tailieunhanh - Engineering Tribology 2E Episode 7

Tham khảo tài liệu 'engineering tribology 2e episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 246 Engineering Tribology Misalignment has surprisingly little effect on the location of the reformation front or even the cavitation front compared to its effect on the pressure peak. This feature is probably due to the large film thickness in the cavitated regions of the hydrodynamic film which ensures a small relative change in film thickness with misalignment. Lubricant flow rates are also relatively unaffected by misalignment which renders unlikely the possibility of lubricant starvation with increasing shaft misalignment. The high values of maximum pressure however are undesirable. VIBRATIONAL STABILITY IN JOURNAL BEARINGS As discussed already in Chapter 4 hydrodynamic bearings are prone to a vibrational instability known as oil whirl . Vibration characteristics of a hydrodynamic film can be modelled by a series of stiffness and damping coefficients. These coefficients can be computed from the solutions of the Reynolds equation. Vibration analysis of hydrodynamic bearings can be directed to the computation of the shaft trajectory in a vibrating bearing. This approach however involves a rigorous analysis of bearing instability and is a specialized task requiring extensive computing. A much simpler mode of analysis for practical engineering applications is discussed in this section. in this approach the limiting shaft speed at the onset of vibration is calculated using the Routh-Hurwitz criterion of stability. The criterion provides a conservative estimate of the shaft speed at which some level of sustained vibration occurs. it has often been found that at moderate shaft speeds shaft vibration may occur but it is limited to a finite and safe amplitude. on the other hand at higher speeds there is no limit to the amplitude of vibration and the shaft will oscillate in ever wider trajectories until it touches the bush which inevitably results in destruction of the bearing. in order to analyze shaft trajectories the non-linear variation in stiffness and .