Back-up means for fluid cylinder and method for using same

The back-up rings of the present invention are seated on opposite sides of a sealing ring within an annular groove of a hydraulic piston. The sealing ring is in frictional engagement with the interior surface of the hydraulic cylinder, and the back-up rings are positioned within the cylinder groove and on opposite sides of the sealing ring. At least one of the back-up rings includes a vent hole extending axially therethrough to permit the alleviation of any fluid pressure accumulating between the two spaced apart back-up rings. As an alternative to the axial vent hole, a groove may be provided on the interior perimeter of the back-up ring or the back-up ring may include a complete break therein for permitting the venting of fluid trapped between the two back-up rings. The method of the present invention includes venting any fluid which is trapped between the two back-up rings during reciprocation of the piston within the cylinder.

BACKGROUND OF THE INVENTION 
This invention relates to a back-up means for a fluid cylinder and a method 
for using the same. 
Back-up rings have been provided in the prior art for reinforcing an 
elastomeric sealing ring in hydraulic and pneumatic cylinders. The sealing 
ring may comprise an O-ring, or it can be in various other shapes such as 
the shape of a U-cup, a D-ring, or a square ring. It is adapted to engage 
the interior surface of the cylinder so as to provide a seal between the 
piston and the cylinder. 
The O-ring is usually fitted within an annular groove extending around the 
perimeter of the piston. During reciprocation of the piston, because of 
the flexible nature of the sealing ring, it is desirable to provide 
reinforcement for the sealing ring so that it does not become extruded 
into the clearance gap between the piston and the cylinder during 
reciprocating movement of the piston within the cylinder. 
A pair of spaced apart back-up rings have been utilized in the prior art to 
provide reinforcement to the sealing ring. The back-up rings are usually 
positioned on opposite sides of the sealing ring wihtin the annular groove 
surrounding the piston. 
FIGS. 8-12 illustrate the use of prior art back-up rings. Cylinder 10 
includes an interior cylinder wall 12. Reciprocatingly mounted within 
cylinder 10 is a piston 14 having an annular groove 16 therein. Groove 16 
includes first and second annular end walls 18, 20 which are inclined 
slightly and an annular bottom wall 22 which extends between end walls 18, 
20. A rounded caming surface 24 is provided at the junctures between end 
walls 18, 20 and the opposite ends of bottom wall 22. 
Seated within annular groove 16 is an elastomeric sealing ring 26 having a 
pair of back-up rings 28, 30 on opposite sides thereof. 
FIG. 8 illustrates the position of the back-up rings 28, 30 and sealing 
ring 26 when the piston is at rest. 
FIG. 9 shows the position of the components during movement of the piston 
in the direction indicated by arrow 32. With piston 14 moving in the 
direction of arrow 32, a fluid pressure builds up to the left in the 
general area designated by the numeral 34. This, together with the 
friction of the sealing ring 26 against the interior cylindrical wall 12 
causes the sealing ring to be urged to the right as viewed in FIG. 9 
within groove 16. This movement of sealing ring 26 also causes back-up 
ring 30 to be cammed radially outwardly by virtue of engagement with 
camming surface 24 and the inclined wall 20 of groove 16. Back-up ring 30 
consequently provides reinforcement for sealing ring 26 and prevents 
extrusion of sealing ring 26 into the clearance 36 between the piston and 
the cylinder on the low pressure side of the sealing ring 26. 
FIG. 10 illustrates the relative positions of the components when the 
piston is moving in the direction as indicated by arrow 38. During this 
movement the relative positions of the components are reversed from that 
shown in FIG. 9. Back-up ring 28 is urged to the left and cams radially 
outwardly in response to engagement of camming surface 24 and inclined 
wall 18 of groove 16. With the piston moving to the left as shown in FIG. 
10, clearance area 34 becomes the high pressure fluid area and clearance 
area 36 becomes the low pressure fluid area. 
FIG. 11 shows the relative position of the components after the piston has 
been reciprocating for sufficient time to cause a heat build-up due to 
friction. As the hydraulic oil or other fluid media heat up due to 
friction, and other factors, the diameter of the back-up rings increases 
due to the expansion of the back-up rings as they are heated. This causes 
the back-up rings to expand outwardly until they engage the interior 
surface 12 of the cylinder bore as shown in FIG. 11. In this position the 
back-up rings 28, 30 function as a seal against the interior surface 12 of 
the hydraulic cylinder. It should be noted that in the position shown in 
FIG. 11, the two back-up rings 28, 30 form a fluid sealed compartment 
designated by the numeral 40 which is defined between the two back-up 
rings 28, 30 and the bottom wall 22 of annular groove 16. 
In this position, fluid is trapped in the fluid sealed compartment 40, and 
continued reciprocation of the cylinder causes a build-up of heat in the 
fluid within compartment 40. Since the fluid cannot escape, it expands and 
the temperature build-up is substantially increased, thereby causing 
continual build-up of the temperature within the back-up rings 28, 30. As 
the back-up rings 28, 30 increase in temperature they continue to expand 
radially outwardly, and the compartment 40 becomes super pressurized due 
to thermal expansion. Ultimately back-up rings 28, 30 are deformed or 
extruded as shown in FIG. 12. Sometimes the expansion of the back-up 
rings, instead of causing extrusion of the back-up rings, causes the 
piston wall to break or crack. When this happens the pressure is relieved 
between the two back-ups, but the system is usually destroyed or the 
equipment that the cylinder operates is damaaged. 
Therefore, a primary object of the present invention is the provision of a 
back-up means for fluid or pneumatic cylinders and method for using the 
same which will minimize the build-up of heat during reciprocating 
movement of the piston within the cylinder. 
A further object of the present invention is the provision of a back-up 
means for fluid and pneumatic cylinders and method for using same which 
minimizes the expansion of the back-up rings in response to heat build-up 
so that the back-up rings do not become extruded or cause damage to the 
cylinder wall or cylinder piston. 
A further object of the present invention is the provision of a back-up 
means for fluid and pneumatic cylinder and method for using same which 
will alleviate fluid pressure between the two back-up rings by venting the 
fluid which is trapped in the space between the back-up rings so that 
pressure cannot build up. 
A further object of the present invention is the provision of a device 
which is economical to manufacture, durable in use and efficient in 
operation. 
SUMMARY OF THE INVENTION 
The present invention minimizes the expansion of the back-up rings in 
response to heat by venting the fluid trapped between the two back-up 
rings so that there will not be a temperature build-up between the two 
back-up rings during continued reciprocating movement of the piston within 
the cylinder. The venting may be accomplished in the preferred embodiment 
by providing a small aperture extending axially through the back-up ring. 
This permits the hydraulic fluid between the two back-up rings to escape, 
thereby minimizing the temperature build-up during reciprocation of the 
piston. An alternative form of the invention utilizes a small notch in the 
interior diameter edge of the back-up ring. This notch is also capable of 
alleviating fluid pressure between the two back-up rings. 
A third modification of the invention utilizes a back-up ring which is a 
split ring having a small break in at least one point around its 
circumference. This small break provides means for venting the fluid 
between the two back up rings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1-7, the parts of the cylinder, piston, annular groove 
and sealing ring previously described in FIGS. 8-12 remainunchanged, and 
therefore corresponding numerals are used to indicate the parts previously 
described. 
Mounted within annular groove 16 on opposite sides of sealing ring 26 are a 
pair of back-up rings 42, 44. The construction of back-up rings 42, 44 is 
identical, and is illustrated by back-up ring 42 shown in FIG. 2. Ring 42 
includes an outer perimetric edge 46, an inner perimetric edge 48, and a 
pair of opposite axially presented faces 50, 52. 
Extending axially through each of back-up rings 42, 44 is a vent aperture 
54 which extends axially as can be seen in FIGS. 3-6. Aperture 54 is 
preferably positioned slightly below the midpoint between inner edge 48 
and outer edge 46 of back-up ring 42. This insures that the aperture 54 is 
positioned slightly below the contact point between the back-up rings 42, 
44 and the outer perimeter of sealing ring 26. 
Back-up rings 42, 44 may be made of a material substantially more rigid 
than the material of elastomeric ring 26. Examples of materials which will 
work satisfactorily are urethane, rubber, Teflon, and polyester 
elastomers. The preferred material is polyester elastomers which, while 
they are substantially more rigid than the material of ring 26, will 
nevertheless cam radially outwardly when pressed against inclined walls 
18, 20 of groove 16. 
FIG. 3 shows the position of the components of the present invention while 
the piston is at rest. In this position, the apertures 54 are positioned 
slightly below the contact points between O-ring 26 and back-up rings 42, 
44. The back-up rings 42, 44 and the O-ring 26, together define a fluid 
compartment 56. Compartment 56, however, unlike the compartment 40 shown 
in FIG. 11, is vented by means of apertures 54 so that fluid pressure 
cannot build up therein. 
FIG. 4 illustrates the relative position of the parts as the piston is 
moving to the left. Back-up ring 44 is cammed radially outwardly so as to 
provide a back-up support for O-ring 26. However, since no pressure is 
applied to back-up ring 42, the back-up ring 42 remains in an unexpanded 
state so that aperture 54 provides communication to the fluid compartment 
56. 
FIG. 5 illustrates the relative position of the components when the piston 
is moving to the left within cylinder 10. In this position, the back-up 
ring 42 is cammed radially outwardly but the back-up ring 44 is not, 
thereby leaving vent aperture 54 of back-up ring 44 in communication with 
the fluid compartment 56. 
FIG. 6 shows the relative positions of the components after extended 
reciprocating movement of the piston within the cylinder. As the result of 
high frequency movement of the piston within the cylinder, friction 
produces heat in the area of groove 16, thereby causing the sealing ring 
26 and the back-up rings 42, 44 to expand radially outwardly. As the fluid 
within compartment 56 is heated, it expands, and exerts an outwardly 
expanding force against the trailing back-up ring 42. This expanding force 
causes the back-up ring 42 to be slightly canted in the position shown in 
FIGS. 6 and 7. This canted movement exposes the aperture 54 in the 
trailing back-up ring 42 to the compartment 56, thereby causing the fluid 
therein to be vented outwardly as indicated by the arrow 58. The venting 
of this fluid prevents a pressure build-up within compartment 56, and 
alleviates the tendency of the fluid therein to be elevated to such an 
extent that it causes expansion of the back-up rings to the point where 
they bind against the interior cylindrical wall 12 of piston 14. Thus, by 
venting the compartment 56 through apertures 54 in back-up rings 42, 44, 
it is possible to keep the temperature low enough that the back-up rings 
do not expand and bind against the cylindrical wall 12. Consequently, the 
back-up rings are not damaged, distorted or extruded in the manner shown 
in FIG. 12, nor do they expand to the point where they cause cracking or 
damage to the piston. 
FIG. 13 shows an alternative form of the back-up ring which is designated 
by the numeral 60. Ring 60 includes a notch 62 which is shown in 
cross-section in FIG. 13A. This notch permits venting of the fluid from 
chamber 56 so that the temperature and pressure do not build up in 
compartment 56. 
Referring to FIG. 14, a modified form of the invention is designated by the 
numeral 64. Ring 64 is a split ring having at least one brake 66 therein. 
This brake 66 provides the function of venting the fluid within 
compartment 56. 
While the form of back-up ring 42 shown in FIG. 2 is the preferred 
embodiemnt of the present invention, the modified forms 60 (in FIG. 13) 
and 64 (in FIG. 14) can be used in the same fashion as shown in FIGS. 3-6, 
and will result in alleviating of the pressure within the compartment 56. 
The present invention therefore prevents heat buildup due to thermal 
expansion and also reduces the likelihood that the back-up rings will 
extrude as shown in FIG. 12. 
Thus, it can be seen that the device accomplishes at least all its stated 
objectives.