Abstract:
A vibration isolator having an upper housing defining an upper fluid chamber, a lower housing defining a lower fluid chamber, a piston resiliently disposed within the upper housing and the lower housing, a tuning passage defined by the piston, for providing fluid communication between the upper fluid chamber and the lower fluid chamber, a tuning fluid disposed within the tuning passage, the upper fluid chamber, and the lower fluid chamber. A reservoir in fluid communication with the upper fluid chamber for providing pressurization control of the vibration isolator. An expanded accumulator region for providing Increased pressure retention during maintenance and operation of the vibration isolator.

Description:
TECHNICAL FIELD 
     The present application relates in general to active vibration control. More specifically, the present application relates to an apparatus for isolating mechanical vibrations in structures or bodies that are subject to harmonic or oscillating displacements or forces. The present application is well suited for use in the field of aircraft, in particular, helicopters and other rotary wing aircraft. 
     DESCRIPTION OF THE PRIOR ART 
     For many years, effort has been directed toward the design of apparatus for isolating a vibrating body from transmitting its vibrations to another body. Such apparatuses are useful in a variety of technical fields in which it is desirable to isolate the vibration of an oscillating or vibrating device, such as an engine, from the remainder of the structure. Typical vibration isolation and attenuation devices (“isolators”) employ various combinations of the mechanical system elements (springs and mass) to adjust the frequency response characteristics of the overall system to achieve acceptable levels of vibration in the structures of interest in the system. One field in which these isolators find a great deal of use is in aircraft, wherein vibration-isolation systems are utilized to isolate the fuselage or other portions of an aircraft from mechanical vibrations, such as harmonic vibrations, which are associated with the propulsion system, and which arise from the engine, transmission, and propellers or rotors of the aircraft. 
     Vibration isolators are distinguishable from damping devices in the prior art that are erroneously referred to as “isolators.” A simple force equation for vibration is set forth as follows:
 
 F=m{umlaut over (x)}+c{dot over (x)}+kx  
 
     A vibration isolator utilizes inertial forces (m{umlaut over (x)}) to cancel elastic forces (kx). On the other hand, a damping device is concerned with utilizing dissipative effects (c{dot over (x)}) to remove energy from a vibrating system. 
     One important engineering objective during the design of an aircraft vibration-isolation system is to minimize the length, weight, and overall size including cross-section of the isolation device. This is a primary objective of all engineering efforts relating to aircraft. It is especially important in the design and manufacture of helicopters and other rotary wing aircraft, such as tilt rotor aircraft, which are required to hover against the dead weight of the craft, and which are, thus, somewhat constrained in their payload in comparison with fixed-wing aircraft. 
     Another important engineering objective during the design of vibration-isolation systems is the conservation of the engineering resources that have been expended in the design of other aspects of the aircraft or in the vibration-isolation system. In other words, it is an important industry objective to make incremental improvements in the performance of vibration isolation systems which do not require radical re-engineering or complete redesign of all of the components which are present in the existing vibration-isolation systems. 
     A marked departure in the field of vibration isolation, particularly as applied to aircraft and helicopters is disclosed in commonly assigned U.S. Pat. No. 4,236,607, titled “Vibration Suppression System,” issued 2 Dec. 1980, to Halwes, et al. (Halwes &#39;607). Halwes &#39;607 is incorporated herein by reference. Halwes &#39;607 discloses a vibration isolator, in which a dense, low-viscosity fluid is used as the “tuning” mass to counterbalance, or cancel, oscillating forces transmitted through the isolator. This isolator employs the principle that the acceleration of an oscillating mass is 180° out of phase with its displacement. 
     In Halwes &#39;607, it was recognized that the inertial characteristics of a dense, low-viscosity fluid, combined with a hydraulic advantage resulting from a piston arrangement, could harness the out-of-phase acceleration to generate counter-balancing forces to attenuate or cancel vibration. Halwes &#39;607 provided a much more compact, reliable, and efficient isolator than was provided in the prior art. The original dense, low-viscosity fluid contemplated by Halwes &#39;607 was mercury, which is toxic and highly corrosive. 
     Since Halwes&#39; early invention, much of the effort in this area has been directed toward replacing mercury as a fluid or to varying the dynamic response of a single isolator to attenuate differing vibration modes. An example of the latter is found in commonly assigned U.S. Pat. No. 5,439,082, titled “Hydraulic Inertial Vibration Isolator,” issued 8 Aug. 1995, to McKeown, et al. (McKeown &#39;082). McKeown &#39;082 is incorporated herein by reference. 
     Several factors affect the performance and characteristics of the Halwes-type isolator, including the density and viscosity of the fluid employed, the relative dimensions of components of the isolator, and the like. One improvement in the design of such isolators is disclosed in commonly assigned U.S. Pat. No. 6,009,983, titled “Method and Apparatus for Improved Isolation,” issued 4 Jan. 2000, to Stamps et al. (Stamps &#39;983). In Stamps &#39;983, a compound radius at the each end of the tuning passage was employed to provide a marked improvement in the performance of the isolator. Stamps &#39;983 is incorporated herein by reference. 
     Another area of improvement in the design of the Halwes-type isolator has been in an effort directed toward a means for changing the isolator&#39;s frequency in order to increase the isolator&#39;s effectiveness during operation. One development in the design of such isolators is disclosed in commonly assigned U.S. Pat. No. 5,435,531, titled “Vibration Isolation System,” issued 25 Jul. 1995, to Smith et al. (Smith &#39;531). In Smith &#39;531, an axially extendable sleeve is used in the inner wall of the tuning passage in order to change the length of the tuning passage, thereby changing the isolation frequency. Another development in the design of tunable Halwes-type isolators was disclosed in commonly assigned U.S. Pat. No. 5,704,596, titled “Vibration Isolation System,” issued 6 Jan. 1998, to Smith et al. (Smith &#39;596). In Smith &#39;596, a sleeve is used in the inner wall of the tuning passage in order to change the cross sectional area of the tuning passage itself, thereby changing the isolation frequency during operation. Both Smith &#39;531 and Smith &#39;596 were notable attempts to actively tune the isolator. 
     Another development in the area of vibration isolation is the tunable vibration isolator disclosed in U.S. Pat. No. 6,695,106, titled “Method and Apparatus for Improved Vibration Isolation,” issued 24 Feb. 2004, to Smith et al, which is hereby incorporated by reference. 
     Referring to  FIGS. 1 and 2 , in  FIG. 1 , a perspective view of isolator  101  is illustrated.  FIG. 2  is a cross-sectional view of isolator  101  taken along section lines II-II in  FIG. 1 . Isolator  101  has an upper housing  113 , a lower housing  115 , a piston  117 , a reservoir  109 , and a sight glass  111 . Reservoir  109  is a volume for containing tuning fluid  105  and gas  103 . Valve  107  is used to pressurize reservoir  109 , as well as to test the pressure within reservoir  109 . In order for isolator  101  to operate effectively without fluid cavitation over the entire operating temperature range, reservoir  109  must remain pressurized. 
     One shortcoming of isolator  101  is the difficulty to keep reservoir  109  pressurized during operation and maintenance procedures. Because the volume of reservoir  109  is so small, a slight pressure leak can cause reservoir  109  to quickly lose pressure, thereby causing isolator  101  to lose effectiveness. During maintenance the pressure within reservoir  109  can be checked through valve  107 , which may also cause a slight leakage of gas  103 , thereby causing the pressure within reservoir  109  to decrease substantially. 
     Although the foregoing developments represent great strides in the area of vibration isolation, many shortcomings remain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the application are set forth in the appended claims. However, the application itself, as well as, a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a prior art vibration isolator; 
         FIG. 2  is a cross-sectional view of the prior art vibration isolator taken from section lines II-II, shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of a vibration eliminator according to the preferred embodiment of the present application; and 
         FIG. 4  is a cross-sectional view of a vibration eliminator according to an alternative embodiment of the present application. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 3  in the drawings, a cross-sectional view of a vibration isolator  201 , according to the preferred embodiment of the present application, is illustrated. Vibration isolator  201  comprises an upper housing  227  and a lower housing  229 . A piston  209  is at least partially disposed within the interior of upper housing  227  and lower housing  229 . Piston  209  is typically coupled to a vibrating body, such as a transmission of an aircraft (not shown). Lower housing  229  is typically coupled to a body to be isolated from vibration, such as a roof structure (not shown) of an aircraft. In such an arrangement, the aircraft structure serves as the body to be isolated from vibration, and the transmission of the aircraft serves as the vibrating body. An upper elastomeric member  213  seals and resiliently locates piston  209  within the interior of upper housing  227 . Similarly, a lower elastomeric member  211  seals and resiliently locates piston  209  within the interior of lower housing  229 . Elastomeric members  211  and  213  function at least as a spring to permit piston  209  to move or oscillate relative to housings  227  and  229 . When no load is applied, elastomeric members  211  and  213  are configured to locate piston  209  generally central to upper housing  227  and lower housing  229 . The interior of piston  209  defines a generally elongated tuning passage  207 . An upper fluid chamber  203  is generally defined by the interior of upper housing  227 , piston  209 , and upper elastomeric member  213 . Similarly, a lower fluid chamber  205  is generally defined by the interior of lower housing  229 , piston  209 , and lower elastomeric member  211 . 
     Tuning passage  207  extends centrally through a longitudinal axis of piston  209 , so that upper fluid chamber  203  and lower fluid chamber  205  are in fluid communication. A tuning fluid  217  is disposed in upper fluid chamber  203 , lower fluid chamber  205 , and tuning passage  207 . Tuning fluid  217  preferably has low viscosity, relatively high density, and non-corrosive properties. 
     Introduction of an axial force into piston  209  translates piston  209  axially relative to upper housing  227  and lower housing  229 . The movement of piston  209  forces tuning fluid  217  to move through tuning passage  207  in a direction opposite to the translation direction of piston  209 . Movement of tuning fluid  217  produces an inertial force that cancels, or isolates, the force from piston  209  at a discrete frequency, i.e., isolation frequency. 
     An accumulator assembly  221  is integral to upper housing  227 , and functions at least to provide fluid  217 , under pressure, to upper fluid chamber  203 , lower fluid chamber  205 , and tuning passage  207 . Reservoir  235  is formed from the interior volume of accumulator assembly  221 . Reservoir  235  is comprised of an expanded accumulator region  231  and a lower region  233 . Passage  225  provides a means of fluid communication between reservoir  235  and upper fluid chamber  203 . Accumulator assembly  221  includes a valve  215  for introducing gas  219  into reservoir  235 . 
     In order for isolator  201  to function as desired, fluid  217  and gas  219  within reservoir  235  should be pressurized, for example to approximately 100 psi (pounds per square inch). During operation of isolator  201 , leakage of either gas  219  or fluid  217  may occur; such a leakage results in a decrease in the pressure of gas  219  and fluid  217  within isolator  201 . If the pressure of gas  219  and fluid  217  substantially decreases, operation of isolator  201  likely results in some of fluid  217  vaporizing into a gas, otherwise known as cavitation of fluid  217 , which results a degradation in isolator  201  performance. In addition, during maintenance of isolator  201 , a measurement of the pressure within isolator  201  is taken from valve  215 . Valve  215  is preferably a two-way valve, such as a Schrader valve; however, it should be appreciated that valve  215  may also be other types of valves, such as a one-way valve. The acquisition of a conventional pressure measurement from valve  215  results in a portion of gas  219  exiting through valve  215 , thus resulting in a pressure loss. However, expanded accumulator region  231  of reservoir  235  provides supplemental gas volume within reservoir  235 . Expanded accumulator region  231  decreases the sensitivity of isolator  201  to gas or fluid leaks, as well as to pressure leaks sustained during maintenance pressure checks. Expanded accumulator region  231  preferably is configured as a cylindrical volume with an interior cylindrical depression  237 . It should be appreciated that it is fully contemplated that expanded accumulator region  231  can be a variety of geometric shapes. Interior depression  237  within expanded accumulator region  231  provides a protective area for valve  215  so as to minimize damage to valve  215  from maintenance personnel and operational hazards. It should be appreciated that even though expanded accumulator region  231  is depicted as being in open fluid communication with reservoir  235 , one or more elements such as pipes, tubes, cylinders, and the like can be used provide fluid communication between expanded accumulator region  231  and reservoir  235 . Because expanded accumulator region  231  provides a supplemental volume for gas  219 , in addition to lower region  233 , the risk of severe pressure loss within isolator  201  is mitigated. In the preferred embodiment, gas  219  is nitrogen, but gas  219  may also be other gas elements and mixtures, such as air. A sight glass  223  is configured to facilitate visually inspection of the amount of fluid  217  within accumulator assembly  221 . 
     Referring now to  FIG. 4  in the drawings, a cross-sectional view of an alternative embodiment of a vibration eliminator  301  is illustrated. Vibration isolator  301  comprises an upper housing  327  and a lower housing  329 . A piston  309  is at least partially disposed within the interior of upper housing  327  and lower housing  329 . Piston  309  is typically coupled to a vibrating body, such as a transmission of an aircraft (not shown). Lower housing  329  is typically coupled to a body to be isolated from vibration, such as a roof structure (not shown) of an aircraft. In such an arrangement, the aircraft structure serves as the body to be isolated from vibration, and the transmission of the aircraft serves as the vibrating body. An upper elastomeric member  313  seals and resiliently locates piston  309  within the interior of upper housing  327 . Similarly, a lower elastomeric member  311  seals and resiliently locates piston  309  within the interior of lower housing  329 . Elastomeric members  311  and  313  function at least as a spring to permit piston  309  to move or oscillate relative to housings  327  and  329 . When no load is applied, elastomeric members  311  and  313  are configured to locate piston  309  generally central to upper housing  327  and lower housing  329 . The interior of piston  309  defines a generally elongated tuning passage  307 . An upper fluid chamber  303  is generally defined by the interior of upper housing  327 , piston  309 , and upper elastomeric member  313 . Similarly, a lower fluid chamber  305  is generally defined by the interior of lower housing  329 , piston  309 , and lower elastomeric member  311 . 
     Tuning passage  307  extends centrally through a longitudinal axis of piston  309 , so that upper fluid chamber  303  and lower fluid chamber  305  are in fluid communication. A tuning fluid  317  is disposed in upper fluid chamber  303 , lower fluid chamber  305 , and tuning passage  307 . Tuning fluid  317  preferably has low viscosity, relatively high density, and non-corrosive properties. 
     Introduction of an axial force into piston  309  translates piston  309  axially relative to upper housing  327  and lower housing  329 . The movement of piston  309  forces tuning fluid  317  to move through tuning passage  307  in a direction opposite to the translation direction of piston  309 . Movement of tuning fluid  317  produces an inertial force that cancels, or isolates, the force from piston  309  at a discrete frequency, i.e., isolation frequency. 
     An accumulator assembly  321  is integral to upper housing  327 , and comprises a reservoir  335  and a sight glass  323 . Reservoir  335  is formed from the interior volume of accumulator assembly  321 . Reservoir  335  functions at least to provide fluid  317 , under pressure, to upper fluid chamber  303 , lower fluid chamber  305 , and tuning passage  307 . Passage  325  provides a means of fluid communication between accumulator assembly  321  and upper fluid chamber  303 . In order for isolator  301  to function as desired, fluid  317  and gas  319  within reservoir  335  should be pressurized, for example, to approximately 100 psi. 
     A partitioned accumulator  331  is coupled to accumulator assembly  321  in order to provide supplemental volume for gas  319 . Partitioned accumulator  331  is preferably bonded to accumulator assembly  321  with adhesive  337 . It should be appreciated that partitioned accumulator  331  can be coupled to accumulator assembly  321  using means other than adhesive bonding; such as, welding, mechanical coupling, or any combination thereof, to name a few. It should be appreciated that the coupling of partitioned accumulator  331  to accumulator assembly  321  may involve at least a mechanical fastener and an associated seal, the seal being configured to aid in leakage prevention. A partitioned chamber  341  is formed from the interior volume of partitioned accumulator  331 . Partitioned chamber  341  is placed in fluid communication with reservoir  335  through an accumulator passage  333 . It should be appreciated that even though accumulator passage  333  is depicted as being an open physical channel, one or more elements such as pipes, tubes, cylinders, and the like can be used provide fluid communication between partitioned chamber  341  and reservoir  335 . 
     During operation of isolator  301 , leakage of either gas  319  or fluid  317  may occur; such a leakage results in a decrease in the pressure of gas  319  and fluid  317  within isolator  301 . If the pressure of gas  319  and fluid  317  substantially decreases, operation of isolator  301  likely results in some of fluid  317  vaporizing into a gas, otherwise known as cavitation of fluid  317 , thus resulting in a degradation of isolator  301  performance. In the current embodiment, valve  315  is preferably a one-way valve; however, it is should be appreciated that valve  315  may also be other types of valves, such as a two-way valve. Because valve  315  is preferably a one-way valve, gas  319  is not able to escape through valve  315 , but gas  319  can be introduced into partitioned accumulator  331  through valve  315 . As such, during maintenance operations, valve  315  is not configured to provide a pressure measurement, but maintenance personnel can introduce gas at a desired pressure until gas no longer flows through valve  315 , meaning that the internal pressure is at the desired pressure. 
     Partitioned chamber  341  within partitioned accumulator  331  provides supplemental gas volume for accumulator assembly  321 . Partitioned chamber  341  decreases the sensitivity of isolator  301  to gas  319  or fluid  317  leaks, as well as to possible gas  319  leaks sustained during maintenance pressure checks. Because Partitioned chamber  341  provides a substantial increase in volume to be filled with gas  319 , the leakage of a certain amount of gas  319  or fluid  317  does not cause a substantial decrease in pressure. It is preferred that gas  319  is nitrogen, but gas  319  may also be other gas elements and mixtures, such as air. Sight glass  323  is configured to facilitate visually inspection of the amount of fluid  317  within reservoir  335 . 
     It should be appreciated that partitioned accumulator  331  is configured to be field retrofitable upon isolator  101 , shown in  FIGS. 1 and 2 . In general, retrofitting isolator  101  into isolator  301  would involve removal of valve  107  and machining away the valve protector around valve  107 . Subsequently, partitioned accumulator  331  can be coupled to reservoir  109  with adhesive  339 , or other appropriate means noted herein. 
     It is apparent that an application with significant advantages has been described and illustrated. Although the present application is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.