Shaft whirling

Screw elevators operate at all inclinations, up to the vertical. Intermediate bearings offer a flow obstruction, necessitate a gap in the flighting, and require access for service, so their use in screw elevators must be avoided where possible. Screw elevators lengths are, therefore, normally limited by the stiffness needed for the centre tube, to prevent casing contact by deflection or shaft whirling. For this reason screw elevators in plant use rarely exceed 8 m overall casing length. 

Kimball, A.L. Internal friction theory of shaft whirling, Phil. Mag. 1925, Vol. 49. 

Fault to be detected Unbalance Misalignment Bent shaft Oil whirl Eccentricity Wear Faults In gears Faults In rolling element bearings... 

Hysteretic whirl. This type of whirl occurs in flexible rotors and results from shrink fits. When a radial deflection is imposed on a shaft, a neutralstrain axis is induced normal to the direction of flexure. From first-order considerations, the neutral-stress axis is coincident with the neutral-strain axis, and a restoring force is developed perpendicular to the neutral-stress axis. The restoring force is then parallel to and opposing the induced force. In actuality, internal friction exists in the shaft, which causes a phase shift in the stress. The result is that the neutral-strain axis and neutral-stress axis are displaced so that the resultant force is not parallel to the deflection. The... 

Whirl from fluid trapped in the rotor. This type of whirl oeeurs when liquids are inadvertently trapped in an internal rotor eavity. The meehanism of this instability is shown in Figure 5-24. The fluid does not flow in a radial direetion but flows in a tangential direetion. The onset of instability oeeurs between the first and seeond eritieal speeds. Table 5-4 is a handy summary for both avoidanee and diagnosis of self-exeitation and instabilities in rotating shafts. 

Dry-friction whirl Any speed Nf = —nN tip clearance effects, balance pistons Shaft in contact with... 

One of the most serious forms of instability encountered in journal bearing operation is known as half-frequency whirl. It is caused by self-excited vibration and characterized by the shaft center orbiting around the bearing center at a frequency of approximately half of the shaft rotational speed as shown in Figure 13-15. 

Modal balaneing is one of the proven methods for flexible rotor balan-eing. Modal balaneing has also been applied to problems of dissimilar lateral stiffness, hysteretie whirl, and to eomplex shaft-bearing problems. In many diseussions on modal balaneing fluid-film damping is not ineluded. In other... 

The Alexandrian seientist Hero (eirea 120 B.C.) would hardly reeognize the modern gas turbine of today as the outgrowth of his aeolipile. His deviee produeed no shaft work—it only whirled. In the eenturies that followed, the prineiple of the aeolipile surfaeed in the windmill (A.D. 900-1100) and again in the powered roasting spit (1600s). The first sueeessful gas turbine is probably less than a eentury old. 

Oil whirl In an abnormal mode of operation, the rotating shaft may not hold the centerline of the sleeve bearing. When this happens, an instability called oil whirl occurs. Oil whirl is an imbalance in the hydraulic forces within a sleeve bearing. Under normal operation, the hydraulic forces such as velocity and pressure are balanced. If the rotating shaft is offset from the true centerline of the bearing, instability occurs. 

Oil whip The most severe damage results if the oil whirl is allowed to degrade into oil whip. Oil whip occurs when the clearance between the rotating shaft and sleeve bearing is allowed to close to a point approaching actual metal-to-metal contact. When the clearance between the shaft and... 

The impellers can be an open-faced (stationary shroud) or closed-face (rotating shroud) design. As long as the tip velocities are below 1000 ft/s, closed-face impellers can be used. The standards allow the impellers to be welded, riveted, milled, or cast. Riveted impellers are unacceptable, especially if the impeller loading is high. Impellers are to be assembled on the shaft with a shrink fit with or without key. Shrink fits should be carefully done because excessive shrink fits can cause a problem known as hysteresis whirl. In compressors where the impellers require their thrust to be balanced, a balance drum is acceptable and preferred. 

This equation shows how the shaft deflection is related to speed of rotation. For low speeds (w o) the radius of whirl (r = BS) is practically zero at the critical speed, when a> = o> , r = BS becomes infinite whilst for very much higher speeds B coincides with G. Thus at very high speeds the centre of mass remains at rest and the shaft rotates about this point, i.e. the shaft is selfcentring. This explains why high-speed shafts are usually designed for operation at speeds above the critical whirling speed in order to achieve stability. 

Another possible deflection mechanism is the occurrence of whirling. Whirling occurs when a shaft reaches a critical speed and becomes dynamically unstable with large lateral amplitudes. This phenomenon is due to the resonance frequency when the rotational speed corresponds to the natural frequencies of lateral vibration of the shaft. For uniform beams vibrating in flexure, the natural frequencies can be expressed as ... 

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