1226 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



This machine can also be used to produce lower G shocks of longer 

 duration and simpler wave form by interposing a resilient rubber pad 

 between the anvil and hammer. The resulting shock resembles the bell 

 shaped output of the MIL-E-IB bump tester. Its magnitude and dura- 

 tion are given by the hammer swing, resilient characteristics of the pad, 

 and the table and specimen weight. The capacity of the machine permits 

 shock testing of heavier tubes (such as the larger magnetrons) by this 

 method. Attainable shock levels are sufficiently high to cover the require- 

 ments placed on these tubes by conditions encountered during transit. 



The L.A.B. Package Tester 



Of particular interest to the packaging engineer, this machine is in- 

 tended to duplicate the destructive vibrations and shocks experienced 

 by a product during transit. It consists of a horizontal table that can 

 be made to vibrate with a circular motion in the vertical plane. Adjust- 

 ments permit variations of this motion to simulate freight car and motor 

 truck movements. Tests performed on this machine, therefore, check on 

 the mechanical strength of the outer shipping container as well as on the 

 adequacy of cushioning materials employed to protect the product. The 

 services are also considering this machine to test equipment designed 

 for use in vehicles. Tests are now in progress by the Signal Corps to 

 determine proper parameters for this application.^ 



Vibration Machines 



A number of vibration machines, made by various manufacturers, are 

 employed for vibration testing of tubes. Due to the high hash output 

 of most mechanically driven machines, which contain gears and linkages, 

 great care must be taken in the selection of these machines. Certain tests, 

 especially the microphonic tests on receiving tubes, require machines 

 with good sinusoidal output, in order to obtain comparable results. It is 

 for this reason that the leaf spring vibration machine (Fig. 16), developed 

 several years ago by Bell Telephone Laboratories, has been recommended 

 as a standard for performing vibration tests. This is a fixed frequency, 

 25-cycle — 2.5 g machine which, due to its construction, has a relatively 

 clean output as shown in Fig. 17. 



Several types of electronically or motor-generator driven vibration 

 machines are on the market. These are variable frequency and variable 

 amplitude machines especially useful for determining resonance fre- 

 quencies of structures and for performing cycling vibration tests. Ac- 

 cessory equipment has been made available lately to conduct these tests 

 on an automatic basis at either constant acceleration or constant am- 



