The present invention generally relates to sealing devices and, more particularly, to step joint rings or packings adapted for use within annular grooves in a piston, rod or the like.  In this regard, an important embodiment of the present invention is particularly directed to step joint piston ring seals adapted for use in piston and cylinder assemblies used in high pressure hydraulic applications. 
The use of step joint piston rings, that is, piston rings which have a one-piece split construction wherein the ring ends are stepped and overlay one another, is well known.  In the past, leakage through the gap in these rings has been solved by employing various joint configurations.  High demands on these seals, such as, for example, in hydraulic sealing applications wherein a broad range of pressures and temperatures are involved, has resulted in joint and other leakage problems which could not be fully met by the previously employed joint configuration solutions. 
In a preferred form, the sealing device of the present invention comprises an outer, relatively stiff ring and an inner resilient back up ring adapted to urge the outer ring into contact with the cylindrical surface in which the piston or rod reciprocates.  According to the invention, a particular form of end gap seal is provided which insures maintenance of the high pressure seal under severe conditions.  The relatively stiff outer ring is of regular, preferably rectangular, cross section and includes any of various forms of end gap seal. 
As noted above, reciprocating rods, pistons, or plungers driven under high forces within fluid containing cylinders present serious sealing problems.  Seal rings used to seal such rods, pistons and plungers (referred to hereinafter for convenience as piston rings) must prevent passage of fluid from the high pressure or sealed region into the low pressure of exterior region, without utilizing unduly high radial biasing forces which would cause excessive friction between the cylinder wall and the axially extending, radially outwardly directed exterior surface ring.  Likewise, these rings must resist cocking or flutter under pressure changes.  Preferably, materials must be used which provide a suitably low coefficient of friction with respect to the cylinder or other sealed surface.  Piston rings of the type in question should also be able to seal effectively at high static pressures as well as at low pressures, and ideally must not be dependent upon the extent of sealed pressure to develop primary sealing forces.  As used herein, "primary" sealing refers to the seal existing between relatively movable parts, such as the outside surface of the piston ring and the cylinder, while "secondary" refers to the seal between parts which undergo little or no relative motion, for example, the piston ring and/or the back up ring with respect to the piston, particularly, the ring groove thereof. 
These objectives are achieved by the gapped piston ring seals of the present invention which include an end gap seal and resilient back up combination wherein the ring ends overlap one another with one end of the reduced sized portions thereof having an elastomeric insert which is compressed in use both radially and axially, and which, in a preferred embodiment, includes surfaces which contact at least the sealed surface (e.g. cylinder wall) itself and the resilient back up ring.  It will be understood that, as used herein, "outer" is used in the sense of being the part closest to the primary seal band or area swept by the seal.  The principles of the invention, however, may also apply to certain constructions wherein the sealing ring is urged inwardly towards the sealed surface rather than outwardly toward it. 
In view of the still existing need for improved seals, particularly seals used in difficult applications, an object of the present invention is to provide an improved seal or packing for variable pressure applications. 
Another object is to provide an improved seal for a piston, rod, or the like. 
Another object is to provide an improved seal which includes an outer, relatively stiff seal ring and an inner elastomeric or spreader ring which acts to urge the outer ring into close contact with the sealed surface, and particularly, to provide such a piston ring with an improved end gap seal. 
A still further object is to provide a piston or rod seal in which the relatively stiff portion thereof contains an end gap seal which includes a pair of overlapping, oppositely directed end portions of reduced axial extent, with one of said portions including an elastomeric end gap seal member adapted to provide an axially directed secondary seal with the other end part in which the elastomeric part is also acted upon radially by the back up ring, with the end seal part having an outer surface directly engaging the cylinder or other surface to be sealed. 
The foregoing objects and advantages of the invention are achieved by providing a sealing assembly which includes a relatively stiff first seal ring with a surface for engaging a counterpart surface to be sealed, additional axial end surfaces for engaging the groove in the member receiving the assembly, and a second, resilient back up seal adapted for reception in the groove and acting radially on the first ring, with the first ring having an end seal comprised of circumferentially overlapping end portions of reduced axial size, one of the reduced size end portions including a third or end gap seal element which provides a secondary seal between the two reduced size end portions and which is engaged on one of its radially directed surface portions by the surface to be sealed and by the resilient back up ring on the radially opposite surface thereof. 
End gap seals in relatively stiff rings have been a continuing problem in piston, rod and like seals.  This is because, with stiff ring materials, provision must be made for expanding the ring over the piston as well as for allowing circumferential growth or shrinkage due to wear, or to thermal or other dynamic factors.  While elastomeric rings may be stretched to snap into the ring grooves, and do not have a problem of circumferential growth or shrinkage, elastomers themselves do not have the mechanical properties necessary to provide a highly effective, low friction seal.