US2468043A - Article supporting structure - Google Patents

Description

P 1949- c. E. CREDE ET AL ARTHI-CLE SUPPORTING STRUCTURE 2 Sheets-Sheet 1 Filed Oct. 51, 1945 LET-E I CHARLES E. CREDE V J. PAUL WALSH Guam,

' April 19491 c. E. cREDE ET AL 2,468,043

ARTICLE SUPPORTING STRUCTURE- Filed Oct. 31, 1945 2 Sheets-Sheet 2 CHARLES E. CREDE J. PAUL WALSXH Patented A r. 26, 1949 ARTICLE SUPPORTING STRUCTURE Charles E.- Crede, Cambridge, Mass., and James Paul Walsh, Washington, D. 0.

Application October 31, 1945, Serial No. 625,922

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) Claims.

This invention relates to article supporting structures, and more particularly to a structure for resiliently supporting machinery or other vibrating parts and for isolating the machinery or vibrating parts from the building in which the machinery is operating, or from the structural parts supporting the machinery.

The desirability of the isolation of rotating 0r reciprocating machinery to reduce the magnitude of the oscillating forces transmitted to adjacent structures is well known to the art. Where operating speeds are high and the oscillating forces are of moderate intensity, an adequate solution is relatively easy to achieve because little care need be taken in design to obtain a sufficiently efficient isolating structure. However, a vibration testing machine requires carefu1 design of the supporting structure, or isolator, because the forces to be isolated are of great magnitude and because such machines operate over a wide range of speed.

The supporting structure of this invention is particularly intended for use with vibration testing machines, but is applicable to other types of machinery where the mass movement, or excursion, is large and as a consequence of which vibrations may be set up in the building and may constitute a source of extensive damage to the foundation of the building and to the supporting structure. Since in vibration testing machines the purpose is to produce vibration in the article being tested, no compensation for or damping of the movement of the vibrating machine may be employed, as this would automatically reduce the effectiveness of the vibration test. Thus it becomes necessary to provide a support for the vibration testing machine which will isolate the vibrating machine from the building and yet one which will not interfere with the operation of the vibration testing machine.

An object of the present invention is to provide an efiicient and effective article supporting structure, and more particularly to provide a structure which will effectively isolate the article supported thereon from a foundation.

In accordance with one embodiment of this invention, an article supporting structure may be provided comprising a large, resiliently supported block, the block being so shaped and of such a mass and the resilient support so designed and so associated with the block that the six natural frequencies of the block are below the minimum operating frequency of the article to be supported thereon and that the six natural frequencies of the supporting structure are substan-' tially equal.

Other objects and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the drawings, wherein:

Fig. 1 is a plan view of a supporting structure constructed in accordance with this invention and having a vibration testing machine mounted thereon;

Fig. 2 is a side view of the structure shown in Fig. 1; and

Fig. 3 is an end view of the structure shown in Fig. 1 and Fig. 2.

While the supporting structure of this invention will :be described primarily in conjunction with a vibration testing machine, it will be understood that the supporting structure of this invention is useful in any application where it is necessary to both support and isolate from its foundation machiner producing recurrent forces of large magnitude.

The requirements for a successful vibration machine installation are first that the vibration amplitude of the supporting structure be small relative to the amplitude of the vibrating machine, and second that the forces transmitted to the supporting foundation be small. The first requirement may be met by employing a mounting structure whose mass is large compared with the mass of the vibrating parts of the testing machine and apparatus undergoing test. This is ordinarily readily accomplished. The second requirement may be met by properly adjusting the various natural frequencies of the mounted machine on its resilient support. This is accomplished in accordance with the present invention.

Various methods have been devised for determining the natural frequencies of a body mounted on springs. While a knowledge of the natural frequencies of the supporting structure of the present invention is essential, these may be determined in accordance with any of the wellknown methods, which form no part of the present invention. The problem to be solved in accordance with this invention, however, is that of controlling the natural frequencies of the mounted machine. For high-speed machines, it has been found that it is generally permissible to have a substantial difference among the several natural frequencies, provided the highest frequency is not above the predetermined allowable maximum. For variable speed machines capable of operating over a wide range of speeds,

I and in this class may be included most vibration testing machines, isolation conditions place an upper limit on allowable natural frequency, while stability conditions, on the other hand, place a corresponding lower limit. Frequently, the upper and lower limits are so close together that all natural frequencies must be substantially equal; this is particularly true of supporting structures for machines capable of operating in low speed ranges.

In determining the natural frequency of the supporting structure and machine supported thereon, the transmissibility, or percentage of impressed force transmitted to the foundation, is an important consideration. The transmitted forces become high unless the natural frequency of the mounting structure is substantially less than the frequency of the forces impressed by the operation of the vibrating machine. This applies to the natural frequencies in any mode of vibration likely to be excited by the impressed force. It frequently happens, particularly in vibration testing work, where the apparatus under test is excited to vibrate in random directions at many different frequencies, that large amplitude vibration of the mounted machine results when the frequency of the impressed force becomes approximately equal to any natural frequency of the machine on its spring support. At this point, the percentage of impressed forces transmitted to the foundation becomes very h gh and may result in damage to the foundation and adjacent structures.

Essentially, in order to provide a successful supporting structure for a vibration testing machine, or other apparatus producing recurrent forces of large magnitude, the supporting structure must be stable in order to isolate the vibration testing machine from the foundation. The stability of any resiliently Supported structure may be determined by" assuming an extreme displacement of the entire system and calculating the energy stored in the springs by this displacement. If the energy that is so stored exceeds the loss of potential energy of the system caused by this mainmum displacement, the installation is then considered stable for operating conditions involving no greater displacements.

In practice, to avoid interfering with the operation of thesupported machine, the supporting structure must have sufiicient mass, and consequently inertia, so that the movement of the supporting structure is insufficient to contribute materially to the absolute motion of the machine supported thereon. This in turn implies that all natural frequencies of the supporting structure must be well below any operating frequency which will be encountered, and also that the ratio of the sum of the vibrating masses to the total mass of the resiliently supported system minus the vibratory masses rnust be equal to the ratio of the allowable amplitude of displacement of the machine and foundation to the amplitude of displacement .of the vibration table.

In accordance with the present invention, a supporting structure has been devised which substantially meets the foregoing requirements. In the drawings, for the purpose of illustrating the application and operation of this invention, a conventional vibration testing machine, has been shown including a frame '4 on which is mounted a variable speed motor 5 connected through a coupling device 6 to a varidrive unit I. Suit able means, such as the pulley '8, associated with the varidrive unit I and belt 9, may be employed for applying driving power to an eccentric driving unit it of a testing platform H, the platform ll being movable both vertically and horizontally at varying rates of speeds and with varying amplitudes of movement. In practice, it has been found that when the frame 4 .of such a machine is mounted directly, or even through a conventional resilient means, on the foundation of a building, the vibrational forces transmitted to the foundation may cause severe damage thereto.

Accordingly, in order to provide a supporting structure which will not only isolate the machine from-the foundation, but which also will in no way interfere with the operation of the machine, a large block 12 is provided, which is T-shaped in cross section, having heavy flanges l3 integrally formed with the .upper portions of either side thereof and extending substantially at right angles thereto. Such a block may be formed conveniently from concrete and ordinarily reinforcing beams willbe incorporated-in the casting as may be required to provide the-necessary structural strength. The base 4 .of the vibration testing machine is rigidly fixedto the upper surface of the block 12 by ,a number of bolts M, or by other suitable means. The dimensions of the block 12 and associated flanges have a definite cooperative relation with the size of the base of the machine to be mounted thereon; while the weight of the block has} a definite cooperative relation with the mass required in the entire spring-supported supporting structure comprised of all parts of said supporting structure except the vibrating parts and thus including all parts which are rigidly fixed to "the block l2. This latter is determined in accordance with the requirements of the formula hereinbefore stated, that is, that the ratio of the sum of the vibrating masses comprised of the table 1.! and any elements movable therewith to the total mass of the supporting system be equal to the ratio of the allowable .amplitude of displacement of the machine and foundation to the amplitude of displacement of the vibration platform. Since this formula defines a minimum permissible ratio, it is, of course, possible-to increase'the weight of the supporting system beyond that indicated, providing thereby a margin of safety. The maximum weight'will, of course, be limited by practical considerations such as the maximum floor loading which is permissible. The primary requirement isthat the supporting structure, including therein the block l2, and all parts of thevibration testing machine except the platform 1-! and parts movable therewith, have sufficient mass so that the movement of the supporting structure does not contribute appreciably to the absolute motion of the vibratiommachin'esplatform and parts associated therewith.

The block 12 is' resiliently supported on the building foundation l5 by a number of helical springs t6, the upper .ends of which bear against the undersidespf the flanges l3, while the lower ends thereof are supported .on beams H by adjustable plates IS, {the beams H being mounted directly on the foundation l5 and spaced apart so that the lower, :or mid-portion, of the block [2 extends downwardly therebetween. Three threaded .members 2!] are associated with each of the spring supportingplates l8 .andare spaced thereon soas to provide a tripodsupport for the plates, thus .permittingadjusting the level .of the springs. While helicalsprings have beenshown, it will be understood that other types of resilient supports may be substituted therefor such as rub- 5. her blocks, provided the required resilience and strength may be obtained therefrom.

The length of each of the springs under normal compression, that is when the vibration testing machine is not operating, is selected so that the base of the block 12 is supported a sufficient distance above the foundation I so that during the operation of the vibration testing machine, the base of the block 12 will not strike the foundation l5. By providing the flanges 13 on the block 12, it will be apparent that the upper ends of the supporting springs I6 may be located in a plane with the center of gravity of the supporting structure including all non-vibrating parts of the machine. Locating the upper ends of the supporting springs in a plane spaced from the lateral plane extending through the center of gravity, correspondingly reduces the stability of the support. At the same time, by reason of the flanges 13, the spacing of the springs from the vertical axis of the block may be selected so that the required leverage is provided in conjunction with the vertical stiffness of the springs to cause the rotational modes of vibration to be equal to the vertical mode. In short, the springs are so spaced in the lateral plane that the distance from the center of gravity equals the radius of gyration about an axis extending through the center of gravity.

The compressional strength of the springs I6 is selected so that the maximum natural frequency for the spring system will be below the minimum frequency of oscillationof the platform I I. Where two springs are used on either side of the block, as shown in the drawings, and the weight of the supporting structure and supported machine is on the order of 4000 pounds, the required vertical stiffness will be on the order of 2600 pounds per inch for a total of four springs, or 650 pounds per inch for each spring. This will provide a vertical natural frequency on the order of 2.5 cycles per second. Since most vibration machines have a minimum operating frequency on the order of 5 cycles per second, this is sufiiciently below the minimum to provide the required isolation.

In accordance with the present invention, it has been found that in order to provide a supporting system which will efficiently isolate the supported structure in all modes, in addition to the requirements heretofore enmerated, the springs l6 must be selected so that their lateral stiffness is such as to cause the system to have natural frequencies in the horizontal planesubstantially equal to the natural frequencies in the vertical plane. Thus in the embodiment shown, where the upper ends of the springs are located in the same plane with the center of gravity of the entire supporting structure and are spaced from the center of gravity a distance equivalent to the radius of gyration of the supporting structure about an axis extending through the center of gravity, it is necessary that the horizontal stiffness of each spring substantially equals 650 pounds per inch, which is the same as the assumed desired vertical stiffness. In this manner the requirement, heretofore stated, that the natural frequencies of the supporting structure be substantially equal in all modes is in effect met, that is to say that the natural frequency of vibration in horizontal translation and the natural frequency in rotation about an axis extending through the center of gravity will both be equal to the natural frequency of translation in the vertical mode.

Where the upper ends of the springs are not located "in aplane with the center ofgravity of of the springs will become necessary to compen sate for the different locations in order to meet these requirements. Such displacement of the location of the springs is highly undesirable in most cases and in all cases renders calculation of the required springs characteristics extremely difficult.

While but one embodiment of this invention has been shown and described, it will be understood that many changes and modifications may be made therein without departing from the spirit or scope of the present invention.

The invention shown and described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. An article supporting structure comprising a base supported on resilient elements each having substantially equal vertical and horizontal stiifnesses, all of said elements being located in a horizontal plane through the center of gravity of said base and positioned so that the rotational natural frequencies of said base are substantially equal to the translational natural frequencies.

2. An article supporting structure comprising a base supported on resilient elements each having substantially equal vertical and horizontal stiffnesses, all of said elements being located substantially in a horizontal plane through the center of gravity of said base and spaced horizontally from said center of gravity a distance substantially equal to the radius of gyration of said base.

3. An article supporting structure comprising a base having a relatively wide upper part and a relatively narrow lower part of substantially equal mass and resilient supporting elements having substantially equal vertical and horizontal stiffnesses, said resilient elements being located below said upper part and beside said lower part and spaced apart a distance substantially equal to twice the radius of gyration of said base.

4. A supporting structure for a driving means and vibrating parts driven by the driving means comprising a supporting frame for the driving means and vibrating parts, a relatively massive support for the said frame, a foundation, and resilient means supporting said massive support on the foundation, the sum of the masses of said massive support, frame and all non-vibrating parts of the driving means and platform having the same ratio to the mass of the vibrating parts, as the ratio of the allowable amplitude of displacement of the foundation to the amplitude of displacement of the vibrating parts, said resilient means being of a resiliency rendering the natural frequencies of the said sum of masses substantially equal in all modes and all less than the minimum operating frequency of the vibrating parts.

5. A supporting structure, for a driving means and vibrating parts driven by the driving means, comprising a supporting frame for the driving means and vibrating parts, a relatively massive support for the said frame, a foundation, and resilient means for supporting said massive sup-

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