Patent Publication Number: US-RE33611-E

Title: Environmentally sealed grounding backshell with strain relief

Description:
BACKGROUND OF THE INVENTION 
     The transmission of high frequency electrically transmitted signals often requires a shielded cable to prevent signal interference or noise resulting from ambient EMI/RFI. The shielding typically may be provided by an electrically conductive braid that surrounds an array of individual insulated conductors within the cable. The braid will extend the entire length of the cable except for the ends where the cable will be appropriately dressed to enable the conductors therein to be electrically connected to the terminals of a connector. 
     The above described shielded cables and associated connectors frequently are used in military applications and other applications requiring high frequency electrical signal transmission. In many such applications the connector/cable interface should be environmentally sealed and should provide a superior strain relief connection to ensure that the quality of the electrical connection is not affected by any physical abuse to which the connector and cable might be subjected. Furthermore, the electrical and mechanical termination of the cable in the connector should ensure that the EMI/RFI shield continues through the connector despite the required termination or dressing of the cable end. The cable/connector interface should also account for the inevitable fact that the termination and connection work often will be performed by field personnel working in substantially uncontrolled environments with possible limited access to tools. 
     The connectors for the above described shielded cable, as used in the above described environments, typically have been provided with backshell and frontshell combinations that are mechanically connectable to one another. The backshell is further mechanically mounted to the cable, while the frontshell is mechanically mounted to a connector housing. Both the backshell and frontshell have been formed from metallic material, and the backshell has further included appropriate electrically conductive means for engaging the cable braid to ensure that the EMI/RFI shield continues through the backshell, the front-shell and the remainder of the connector assembly. Although many prior art backshell assemblies have provided adequate EMI/RFI shielding for the above described braided cables, they have either not provided adequate strain relief and environmental sealing, or have relied upon complex multi-component structures for achieving strain relief and environmental sealing. 
     One grounding backshell is shown in U.S. Pat. No. 3,739,076 which issued to Schwartz on June 12, 1973. The connector shown in U.S. Pat. No. 3,739,076 comprises a rearwardly projecting neck portion having an array of externally disposed threads thereon. An end member or backshell is provided with an array of internal threads for threaded engagement over the rearwardly projecting neck of the connector. A coil spring formed into a generally toroidal configuration is disposed intermediate the rearwardly projecting neck of the connector and the backshell. The toroidal spring is further disposed to be aligned with the exposed conductive braid of the dressed cable. The threaded engagement of the rearwardly projecting neck of the connector with the backshell radially compresses the toroidal spring into electrical contact with the braid. The toroidal spring thus electrically grounds the braid to the backshell and connector housing to achieve a continuous EMI/RFI shielding across the connector. The connector shown in U.S. Pat. No. 3,739,076 is further provided with an O-ring disposed intermediate the toroidal spring and the backshell. The O-ring is intended to achieve environmental sealing of the connector. However, the O-ring is disposed at a location within the connector that would permit the accumulation of moisture or the like inside a portion of the connector assembly. It generally is desirable to keep moisture and other such environmental contaminants entirely out of the connector structure if possible. In many instances, a single O-ring disposed within a connector is not considered sufficient environmental sealing. Furthermore, the O-ring is not intended to and cannot achieve strain relief, and the connector shown in U.S. Pat. No. 3,739,076 does not provide any alternate means for achieving the necessary strain relief. 
     Other prior art connectors have included variations of the structures shown in U.S. Pat. No. 3,739,076. For example, one such prior art connector includes a metallic ground ring secured intermediate the backshell and a threaded adapter. The ground ring is dimensioned to electrically ground the braid of the cable. These other prior art backshell connector assemblies have relied upon internally disposed O-rings, grommets, washers or the like to achieve some degree of environmental sealing. Strain relief has been provided by separate cable clamps which are connectable to the prior art backshell. For example, an adapter which is engageable with the prior art backshell may include a rearwardly directed array of external threads. A strain relief clamp may include a corresponding array of internally disposed threads which are engageable with the threads of the adapter. The strain relief clamp further comprises a rearwardly disposed cable clamp which comprises a pair of bolts extending generally transverse to the cable and on opposite sides thereof. The strain relief clamp is first threaded onto the adapter and then the transversely extending bolts are tightened to urge clamping members into a tight strain relief clamping engagement with the cable. Prior art backshell connector assemblies of this general type are undesirable because of the complex plural components required. Furthermore, the assembly is time-consuming and complex and requires field personnel to utilize several different tools. 
     In view of the above, it is an object of the subject invention to provide a grounding backshell with improved strain relief. 
     Another object of the subject invention is to provide a grounding backshell which achieves exceptional environmental sealing. 
     Still another object of the subject invention is to provide a grounding backshell that can be easily assembled by field personnel. 
     An additional object of the subject invention is to provide a grounding backshell with means for simultaneously providing environmental sealing and strain relief. 
     A further object of the subject invention is to provide an environmentally sealed strain relief backshell which simultaneously provides grounding to a shielded cable in one single connecting operation. 
     SUMMARY OF THE INVENTION 
     The subject invention is directed to an environmentally sealed grounding backshell assembly that achieves superior strain relief automatically as part of the mounting of the assembly to the cable. The backshell assembly includes forwardly disposed means for connecting the backshell to the frontshell of the connector assembly. For example, the forward end of the backshell may comprise an array of threads which are engageable with a corresponding array of threads on the frontshell. 
     The backshell assembly comprises a generally cylindrical metallic backshell having a central through aperture for receiving the cable. The assembly further comprises a grounding coil spring disposed in a generally toroidal configuration such that the axis of the coiled spring defines a circle. The toroidally configured spring is dimensioned to be disposed in the central through aperture of the backshell. The backshell may be configured to permit the toroidal grounding spring to be inserted therein from the rearward end of the backshell. However, the backshell assembly further comprises restriction means to prevent complete passage of the toroidal grounding spring the entire distance therethrough. The coiled configuration of the grounding spring enables the toroidal structure to be compressed radially inwardly. The dimensions of the toroidal grounding spring are such that the cable can be readily passed therethrough prior to compression of the spring. However, compression of the grounding spring, as explained herein, will be sufficient to achieve a grasping electrical ground to the braid or other shield of the cable. 
     The restriction means within the backshell against which the toroidal grounding spring is urged may comprise a reversible washer of generally stepped cylindrical configuration. The reversal of this washer can change the axial position along the backshell at which the grounding spring begins its compression. Thus, the amount of compression can be varied by the orientation of the reversible washer, as explained further below. 
     The backshell assembly further comprises means for radially compressing the grounding spring, such as a plunger having a central through aperture dimensioned to receive the cable. The outer surface of the plunger is dimensioned for telescoping engagement within the rearward portion of the backshell. The forward end of the plunger may define a cam surface of generally concave frustoconical configuration. More particularly, the orientation of the cam surface is such that radially outward portions thereof are at axially forward positions relative to the radially inward portions. As a result of this construction, the axial telescoping movement of the plunger within the backshell will urge the toroidal grounding spring radially inwardly into electrical grounding contact with the braid of the cable. 
     The backshell assembly further comprises a second coil spring formed into a toroidal configuration as described above. This second toroidal spring defines a cable gripping strain relief spring having dimensions approximately equal to or slightly greater than the dimensions of the grounding spring. The strain relief spring is disposed adjacent the rearward end of the plunger. 
     A compression means is provided for radially compressing the strain relief spring and urging the strain relief spring forward. The compression means may be a nut threadably engageable with the rearward end of the backshell. The compression nut may comprise a central through aperture with a concave forwardly facing cam surface which is engageable with the strain relief spring. The strain relief spring may be disposed between the cam surface of the compression nut and the rearwardly facing surface of the plunger. Thus, the forward movement of the strain relief spring will generate an equivalent forward movement of the plunger and a resulting compression of the grounding spring. The length of the plunger and the location of the compression nut cam surface preferably are such that the strain relief spring will be urged into a portion of the dressed cable on which the outer insulation is disposed. In particular, each coil of the strain relief spring will deform the cable insulating jacket to ensure a strain relief connection that cannot be achieved by O-rings and the like. The inward compression of the toroidal strain relief spring will securely grippingly engage the cable at a location thereon spaced from the electrical grounding connection of the grounding spring to the braid. Thus, the strain relief spring is structually and functionally distinct from the prior art which provides a single spring for electrical grounding. 
     The compression means of the backshell assembly may further comprise means for retaining an elastomeric seal. For example, the outer circumference of the compression nut may be defined by a generally annular groove or ridge. 
     An elastomeric seal is provided for secure environmentally sealed connection to the backshell assembly. For example, the elastomeric seal may comprise in inwardly directed flange extending thereabout and dimensioned to engage the groove or ridge of the compression means. The rearward end of the elastomeric seal extends rearwardly beyond the compression nut and radially inwardly toward the cable. A through aperture at the rearward end of the seal is dimensioned to resiliently grip the cable for secure environmental sealing. Preferably, the rearwardmost portion of the seal is of generally frustoconical convex configuration. As a result of this configuration, the forward movement of the compression means relative to the backshell will pull the forward portions of the elastomeric seal in an axially forward direction. Therefore, the frustoconical convex rearward portion of the seal will gradually move into a more acute frustoconical configuration with a correspondingly tighter resilient gripping engagement around the cable. This interengagement of the elastomeric seal with the cable ensures both an efficient environmental sealing and further contributes to the strain relief of the backshell assembly. It will be appreciated that both the strain relief and the environmental sealing are achieved as part of the interengagement of the compression means and backshell for the purpose of achieving the grounding electrical connection with the braid of the cable. In particular, the simple rotational interengagement of the compression nut with the backshell simultaneously achieves the grounding electrical connection, the required strain relief and an efficient environmental seal. No additional tools or structures are needed to achieve these important objectives. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of the backshell assembly of the subject invention. 
     FIG. 2 is a perspective view of the assembled backshell assembly. 
     FIG. 3 is a cross-sectional view of the backshell assembly shown in FIG. 2 prior to secure interengagement with the cable. 
     FIG. 4 is a cross-sectional view taken along line 4--4 in FIG. 3. 
     FIG. 5 is a cross-sectional view similar to FIG. 3 but showing the backshell assembly fully engaged with the cable. 
     FIG. 6 is a cross-sectional view taken along line 6--6 in FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The backshell assembly of the subject invention is identified generally by the numeral 10 in FIGS. 1-6. As shown most clearly in FIG. 1, the backshell assembly 10 comprises a generally cylindrical, electrically conductive metallic backshell 12 having an externally threaded forward end 14 and an externally threaded rearward end 16. A generally cylindrical through aperture 18 extends entirely through the backshell 12 from the forward end 14 thereof to the rearward end 16. The specific construction and configuration of the central through aperture 18 is described in greater detail below. 
     The assembly further comprises a metallic reversible washer 20 having a small diameter end 22 and a large diameter end 24 which is defined by a generally annular outwardly extending flange 26. A cable receiving aperture 28 extends entirely through the washer 20. The flange 26 is dimensioned to be slidably inserted into the through aperture 18 of the backshell 12 from the rearward end 16 thereof. More particularly, the flange 26 is diametrically dimensioned to be securely retained against a shoulder in the backshell 12, while the small diameter end 22 of the reversible washer 20 is dimensioned to pass beyond the shoulder in the backshell 12. Thus, as explained below, the axial position of the washer 20 relative to the rearward end 16 of the backshell 12 can be changed by reversing the washer 20. 
     The backshell assembly 10 further comprises a grounding spring 30 which is a coiled spring formed into a generally toroidal shape. The toroidal configuration of the coiled grounding spring 30 enables the grounding spring 30 to be compressed radially inwardly. The dimensions of the coiled toroidal grounding spring 30 are such that the cable can be readily inserted therethrough in the uncompressed condition of the grounding spring 30. However, the inward compression of the grounding spring 30 is sufficient to enable the grounding spring 30 to graspingly engage the EMI/RFI shield of the cable. The dimensions of the toroidal grounding spring 30 also are such that the grounding spring 30 will achieve axial engagement with the reversible washer 20 in either of the reversed orientations of the washer 20, and in both the unbiased and compressed conditions of the grounding spring 30. As will be shown below, the grounding spring 30 is disposed at a longitudinal location in the backshell assembly 10 to achieve an electrically grounding connection with the electrically conductive braid of a shielded cable. 
     The assembly 10 further comprises a plunger 32 which also preferably is formed from a metallic material and is of generally cylindrical configuration. More particularly, the plunger 32 includes a cylindrical outer surface 34 which is dimensioned to be slidingly telescopingly received within the central through aperture 18 of the backshell 12. The plunger 32 further comprises a generally cylindrical interior surface 36 extending entirely therethrough. The cylindrical interior through aperture 36 is dimensioned to slidingly receive the cable therethrough, as illustrated and described below. The plunger 32 includes opposed rearward and forward ends 38 and 40. The forward end 40 of the plunger 32 defines a camming surface as explained below for compressing the toroidal grounding spring 30 radially inwardly and into grounding contact with the braided shield of the cable secured in the backshell assembly 10. 
     The backshell assembly 10 further comprises a coiled strain relief spring 42 which is formed into a generally toroidal configuration and which preferably, but not necessarily, is formed from metal. The toroidal strain relief spring 42 in its unbiased condition is dimensioned to receive the insulated cable axially therethrough. However, the strain relief spring 42 is radially compressible to achieve a secure strain relief connection to the jacketed cable. The strain relief spring 42 is further dimensioned to be placed in abutting end-to-end relationship with the rear end 38 of the plunger 32 in both the unbiased and compressed conditions of the strain relief spring 42. 
     A generally cylindrical compression nut 44 which may be formed from metallic or plastic material is further provided as part of the backshell assembly 10. The compression nut 44 includes opposed forward and rearward ends 46 and 48. The forward end 46 is provided with an internal array of threads interengageable with the externally threaded rearward end 16 of the backshell 12. The interior of the compression nut 44 is further provided with a forwardly facing cam surface for engaging and compressing the strain relief spring 42. The axial movement of the compression nut 44 to achieve this radially inward compression of the strain relief spring 42 is achieved by the simple threaded interconnection of the compression nut 44 with the backshell 12. The rearward end 48 of the compression nut 44 is provided with an inwardly directed annular groove 52 disposed around the exterior thereof. The annular groove 52 is dimensioned to lockingly engage a seal as explained further below. 
     The backshell assembly 10 further comprises an elastomeric seal 54 having opposed forward and rearward ends 56 and 58 and having a central through aperture 60 extending entirely therethrough. The through aperture 60 adjacent the rearward end 58 of seal 54 is dimensioned to resiliently engage the jacketed cable to which the backshell assembly 10 is mounted. As will be explained in greater detail, the rearward end 58 of the seal 54 is of generally convex frustoconical configuration. The forward end 56 of the seal 54 includes an inwardly directed generally annular ridge (not shown in FIG. 1) which is dimensioned to engage the annular groove 52 of the compression nut 44. 
     The backshell assembly 10 is illustrated in its assembled condition in FIGS. 2-6. In particular, FIGS. 3 and 4 show the backshell assembly 10 in its assembled but not fully tightened position relative to a cable 62. More particularly, the cable 62 is provided with an insulating jacket 64 extending thereabout. A braided electrically conductive EMI/RFI shield 66 is disposed intermediate the outer insulating jacket 64 and a plurality of separate insulated conductors 68 are disposed therein as shown in FIGS. 4 and 6. The cable 62 is prepared or dressed by removing both the outer insulating jacket 64 and the conductive braid 62 from an extreme end thereof to permit the conductors 68 therein to be securely connected to terminals in a connector (not shown). The dressing of the cable 62 further provides a portion of the braided EMI/RFI shield 66 to be exposed to complete the grounding throughout the metallic backshell 12 and the connector (not shown) as explained further below. 
     The backshell assembly 10 is assembled by first disposing the reversible washer 20 in the backshell 12 such that the flange 26 of the reversible washer 20 is firmly seated against the annular ledge 70 within the backshell 12. It will be noted that the annular ledge 70 defines a minor diameter portion 72 having an internal diameter substantially equal to or slightly greater than the external diameter of the small end 22 of the reversible washer 20. However, the internal diameter defined by portion 72 of ledge 70 is less than the diameter defined by the annular flange 26 at the opposed end 24 of the reversible washer 20. The ledge 70 further defines a major diameter portion 74 having an internal diameter substantially equal to or slightly greater than the diameter defined by the annular flange 26 at end 24 of the reversible washer 20. As shown in FIG. 3, the reversible washer 20 is positioned in the backshell 12 such that the flange 26 thereof is firmly seated in the large diameter portion 74 of ledge 70, and such that the small end 22 of washer 20 is directed toward the rear 16 of the backshell 12. It will be appreciated, however, that the washer 20 can be reversed such that the small end 22 thereof is slidably inserted through the minor diameter portion 72 of the ledge 70. This reversal of the washer 20 will change its axial position relative to the rearward end 16 of the backshell 12, and will thereby change the amount of camming achieved by the plunger 32. Thus, the reversed orientations of the washer 20 enable the assembly 10 to be used with cables of different diameters. 
     The backshell assembly 10 is further assembled by inserting the generally toroidally configured grounding coil spring 30 into the central aperture 18 of backshell 12 from the rearward end 16 thereof. More particularly, the grounding spring 30 is advanced axially into contact with the rearwardly disposed portion of the reversible washer 20. The particular axial location of the grounding spring 30 will depend upon the reversible orientation of the washer 20. 
     The plunger 32 is next slidably inserted into the backshell 12 such that the forward end 40 thereof is urged toward the grounding spring 30. As shown most clearly in FIG. 3, the forward end 40 of the plunger 32 is characterized by a concave frustoconical cam surface 76 which is oriented such that the outer cylindrical surface 34 of the plunger 32 leads the inner cylindrical surface 36 thereof at the forward end 40. As will be explained further below, the axial movement of the plunger 32 causes the cam surface 76 thereof to urge the grounding spring 30 in a generally radially inward direction. 
     The backshell assembly 10 next includes the generally toroidal strain relief coil spring 42 disposed in abutting generally coaxial contact with the rear end 38 of the plunger 32. The compression nut 44 is then disposed over the strain relief spring 42 and over the rear end 16 of the backshell 12. More particularly, the compression nut 44 includes an internally disposed array of threads 78 adjacent the forward end 46 thereof which are engageable with the externally threaded rearward end 16 of the backshell 12. The compression nut 44 further comprises a concave generally frustoconical cam surface 80 disposed on an interior portion intermediate the opposed front and rear ends 46 and 48. The cam surface 80 is disposed to engage the toroidal strain relief spring 42 upon the threaded engagement of the compression nut 44 with the threaded rearward end 16 of the backshell 12. The inward compression of the strain relief spring 42 progresses with additional threaded interengagement of the compression nut 44 with the backshell 12. 
     As shown in FIG. 3, the elastomeric seal 54 of the backshell assembly 10 includes an inwardly directed flange 82 at the forward end 56 thereof. The flange 82 is disposed and dimensioned to resiliently engage the inwardly directed groove 52 of the compression nut 44. 
     As illustrated more clearly in FIG. 3, the cable 62 is directed axially through the above described components of the backshell assembly 10. The cable 62 is dressed or prepared such that the conductive braid 66 thereof is exposed at a location to be substantially in line with the grounding spring 30. The extreme end of the cable 62 is prepared to expose the conductors 68 thereof and to enable electrical connection to terminals in a connector assembly (not shown). In this untightened condition, as shown in FIGS. 3 and 4, it will be noted that the cable 62 can readily move in an axial direction relative to the backshell assembly 10, and in particular with respect to the strain relief spring 42 and the grounding spring 30. 
     The grounding, strain relief and environmental sealing for the backshell assembly 10 is simply accomplished by merely threadably completing the interconnection of the compression nut 44 to the backshell 12 as shown in FIGS. 5 and 6. More particularly, complete threaded interengagement of the compression nut 44 with the backshell 12 causes the concave frustoconical cam surface 80 of the compression nut 44 to move in an axial direction into contact with the strain relief spring 42. The forces generated by this camming action urge the strain relief spring in both an axial direction and a radially inward direction. The axial movement of the strain relief spring 42 generates axial forces on the plunger 32, which in turn causes the concave frustoconical cam surface 76 thereof to contact the grounding spring 30. The grounding spring 30 is firmly seated against the reversible washer 20 and cannot move in an axial direction. Therefore, the grounding spring 30 reacts to the camming forces exerted by the plunger 32 by compressing radially inwardly and into a graspingly secure electrically grounding contact with the conductive braid 66 of the cable 62. As the grounding spring 30 tightens against the braid 66, continued longitudinal movement of the plunger 32 is reduced. Therefore, the further threaded interengagement of the compression nut 44 with the backshell 12 causes the cam surface 80 of compression nut 44 to urge the strain relief spring 42 primarily in a radially inward direction and into a tight strain relief circumferential engagement with the jacketed portion of the cable 62. 
     As noted above, the elastomeric seal 54 is dimensioned such that the rearward aperture 60 thereof resiliently engages the jacket 64 of the cable 62. The threaded interengagement of the compression nut 44 to the backshell 12 effectively moves the compression nut 44 in an axial direction relative to the backshell 12. This longitudinal movement of the compression nut 44 deflects the frustoconical end 58 of the elastomeric seal 54 into a more acute alignment relative to the cable 62. This deflection tightens the elastomeric seal 54 around the cable 62 thereby enhancing the environmental sealing effect and further contributing to strain relief. As a result, the simple threaded interengagement of the compression nut 44 onto the backshell 12 achieves a high quality grounding of the braided cable 62 by virtue of the radially inward compression of the grounding spring 30. Furthermore, the simultaneous radially inward compression of the strain relief spring 42 against the jacketed portion of the cable 62 provides effective strain relief for the cable 62. This same threaded interengagement causes a deformation of the elastomeric seal 54 which enhances the environmental sealing effect and further contributes to strain relief. 
     While the invention has been described with respect to certain preferred embodiments, it is apparent that various changes can be made without departing from the scope of the invention as defined by the appended claims.