Abstract:
Vibration absorbers are provided which utilize a torsional spring and a dynamic mass to control vibrations that occur within the structure to which they are attached. Additionally, pipeline systems are provided which utilize such vibration absorbers including a mass and a torsional spring to control the vibrations that occur within the pipeline system. In various embodiments of the present invention, the torsional spring is one or more elastomer elements coupled between the dynamic mass and a support frame to produce a spring force responsive to pivotal rotation of the dynamic mass relative to the support frame. In various embodiments, one or more elastomer elements arranged in series or in parallel may be used as the torsional spring to provide a desired vibratory dampening characteristic for applications, such as pipelines. Use of such torsional springs may provide a more simplified design with less mechanical joints which may be less susceptible to failure in harsh environments.

Description:
FIELD OF INVENTION 
     The present invention relates to vibration control systems, and more particularly to vibration absorbers for reducing structural vibrations. 
     BACKGROUND OF INVENTION 
     Vibration absorbers generally include a mass coupled with a spring and are used to control vibrations that occur within the member to which they are attached. They are typically resonant devices and their natural frequencies are generally tuned to coincide with a predominant disturbance frequency of the vibrating member. Examples of vibration absorbers may be found in U.S. Pat. Nos. 3,288,419; 3,322,379; 3,388,772; 3,490,556; 3,668,939; 3,767,181; 4,230,291; 4,697,781; 4,852,848; 5,052,530 and 5,072,801. 
     One known beneficial application of vibration absorbers is on above-ground pipelines. Pipelines are often used to transfer fluids, such as oil and natural gas, across large distances. In many instances, these pipelines are installed above-ground by suspending the pipeline on spaced-apart supports. Pipelines, however, may be susceptible to damage from vibrations that may occur due to such things as wind-induced vortex shedding and earthquakes. An example of an application of a vibration absorber to a pipeline is described in U.S. Pat. No. 5,193,644 to Hart et al. Hart et al. describes a vibration damper that includes a mass suspended from a pipeline by interconnected damper members which generally act as a linear spring. Other examples of a vibration absorbers used in conjunction with a pipe are found in U.S. Pat. Nos. 4,576,356 and 4,716,986. 
     SUMMARY OF INVENTION 
     According to the present invention, vibration absorbers are provided which utilize a torsional spring and a dynamic mass to control vibrations that occur within a structure to which they are attached. Additionally, pipeline systems are provided which utilize such vibration absorbers to control vibrations that occur within the pipeline system. In various embodiments of the present invention, the torsional spring is one or more elastomer elements connected between the dynamic mass and a support frame to produce a spring force responsive to pivotal rotation of the dynamic mass relative to the support frame. In various embodiments, one or more elastomer elements arranged in series or in parallel may be used as the torsional spring to provide a desired vibratory dampening characteristic for applications, such as pipelines. Use of such torsional springs may provide a more simplified design with less mechanical joints which may be less susceptible to failure in harsh environments. 
     In embodiments of the present invention, the vibration absorber includes a support frame, a dynamic mass pivotally connected to the support frame for movement about a pivotal axis and a torsional spring connected between the dynamic mass and the support frame that provides a spring force responsive to pivotal rotation of the dynamic mass relative to the support frame. The torsional spring may be positioned along the pivotal axis. The torsional spring may be a coil spring or an elastomer element. The elastomer element may include a elastomeric disc. The elastomeric disc may be made from a material selected from the group consisting of natural rubber elastomer, synthetic elastomer or a blend of natural rubber elastomer and synthetic elastomer. 
     In other embodiments of the present invention, the support frame includes a first side member and a second side member offset from the first side member along the pivotal axis. The dynamic mass may be positioned between the first side member and the second side member along the pivotal axis. 
     In further embodiments of the present invention, the elastomer element further includes a first plate. The first plate may be connected to the elastomeric disc between the elastomeric disc and the first side member along the pivotal axis. The first plate may be connected to the first side member at a radial position offset from the pivotal axis. The first side member may include a slot configured to provide selectable rotational orientation of the dynamic mass relative to the support frame. 
     In still further embodiments of the present invention, the dynamic mass includes an arm and an adjustment mass movably mounted on the arm. The adjustment mass may include a plurality of metal plates. The arm may include a channel having a track formed therein such that the adjustment mass may be moved along the track to adjust a dynamic characteristic of the vibration absorber. The elastomer element may be connected between the first side member and the arm. The elastomer element may include a second plate that may be connected to the elastomeric disc. The second plate may also be connected to the side of the arm. 
     In still further embodiments of the present invention, the torsional spring further includes a second elastomer element connected between the second side member and the arm. The second elastomer element may, alternatively, be connected between the first elastomer element and the arm. 
     In still other embodiments of the present invention, the vibration absorber includes a support frame including a first side member and a second side member offset from the first side member, a dynamic mass pivotally connected to the support frame for movement about a pivotal axis, and an elastomer element connected between the dynamic mass and the support frame that provides a spring force responsive to pivotal rotation of the dynamic mass relative to the support frame. The elastomer element and the dynamic mass may be positioned between the first side member and the second side member along the pivotal axis. The vibration absorber may include a plurality of elastomer elements positioned between the first side member and the second side member along the pivotal axis. The elastomer elements may all be connected between the first side member and the arm. Alternatively, at least one elastomer element may be connected between the first side member and the arm and at least one elastomer element may be connected between the second side member and the arm. 
     In still further embodiments of the present invention, the vibration-reduced pipeline system includes a pipeline section and a vibration absorber connected to the pipeline section. The vibration absorber may include a support frame connected to the pipeline section, a dynamic mass pivotally connected to the support frame for movement about a pivotal axis, and a torsional spring connected between the dynamic mass and the support frame that provides a spring force responsive to pivotal rotation of the dynamic mass relative to the support frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of embodiments of a vibration absorber of the present invention. 
     FIG. 2 is an exploded perspective view of the vibration absorber of FIG.  1 . 
     FIG. 3 is a cross-sectional view of the vibration absorber of FIG. 1 taken along line  3 — 3 . 
     FIG. 4 is a perspective view of other embodiments of a vibration absorber according to the present invention. 
     FIG. 5 is a cross-sectional view of the vibration absorber of FIG. 4 taken along line  5 — 5 . 
     FIG. 6 is a perspective view of further embodiments of a pipeline system including a vibration absorber according to the present invention. 
     FIG. 7 is a cross-sectional view of the vibration absorber of FIG. 6 taken along line  7 — 7 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Instead, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. It will be understood that when an element such as an arm, elastomer element or side member is referred to as being “connected to” another element, it can be directly connected to the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly connected to” another element, there are no intervening elements present. Like numbers refer to like elements throughout. 
     With reference to FIGS. 1 and 2, one embodiment of a vibration absorber  100  according to the present invention will now be described. The illustrated vibration absorber  100  includes a support frame  110  and a dynamic mass  130  movably, preferably pivotally, connected to the support frame  110  for movement about a pivotal axis  102 . While pivotal mounting and rotation are preferred, the arm  132  may be connected to an end of the torsional spring  150  opposite an end of the torsional spring  150  connected to the support frame thereby resulting in a variety of movement components of the arm  132 , at least one of which includes a rotational component relative to a pivotal axis of the torsional spring  150 . The dynamic mass  130  in the illustrated embodiment of FIG. 1 includes an arm  132  and an adjustment mass  134 . A torsional spring  150  is coupled between the dynamic mass  130  and the support frame  110 . The torsional spring  150  provides a spring force responsive to pivotal rotation of the dynamic mass  130  relative to the support frame  110  about the pivotal axis  102 . 
     The support frame  110  in the illustrated embodiment includes a base portion  112  and side members  114   a ,  114   b  extending therefrom. The side members  114   a ,  114   b  each include a hole  116  positioned along, and substantially centered about, the pivotal axis  102  and configured to receive a shaft  117 . The shaft  117  may be a rod threaded at each end or a bolt threaded at one end and may be secured in position by a nut  122 . However, the shaft  117  may be retained in position to define the pivotal axis  102  by other means generally known to those of skill in the art such as riveting, welding, retaining rings, cotter pins, spring pins, etc. 
     Preferably, at least one of the side members  114   a ,  114   b  further includes a means for providing selectable rotational orientation of the dynamic mass  130  relative to the support frame  110 . This allows the initial angular orientation of the dynamic mass  130  to be set as desired and further may provide for repositioning of the angular orientation of the dynamic mass  130  to accommodate spring drift in the torsional spring  150  that may occur, for example, due to stress relaxation. Preferably, the means for providing selectable rotational orientation of the dynamic mass  130  relative to the support frame  110  is a semi-circular slot  118 . Other means for providing selectable rotational orientation of the dynamic mass  130  relative to the support frame  110 , such as individual holes, a gear system, cams or clamps, will be apparent to those of skill in the art and are included within the scope of this aspect of the present invention. 
     The side members  114   a ,  114   b  may further include slots  120  which are configured to receive straps  382  (FIG. 6) to secure the vibration absorber  100  to a structural member such as a pipeline section  392  (FIG.  6 ). Other suitable means for securing the vibration absorber  100  will be understood by those of skill in the art such as adhesives, welding, velcro, flexible straps with buckles or rachets, etc. The side members  114   a ,  114   b  may also include openings  124  for securing a cover which may protect the components of the vibration absorber  100  from exposure to sun, rain, snow, ice, etc. An example of such a cover is illustrated in U.S. patent application Ser. No. 09/178,003 entitled “Pivoting Tuned Vibration Absorber and System Utilizing Same,” which application is hereby incorporated by reference as if fully set forth herein. Such a cover, however, need not be utilized with the present invention because, unlike previous vibration absorbers that generally have multiple pivots and more closely-spaced, moving components, various embodiments of the present invention may have only one pivot and fewer components constructed in a more open architecture. The lesser number of components and open architecture may allow the elements (such as snow and ice) to flow more freely through the components without collecting on them and the reduction in pivots may reduce the number of mechanical joints that might be adversely affected by the buildup of snow and ice. 
     As illustrated in FIGS. 1 and 2, the dynamic mass  130  includes an arm  132  extending away from the pivotal axis  102  and an adjustment mass  134  moveably mounted on the arm  132 . Preferably, the arm  132  extends transversely, preferably perpendicularly, away from the pivotal axis  102  as illustrated in FIG.  1 . The shaft  117  extends through holes  142  in the arm  132  and is configured to allow the arm  132  to pivot about the pivotal axis  102 . As will be understood by those of skill in the art, in keeping with the present invention, the shaft  117  may be rigidly connected to the side members  114   a ,  114   b  or the arm  132  so long as the arm  132  remains coupled to the torsional spring  150  in a manner such that the torsional spring  150  produces a spring force when the arm  132  is rotated relative to the support frame  110 . 
     As shown in FIGS. 1 and 2, the arm  132  comprises a channel including a track  136  formed therein. The arm  132  may include drainage openings  133  that may provide for drainage of the channel. Channel nuts  138  are slidably received in the track  136  and, together with bolts  140 , secure the adjustment mass  134  to the arm  132  at a selected position. The adjustment mass  134  in the illustrated embodiment may be repositioned along the track  136  by loosening the bolts  140  slightly and sliding the adjustment mass  134  inwardly or outwardly along the arm  132  to adjust the natural frequency f n , of the vibration absorber  100 . However, it is to be understood that the position of the adjustment mass  134  need not be adjustable and may be pre-configured with a desired natural frequency and a fixed position. Other means for providing adjustability for the adjustment mass  134  may also be utilized including gears, clamps, friction locks, clevis pins, spring pins and other mechanisms as will be understood by those of skill in the art. 
     As shown in FIG. 1, the adjustment mass  134  includes a bumper  146  that may protect the pipeline section  392  (FIG. 6) from damage if contacted by the adjustment mass  134 . The adjustment mass  134  may be made from a wide variety of known materials, but is preferably made from denser materials typically used for weights in order to limit the size of the adjustment mass  134 . The adjustment mass  134  may, for example, be formed from cast iron. Alternatively, the arm  132  and adjustment mass  134  could be cast or formed as an integral unit. Preferably, the adjustment mass  134  is formed from one or more metal plates  135  which allows the total mass, and, thus, the natural frequency f n , of the vibration absorber  100  to be adjusted by adding or removing individual metal plates  135 . 
     The torsional spring  150  as shown in FIG. 1 is positioned along the pivotal axis  102 . As seen in the embodiment of FIG. 3, the torsional spring  150  is an elastomer element. The elastomer element  150  in the illustrated embodiment includes an elastomeric disc  155 , a first plate  156   a  and a second plate  156   b . The elastomeric disc  155  as shown in FIG. 3 includes a bore  151  extending longitudinally through a central portion thereof and configured to receive the shaft  117 . The elastomer element  150  is preferably loaded in torsion, and, more preferably, pure torsion, about the pivotal axis  102 . Alternatively, other types of torsional springs, such as coil springs, torsion bars and linear springs configured to act torsionally may be used as will be understood by those of skill in the art. In addition, springs configured to act in cocking (e.g. putting a block of elastomer material under the arm  132  which then produces a spring force when the arm  132  is forced into the elastomer material, also referred to as a spring acting in coning, or configured to act in bending) may be used as will be understood by those of skill in the art. 
     The first plate  156   a  is preferably bonded to one end of the elastomeric disc  155 , and the second plate  156   b  is preferably bonded to an opposing end of the elastomeric disc  155  via bonding means such as injection or transfer bonding. However, as will be understood by those of skill in the art, a variety of other connecting means such as post-vulcanization bonding may also be utilized. The elastomer element  150  may be formed in part by injecting the elastomer material through the holes  157  into the region between the first plate  156   a  and the second plate  156   b . The first and second plates  156   a ,  156   b  are preferably square and include corner holes  158  and a center hole  153 . As best seen in FIG. 3, the shaft  117  is received through the hole  116  of the first side member  114   a , the center hole  153  of the first plate  156   a , the bore  151 , the center hole  153  of the second plate  156   b , the holes  142  in the arm  132 , and the hole  116  of the second side member  114   b  to defme the pivotal axis  102 . As best seen in FIG. 2, the shaft  117  may also be inserted through bearings  148  positioned in the holes  142  in order to reduce the wear on the holes  142  and the shaft  117 . A spacer  149  may also be used in order to limit the lateral movement of the elastomer element  150  and the dynamic mass  130  along the pivotal axis  102 . A washer  147  may also be used. 
     Referring now to FIG. 2, the arm  132  may be attached to the second plate  156   b  via the corner holes  158  by fasteners such as a bolt  144  and nut  145 . The bolt  128  in the illustrated embodiment is inserted through the semi-circular slot  118  of the first side member  114   a  and another corner hole  158  of the first plate  156   a  and threadedly secured by the nut  129  thus fixing the position of the first plate  156   a  relative to the first side member  114   a . Positioning of the secured point of the bolt  128  in the semi-circular slot  118  further fixes the initial (or rest) rotational position of the arm  132  relative to the support frame  110 . A washer  127  may also be used. 
     With reference to FIGS. 4 and 5, a vibration absorber  200  according to a further embodiment of the present invention will now be described. The vibration absorber  200  includes a support frame  210  and a dynamic mass  230  pivotally connected to the support frame  210  for movement about a pivotal axis  202 . The dynamic mass  230  in the illustrated embodiment of FIG. 4 includes an arm  232  and an adjustment mass  234 . A torsional spring  250  is coupled to the dynamic mass  230  and the support frame  210 . The torsional spring  250  provides a spring force responsive to pivotal rotation of the dynamic mass  230  relative to the support frame  210  about the pivotal axis  202 . The support frame  210 , the dynamic mass  230  and the pivotal axis  202  may generally be constructed in the same manner as the support frame  110 , the dynamic mass  130  and the pivotal axis  102  described above with reference to the vibration absorber  100 . 
     In the vibration absorber  200 , the torsional spring  250  is positioned along the pivotal axis  202 . As shown in the illustrated embodiment, the torsional spring  250  includes a first elastomer element  254  and a second elastomer element  274 . The first elastomer element  254  and the second elastomer element  274  may both be generally constructed in the same manner as the elastomer element  150  described above with reference to the vibration absorber  100 . 
     Referring to FIGS. 4 and 5, the support frame  210  includes side members  214   a ,  214   b . The side members  214   a ,  214   b  each include a hole  216 , and at least one side member  214   a ,  214   b  includes a semi-circular slot  218 . The dynamic mass  230  includes an arm  232 , and the arm  232  has holes  242 . The first elastomer element  254  includes an elastomeric disc  255 , a first plate  256   a  and a second plate  256   b . The elastomeric disc  255  includes a bore  251 . The first and second plates  256   a ,  256   b  include corner holes  258  and a center hole  253 . The second elastomer element  274  includes an elastomeric disc  275 , a third plate  276   a  and a fourth plate  276   b . The elastomeric disc  275  includes a bore  271 . The third and fourth plates  276   a ,  276   b  include comer holes  278  and a center hole  273 . 
     As best seen in FIG. 5, the shaft  217  is received through the hole  216  of the first side member  214   a , the center hole  253  of the first plate  256   a , the bore  251 , the center hole  253  of the second plate  256   b , the center hole of the third plate  276   a , the bore  271 , the center hole  273  of the fourth plate  276   b , the holes  242  in the arm  232 , and the hole  216  of the second side member  214   b  to define the pivotal axis  202 . The shaft  217  may also be inserted through bearings positioned in the holes  242  in order to reduce the wear on the holes  242  and the shaft  217 . A washer  247  may also be used. 
     The second plate  256   b  in the illustrated embodiment is secured to the third plate  276   a  via the comer holes  258 ,  278  by bolts  279 . As will be appreciated by those of skill in the art, the second plate  256   b  and the third plate  276   a  could be combined into one plate thus integrating the first and second elastomer elements  254 ,  274  into a single assembly including two elastomer discs  255 ,  275 . The arm  232  may be attached to the fourth plate  276   b  via the comer holes  278  by fasteners  277 . The bolt  228  in the illustrated embodiment is inserted through the semi-circular slot  218  and another comer hole  258  of the first plate  256   a  and threadedly secured by a nut thus fixing the position of the first plate  256   a  relative to the first side member  214   a . A washer  227  may also be used. Positioning of the secured point of bolt  228  in the semi-circular slot  218  further fixes the initial (or rest) rotational position of the arm  232  relative to the support frame  210 . 
     The elastomer elements  254 ,  274  in the vibration absorber  200  are in series as that term is used herein. Assuming substantially identical elastomer elements, using multiple elastomer elements in series may produce a vibration absorber with a lower range of possible natural frequencies f n  than a vibration absorber that uses only one such elastomer element (e.g., vibration absorber  100 ). This is the expected result as connecting elastomer elements in series generally provides a softer effective spring rate than an individual elastomer element. 
     With reference to FIGS. 6 and 7, a vibration absorber  300  according to a further embodiment of the present invention will now be described. The vibration absorber  300  includes a support frame  310  and a dynamic mass  330  pivotally connected to the support frame  310  for movement about a pivotal axis  302 . The dynamic mass  330  in the illustrated embodiment of FIG. 6 includes an arm  332  and an adjustment mass  334 . A torsional spring  350  is coupled to the dynamic mass  330  and the support frame  310 . The torsional spring  350  provides a spring force responsive to pivotal rotation of the dynamic mass  330  relative to the support frame  310  about the pivotal axis  302 . The support frame  310 , the dynamic mass  330  and the pivotal axis  302  may generally be constructed in the same manner as the support frame  110 , the dynamic mass  130  and the pivotal axis  102  described above with reference to the vibration absorber  100 . 
     In the vibration absorber  300 , the torsional spring  350  is positioned along the pivotal axis  302 . As shown in the illustrated embodiment, the torsional spring  350  includes a first elastomer element  354  and a second elastomer element  374 . The first elastomer element  354  and the second elastomer element  374  may both be generally constructed in the same manner as the elastomer element  150  described above with reference to the vibration absorber  100 . 
     The support frame  310  includes side members  314   a ,  314   b . The side members  314   a ,  314   b  each include a hole  316 . The first side member  314   a  includes a semi-circular slot  318   a  and the second side member  314   b  includes a semi-circular slot  318   b . The dynamic mass  330  includes an arm  332 , and the arm  332  has holes  342 . The first elastomer element  354  includes an elastomeric disc  355 , a first plate  356   a  and a second plate  356   b . The elastomeric disc  355  includes a bore  351 . The first and second plates  356   a ,  356   b  include comer holes  358  and a center hole  353 . The second elastomer element  374  includes an elastom eric disc  375 , a third plate  376   a  and a fourth plate  376   b . The elastomeric disc  375  includes a bore  371 . The third and fourth plates  376   a ,  376   b  include comer holes  378  and a center hole  373 . 
     As best seen in FIG. 7, the shaft  317  extends through the hole  316  of the first side member  314   a , the center hole  353  of the first plate  356   a , the bore  351 , the center hole  353  of the second plate  356   b , the holes  342  in the arm  332 , the center hole  373  of the third plate  376   a , the bore  371 , the center hole  373  of the fourth plate  376   b , and the hole  316  of the second side member  314   b . In other words, in the vibration absorber  300  the elastomer elements  354 ,  374  are placed in parallel (i.e., with the arm  332  intervening) as contrasted with the serial sequence of the vibration absorber  200 . The shaft  317  may also be inserted through bearings positioned in holes  342  in order to reduce the wear on the holes  342  and shaft  317 . A washer  347  may also be used. The arm  332  may be attached to the second plate  356   b  and the third plate  376   a  via the comer holes  358 ,  378  by fasteners  377 . The bolt  328  in the illustrated embodiment is inserted through the semi-circular slot  318   b  and another comer hole  378  of the fourth plate  376   b  and threadedly secured by a nut thus fixing the position of the fourth plate  376   b  relative to the second side member  314   b . A washer  327  may also be used. The first plate  356   a  may be similarly secured. Positioning of the secured point of the bolt  328  in the semi-circular slot  318   b  and the corresponding attachment of the first plate  356   a  further fixes the initial (or rest) rotational position of the arm  332  relative to the support frame  310 . 
     The elastomer elements  354 ,  374  in the vibration absorber  300  are in parallel as that term is defined herein. Assuming substantially identical elastomer elements, using multiple elastomer elements in parallel may produce a vibration absorber with a higher range of possible natural frequencies f n  than a vibration absorber whose elastomer elements are in series (e.g., vibration absorber  200 ). This is the expected result as connecting elastomer elements in parallel generally provides a stiffer effective spring rate than such elastomer elements connected in series. 
     The vibration absorber  300  is illustrated in FIGS. 6 and 7 connected to a pipeline section  392  to form a pipeline system  390 . Such a system may similarly be provided using the vibration absorbers  100 ,  200  of FIGS. 1 through 5. The support frame  310  may be secured to a pipeline section  392  in order to receive vibrations therefrom. The support frame  310  as illustrated in FIG. 6, is secured to the pipeline section  392  by at least one strap assembly  380 . Each illustrated strap assembly  380  includes first and second straps  382  received into the slots  320  at one end and secured together by fasteners  384  at an opposite end thereof. A liner  186  (FIG.  2 ),  386  may be used between the base portion  312  and the pipeline section  392  to protect the pipeline section  392 . 
     In operation, vibrations, such as wind-induced vertical vibrations, of the pipeline section  392  may cause the dynamic mass  330  to move, more particular, to oscillate relative to the support frame  310  by rotating about the pivotal axis  302 . These oscillations load and unload the torsional spring  350 . The torsional spring  350  and dynamic mass  330  are preferably chosen and configured to provide a natural frequency f n  of the vibration absorber  300  that is substantially coincident with the disturbance frequency f d  of the pipeline section  392  that is excited, for example, by wind passing over the pipeline section  392  while further providing sufficient reactive forces for particular applications. Most preferably, the vibration absorber  300  is configured such that it exhibits the desired natural frequency f n  when subjected to the average daily temperature of the environment in which the vibration absorber  300  will operate in order to reduce any drift in the natural frequency that may occur as a result of temperature variations. 
     In various embodiments of the present invention, adjustments to the natural frequency f n  may be made in the field by readjusting the position of the adjustment mass  134 ,  234 ,  334  on the arm  132 ,  232 ,  332  by loosening the bolts  140 ,  240 ,  340  and sliding the adjustment mass  134 ,  234 ,  334  in or out along the arm  132 ,  232 ,  332  and then resecuring it or by adding or removing metal plates  135 ,  235 ,  335 . Moving the adjustment mass  134 ,  234 ,  334  outwardly (i.e., away from the pivotal axis  102 ,  202 ,  302 ) on the arm  132 ,  232 ,  332  or adding metal plates  135 ,  235 ,  335  generally lowers the natural frequency f n  and, conversely, moving the adjustment mass  134 ,  234 ,  334  inwardly on the arm  132 ,  232 ,  332  or removing metal plates  135 ,  235 ,  335  generally increases the natural frequency f n . As will be understood by those of skill in the art, the range of possible natural frequencies f n  may also be increased by increasing the length of the arm  132 ,  232 ,  332  to allow a greater range of displacements of the adjustment mass  134 ,  234 ,  334  from the pivotal axis  102 ,  202 ,  302 . Further adjustments in the natural frequency f n  may be made by adjusting the stiffness of at least one elastomer element  150 ,  254 ,  274 ,  354 ,  374 . Such adjustment to an elastomer element  150 ,  254 ,  274 ,  354 , 374  may be provided through use of an elastomeric disc  155 ,  255 ,  275 ,  355 ,  375  manufactured from a different durometer material or in a different geometry (for example but not limited to, varying the thickness). Using elastomer discs  155 ,  255 ,  275 ,  355 ,  375  of higher durometer or thinner construction generally increases the natural frequency f n . Conversely, using elastomer discs  155 ,  255 ,  275 ,  355 ,  375  of lower durometer or thicker construction generally decreases the natural frequency f n . Even further adjustments in the natural frequency f n  may be made by combining additional elastomer elements in series (FIG. 4) or in parallel (FIG. 6) with the first elastomer element  150 ,  254 ,  354 . 
     By way of example and not limitation, the elastomer element  150 ,  254 ,  354  may be comprised of a natural rubber elastomer, a synthetic elastomer or a blend of natural rubber elastomer and synthetic elastomer which is formulated for low temperature spring characteristics. The durometer of the elastomer element  150 ,  254 ,  354  when the vibration absorber  100 ,  200 ,  300  is applied to a pipeline section  392  is preferably between about 46 and 65 Shore A. It is to be understood that variations in the characteristics of the apparatus to be controlled may affect the preferred range of durometer readings. By way of example and not limitation, the adjustment mass  134 ,  234 ,  334  may weigh between about 10 pounds to 90 pounds. Preferably, the arm  132 ,  232 ,  332  is manufactured from standard steel and the straps  382  and support frame  110 ,  210 ,  310  are made of stamped steel. 
     As will be understood to those of skill in the art, the vibration absorbers  100 ,  200 ,  300  may be referred to as tuned vibration absorbers. 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.