Vibration table

A vibration table for testing products employs threaded inserts for mounting the test products or fixtures therefor on the table. The threaded inserts have enlarged and elongated threaded shafts that engage threaded openings in the upper surface of the table, with enlarged heads on the inserts being positioned adjacent the outer surface of the table or the outer surface of an insulation layer mounted on the table. The inserts have elongated internally threaded openings therein, to which the test products or fixtures are bolted by elongated bolts. The inserts enhance the transmission of vibration forces to the test product with or without an insulating layer. They also provide improved temperature uniformity by providing air circulation between the test product and the table.

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'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'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.

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 heating of a test product.

SUMMARY OF THE INVENTION

In accordance with the present invention, a vibration table having an upper surface, which may be covered by an insulation layer, employs a plurality of inserts for mounting a test product or a mounting fixture thereof to the upper surface. The insert comprises an enlarged externally threaded shaft having an enlarged head at an upper end. The insert also has a threaded internal opening therein extending in an axial direction inwardly from an open end in the upper end of the head. The externally threaded shaft extends through an opening in any insulation layer and into engagement with an internally threaded opening in the upper surface of the table. The test product or mounting fixture is mounted to the upper surface of the insert by a bolt that engages the threaded internal opening in the insert. The insert improves the transmission of vibration forces to the test product, notwithstanding the presence or absence of an insulation layer. The insert also provides improved temperature uniformity by providing for air circulation between the test product and the vibration table.

The externally threaded shaft on the insert has an outer diameter that is sufficiently large that the insert transmits acceleration forces from the table to the test product without substantial attenuation caused by the insert. Desirably, the external diameter of the threaded shaft on the insert is greater than ⅜ inches and preferably is at least about ¾ inches. The threaded shaft on the insert can be locked in the opening in the upper plate by a bolt locking compound.

The preferred insert not only has an external diameter larger than a conventional ⅜ inch plate opening but it also has an internal threaded opening that is longer than the threaded opening in the mounting plate, so that longer attachment bolts can be employed. Both features improve thread contact area and thereby improve the force transmission properties of the table, even without the use of an insulating layer on the table.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the drawings, a shaker table10constructed in accordance with the present invention is shown inFIG. 1. Shaker table10includes an aluminum upper plate12(typically about one-half inch thick), lower plate14, and edge spacers16separating the upper and lower plates. Edge spacers16are attached to the upper and lower plates by bolts18and typically space the plates about one-half inch apart. Bolts18typically 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 nuts19on 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.

An insulating sheet or layer20, preferably a ceramic material, is bonded on the top of upper plate12. 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 insulating sheet and upper surface of plate12include a plurality of openings36and24, 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 inserts26of the present invention.

Vibrators28of conventional design are mounted on the underside of the lower plate. 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. Bolts44extend upwardly through and into ends of vibrators28and into the lower plate to mount the vibrators securely to the bottom of lower plate14.

The inserts26include an externally threaded shaft30with a head32on one end. The head can have a hexagonal shape. The inserts have internal threaded openings34therethrough. Inserts26extend through openings36in the insulator sheet20and then are threaded into internally threaded openings24in the upper plate. The inserts are threaded all the way into and preferably through the openings in the upper plate until the heads32of the inserts securely abut the insulator sheet20. Desirably the insert shafts extend past the bottom of plate12(typically about ¼ inch).

Inserts26are mounted securely in threaded openings24in 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 openings24in 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.

Inserts26in the preferred embodiment are ¾×1 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 surface48of the insert by means of attachment bolts50that engage internal threaded openings34in the insert to attach the test product or a mounting fixture for the test product (both referred to by numeral52) to the upper surface of the spacer. Some parts can be screwed directly to the table. Other parts require a mounting fixture. Attachment bolts50in the exemplary embodiment are ⅜ inch (10 mm) bolts. Bolts50can 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 bolt50. 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.