaft speed | 300 to 6,000 rpm | 20 | 400 | 380 | +10% | +10 % |
Sweep of Shaft speed
| Shaft speed (rpm) | Excitation frequency (Hz) |
|---|---|
| 300 | 20 |
| 1,013 | 67.5 |
| 1,725 | 115 |
| 2,438 | 162.5 |
| 3,150 | 210 |
| 3,863 | 257.5 |
| 4,575 | 305 |
| 5,288 | 352.5 |
| 6,000 | 400 |
Sweep one variable
Variables and how each one contributes
Excitation frequency
What the equation gives you. Everything else feeds it.
Order number
The order number, a plain number: 1 for once per revolution, 2 for twice, and so on. Raises the frequency in proportion. Raising it by 1 (to 5) moves the result by +25 %, from 100 to 125 Hz.
Shaft speed
The shaft speed of the source, in rpm. Raises the frequency in proportion. Raising it by 10 % (to 1,650) moves the result by +10 %, from 100 to 110 Hz.
Worked example with the values above
- Rotation frequency: n/60 = 1,500/60 = 25 Hz.
- Multiply by the order: 4 × 25.
- forder = 100 Hz.
Assumptions and limits
- The source rotates at a steady speed, with the order fixed by the mechanism.
- The speed n is the actual speed of the source shaft, not the vehicle’s engine speed if a gear intervenes.
Common mistakes
- Using the engine speed for a shaft that turns at a different speed.
- Entering radians per second in place of rpm.
Statistics and uncertainty
Engine speed varies by a few per cent under load, so the order frequency drifts by the same fraction. A resonance within ±5 % of the frequency is within reach of normal speed variation, so allow that margin when checking for clashes.
Where it is used
- Matching measured vibration peaks to their source.
- Finding speeds at which an order meets a structural resonance.
Maths you need
- Frequency: cycles per second, hertz
- Revolutions per minute: divide by 60 for revolutions per second