tailieunhanh - Hydrodynamic Lubrication 2009 Part 5

Tham khảo tài liệu 'hydrodynamic lubrication 2009 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 64 5 Stability of a Rotating Shaft Oil Whip Item 4 is the subject of this chapter and is a self-excited whirling vibration due to oil film action in journal bearings. Since this may happen over a wide range of speeds above a certain threshold and may become a severe vibration precautions must be taken. Its frequency is almost constant and is equal to the natural frequency of the shaft. Item 5 is a self-excited whirling vibration due to fluid forces in the seal gap or blade tip gap of turbines and turbocompressors and its frequency is equal to the natural frequency of the shaft. Oil whip can be included in flow-induced vibrations in a wide sense. Oil Whip In experiments on a rotating shaft supported by journal bearings Newkirk and Taylor 1925 found under certain conditions a new kind of severe vibration or whirling that was different from the vibration at the critical speed 2 . Since the vibration disappeared when the oil supply to the bearings was stopped and it resumed when the oil was supplied again they concluded that the vibration was caused by the oil film in the bearing and named it oil whip. Whip means a severe vibration of a shaft. The phenomenon is described in more detail in Fig. . When the rotating speed of a shaft is gradually increased a large resonant vibration occurs in the shaft at the critical speed W1. The vibration diminishes however when the rotating speed passes the critical speed. Next when twice the critical speed 2w1 is reached a large vibration or whirling will appear as shown in Fig. under certain conditions. When the shaft speed is increased further this vibration will not diminish but it may continue as it was or it may become even larger unlike resonant vibrations. This is typical of oil whip. Oil whip was simply said to start at twice the critical speed in the early days. Later however some examples were reported in which the oil whip onset speed was somewhat or significantly higher than twice the critical speed as shown in