What is oil whirl in journal bearings?

Oil whirl is a self-excited vibration in fluid-film (journal) bearings caused by the oil wedge pushing the shaft in a forward precessional orbit. It occurs at approximately 0.42–0.48× shaft speed (often cited as ~0.43×). Oil whirl amplitude increases with speed and can become unstable if uncorrected.

What is oil whip and how does it differ from oil whirl?

Oil whip occurs when oil whirl frequency coincides with the rotor's first natural frequency (first critical speed). At this point, the whirl 'locks on' to the natural frequency and does not increase further with speed. Oil whip is a severe instability condition with potentially destructive amplitude levels.

How do I distinguish oil whirl from other sub-synchronous vibrations?

Oil whirl is identified by: (1) frequency at ~0.42–0.48× shaft speed, (2) forward precession orbit, (3) frequency tracks shaft speed proportionally, (4) amplitude typically increases with speed. Other sub-synchronous sources like rub or loose components typically occur at exact sub-harmonics (1/2×, 1/3×) and may show reverse precession.

What causes oil whirl onset?

Oil whirl onset depends on: (1) shaft speed exceeding the stability threshold, (2) light bearing load (lightly loaded bearings are more prone), (3) low oil viscosity (from high temperature or wrong oil grade), (4) excessive bearing clearance, and (5) misalignment or unbalance affecting the bearing load distribution.

How to prevent or fix oil whirl and oil whip?

Solutions include: (1) increase bearing load or reduce clearance, (2) use anti-whirl bearing designs (tilting pad, pressure dam, lemon bore), (3) change oil viscosity (grade or temperature control), (4) reduce operating speed below instability threshold, (5) improve alignment and balance to stabilize the bearing load pattern.

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滑動軸承故障頻率

計算滑動/套筒軸承的油膜渦動頻率範圍、油膜振盪指標、軸頻率以及次同步振動特性。

油膜渦動油膜振盪次同步

結果

油膜渦動頻率範圍

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典型油膜渦動（按選定比例）

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軸頻率 (1×)

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2× 軸（不對心檢查）

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關鍵頻率摘要

頻率	Hz	CPM	指標

⚠️ 油膜振盪： 若運轉轉速超過第一臨界轉速的約 2 倍，油膜渦動可能會鎖定轉子自然頻率，產生油膜振盪——一種嚴重的失穩狀態。請密切監控次同步分量。

油膜渦動

油膜渦動是液膜軸承中的一種自激振動。油膜楔產生的力會驅動軸以低於軸轉速的頻率進行前向進動軌道：

典型渦動比約為 0.43，但根據軸承幾何形狀、負載、油黏度及游隙的不同，範圍介於 0.42 至 0.48 之間。

油膜振盪

當油膜渦動頻率與轉子的第一彎曲臨界轉速重合時，就會發生油膜振盪。此時，次同步振動會「鎖定」在臨界轉速上，不再跟隨軸轉速變化：

油膜振盪是一種可能具有破壞性的狀態，需要立即處理。

軸頻

關鍵振動指標

頻率	模式	指示
0.42–0.48× 軸	次同步，前向進動	油膜渦動——軸承失穩初期
鎖定次同步	頻率固定，不受轉速影響	油膜振盪——鎖定於第一臨界轉速
1× 軸	同步	正常不平衡響應
2× 軸	二次諧波	不對心、鬆動或非線性響應

範例 — 3000 RPM 汽輪機

已知： 軸轉速 = 3000 RPM，渦動比 = 0.43

軸頻率 = 3000 / 60 = 50.0 Hz

油膜渦動範圍 = 0.42×50 至 0.48×50 = 21.0 – 24.0 Hz

典型渦動 = 0.43 × 50 = 21.5 Hz (1290 CPM)

2× 軸 = 100.0 Hz

診斷提示： 油膜渦動會跟隨軸速（頻率成比例變化）。油膜振盪則不會跟隨速度——它會鎖定在臨界速度上。這是在升速或惰轉降速期間區分這兩種狀況的主要方法。