Tham khảo tài liệu 'rotating machinery vibration 2011 part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Signal Analysis 283 4. Rotor mb force magnitude is modulated in synchronization with the once-per-rev component of rotor vibration orbit providing an asymmetric friction-induced heat input to the rotor causing it to develop a thermal bow which initiates a slowly precessing vibration phase angle forward or backward because the rotor high spot and hot spot do not coincide due to thermal inertia and phase lead or lag between the unbalance and the synchronous orbital response it excites. In effect the hot spot tries to catch up to the high spot but as the hot spot migrates so does the high spot in response. This phenomenon is sometimes referred to as vector turning because when a representative vibration signal is polar plotted over time see Fig. 8 the vibration vector slowly rotates . couple of hours per turn in one direction as further explained in the corresponding case study of Chapter 12. When insufficient heat-removal capacity is available the turning vector does not stay within an acceptable vibration amplitude but instead continues to spiral outward slowly. In brush-type exciters of large AC generators this vector turning phenomenon can be instigated by the brushes rubbing contact friction forces being synchronously modulated by brush inertia . brushes are spring preloaded to track rotor contact-surface orbital motions. This type of rubbing friction-induced vibration is also known to occur from rotor rubbing of radial seals where the rubbing contact friction force is synchronously modulated by the effective support stiffness of the seal. The field measurement case explained earlier in this chapter pertaining to Fig. 6 and 10 is a prime example of category 3 in which the rub-impacting significantly worsens subsynchronous vibration on coast-down. Figure 22 from tests and corresponding computer simulations 4 demonstrates transition from category 1 to category 2 . Referring to the orbital responses in Fig. 22 the stronger the rub-impacting the higher the .
