Abstract:
A vibration table having an upper surface defining a plane and on which test products are mounted by bolts that engage spaced internally threaded openings in the upper surface. The vibration table has mounted vibrators for generating vibrations in x-, y- and z-axes lying in the plane of the upper surface. Variation in the magnitude of the vibrations generated in the x-, y- and z-axes over the surface of the table are minimized by providing an upper surface that is circular in the plane of the upper surface.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     Vibration tables are used primarily in connection with test fixtures designed to test products under vibrating conditions. The general construction of vibration tables is well known. Generally they include upper and lower plates separated by spacers, with a vibrator mounted on the bottom of the lower plate and a test product mounted on the top of the upper plate. Representative vibration tables are shown in Applicant&#39;s U.S. Pat. No. 5,594,177 (which is incorporated by reference) and U.S. Pat. No. 5,402,991. 
     An object in the design of a vibration table is to provide a table that transmits a high proportion of the vibration energy to the test product over a desired frequency range. In order to accomplish these goals, it is generally desirable to make the table as rigid as possible and yet as light as possible. Rigidity maximizes higher frequency and higher energy transmission and minimizes resonant frequencies at low frequency ranges, while a light weight table minimizes energy loss that results from the mass of a heavy table. To accomplish these goals, tables are frequently made with a hollow or semi-hollow core. In Applicant&#39;s U.S. Pat. No. 5,594,177, the table includes spaced upper and lower plates connected together at the periphery by edge spacers and having intermediate spacers interconnecting the plates at intermediate positions. 
     Another feature of vibration tables is that they are frequently used to test products in high and low temperature environments. Thus, an insulating sheet is often bonded to the top of the upper plate. A metal sheet formed of titanium or the like can be mounted on top of the insulation. A problem with the incorporation of insulation in a shaker table is that the insulation can have a dampening effect on vibration that reduces the energy transmitted by the vibration table and the frequency response of the table. Prior tables typically have been limited to transmission of forces of under about 70 Gs RMS, with the great majority of the force being transmitted in a frequency range of about 1000-3000 Hz. This issue was addressed by Applicant&#39;s U.S. Pat. No. 7,886,606, incorporated herein by reference. 
     A further issue that affects the use of vibration tables, and the consistency of results obtained, lies in the variability of vibrations from point to point over the surface of the table. In a table having a conventional square or rectangular upper surface, the vibration profile (i.e., the amplitude per frequency of the vibrations over a frequency range in each of the x-, y-, and z-axes) can vary up to 20% over the surface of the table (i.e., at various points in the x-y axis) due to resonance of the vibrations reflected and concentrated by the sides and corners of the upper surface. 
     An object of the present invention is to provide a lightweight table that has improved energy responsiveness and a broader frequency spectrum and provides a means for uniform vibration profiles over the surface of the table. Specifically, it is an object of the present invention to provide a vibration table demonstrating a vibration profile over the surface of the table having a variability of less than 5%, and preferably +/−3%. 
     SUMMARY OF THE INVENTION 
     In a vibration table having an upper surface defining a plane and on which test products are mounted by bolts that engage spaced internally threaded openings in the upper surface, the vibration table having mounted vibrators for generating vibrations in x-, y- and z-axes lying in the plane of the upper surface, an improvement for reducing a variation in magnitude of the vibrations generated in the x-, y- and z-axes over the surface of the table, wherein the upper surface is circular in the plane of the upper surface whereby variation in vibrations generated in the x-, y- and z-axes are minimized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the improved vibration table with circular mounting surface of the present invention. 
         FIG. 1A  is a perspective view of a vibration table having a squared surface and an insulating layer. 
         FIG. 2  is a fragmentary side elevational view taken along line  2 - 2  of  FIG. 1A , showing the spacer mounted in a shaker table and with a test product mounted thereon. 
         FIG. 3  is a plan view of an improved spacer of the present invention. 
         FIG. 4  is an end view of the spacer of  FIG. 3 . 
         FIGS. 5 and 6  are perspective views of one embodiment of the present invention. 
         FIG. 7  is a side elevational view of the embodiment of  FIGS. 5 and 6 . 
         FIG. 8  is a top plan view of the embodiment of  FIG. 7 . 
         FIG. 9  is a bottom view of the embodiment of  FIG. 7 . 
         FIG. 10  is a top plan view of the vibration table with circular mounting surface of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring now to the drawings, a shaker table  10  constructed in accordance with the present invention is shown in  FIG. 1 . Shaker table  10  includes an aluminum upper plate  12  (typically about one-half inch thick), lower plate  14 , and edge spacers  16  separating the upper and lower plates. A center support  17  ( FIG. 10 ) is secured in the center of the table surface between the upper and lower plates. Edge spacers  16  are attached to the upper and lower plates by bolts  18  and typically space the plates about one-half inch apart. Bolts  18  typically are ⅜ inch internal hex head stainless steel bolts that extend all the way through the upper and lower plates and edge spacers and are secured by nuts  19  on the lower ends. Lock washers are positioned adjacent the heads. These bolts are tightened to a torque of about 30-45 ft lbs for desired rigidity. The higher torque limit is determined by the bolt strength. The lower limit is significant for energy transmission. The G forces transmitted by the table drop off significantly when a lower torque setting is used. 
     In one embodiment, shown in  FIG. 1A , the vibration table  10 ′ includes an insulating sheet or layer  20 , preferably a ceramic material, is bonded on the top of upper plate  12 ′. A metal top plate formed of titanium or the like can be positioned on the top of the insulating sheet but is not essential in the present invention. An insulating sheet is generally employed, but there are some applications where an insulating sheet is not used. The table of  FIG. 1A  is illustrated as square, but the modification is applicable to the round table  10  of the present invention. 
     The insulating sheet and upper surface of plate  12  include a plurality of openings  36  and  24 , respectively, spaced in a conventional pattern (typically a rectangular grid on four inch or 100 mm centers) over the top of the vibrator table. The openings in the upper plate are internally threaded. These openings are larger than in a conventional vibration table to accommodate novel inserts  26  of the present invention. 
     Vibrators  28  of conventional design are mounted on the underside of the lower plate. The vibrators can be of a pneumatic design, or of an electro-dynamic (ED) configuration. An ED vibrator is capable of generating vibrations up to 250 Gs RMS and 50-100 kHz. At least some of the vibrators are mounted at an angle with respect to the bottom plate and skewed in different directions in order to provide uniform multi-axis excitation. The vibrations generated in the x-, y- and z-axes defined on the upper surface of the vibration table  10  with a circular upper plate  12  will generally vary less than 5% over the surface of the round table surface according to the invention, and can vary as little as +/−3%. Bolts  44  extend upwardly through and into ends of vibrators  28  and into the lower plate to mount the vibrators securely to the bottom of lower plate  14 . 
     The inserts  26  include an externally threaded shaft  30  with a head  32  on one end. The head can have a hexagonal shape. The inserts have internal threaded openings  34  therethrough. Inserts  26  extend through openings  36  in the insulator sheet  20  and then are threaded into internally threaded openings  24  in the upper plate. The inserts are threaded all the way into and preferably through the openings in the upper plate until the heads  32  of the inserts securely abut the insulator sheet  20 . Desirably the insert shafts extend past the bottom of plate  12  (typically about ¼ inch). 
     Inserts  26  are mounted securely in threaded openings  24  in the upper plate. The inserts thus serve to provide a rigid extension of the upper plate above the insulator sheet. The torque applied to the inserts should be sufficient to snugly clamp the inserts to the upper plate. It is desired that the inserts be tightened in the openings  24  in the upper plate with a torque of approximately 120-130 foot pounds. The upper torque limit is set below the torque limit of the insert threads. 
     Inserts  26  in the preferred embodiment are ¾ by 1 inch stainless steel bolts with the head being 1¼ inch wide and about ¼ to ⅜ inches thick. In the exemplary embodiments, the insert bolts have 16 threads per inch, but this does not appear to be critical. Bolt diameter, however, is important. Smaller bolts of ⅜ inch diameter do not appear to transmit G forces as effectively. Bolt sizes greater than ⅜ inch should be used. The bolt diameter should be large enough so that there is a large enough thread contact area to transmit acceleration forces without substantial attenuation caused by the inserts. Bolt diameters larger than ¾ inch are generally less practical to produce and are not essential. The inserts are preferably locked in position by means of a bolt locking fluid or compound, such as a fluid sold under the brand name Locktite or the like. 
     With the inserts inserted as described, a test product may be mounted on the upper surface  48  of the insert by means of attachment bolts  50  that engage internal threaded openings  34  in the insert to attach the test product or a mounting fixture for the test product (both referred to by numeral  52 ) to the upper surface of the spacer. Some parts can be screwed directly to the table. Other parts require a mounting fixture. Attachment bolts  50  in the exemplary embodiment are ⅜ inch (10 mm) bolts. Bolts  50  can extend all the way through internal insert thread area. The extended thread area of the insert makes it possible to obtain a longer thread contact area with attachment bolt  50 . This increased thread contact area improves the transmission of acceleration forces through the test part. The larger external diameter of the insert and the longer length of the internal threaded opening in insert and attachment bolt both are believed to contribute to the improved force transmission properties of the invention, such that the force transmission properties of the present invention are better than the force transmission properties of a conventional uninsulated aluminum plate where the test product is bolted directly to the plate with conventional ⅜ inch bolts. 
     There are a number of advantages to a shaker table constructed in this manner. The table is very rigid and responsive to vibration, such that acceleration forces of up to 120 Gs RMS or more can be transmitted to the table through the insert over a frequency range of up to about 10,000 Hz or more, with the vibration forces being distributed primarily over a range of about 1000-4000 Hz. A typical vibration table employing a comparable actuation force transmits vibrations of 70 Gs RMS or less, concentrated primarily in a frequency range of about 1000-3000 Hz. The new table also transmits substantially higher peak acceleration forces than prior vibration tables, and the forces are distributed more evenly over the concentrated frequency range. 
     Another advantage of the inserts of the present invention is that they space the test product or test product mounting fixture away from the insulation sheet on the upper plate. This makes it possible to maintain a more uniform temperature control around all the upper and lower surfaces of the test product. Test products are sometimes subjected to temperatures ranging from −100 to +200 C. In prior vibration tables, the product or its mounting fixture is mounted directly on the upper surface of the vibration table, where the upper surface prevents air flow under the test product and therefore produces different temperatures on the lower and upper side of the test product. 
     It should be understood that the foregoing is merely exemplary of the preferred practice of the present invention and that various changes in the details and arrangements of the embodiments disclosed herein may be made without departing from the spirit and scope of the present invention.