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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 8, Vibration in Bearings

Section 12, Vibration Due to Oil Whirl (in Plain or Sleeve Bearings)

Oil whirl (sometimes called "oil whip") is one of the easiest vibrations to recognize as it is one of those rare vibrations with a frequency of well below 1 x rpm running speed. Its frequency has been reported to be anywhere from approximately 45 to almost 50 percent of rpm. Simply view oil whirl vibration frequency as "slightly less than ½ x rpm.

Referring to the diagram showing the normal running conditions between a shaft and a plain bearing, notice that the shaft is very rarely operating and running with its own centerline coincident with the bearing's centerline. Therefore, there is a section wherein the clearance for lubricant would be less than in other areas. This could be the normal result of the weight of the rotor, the always present partial coupling and shaft misalignment, and so on. The narrow gap area acts to build up the pressure of the rotating oil, forming a higher pressure point between the shaft and bearing. As the speeds increase, pressure increases, thereby pushing the shaft and rotor further upward (in most cases), and/or toward one side. This allows the gap to widen at the pressure point and move the new pressure point to a position closer to the shaft's bottom. All described so far apply to rotors without oil whirl and are considered normal.

However, if the load is too light (rare), the clearance too great (also rare), or if there is any other reason why the pressure point proceeds to a place whereby the higher pressure can lift the shaft high enough so that the higher pressure section can "escape," then oil whirl results. The shaft that is lifted high enough to allow the escape of the higher pressure section is now no longer supported in this position. The shaft suddenly drops -- the gap narrows again -- and the higher pressure again develops. The process repeats itself cyclically; pressure lifting the shaft -- escape of pressure -- dropping of shaft -- pressure buildup -- lifting of shaft -- escape of pressure -- dropping of shaft -- pressure buildup -- lifting of shaft -- and so on. The cyclical frequency of all this is the average oil velocity. The oil's velocity right on the shaft's surface is equal to that of the shaft's surface speed. The oil's velocity right on the bearing's surface is zero. The average velocity is 50 percent of the rotor's rpm. With a little slippage, the actual velocity is slightly less than ½ x rpm. Do not confuse it with vibration at exactly ½ x rpm, which does not originate with oil whirl but instead is most often associated with bearing looseness or a rub.

The most common published reason for oil whirl is that the bearing loads are too light, relative to the oil pressures built up by higher speeds. Yet in maintenance work of already built machinery with more established design, it has been found that this is the least probable source. (Yet, it is a common source for newly designed machinery that hasn't been de-bugged.)

Another reason given is bearings that have too much clearance. Again, with established designs, it is doubted this is very common. An approach used by Update is to first remember that when rotors vibrate for any reason, the centerline of the shaft will be tracing an orbit around the actual axis of rotation. Visualizing the shaft's surface relative to the bearing's inside diameter, the distance between the two will be changing at a rate equal to the vibration frequency. When shaft vibration reaches several mils, the opening and closing of the oil gap for the film will also be equal to several mils.

The orbit that seems to create oil whirl most often is from large coupling or shaft misalignment. The misalignment, for example, can position the shaft at a location so as to more easily lift the shaft and thereby allow escape . Misalignment can cause bearings and shafts that do not normally result in oil whirl to be in a threshold position for it.

The solution is not to change the bearing, decrease the clearance, or increase the load (although each of these could work), but to first analyze the vibration at the lower end of the frequency range, such as at 1 x and 2 x rpm and so on, so as to determine if there is a large unbalance or large misalignment. If there is, the unbalance or misalignment can usually be corrected more easily than making bearing alternations.

For most process plant machinery that can't readily be shut down for bearing changes, rebalancing or realignment, there are some relatively successful methods for curing oil whirl temporarily until permanent changes can be made. The most common successful method reported is to increase the oil's temperature, usually by approximately 10 percent. Although this seems to work more frequently than cooling the oil, there are cases reported whereby the oil whirl problem worsened with increased oil temperature, instead cooling the oil eliminated the oil whirl.

Some have accomplished the desired results by decreasing the oil's viscosity (most common success), whereas certain situations required raising the viscosity to eliminate the oil whirl.

For altering the bearing itself, several methods have been reported. One is to make the bearing ID egg-shaped, usually scraping material away in the 9:00 and 3:00 o'clock positions. Another way is to form a dam or step on the top of the bearing's ID so that the oil will produce a back pressure -- and therefore higher pressure on the top of the shaft -- preventing it from lifting enough to cause the previously described oil pressure section escape. Another has been to reduce the surface area with grooves, especially at the bottom half, so as to increase the bearing load.

This section is not complete. Bearing and rotating machinery manufacturers have much more experience and knowledgeable details on the subject. Considering the high cost of shutdown and the high cost of a mistake, it is suggested that such sources be consulted in the event bearings are to be altered as a solution.

 

 

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