tailieunhanh - Process Engineering Equipment Handbook 2009 Part 7

Tham khảo tài liệu 'process engineering equipment handbook 2009 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | C-322 Condition Monitoring Condition-Monitoring System s Engine Condition Monitoring Engine Condition-Monitoring System s FIG. C-348 Critical speed then w Wt Therefore we see that flexibility lowers the natural frequency of a system. This is diagrammatically represented in Fig. C-348 which can be extended to include 2nd 3rd and higher critical speeds. Figure C-349A shows vibration modes of a uniform flexible shaft supported at both ends by rigid supports. Figure C-349B shows a rigid rotor supported by flexible supports. The critical speed concept theory outlines the identification of the operating range of a rotor-bearing system probable mode shapes and approximate locations of peak amplitudes. Forced vibration. In forced vibration the usual driving force in rotating machinery is the shaft speed or multiples of this speed. Note the characteristics of forced and self-excited vibration in Table C-29. The speed becomes critical when excitation frequency is equal to one of the natural frequencies of the vibration. In forced vibration the system is a function of frequencies that can be multiples of rotor speed excited by frequencies other than 1x rpm. They might be bladepassing frequency gear-teeth frequency and so forth. For forced vibration critical frequency remains the same for any shaft speed. Damping reduces the amplitude of forced vibration but does not change critical speed values. Condition Monitoring Condition-Monitoring System s Engine Condition Monitoring Engine Condition-Monitoring System s C-323 1st critical 2nd critical 3rd critical FIG. C-349A Rigid supports and a flexible 1st critical 2nd critical 3rd critical FIG. C-349B Flexible supports and rigid TABLE C-29 Characteristics of Forced and Self-Excited Vibration13 Forced or Resonant Vibration Self-excited or Instability Vibration Frequency rpm relationship Nf Nrpm or N or rational fraction Constant and relatively independent of rotating speed Amplitude rpm relationship Peak in