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CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. nationalization under 35 U.S.C. §371 of International Application No. PCT/US2010/058962, filed Dec. 3, 2010, which claims priority to U.S. provisional patent application No. 61/266,814, filed Dec. 4, 2009. The disclosures set forth in the referenced applications are incorporated herein by reference in their entireties, including all information as originally submitted to the United States Patent and Trademark Office. 
    
    
     BACKGROUND 
     The present disclosure relates to a latch mechanism for latching a first panel to a second panel to prevent the first panel from opening with respect to the second panel, and in particular to a pressure relief latch mechanism used for securing dual purpose aircraft doors which can open at a predetermined internal pressure or opened from the exterior for routine maintenance and inspection purposes. 
     One application for such pressure relief latches is on aircraft engine cowls. Aircraft engines have high internal operating pressures, and aircraft designers wish to have the pressure release latch not operate until the internal pressure approaches very close to the maximum permitted pressure. Thus, the tolerances on adjustment and operation of the release latch become tighter and the requirements for repeatability become more intensive. The pressure relief latch is adjusted at the factory to operate at a specific pressure. Failures are sometimes encountered in the field after many hours of usage, and the aircraft operator desires assurance that the pressure release latch will operate when needed and will not operate prematurely. Variations in friction and contamination from use can affect the operation of the latches. Temperature changes can also affect latch operation by causing variations in spring rate, which may affect the latch effective release pressure. 
     SUMMARY 
     According to the present disclosure, a pressure relief latch mechanism comprises a housing for mounting on a first panel, a spring loaded detent carried within the housing, and a bolt having a pivot axis and pivotally mounted on the housing for latching engagement with a second panel. The bolt of the pressure relief latch mechanism is configured to open at a predetermined internal pressure and can be quickly opened from the exterior when it is desired to open the panels. 
     In illustrative embodiments, the detent includes a detent ball and a compression spring positioned along a central spring axis perpendicular to the pivot axis of the bolt. The bolt includes a curved exterior surface that is provided with a detent for receiving the detent ball. The curved exterior surface of the bolt is formed to include a groove that is configured to accept the detent ball to permit the detent ball to roll along the surface of the groove during movement of bolt from a locked first position to an open second position. The position of the detent ball is retained by a detent cup that includes a concave surface that engages the detent ball. The use of the groove and detent cup permits the detent ball to roll along the surface of the bolt during the operating cycle of the bolt. 
     In illustrative embodiments, the housing includes a pair of cylindrical bores that include a threaded opening at one end and a detent ball opening at the other end. The cylindrical bores are configured to house a pair of detent assemblies, each of which includes a detent ball, a detent cup, a detent spring, a thermo-expansion spacer and a threaded plug. The thermal-expansion spacer is made from a material that has a different coefficient of thermal expansion than the housing. The thermal-expansion spacer is configured to compensate for the change in detent spring rate at different temperatures. 
     Additional features of the disclosure will become apparent to those skilled in the art upon consideration of the following detailed description of illustrative embodiments exemplifying the best mode of carrying out the disclosure as presently perceived. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The detailed description particularly refers to the accompanying figures in which: 
         FIG. 1  is a perspective view of an airplane showing an engine provided with a cowling having access panels including pressure relief latches; 
         FIG. 2  is a enlarged partial view of an access panel showing a pressure relief latch mounted to the panel to retain the panel in a closed position; 
         FIG. 3  is a perspective view of the pressure relief latch showing a housing and a bolt pivotally coupled to the housing; 
         FIG. 4  is an exploded view of the pressure relief latch showing the housing, a pair of detent assemblies and the bolt; 
         FIG. 5  is a front view of the pressure relief latch showing the bolt pivotally coupled to the housing; 
         FIG. 6  is a side view of the pressure relief latch showing the bolt pivotally coupled to the housing; 
         FIG. 7  is another side view of the pressure relief latch; 
         FIG. 8  is a bottom view of the pressure relief latch showing a pair of parallel grooves formed in the bottom surface of the bolt and showing a pair of detent balls positioned within the grooves; 
         FIG. 9  is a perspective view of the bolt showing a detent ball positioned within a detent of the bolt and also showing a second detent and a pair of curved grooves extending along the bottom surface of the bolt; 
         FIG. 10  is a bottom view of the pressure relief latch of  FIG. 9  showing the detent balls of the detent assemblies positioned with in the grooves of the bolt; 
         FIG. 11  is a perspective view of the pressure relief latch with the bolt removed showing the detent balls extending through a pair of openings formed in the housing; and 
         FIG. 12  is a sectional view taken along line  12 - 12  of  FIG. 11  showing the cylindrical bores of the housing for containing the detent assemblies. 
     
    
    
     DETAILED DESCRIPTION 
     A pressure relief latch  10  is shown coupled to the cowl  12  of an airplane engine  14 , as shown in  FIG. 1  to secure first and second access panels  16 ,  18  of an airplane  20 , as shown in the illustrative embodiment of  FIG. 2 . Bolt  24  of pressure relief latch  10  is configured to open at a predetermined internal pressure and can be quickly opened from the exterior of the plane for routine maintenance and inspection purposes. 
     Pressure relief latch  10  includes a housing  22  for mounting on the first panel  16 , a pair of spring loaded detent assemblies  26  carried within housing  22 , as shown, for example, in  FIGS. 2 and 4 . Pressure relief latch also includes a bolt  24  having a pivot axis and is pivotally mounted to the housing  22  for latching engagement with the second panel  18 . Bolt  24  of the pressure relief latch  10  is configured to open at a predetermined internal pressure and can be quickly opened from the exterior when it is desired to open the panels  16 ,  18 . 
     Housing  22  of pressure relief latch  10  houses detent assemblies  26  and permits the pivotal connection of bolt  24 . Housing  22  includes a flange plate  28  that is configured to be coupled to first access panel  16 . Flange plate  28  is formed to include a series of openings  30  that are used to secure housing  22  to first access panel  16 . Rivets or other fasteners, such as bolts can be used to secure housing  22  to first access panel  16 . 
     Housing  22  is formed to include a bolt passageway  32 , as shown in  FIG. 4 . Bolt passageway  32  is configured to allow bolt  24  to move through bolt passageway  32  when bolt  24  pivots about pin  34  and bushing  48  from a closed position to an open position. Bolt passageway  32  of housing  22  is defined by sidewalls  35 - 37 . Sidewall  36  is formed to include openings  38 ,  40  that are configured to permit the passage of detent balls  42  of detent assemblies  26 . 
     Housing  22  also includes bolt flanges  44 ,  46 , as shown in  FIGS. 4 ,  11 , and  12 . Bolt flanges  44 ,  46  include apertures  50 ,  52  that are configured to accept pin  34  for mounting bolt  24  to housing  22 , as shown in  FIGS. 11 and 12 . Housing  22  also includes a pair of cylindrical bores  54 ,  56 , which house detention assemblies  26 . Cylindrical bores  54 ,  56  pass through end wall  58  of housing  22  and terminate at openings  38 ,  40  in side wall  36 . Cylindrical bores  54 ,  56  include internal threads  60  used to secure detention assemblies  26 . Threads  60  also allow for adjustability of detent assemblies  26 . 
     Each of the detention assemblies  26  of pressure relief latch  10  include detent ball  42  and a retention cup  62  having a concave face  64  that is configured to engage the detent ball  42 , as shown, for example, in  FIG. 4 . Retention cup  62  also includes post  66  that opposes concave face  64 . Each detention assembly  26  also includes a compression spring  68  positioned along a central spring axis perpendicular to the pivot axis of bolt  24 . Concave face  64  cups detent ball  42  to reduce friction and allow detent ball  42  to roll more freely. 
     The detention assemblies  26  also include expansion spacer  70 , as shown in  FIG. 4 . Expansion spacer is made from a material that has a coefficient of thermal expansion that is different than housing  22 . The use of an expansion spacer  70  that is made from a material with a different coefficient of thermal expansion allows for compensation in the spring rate of spring  68  at different temperatures. Use of expansion spacer  70  allows pressure relief latch  10  to yield consistent release loads from cycle to cycle and adjust for variations in temperature. As an example, if housing  22  is made from steel, expansion spacer  70  may be made form aluminum to allow for compensation during changes in temperature. 
     Expansion spacer  70  includes a post  72  that is co-linear with post  66  of retention cup  62 . Detention assemblies  26  also include adjustment cap  74 , as shown in  FIG. 4 . Adjustment cap  74  includes external threads that are configured to threadably engage threads  60  of cylindrical bores  54 ,  56 . Adjustment cap  74  allows for adjustment of preload on detent ball  42 . Adjustment cap includes post  76  that is co-linear with posts  66  and  72  and is configured to be positioned within a cavity  78  of expansion spacer  70 . Use of expansion spacers  70  allows preload of detention assemblies  26  to be set at the factory and the preload to remain relative constant at varying temperatures. 
     Bolt  24  is pivotally coupled to housing  22  by use of pin  34  to move from a closed position to an open position, as shown, for example in  FIGS. 3 ,  4  and  9 . Bolt  24  includes a curved lower surface  80  and is provided with a pair of detents  82 ,  83  for receiving the detent ball  42 . The curved lower surface  80  of the bolt is formed to include a pair of generally parallel grooves  84 ,  86  that are positioned to lie near detents  82 ,  83 . Grooves  84 ,  86  are configured to accept detent balls  42  to permit detent balls to roll along the surface of the grooves  84 ,  86  during movement of bolt  24  from a locked first position to an open second position. Bolt also includes lower detents  112 ,  114  that are used to hold bolt in an open position. Lower detents  112 ,  114  are positioned to lie near a second end of grooves  84 ,  86  of bolt  24 . 
     Bolt  24  includes mounting flange  88 , as shown for example, in  FIG. 9 . Mounting flange  88  includes aperture  90  that is configure to accept sleeve  48  and pin  34 . Bolt  24  also includes flush mounted release member  92  formed to include tool port  94 . Tool port  94  is configured to accept a tool, such as a screwdriver, to allow bolt  24  to be pivoted to an open position. Bolt  24  further includes end fitting  96  having adjustable engagement post  98 . 
     Engagement post  98  includes a head portion  100  and a threaded shaft  102  coupled to the head portion  100 . Threaded shaft  102  is configured to be positioned within opening  104  formed in bolt  24 . Threaded shaft  102  is secured to bolt  24  by use of nut  106  and the position of head portion  100  is adjusted by use of washers  108 . Washers  108  allow for the proper positioning of head portion  100  to ensure that access panels are held closed tightly. Bolt  24  also includes arcuate flanges  110  that assist in guiding bolt  24  during movement. 
     Various features of the invention have been particularly shown and described in connection with the illustrated embodiment of the invention, however, it must be understood that these particular arrangements merely illustrate, and that the invention is to be given its fullest interpretation within the terms of the appended claims.

Summary:
A pressure relief latch for use in securing panels on an air-craft. The latch includes an apparatus and method to retain the latch in a latched condition under certain conditions. The latch includes a detention assembly which includes at least a portion which has a coefficient of thermal expansion which differs from a housing of the latch.