了解轉子油膜渦動與油膜振盪不穩定現象
渦動 與 油膜振盪 — most often encountered as 油膜渦動 and oil whip — are two related and highly dangerous forms of self-excited, 次同步 振動 that occur in high-speed rotating machinery running in fluid-film (軸頸) bearings. They are not 強制振動 driven by faults such as 不平衡 或 不對心; instead they are 轉子不穩定現象 in which the motion of the rotor itself generates the very forces that sustain and amplify the vibration. In both cases the shaft “whirls” — it precesses forward in a large orbit within its bearing clearance, tracing a path quite separate from its own spin.
1. Definition: What Are Whirl and Whip?
It is worth separating two ideas that the everyday term “whirl” blurs together. Spin is the rotor turning about its own geometric axis. 渦動 (or precession) is the orbiting of that axis as a whole around a larger circle inside the bearing — picture a spinning coin whose centre also loops around the table. All rotors whirl a little; the trouble starts when the whirl stops being a benign response to 剩餘不平衡 and becomes self-excited, drawing its energy from the steady rotation rather than from any external forcing. Oil whirl is the self-excited precession driven by the bearing oil film; oil whip is the violent resonance it can mature into. Because the energy source is the rotation itself, these instabilities cannot be balanced out — a defining contrast with synchronous problems.
2. The Mechanism: How Does It Happen?
In a fluid-film bearing the rotating shaft is supported not by metal-to-metal contact but by a high-pressure wedge of oil. The shaft does not sit in the centre of the bearing; it rides up one side, displaced by the load it carries. As the journal surface drags oil around the annular gap, the lubricant circulates at an average speed of slightly less than half the shaft’s surface speed — the fluid touching the shaft moves at shaft speed, the fluid against the stationary bearing wall is nearly still, and the bulk average lands just under 0.5×.
Oil whirl occurs when this circulating film begins to “push” the lightly loaded shaft ahead of itself, sweeping it into a large forward orbit around the bearing. The frequency of the whirl is set by the average velocity of the oil film, which falls typically between 42% and 48% of running speed (0.42× to 0.48×). That distinctive sub-synchronous signature — close to, but never exactly, half of 運轉轉速 — is the fingerprint analysts look for. (The “slightly less than half” figure is also why oil whirl is sometimes loosely called “half-speed whirl,” though the true value never quite reaches 0.5×.)
3. Oil Whirl: The Precursor
Oil whirl is usually the opening stage of the instability — a warning, not yet a catastrophe. Its characteristics are:
- 頻率： appears as a distinct peak in the FFT 頻譜 between 0.42× and 0.48× of RPM.
- 行為： the whirl frequency increases as the machine speeds up, always tracking that ~45% proportion of running speed. On a run-up it climbs as a sub-synchronous shadow beneath the 1× line.
- 嚴重度： it can produce high but sometimes stable vibration, and it may appear or vanish as load, speed, or oil temperature change. Undesirable, certainly — but not always immediately destructive.
- 靈敏度： lightly loaded, oversized, or worn bearings are the usual culprits, because a low specific load lets the oil wedge dominate the shaft’s position.
4. Oil Whip: The Critical Danger
Oil whip is a far more severe condition that grows directly out of oil whirl. It strikes when the machine accelerates to the point where the oil-whirl frequency (at roughly 45% of running speed) climbs up to meet the rotor’s 首先 自然頻率 — its first 臨界轉速. At that moment the whirl “locks onto” the natural frequency and excites a full-blown 共振. Its characteristics are:
- 頻率： the vibration locks at the rotor’s first natural frequency and does not rise any further, even as the machine keeps speeding up — so the sub-synchronous peak “flatlines” while the 1× peak marches on.
- 振幅： the vibration grows very large, becoming violent and unstable.
- 行為： oil whip is extremely destructive and will 不 clear by speeding up further. It can wreck bearings, seals, and the rotor itself within a very short time, sometimes through severe 轉子摩擦 as the orbit fills the clearance.
The speed at which whip sets in is typically just over twice the rotor’s first critical speed — the point where the ~0.5× whirl line crosses the first natural frequency. A machine in the grip of oil whip needs an immediate 停機; this is precisely the scenario that 機械保護 systems are built to trip on.
5. 如何辨識油膜渦動與油膜振盪
- 頻譜分析： 尋找強烈的次同步峰值。在升速過程中，若該峰值的頻率隨轉速上升，則為油膜渦動；若其在固定值處「平直化」，而 1× 峰值持續攀升，則已轉變為油膜振盪。
- 軌跡圖: 軸心軌跡為一個大型的前進進動圓形或橢圓形，通常疊加 1× 分量，呈現特徵性的「環狀回環」圖案。
- 瀑布圖: 瀑布圖（或 瀑布圖）圖從啟動過程提供最清晰的圖景，顯示渦動頻率隨轉速上升，直到與第一自然頻率相交並鎖定為油膜振盪。繪製這些交叉點正是 坎貝爾圖 is for.
由於油膜渦動與油膜振盪發生在 1× 以下，分析儀必須能涵蓋遠低於運轉轉速的範圍，並精確解析相位。像 Balanset-1A 這樣的便攜式雙通道儀器，能在升速或惰轉降速過程中擷取運轉轉速分量的同步 振幅 與 相位 ，讓工程師在現場確認頑固的低頻峰值是真正的軸承不穩定，而非一般的不平衡——同樣重要的是，在嘗試無效的修正前，排除動平衡問題。
6. 成因與解決方案
這些不穩定現象受軸承設計、轉子幾何形狀、油黏度、溫度與負荷支配——這是一組複雜的交互作用，正式記錄於 轉子動力學。它們並非由不平衡引起，也無法透過 動平衡校正來治癒；解決方案在於設計層面的變更：
- 改用更穩定的軸承幾何形狀，例如傾斜瓦滑動軸承。
- 調整油黏度或運轉溫度，以改變油膜行為。
- 增加軸承比壓，使軸穩固定位，油楔不再主導。
- 添加溝槽、軸向壩或檸檬孔剖面，以破壞滋養渦動的環向油流。
一種密切相關的不穩定現象， 蒸汽渦動，源於渦輪機中的氣動力而非油膜力，但產生類似的自激次同步圖譜——這提醒我們，「渦動」是一系列現象的總稱，其共同特徵為：轉子將能量饋入自身的軌跡。
