Gas spring with automatic locking mechanism

A gas spring includes a cylinder having two axial portions of different cross section and two pistons mounted on a common piston rod and respectively matching the cross sections of the cylinder portions. The pistons are spaced on the piston rod so that they are either both in the wider cylinder portion or respectively received in the two portions. A first throttling passage permits flow of fluid between the two cylinder compartments separated by the larger piston, and a second throttling passage by-passes the smaller piston only during movement inward of the smaller cylinder portion to permit flow of fluid out of that cylinder portion. A valve arrangement is biased for sealing the two parts of the cylinder cavity separated by the smaller piston when the latter is in the smaller cylinder portion, but does not move inward of that portion. The valve arrangement responds to a sufficient, axially applied force for connecting the two cavity parts.

This invention relates to pneumatic and hydropneumatic springs, commonly 
referred to as gas springs, and particularly to a gas spring with 
automatic locking mechanism and to arrangements, such as pivotally mounted 
parts on the body of an automotive vehicle combined with a gas spring of 
the invention. 
Gas springs have found a rapidly expanding field of application in 
automotive vehicles in which they compensate for the weight of motor 
hoods, trunk lids, and particularly rear windows in station wagons and 
passenger cars which swing about a horizontal axis. The pivotally mounted 
masses need to be held in the open position against the force of gravity, 
and it was proposed in the commonly owned U.S. Pat. No. 3,938,793 to build 
a mechanical stop into the gas spring. The stop is released by further 
lifting the hood, lid, or window, and this is inconvenient to some people, 
particularly those not familiar with this specific type of gas spring. 
It has now been found that certain features known from the two-stage shock 
absorber disclosed in U.S. Pat. No. 3,447,644 may be modified for use in a 
gas spring to provide an automatic locking mechanism which can be released 
by gentle manual pressure applied to the pivotally mounted mass in a 
downward direction. 
According to one more specific aspect of this invention, there is provided 
a gas spring whose cylinder has an axis and bounds a sealed, fluid filled 
cavity therein. A first portion of the cavity has a greater cross section 
at right angles to the axis than a second cavity portion axially offset 
from the first portion in a predetermined direction. A piston rod axially 
movable into and out of the cavity in sealing engagement with the cylinder 
carries a first piston and a second piston in axially spaced relationship. 
The pistons have respective cross sections matching those of the two 
cavity portions. They move with the piston rod between a first position in 
which both pistons are received in the greater, first cavity portion and a 
second position in which they are received respectively in the matching 
cylinder portions. A first throttling passage axially by-passing the 
larger first piston permits flow of fluid between the two compartments of 
the cylinder cavity which are separated by the first piston. A second 
throttling passage permits flow of fluid between the two parts of the 
cylinder cavity separated by the second, smaller piston in the second 
position of the latter only when the second piston moves relative to the 
cylinder in the afore-mentioned predetermined direction. A valve 
arrangement normally seals the two cavity parts from each other in the 
absence of such piston movement in the predetermined direction, but 
responds to a sufficient force axially applied to the piston rod for 
connecting the cavity parts. 
In another aspect, the invention also provides a gas spring arrangement in 
which one or more gas springs of the type described above, whose cavities 
are charged with a fluid under superatmospheric pressure, are interposed 
between a support and a solid mass mounted on the support for movement in 
a direction having a vertical component, and therefore biased by gravity 
from a raised toward a low position. The cylinder and piston rod of the 
spring or of each spring is secured to the support and the mass in such a 
manner that the combined axial length of cylinder and piston rod increases 
during movement of the mass from the low to the raised position, and the 
second piston simultaneously moves into the smaller, second portion of the 
cylinder. The fluid includes a gas in the cylinder cavity under such 
pressure that the vertical component of the force of the gas biasing the 
piston rod outward of the cylinder cavity is greater than one half of the 
force of gravity biasing the mass toward its low position, but smaller 
than the force of gravity. In a typical application of such a gas spring 
arrangement, the support is the body of an automotive vehicle, and the 
mass is an element pivotally fastened to the body for movement between low 
and raised positions.

Referring now to the drawing in detail, and initially to FIG. 1, there is 
seen a gas spring whose normally visible elements are a cylinder 1 and a 
piston rod 2 sealed in an annular end wall of the cylinder 1 in axially 
slidable engagement. The end of the piston rod 2 in the sealed cavity of 
the cylinder 1 carries a main piston 3 which axially separates two 
compartments 4,5 of the cavity. A restricted, axial bore 6 in the piston 3 
permanently connects the compartments. The cross section of the piston 3 
matches that of the main portion of the cylinder 1. 
A second piston 7 matches in cross section the radially reduced portion of 
the cavity in a part 8 of the cylinder near the annular end wall. The two 
pistons 3,7 are mounted on the piston rod 2 in fixed axial positions 
maintained by a tubular spacer 9 in such a manner that the pistons move 
with the piston rod between the illustrated position, in which both 
pistons are received in the main portion of the cylinder cavity, and a 
non-illustrated position in which the piston 3 is still in the main 
portion of the cavity while the smaller piston 7 is received in the 
reduced cylinder portion 8. 
The orifices of large, axial bores 10 of the piston 7 in the radial bottom 
face of the piston are sealed by a pressure relief valve including an 
annular, flat valve disc 11 of elastomeric material backed by a metal disc 
12 and a helical compression spring 13 whose ends abut against the disc 12 
and a shoulder of the piston rod 2. 
The free end of the piston rod 2 outside the cylinder cavity and the 
imperforate end wall of the cylinder 1 which axially bounds the cylinder 
cavity carry fastening eyes 19, as is conventional. 
The two pistons 3, 7 are equipped with check valves, or one-way valves 
constituted by respective piston rings 20, 25 capable of axial movement 
relative to the piston rod 2 and the main parts of the respective pistons 
3, 7. Such movement is limited by retaining rings 21, 24 attached to the 
piston rod 2 by angularly spaced radial arms 23, 26. 
During movement of the piston rod 2 inward of the cylinder 1, the piston 
ring 20 which frictionally engages the inner cylinder face abuts against 
the top face of the piston 3 and seals the narrow, annular clearance gap 
22 between the main piston 3 and the cylinder 1. During outward movement 
of the piston rod 2, that is, during movement in the same direction in 
which the piston 7 is offset from the piston 3, frictional drag holds the 
piston ring 20 against the retaining ring 21, thereby opening an 
additional, wider flow path between the two compartments 4,5 through the 
gaps between the arms 23 and the gap 22 supplementing the flow path 
through the throttling bore 6. 
As long as the second piston 7 is received in the wide, main portion of the 
cylinder cavity, the associated devices described above are inoperative. 
Fluid may flow freely between the cylinder wall and the piston 7. When the 
piston 7 enters the cylinder portion 8, the piston ring 25 operates as 
described above with reference to the ring 20. It opens a slightly 
restricted path for flow of liquid between the two cavity parts otherwise 
separated by the piston 7 in the cylinder part 8 during downward movement 
of the piston rod 2, but it seals this path when the piston assembly of 
piston rod 2, pistons 3 and 7, and associated elements moves inward of the 
cylinder cavity, that is upward, as viewed in FIG. 1. 
Except for a small body of highly compressed air or nitrogen, the cylinder 
cavity is filled with liquid, such as hydraulic brake fluid or oil. When 
the by-pass through the ring 25 is closed and no liquid can flow between 
the cavity parts on opposite sides of the piston 7 in the cylinder part 8, 
the piston assembly is locked in position as long as the spring 12 holds 
the valve disc 11 against the orifices of the bores 10. The strength of 
the spring and the gas pressure in the cylinder cavity are matched to the 
normal operating forces tending to push the piston rod 2 inward of the 
cylinder 1 so that the valve disc 11 maintains its sealing position. 
However, an inward force applied to the piston rod 2 which is greater than 
the normal operating force for which the spring is designed can overcome 
the spring pressure and open the bores 10 to flow of liquid through the 
piston 7. The piston 7 thereafter can be moved out of the cylinder portion 
8 against the pressure of the gas cushion which tends to push the piston 
rod 2 out of the cylinder 1. 
The gas spring 34 illustrated in FIG. 2 differs from that described with 
reference to FIG. 1 by a gas-filled cylinder 1' which has a radially 
expanded terminal portion 15 adjacent to the imperforate end wall 14, and 
by a piston rod 2' modified to accommodate an axial bore 16 communicating 
with a radial bore 17. The passage formed by the bores 16, 17 by-passes 
the unchanged piston 3 as well as the piston 7' which lacks the axial 
bores 10 of the otherwise identical piston 7. A valve sleeve 18 of 
oil-resisting rubber normally seals the orifice of the radial bore 17 
subjacent the piston 7'. It is to be understood that the expanded terminal 
portion 15, while shown only in FIG. 2, may in fact be associated with the 
gas spring of each figure. 
When moving outward of the cylinder cavity from the illustrated position, 
the piston assembly shown in FIG. 2 operates as described above with 
reference to FIG. 1. The piston rings 20, 25 permit fluid flow past the 
pistons 3, 7'. When outward movement stops, and an applied external load 
causes incipient inward movement of the piston rod 2, the piston ring 25 
seals all available by-pass connections between the parts of the cylinder 
cavity on opposite axial sides of the piston 7', and the axial length of 
the spring, that is, the combined length of the cylinder 1' and the piston 
rod 2' cannot be changed until a greater external force causes the rubber 
sleeve 18 to be lifted from the orifice of the bore 17 by the pressure of 
the gas entering the bore 16. 
When the piston assembly moves inward of the cylinder 1 from the 
illustrated position, gas flow initially is restricted to the throttling 
passage through the piston 3 because the clearance space between the 
piston 3 and the cylinder wall is sealed by the piston ring 20. When the 
piston ring clears the main portion of the cylinder 1' and enters the 
expanded cylinder portion 15, so wide an annular passage is opened between 
the piston and the cylinder as not to present significant resistance to 
further movement of the piston assembly to the upper end of its stroke. 
The downward stroke is limited by an internal rib 29 in all illustrated 
embodiment of the invention short of abutting engagement of the piston 7' 
with the annular end wall of the cylinder. 
FIG. 3 shows as much of the otherwise conventional body 30 of a motorcar as 
is needed for illustrating a typical application of the gas springs of the 
invention, in this instance the spring 34 illustrated in FIG. 2. A trunk 
lid 31 is attached to the vehicle body 30 by pivots 32 whose common axis 
is horizontal and transverse to the normal direction of vehicle movement. 
The cylinders of two gas springs 34, of which one obscures the other in 
the view of FIG. 3, are hinged to the body 30 below the pivots 32 on 
coaxial pins 33, each gas spring being approximately in line with a side 
wall of the car body. The fastening eyes on the piston rods 2' are 
attached to the lid 31 by hinge brackets 35 near the lower lid edge. In 
the closed lid position shown in fully drawn lines, the piston assembly in 
the spring 34 assumes a position similar to that seen in FIG. 2 in which 
the compressed gas in the spring cylinder near the hinge pin 33 tends to 
expel the piston rod 2'. 
When a lock 36 is opened, the lid may be lifted manually into the raised 
position indicated in broken lines. The bracket 35 travels in a path 
indicated in chain dotted line which, in the fully drawn low position, is 
almost perpendicular to the spring axis so that the gas pressure in the 
spring 34 initially contributes little to overcoming the force of gravity 
acting on the window. Because of the illustrated relative positions of the 
respective pivot axes of the lid and of the gas spring on the vehicle body 
30, the angle between the axis of the spring and the path of the bracket 
35 increases as the lid 31 is raised, and the force exerted by the gas 
pressure on the piston rod 2 increasingly counteracts the force of gravity 
acting on the lid 31. 
The gas pressure is preferably chosen sufficient for almost balancing the 
force of gravity over at least a portion of the upward lid movement while 
the piston assembly moves outward of the cylinder 1 in a direction which 
is downward in the view of FIG. 2, but is gradually inverted as is evident 
from FIG. 3. Lightly applied manual force causes the lid to be raised. 
During the outward movement of the piston rod 2' from the cylinder 1' and 
the simultaneous movement of the lid 31 from the low to the raised 
position, the one-way valves constituted by the piston rings 20, 25 and 
associated elements are open and present minimal resistance to the 
necessary fluid flow as long as both pistons 3, 7' are in the wide main 
section of the cylinder cavity. The upward movement in braked only 
slightly by the restricted flow path through the annular clearance between 
the second piston 7' and the narrower cylinder portion 8. When the piston 
7' ultimately is stopped by abutment against the rib 29, or by cooperating 
abutments on the body 30 and the lid 31, or is released from manual 
lifting force before the end of its available stroke is reached, the lid 
starts descending, and the piston ring 25 is shifted almost 
instantaneously into the position on the piston 7' which is illustrated in 
FIG. 2 and prevents the flow of liquid necessary for inward movement of 
the piston rod 2'. The lid is locked in its raised position. 
Gravity alone cannot open the bore 17. However, light manual downward 
pressure applied to the lid and assisted by the weight of the latter 
overcomes the combined resistance of gas pressure and of the resiliency of 
the sleeve 18 to expand the latter and to permit fluid flow through the 
bore 16 inward of the cylinder portion 8 while the lid 31 swings downward 
through the arc A (FIG. 3). Even less manual pressure is required as soon 
as both pistons are received in the wider main portion of cylinder 1'. 
When the main piston 3 moves into the enlarged cylinder portion 15 at the 
end of the arc B (FIG. 3), viscosity of the gas no longer contributes a 
significant braking effect. The gas pressure being insufficient for 
balancing the weight of the lid 31, the latter moves through the arc C at 
increasing velocity until the lock 36 is engaged. 
When the gas springs of the type illustrated in FIG. 1 are substituted in 
the spring arrangement illustrated in FIG. 3 for the springs 34, the mode 
of operation is modified only to the extent that slight manual pressure 
needs to be applied until the lock 36 engages the lid 31 in the low 
position. Similar operation is necessary with the additional gas spring 
illustrated in FIG. 4. 
It differs in external appearance from the structure illustrated in FIG. 1 
by a cylinder 1" which is cylindrical over its entire length except for an 
annular groove corresponding to the rib 29. The piston assembly shown in 
FIG. 4 includes a second piston 7' free from axial bores in the same 
manner as in FIG. 2. The reduced portion of the cylinder cavity matching 
the cross-section of the piston 7' is formed by a generally cylindrical 
sleeve 140 axially fixed in the cylinder 1" by a shrink fit and provided 
with axial, external grooves 141 sealed in a radially outward direction by 
the inner face of the cylinder 1". The grooves 141 are normally sealed 
toward the rib 29 and the annular end wall of the cylinder 1" by a plastic 
valve plate 142 and a helical compression spring 143 interposed between 
the valve plate and a washer 144 on the rib 29. 
During outward movement of the piston assembly of FIG. 4 from the cylinder 
cavity which is assisted by the compressed gas in the cylinder, the 
one-way valves at the piston rings 20, 25 are open. When the piston ring 
25 is shifted into the illustrated position by the frictional drag of the 
sleeve 140, i.e., when the second piston 7' is within the sleeve 140 and 
the piston rod 2 is urged upwardly, the piston assembly is axially locked 
until sufficient inward pressure is applied to the piston rod 2 to open 
the valve plate 142 against the biasing restraint of the spring 143. As 
will be appreciated, when the piston assembly moves upward the pressure 
above the second piston 7' is increased, and this pressure acts downward 
through the grooves 141 to lift the edge of the valve plate off the lower 
end of the sleeve 140. 
While the gas springs of the invention have their greatest utility at this 
time in automotive applications of the kind illustrated in FIG. 3, they 
may be employed to advantage wherever a solid mass is mounted on a support 
for movement in a direction having a vertical component, and those skilled 
in the art will know how to select internal gas pressures and suitable 
resilient valve elements to achieve a desired mode of operation in which 
the piston assembly is locked automatically when its movement is stopped 
or reversed, but can be released by pressure applied in the direction of 
the reversed movement. 
It should be understood, therefore, that the foregoing disclosure relates 
only to preferred embodiments of the invention, and that it is intended to 
cover all changes and modifications of the examples of the invention 
chosen herein for the purpose of the disclosure which do not constitute 
departures from the spirit and scope of the invention set forth in the 
appended claims.