Tham khảo tài liệu 'gear noise and vibration episode 2 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 260 Chapter 16 Fig Sketch of vibration responses and paths. In practice because measuring inside the gearbox is very difficult it is probably better to rely on . excitation when running or to estimate the internal resonances. The reciprocal theorem is of limited help since although it may help for cross receptances P13 and p23 it does not assist access for p 2 or the local responses pH and p22 which need access inside the box in zero space. In some cases the wheel is so massive and its support is so stiff that wheel response may be ignored simplifying the algebra considerably. Coherence Whichever method b c or d is used for measuring a transfer function with a transfer function analyser it is worthwhile checking coherence if there is any possibility of background noise whether mechanical or electrical. The idea of coherence is that if we take a single transfer function measurement we can deduce a transfer function. However we do not know how much of the output is really due to the input and how much is due to random external or internal disturbances. Repeating the test many times and getting exactly the same result in both amplitude and phase would suggest that there is little random effect. Any variation would suggest randomness. Coherence analysis routines carry out this check and compare how much of the measured output power at a Copyright Marcel Dekker Inc. All rights reserved. Marcel Dekker Inc. 270 Madison Avenue New York New York 10016 Vibration Testing 261 particular frequency can be attributed to the consistent transfer function. A coherence of 1 suggests that output is firmly connected to input but suggests that random noise is dominating the measurements. Any results with coherence should be viewed with suspicion. Even if there are two vibrations whose coherence is 1 it is not necessarily true to say that the output is due to the input since both vibrations may have been generated by another unknown input. In particular a panel .
