Gas spring with an anti-rattle piston assembly

Rattling of the piston against the tube in a gas spring due to transient lateral loads on the piston are inhibited by engagement of the base of the piston ring groove with internal projections of a serrated inner edge of the sealing ring. Assurance against seizure of the sealing ring to the piston and preservation of a minimum width of the face seal between the sealing ring and the ringland to ensure against leakage are provided.

FIELD OF THE INVENTION 
The present invention relates to a gas spring of the type used to lift and 
partially or wholly counter-balance a load and, in particular, a gas 
spring having an anti-rattle piston unit. 
BACKGROUND OF THE INVENTION 
Gas springs are widely used in automotive vehicles to lift and partly or 
wholly counterbalance engine compartment hoods, trunk lids, tail gates and 
the like (hereinafter referred to as "loads"). To prevent the loads from 
moving toward open at an undesirably rapid speed, most gas springs used in 
automotive and similar applications incorporate a dampening feature in the 
form of a piston unit having a restrictive bypass that throttles the flow 
of fluid across the piston unit when the piston rod is moving out to lift 
the load. At the same time it is usually desirable to have an open bypass 
across the piston unit when the load is being moved toward closed in order 
to keep the handle load low. Gas springs having a restricted bypass that 
operates when the piston rod moves out and an open bypass that operates 
when the piston rod moves in are described and shown in U.S. Pat. Nos. 
4,438,833 (Schafer, Mar. 27, 1984), 4,466,514 (Molders et al., Aug. 21, 
1984), and 4,467,899 (Molders et al., Aug. 28, 1984). 
The gas springs of those patents comprise an elongated tube that is closed 
at one end and receives a piston rod in sealed and sliding relation at the 
other end and a piston unit affixed to the piston rod within the tube and 
defining a closed end chamber within the tube adjacent said one end and a 
rod end chamber within the tube adjacent said other end. The tube contains 
a gas at a pressure greater than atmospheric pressure and usually contains 
some liquid, such as brake fluid. A restricted bypass through the piston 
unit throttles the flow of the gas and liquid between the closed end and 
rod end chambers, and a one-way open bypass defined by an annular 
clearance space between a perimeter wall of the piston unit and the tube 
adjacent the closed end chamber and at least one passage in the piston 
unit adjacent the rod end chamber allows essentially unrestricted fluid 
flow from the closed end chamber to the rod end chamber A sealing ring 
received in a ring groove in the piston unit intermediate the clearance 
space and passage and engaged in sealed and sliding relation with the 
inner wall of the tube is movable in the ring groove axially with respect 
to the tube between a sealed position in engagement with a ringland 
adjacent the clearance space such as to close the open bypass and an open 
position spaced apart from the ringland such as to open the bypass. 
To enable the fluid in the closed end chamber to flow through the open 
bypass when the sealing ring is in the open position, the inner edge of 
the sealing ring is relatively widely spaced apart from the base of the 
ring groove to provide an annular flow passage that forms part of the open 
bypass--i.e., communicates the clearance space between the piston unit and 
the inner wall of the tube adjacent the closed end chamber with the 
passages in the piston unit adjacent the rod end chamber. 
To ensure a good sealing engagement between the sealing ring and the 
ringland in the closed position of the sealing ring so that fluid does not 
leak through the open bypass when the rod is moving out and instead must 
flow through the restricted bypass, it is necessary for the outer 
circumference of the ringland to be in close clearance from the inner wall 
of the tube and for the radial dimension of the zone of engagement between 
the ringland and the sealing ring to exceed a certain value at all points 
circumferentially. 
In many applications of gas springs the tube diameter must be kept as small 
as possible to conserve space. Keeping the diameter small presents various 
difficulties, such as providing an adequate land area width for the 
sealing ring and sufficient clearance between the piston and the tube to 
keep the piston from rattling against the tube wall 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a gas spring in which 
rattling of the piston against the tube is inhibited. Another object is to 
enable piston-rattling to be inhibited without requiring significant 
changes to be made in known and proven designs. It is also an objective to 
prevent rattling in gas springs of a small diameter, i.e., those in which 
constraints on dimensional relationships present special design problems. 
The present invention is a gas spring which is largely the same as 
presently known designs, such as the gas springs described and shown in 
the above-referred to patents. The invention is characterized in that the 
sealing ring has a serrated inner edge formed by spaced-apart internal 
projections and spaces between the projections defining with the base of 
the ring groove passages that constitute part of the open bypass when the 
sealing ring is in the open position The internal projections are in 
sufficient clearance with the base of the ring groove such that they do 
not bind or seize in the ring groove. At the same time, however, the 
projections are engageable by the base of the ring groove upon small 
deflections of the piston and resist lateral movement of the piston unit 
into contact with the inner wall of the tube under transient lateral 
loads. In preferred embodiments of the present invention, the projections 
have inner wall surfaces that are segments of a circular cylindrical 
surface that is concentric with the base of the ring groove 
For a better understanding of the present invention, reference may be made 
to the following description of an embodiment, taken in conjunction with 
the accompanying drawings.

DESCRIPTION OF THE EMBODIMENT 
The embodiment comprises a tube 10 that is closed at one end, such as by a 
cap 12 or a rolled closure formed on the tube, and receives through its 
other end a piston rod 14. The piston rod is slidably supported in the 
tube end and sealed to the tube, such as by a guide 16 and a main seal 18 
retained in place by a retainer washer 20. 
Affixed to end of the piston rod 14 within the tube 10 is a piston unit 22, 
which is a sub-assembly with the rod composed of a piston body 24, a 
plain, round washer 26, and a notched washer 28 (i.e., a washer having 
spaced-apart notches 30 along its perimeter). The piston body 24 has small 
grooves 32 and 34 in each of its end faces that in the assembled piston 
unit are closed off by the washers 26 and 28 to form a restricted bypass 
between the closed end chamber 36 defined within the tube 10 between the 
piston unit 22 and the cap 12 and the rod end chamber 38 defined within 
the tube by the seal 18 and the piston. The details of the restricted 
bypass are not described here and are not fully shown in the 
drawings--instead reference is made to the three patents referred to near 
the beginning of this specification, which patents are hereby incorporated 
herein by this reference to them. Suffice it to say here that the fluid 
contained in the gas spring can flow in both directions through the 
restricted bypass between the rod end chamber 38 and the closed end 
chamber 36, but the restricted bypass has a high flow resistance that 
throttles the rate of fluid flow. Accordingly, when the only way that 
fluid can flow between the chambers 36, 38 is via the restricted bypass, 
the velocity of the movement of the piston unit 22 and piston rod 14 along 
the tube is limited. 
Because it is usually not desirable to retard significantly the movement of 
the rod/piston unit toward the closed end chamber ("rod-in movement") 
under a handle load applied to the load, the gas spring has an essentially 
open, one-way bypass through which fluid can flow with only a 
comparatively small restriction from the closed end chamber 36 to the rod 
end chamber 38. The open bypass comprises an annular clearance space 
between the larger diameter portion 40 of the piston body 24 and the inner 
wall of the tube 10 and the notches 30 of the notched washer 28. The open 
bypass is made one-way by a sealing ring 42 that is received in an axially 
over-sized ring groove formed in the piston unit by a smaller diameter 
portion 44 of the piston body 24 and by the projections 46 between the 
notches 30 of the notched washer 28. The sealing ring 42 functions as a 
one-way valve. Its outer edge is in sliding/sealing engagement with the 
inner wall of the tube 10, and the friction of that engagement produces a 
frictional force that moves the sealing ring to a closed position in 
sealing engagement with a ringland 48 of the ring groove formed by a 
shoulder between the larger and smaller diameters 40 and 44 of the piston 
body when the rod is being forced out of the tube by the gas pressure 
("rod-out movement"). Upon rod-in movement of the rod/piston the 
frictional force on the seal moves it in the ring groove to an open 
position spaced apart from the ringland 48, namely into the engagement 
with the projections 46 of the notched washer 28. 
In the closed position it assumes upon rod-out movement of the rod/piston, 
the sealing ring forms with the ringland 48 a face seal that prevents 
fluid from flowing past the sealing ring to the annular clearance space 
between the portion 40 of the piston body and the tube, thereby closing 
the open bypass. Accordingly, the only way the fluid can flow past the 
piston from the rod end chamber to the closed end chamber is through the 
restricted bypass, and the speed of the rod-out movement is limited. 
As described thus far, the embodiment is well-known to those skilled in the 
art from, for example, the three patents previously referred to. In some 
circumstances gas springs of the previous designs rattle as a result of 
transient side loads on the piston when the gas spring is inactive and the 
piston resides near the closed end. The sliding fit between the rod 14 and 
the rod guide 16 (or other mounting arrangement) allows the rod to cock 
slightly, and when the rod is in, even the slight cocking of the rod can 
be enough to allow contact of the piston with the tube. 
To prevent rattling, the present invention provides a special sealing ring 
42 that has a serrated inner edge constituted by spaced-apart internal 
projections 50. The spaces 52 between the projections 50 provide passages 
between the base 48 of the ring groove (the smaller diameter portion of 
the piston body) and the sealing ring 42 through which fluid can flow when 
the sealing ring is in the open position during rod-in movement The 
passages formed by the spaces 52 complete (form part of) the open bypass; 
fluid can flow without significant restriction through the annular 
clearance space between the larger diameter portion 40 of the piston body 
24 and the inner wall of the tube, into the ring groove, through the 
spaces 52 and finally through the notches 30 in the notched washer 28. 
The inner ends of the internal projections 50 of the serrated edge of the 
sealing ring 42 are in close clearance from the base 44 of the ring 
groove. In particular the internal ends have, in the embodiment and by 
preference according to the invention, surfaces that are segments of a 
circular cylindrical surface concentric with and of a diameter slightly 
greater than that of the base 44 of the ring groove. The clearance is 
necessary to prevent the sealing ring from binding with the ring groove 
base and seizing, but the clearance is close in order that under side 
loads acting on it the piston is inhibited from being forced out into 
contact with the tube by engagement with one or more of the projections 50 
of the sealing ring--under side loads the piston is laterally supported by 
the sealing ring. The sealing ring is moderately resilient, so lateral 
movements of the piston unit are also dampened by the engagement. 
There are three constraints on the dimensioning of the gas spring of the 
present invention: 
(1) the close clearance between the ring groove base and the end surfaces 
of the sealing ring projections 50 should be such as to prevent the 
possibility of the sealing ring binding with the piston; 
(2) the clearance between the larger diameter portion 40 of the piston and 
the inner wall of the tube 10 should be such as to ensure that the piston 
will not contact the tube without significant radial compression of the 
sealing ring; and 
(3) the radial dimension of the face seal (engagement area) between the 
sealing ring 42 and the ringland 48 should be such as to ensure against 
leakage and, therefore, provide the desired dampening of rod out movement. 
In fulfilling the above three constraints, the upper and lower temperature 
extremes in which the gas spring will be operating must be taken into 
account. Selection of the material of the sealing ring to meet the 
requirements for durability, resiliency and sealability is also important.