Sleeve Bearings

Typical Spectrum

Latter stages of sleeve bearing wear are normally evidenced by the presence of whole series of running speed harmonics (up to 10 or 20). Wiped sleeve bearings often allow high vertical amplitudes compared to horizontal. Sleeve bearings with excessive clearance may allow a minor unbalance and/or misalignment to cause high vibration which would be much lower if bearing clearances were to specification.

Oil Whirl Instability
Typical Spectrum	Shaft Diagram
Oil Whirl instability occurs at 0.42 - 0.48 x RPM and is often quite severe. Considered excessive when amplitude exceeds 50% of bearing clearances. Oil whirl is an oil film excited vibration where deviations in normal operating conditions (attitude angle and eccentricity ratio) cause oil wedge to "push" the shaft around within the bearing. Destabilising force in the direction of rotation results in a whirl (or precession). Whirl is inherently unstable since it increases centrifugal forces which increase whirl forces. Can cause oil to no longer support the shaft, or can become unstable when whirl frequency coincides with a rotor natural frequency. Changes in oil viscosity, lube pressure and external pre-loads can affect oil whirl.
Oil Whip Instability
Typical Spectrum
A Spectral Map showing Oil Whirl becoming Oil Whip Instability as shaft speed reaches twice critical.

Oil Whip may occur if a machine is operated at or above 2x rotor critical frequency. When the rotor is brought up to twice critical speed, whirl will be very close to rotor critical and may cause excessive vibration that the oil film may no longer be capable of supporting. Whirl speed will actually "lock onto" rotor critical and this peak will not pass through it even if the machine is brought up to higher and higher speeds.

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