13t6] vibration and shock 695 



vibration and shock constitute a challenge which engages much of the 

 attention of designers, specification writers, and environmental test 

 specialists. Vibration and shock problems are the subject of several books 

 and a vast number of technical papers and articles. 



A vibration is an oscillatory motion continuing over an extended period of 

 time. This oscillation may be sinusoidal, having a definite frequency and 

 amplitude; it may be a complex periodic motion which can be described by 

 a Fourier series; or it may be a random motion describable only in statistical 

 terms. When an oscillatory force supplies to a vibrating system sufficient 

 energy to compensate for that lost by damping, the vibration is termed 

 forced vibration. When vibrations are maintained only by the elastic 

 and kinetic energy initially in the system, they are called free or natural 

 vibrations. Since some friction will always be present in a mechanical system, 

 free vibration is a transient condition. Forced vibration, on the other 

 hand, is normally a steady-state condition. 



Mechanical shock is a transient condition wherein a body or system is 

 distributed by a sudden impulse of force or a sudden change in velocity. 

 This disturbance is followed by a damped free vibration in which the sys- 

 tem returns to equilibrium or to its steady-state vibration, depending on 

 the environment. 



The dynamic loads of shock and forced vibration can occur during the 

 transportation and handling of the radar and during the various phases of 

 aircraft flight operation, including taxiing, takeoff, flight maneuvers, and 

 landing. Aerodynamic effects and power-plant reactions in flight contribute 

 to the generation of potentially dangerous vibration and shock loads. The 

 firing of missiles, rockets, and guns adds further excitation. Equipment 

 destined for naval use will usually be subjected also to the vibrations and 

 shocks characteristic of ships, catapult takeoffs, and arrested landings. 



Vibration may cause failures in a radar that are either temporary or 

 permanent. The temporary failures are the malfunctions which persist 

 during vibration but leave no mechanical damage. A common example of 

 a temporary failure would be a vibrating relay contact. The permanent 

 failures occur in components, structural members, and fasteners which 

 sustain various types of mechanical damage leading to equipment failure. 

 The loosening of threaded fasteners during vibration may be considered a 

 third class of failure. Since the reliability of the radar system operation is 

 a measure of design success, it must be remembered that a temporary 

 malfunction of the equipment, even though caused by deflections well 

 within the elastic limit of the materials used, is a real failure of mechanical 

 design or construction, just as surely as a broken bolt or a cracked casting. 



In general, vibration failures of a permanent nature are fatigue failures 

 resulting from the repetition of many cycles of stress within the elastic limit 

 of the material used. Thus vibration failures are a function of id) the 



