Patent Publication Number: US-2009236492-A1

Title: Precision alignment assembly for rack mount guidance and navigation system

Description:
BACKGROUND OF THE INVENTION  
     Rack mount inertial guidance and navigation (G&amp;N) systems typically have an alignment repeatability requirement imposed on them. A rack mount system allows for convenient installation and removal of G&amp;N systems thus easing maintenance operations and troubleshooting. The challenge with a rack mount system is ensuring alignment repeatability for a given unit or units upon removal and installation. Restraint stability equivalent to a “hardmounted” bolted down system is difficult to attain. 
     A combination of precision fit daggers  10  and bushings  20  are typically used in rack mount systems, but some “slop” (i.e., undesired clearance between the dagger and bushing allowing radial movement of the dagger relative to the bushing) remains when the dagger and bushing are mated, as indicated by arrow  30 , for fit allowance and machining limitations. This “slop” between the dagger and bushing interface is the main source of uncompensated alignment error in the G&amp;N system. 
     SUMMARY OF THE INVENTION 
     In an embodiment, a system includes a pin assembly having a mating portion of a first axial cross-sectional area, a hilted portion of a second axial cross-sectional area, and a chamfered portion disposed between the mating portion and the hilted portion. The chamfered portion has third and fourth axial cross-sectional areas, and axially tapers at a first angle from the third cross-sectional area to the fourth cross-sectional area. A bushing assembly has a mating chamber and receiving port configured to receive the mating portion, and a countersink portion circumscribing the receiving port. The countersink portion has fifth and sixth axial cross-sectional areas, and axially tapers at a second angle from the fifth cross-sectional area to the sixth cross-sectional area. When the pin assembly and bushing assembly are mated, the countersink portion is configured to constrain radial movement, and movement in at least one axial direction, of the chamfered portion. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  is a schematic illustration of a prior art pin-and-bushing arrangement; 
         FIG. 2  illustrates a perspective view of a pin assembly according to an embodiment of the present invention; 
         FIG. 3  illustrates a perspective view of a bushing assembly according to an embodiment of the present invention; 
         FIG. 4  illustrates in partial cross-section mating of the pin and bushing assemblies of  FIGS. 2 and 3 ; 
         FIG. 5  illustrates a perspective view of a system according to an embodiment of the invention including an inertial guidance and navigation chassis and a mounting rack; 
         FIG. 6  illustrates a perspective view of mounting of the chassis to the mounting rack of  FIG. 5 ; and 
         FIG. 7  illustrates a perspective view of the chassis mounted to the rack of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As will be described more fully hereinafter, A countersink feature added to the bushing which conforms to a specified chamfer on the base of the dagger side will absorb the remaining “slop” between the differences in the diameter of the dagger pin to the diameter of the bushing. The coupling between the countersink feature of the bushing and chamfer on the dagger pin base is essentially a wedge type interface which intimately joins the parts as they are mated together. 
       FIG. 2  illustrates a pin assembly  40 , according to an embodiment, having a mating portion  50  of a first axial cross-sectional area. The mating portion  50 , in an embodiment, may be cylindrical in configuration and may include a conical point  55 . The pin assembly  40  further includes a hilted portion  60 . As can be seen in  FIG. 2 , the diameter of the hilted portion  60  is greater than the diameter of the mating portion  50  and, consequently, the cross-sectional area of the hilted portion is greater than the cross-sectional area of the mating portion along an axis A-A. The pin assembly  40  further includes a chamfered portion  70  disposed between the mating portion  50  and the hilted portion  60 . The chamfered portion  70  defines a major edge  80  and a minor edge  90 . The chamfered portion  70  tapers at an angle along the axis A-A from the major edge  80  to the minor edge  90 . The taper angle of the chamfered portion  70  is preferably 45°, but in varying embodiments can be any angle greater than 0° and less than 90°. As such, the diameter of the chamfered portion  70  at the major edge  80  is greater than the diameter of the chamfered portion at the minor edge  90  and, consequently, the cross-sectional area of the chamfered portion at the major edge is greater than the cross-sectional area of the chamfered portion at the minor edge along the axis A-A. The diameter of the major edge  80  may be, but is not necessarily, equal to the diameter of the hilted portion  60 . 
       FIG. 3  illustrates a bushing assembly  100 , according to an embodiment, having a mating chamber  110  and a receiving port portion  120 . The mating chamber  110  is configured to receive the mating portion  50 , through the receiving port  120 , to enable mating of the pin assembly  40  and bushing assembly  100 . The bushing assembly  100  further includes a countersink portion  130  circumscribing the receiving port  120 . The countersink portion  130  defines a major edge  140  and a minor edge  150 . The countersink portion  130  tapers at an angle along the axis A-A from the major edge  140  to the minor edge  150 . The taper angle of the countersink portion  130  is preferably 45°, but in varying embodiments can be any angle greater than 0° and less than 90°, and complementary to the taper angle of the chamfered portion  70 . As such, the diameter of the countersink portion  130  at the major edge  140  is greater than the diameter of the countersink portion  130  at the minor edge  150  and, consequently, the cross-sectional area of the countersink portion  130  at the major edge  140  is greater than the cross-sectional area of the countersink portion  130  at the minor edge  150  along the axis A-A. 
       FIG. 4  illustrates, in partial cross-section, the pin assembly  40  mated with the bushing assembly  100 . Because the interface between the chamfered portion  70  and countersink portion  130  is wedged, the countersink portion  130  functions to severely limit, if not prevent, radial movement of the pin assembly  40  relative to the bushing assembly  100 , as well as axial movement of the pin assembly toward the bushing assembly. 
       FIG. 5  illustrates a system according to an embodiment of the invention including an inertial guidance and navigation chassis  200  and a mounting rack  300 . As best illustrated in  FIG. 6 , the chassis includes a plurality of bushing assemblies  100 , and the rack includes a corresponding plurality of pin assemblies  40 . The chassis  200  may be mounted to the rack  300  by sliding the chassis along the rack in the direction of arrow B until the bushing assemblies  100  and pin assemblies  40  are mated. As illustrated in  FIG. 7 , the rack  300  further includes at least one swingbolt  400  disposed on the side opposite of the rack to that of the pin assemblies  40 . The swingbolts  400  are configured to be coupled to the chassis  200 , such that, when the chassis is mounted to the rack  300 , the swingbolts prevent the chassis from sliding relative to the mount, and constrain axial movement of the bushing assemblies  100  with respect to the pin assemblies  40 . 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.