1、翻译： 英文原文 Definitions and Terminology of Vibration vibration All matter-solid, liquid and gaseous-is capable of vibration, e.g. vibration of gases occurs in tail ducts of jet engines causing troublesome noise and sometimes fatigue cracks in the metal. Vibration in liquids is almost always longitudina
2、l and can cause large forces because of the low compressibility of liquids, e.g. popes conveying water can be subjected to high inertia forces (or “water hammer”) when a valve or tap is suddenly closed. Excitation forces caused, say by changes in flow of fluids or out-of-balance rotating or reciproc
3、ating parts, can often be reduced by attention to design and manufacturing details. Atypical machine has many moving parts, each of which is a potential source of vibration or shock-excitation. Designers face the problem of compromising between an acceptable amount of vibration and noise, and costs
4、involved in reducing excitation. The mechanical vibrations dealt with are either excited by steady harmonic forces ( i. e. obeying sine and cosine laws in cases of forced vibrations ) or, after an initial disturbance, by no external force apart from gravitational force called weight ( i. e. in cases
5、 of natural or free vibrations). Harmonic vibrations are said to be “simple” if there is only one frequency as represented diagrammatically by a sine or cosine wave of displacement against time. Vibration of a body or material is periodic change in position or displacement from a static equilibrium
6、position. Associated with vibration are the interrelated physical quantities of acceleration, velocity and displacement-e. g. an unbalanced force causes acceleration (a = F/m ) in a system which, by resisting, induces vibration as a response. We shall see that vibratory or oscillatory motion may be
7、classified broadly as (a) transient； (b) continuing or steady-state； and (c) random. Transient Vibrations die away and are usually associated with irregular disturbances, e. g. shock or impact forces, rolling loads over bridges, cars driven over pot holes-i. e. forces which do not repeat at regular
8、intervals. Although transients are temporary components of vibrational motion, they can cause large amplitudes initially and consequent high stress but, in many cases, they are of short duration and can be ignored leaving only steady-state vibrations to be considered. Steady-State Vibrations are oft
9、en associated with the continuous operation of machinery and, although periodic, are not necessarily harmonic or sinusoidal. Since vibrations require energy to produce them, they reduce the efficiency of machines and mechanisms because of dissipation of energy, e. g. by friction and consequent heat-
10、transfer to surroundings, sound waves and noise, stress waves through frames and foundations, etc. Thus, steady-state vibrations always require a continuous energy input to maintain them. Random Vibration is the term used for vibration which is not periodic, i. e. has no made clear-several of which
11、are probably known to science students already. Period, Cycle, Frequency and Amplitude A steady-state mechanical vibration is the motion of a system repeated after an interval of time known as the period. The motion completed in any one period of time is called a cycle. The number of cycles per unit
12、 of time is called the frequency. The maximum displacement of any part of the system from its static-equilibrium position is the amplitude of the vibration of that part-the total travel being twice the amplitude. Thus, “amplitude” is not synonymous with “displacement” but is the maximum value of the
13、 displacement from the static-equilibrium position. Natural and Forced Vibration A natural vibration occurs without any external force except gravity, and normally arises when an elastic system is displaced from a position of stable equilibrium and released, i. e. natural vibration occurs under the
14、action of restoring forces inherent in an elastic system, and natural frequency is a property of he system. A forced vibration takes place under the excitation of an external force (or externally applied oscillatory disturbance) which is usually a function of time, e. g. in unbalanced rotating parts
15、, imperfections in manufacture of gears and drives. The frequency of forced vibration is that of the exciting or impressed force, i. e. the forcing frequency is an arbitrary quantity independent of the natural frequency of the system. Resonance Resonance describes the condition of maximum amplitude.
16、 It occurs when the frequency of an impressed force coincides with, or is near to a natural frequency of the system. In this critical condition, dangerously large amplitudes and stresses may occur in mechanical systems but, electrically, radio and television receivers are designed to respond to reso
17、nant frequencies. The calculation or estimation of natural frequencies is, therefore, of great importance in all types of vibrating and oscillating systems. When resonance occurs in rotating shafts and spindles, the speed of rotation is known as the critical speed. Hence, the prediction and correcti
18、on or avoidance3 of a resonant condition in mechanisms is of vital importance since, in the absence of damping or other amplitude-limiting devices, resonance is the condition at which a system gives an infinite response to a finite excitation. Damping Damping is the dissipation of energy from a vibr
19、ating system, and thus prevents excessive response. It is observed that a natural vibration diminishes in amplitude with time and, hence, eventually ceases owing to some restraining or damping influence. Thus if a vibration is to be sustained, the energy dissipated by damping must be replaced from a
20、n external source. The dissipation is related in some way to the relative motion between the components or elements of the system, and is caused by frictional resistance of some sort, e.g. in structures, internal friction in material, and external friction caused by air or fluid resistance called “v
21、iscous” damping if the drag force is assumed proportional to the relative velocity between moving parts. One device assumed to give viscous damping is the “dashpot” which is a loosely fitting piston in a cylinder so that fluid can flow from one side of the piston to the other through the annular clearance space. A dashpot cannot store energy but can only dissipate it.
