Self-excited vibration

Figure 5-22a. Hysteretic whirl. (Ehrich, F.F., Identification and Avoidance of Instabilities and Self-Excited Vibrations in Rotating Machinery, Adopted from ASME Paper 72-DE-21, General Electric Co., Aircraft Engine Group, Group Engineering Division, May 11, 1972.)...

Self-Excited Vibration Hyslerelic whirl N> Nt Nf A equipment misaligned Shrink fits and... 

One of the most serious forms of instability encountered in journal bearing operation is known as half-frequency whirl. It is caused by self-excited vibration and characterized by the shaft center orbiting around the bearing center at a frequency of approximately half of the shaft rotational speed as shown in Figure 13-15. 

Machine tool vibrations Self-excited vibrations in metal cutting... 

Chatter is a self-excited vibration type resulting from the dynamic interaction between the cutting tool and the work material (Tobias 1965 Koenigsberger and Tlusty 1967). For forced vibrations arising in mechanical systems, the excitation is independent of the response, i.e., the vibrations do not affect the forces. In self-excited chatter vibrations, on the other hand, the chip thickness becomes modulated... 

Self-excited vibrations are vibrations caused by some mechanism inherent in the system such that the energy supplying these vibrations is obtained from a uniform source of power. For the case of self-excited vibration, the alternating force that sustains the motion is created or controlled by... 

An example for self-excited vibrations is the stick-slip phenomenon. In machining applications, stick-slip typically arises at the glides. The mechanical model can be seen in Fig. 12. The block of mass m is fixed to the moving wall by a spring of stiffness k and a dashpot of damping c. The wall is moving with velocity vq-The friction force acting on the block is... 

Regenerative Self-excited Vibrations in Machining Processes... 

Figure 16 shows the practical region of the stability chart (i.e., for positive depths of cut) and the associated frequency ratio diagram, where / = coHn is the frequency of the arising self-excited vibrations at the stability boundaries in [Hz]. The stabihty lobes are usually characterized by their index the lobe of index j is called theyth lobe. The minimum points of the lobes can be given as... 

There are many phenomena giving rise to loads that are a function of the delayed stmctural vibration. These loads can produce self-excited vibration of the structure, like in the flutter of airplane wings or the chatter of machine tools. Damping is the most important stmctural property to impede that kind of vibration. 

Currently, there are no established methods for estimation of damping at the design stage. The engineers have to make an experience-based guess, or measure it when the prototype is built. This is a very undesirable situation, as the size of the stracture required for a particular application can well be mainly dependent on the required stability against self-excited vibration, directly dependent on the damping that the stracture will have. 

Chatter Forced vibration Self-excited vibration... 

In general, vibration is a phenomenon where motion or oscillation occurs around an equilibrium point. This motion may be periodic or random. In - machine tools, the vibrations are induced from various sources such as the process, the environment, or the machine itself. They are usually undesirable. However, there are also self-excited vibrations which are commonly experienced in a wide variety of machining processes. See chatter and stability for details on those self-excited vibrations generated in machine tools. 

If the vibration of the system (X(cd)) affects the excitation force F(co), this system can be considered to be in closed loop where self-excited vibrations such as chatter and friction-induced vibration may occur. Unstable vibration in servomechanism, which occurs at an excessive serve gain ( Control), can also be considered as the self-excited vibration. Figure 2 shows the closed loop dynamics of this type of vibrating system. 

At about 1450 rpm excessively large oscillations due to self-excitated vibrations were observed. The measured first resonance frequency was about 1950 rpm (as against 2050 rpm in the precalculation). The conclusion is, that approximately 100 to 2(X) rpm must be deducted from the theoretical calculations for stiff bearings to meet the realistic conditions. 

Fig. 11 shows a record of the overall amplitude of the rotor oscillations measured at two measuring points staggered by 90°. These measured oscillations were typical for the vibrations observed with the EVO-turbomachinery and indicated self-excitated vibrations. Taking into account the known effects that influence a self-excited vibration, various countermeasures were taken including providing transverse strips in the labyrinth seals. 

Kramer, E. Self excited vibration of shafts as a result of transverse forces, Brennstoff-Warne-Kraft, 1968,... 

F Ehric. A state-of-the-art survey in rotor dynamics - nonlinear and self-excited vibration phenomena. Proceedings 2nd International Symposium on Transport Phenomena, Dynamics and Design of Rotating Machinery. Hemisphere Publishing Corporation (1989)... 

An important area of dynamics has to do with problems where bodies oscillate in response to external disturbances. The exciting disturbances may be due to imbalance of rotating machinery a sudden shock, as in an earthquake or a periodic series of forces, as in an internal combustion engine. Self-excited vibrations exist when a system, once disturbed, vibrates indefinitely. This occurs when the energy stored exceeds that which is released during a cycle of vibration. In such cases, the amplitude of vibration continues to increase imtil the increase in energy lost per cycle in... 

In contrast, self-excited vibration called chatter vibration often occurs on the boring bar of a cutting tool being used to expand the bores in metal parts. It is especially disturbing in iron boring operations. When chatter vibration is excited, it not only affects the finishing... 

The sliding nature of the contact in lead screw drives puts great importance on the role of friction on their performance. In addition to efficiency concerns, driving torque requirements, or wear, friction can be the cause of dynamic instabilities, resulting in self-excited vibrations which deteriorates the performance of the system and may cause unacceptable levels of audible noise. 

Numerous researchers have studied self-excited vibration phenomena in a variety of frictional mechanisms (see, e.g., [4-9]). A considerable portion of the research in the field of friction-induced vibration is devoted to the brake systems. See, for example, the review paper by Kinkaid et al. [7]. The major self-excited vibration mechanisms in the systems with friction can be categorized into three types ... 

The results presented throughout this book will help designers better understand the intricacies of the lead screw dynamics with friction and will provide guidelines to prevent friction-induced self-excited vibrations at the design stages as well as in the practical situations. 

Different models have been proposed to reproduce this type of velocity-dependent friction (see, e.g., [16, 17]). The important feature of these models is the existence of a region of negative slope in the friction-velocity curve, which may lead to self-excited vibrations. This type of instability is discussed in Sect. 4.1 and Chap. 6. 

Olofsson and Ekerfors [18] investigated the friction-induced noise of screw-nut mechanisms. They discussed the tribological aspects of lubricated interaction between lead screw and nut threads, which accounts for the Stribeck friction. Based on experimental results, they have concluded that (a) the squeaking noise is the result of self-excited vibration between lead screw and nut threads (b) in the system studied (consisting of a long and slender screw), these vibrations excite... 

Thomsen and Fidlin [62] also used averaging techniques to derive approximate expressions for the amplitude of stick-slip and pure-slip (when no sticking occurs) vibrations in a model similar to Fig. 4.1. They used a third-order polynomial to describe the velocity-dependent coefficient of friction. Other researchers have shown that in cases where the coefficient of friction is a nonlinear function of sliding velocity (e.g., humped friction model), the presence of one or more sections of negative slope in the friction-sliding velocity curve can lead to self-excited vibration without sticking [4, 70, 71]. 

Gallina P, Giovagnoni M (2002) Design of screw jack mechanism to avoid self-excited vibrations. J Dyn Syst Meas Control 124 477-480... 

D Souza AF, Dweib AH (1990) Self-excited vibrations induced by dry friction, part 2 stability and limit-cycle analysis. J Sound Vib 137(2) 177-190... 

Chatterjee S (2007) Non-linear control of friction-induced self-excited vibration. Int J Non Lin Mech 42 459 69... 

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