Hydro-pneumatic shock absorber of the kind comprising a fluid-containing cylinder which is divided into two chambers of variable volume by a piston mounted on a piston rod, there being also provided a compensation chamber for displaced fluid, a compressed gas pressure being maintained within said compensation chamber, wherein the aforesaid piston is secured to the middle of the piston rod which projects through a gland at each end of the cylinder, at least one of said two chambers of variable volume communicating with said compensation chamber by means of a communication port.

This invention relates to a hydro-pneumatic shock absorber, particularly 
for use in vehicles (such as road or rail vehicles), of the kind wherein a 
piston mounted on a piston rod subdivides the interior of a 
fluid-containing cylinder into two chambers of variable volume, and 
wherein an equalising or compensation chamber is provided for the absorber 
fluid and a raised gas pressure is maintained within said chamber. 
Numerous forms of shock absorbers of the kind specified are known. These 
include devices wherein the compensation chamber is concentric with the 
cylinder, for example as in the so-called double-tube absorber device 
described in German PS No. 1 284 693. In this device the compensation 
chamber is annular and surrounds the cylinder and communicates with the 
absorber fluid in the cylinder by means of a fluid duct. The piston, which 
is equipped with damper valves, is secured to the end of the piston rod so 
that the compensation chamber accepts the fluid displaced by the piston 
rod when the piston rod enters into the cylinder and returns the fluid 
when the piston rod moves outwardly again. Furthermore, a raised pressure 
level is maintained in the compensation chamber by means of a compressed 
gas in order to prevent cavitation effects in the region of the valve 
ports of the piston valves through which the fluid passes, which effects 
would cause considerable modifications to the damper characteristics of 
the valves. 
However these conventional devices are afflicted by a disadvantage which 
resides in the fact that the piston rod is subjected to considerable 
outwardly directed thrust or expulsion forces created by the pressurised 
gas which may have an excess pressure of between approximately 20 to 40 
atmospheres, and in that these forces increase further still with rising 
temperature levels of the whole system after prolonged service. As a 
result of this, the shock absorber becomes comparatively rigid and this 
will modify the elasticity characteristics of the shock absorber in 
service. There are also various undesirable side effects. It is true that 
these side effects may be somewhat moderated by using very thin piston 
rods, yet this again raises the danger of such piston rods bending or even 
buckling out of true during service. On the other hand, if the 
cross-sectional dimensions of the piston rod are selected so as to be 
relatively large in order to preclude this type of damage, the 
above-mentioned impairment with regard to elasticity must be inevitably 
accepted. For this reason it has hitherto been possible to make 
sufficiently rigid hydro-pneumatic shock absorbers only at the expense of 
considerable additional constructional outlay. 
The object of the present invention is to provide an improved 
hydro-pneumatic shock absorber which does not suffer from the 
aforementioned disadvantages. 
In accordance with the invention there is provided an hydro-pneumatic shock 
absorber of the kind wherein a piston mounted on a piston rod subdivides 
the interior of a fluid-containing cylinder into two chambers of variable 
volume, and wherein a compensation chamber is provided for the absorber 
fluid and a compressed gas pressure is maintained within said chamber, 
characterised in that the piston is secured to the piston rod 
approximately in the middle of said rod which in turn projects through a 
gland at each end of the cylinder and in that at least one of said two 
chambers of variable volume communicates with said compensation chamber by 
means of a communication port. 
If the piston rod is pushed inwardly into a shock absorber constructed as 
above defined this will not cause any change of volume in the fluid 
contained within the cylinder. On the contrary, the sum of volumes on 
either side of the piston always remains constant. Inasmuch as absorber 
fluid is exchanged through the communicating ports with the common 
compensation chamber, the amount of fluid entering on one side will always 
correspond to that displaced on the other. However, this compensation 
chamber is by no means dispensable altogether because the temperature of 
the fluid will progressively increase during working. Moreover, as this 
chamber is constantly maintained under pressure, there can be no 
cavitation effects in the region of any damping valves. The pressure 
within the compensation chamber is conveniently established in a per se 
conventional manner with the aid of a compressed gas, and its value may be 
selected virtually as high as may be desired because it does in no way 
affect the piston rod displacement. On the contrary, the such piston rod 
displacement can be obtained at relatively different rates in the two 
directions of piston rod movement solely through the action of the damping 
valves. 
In a shock absorber according to this invention, the piston may be a 
conventional piston with valves of relatively different damping force 
opening in both directions of flow of the absorber fluid. On the other 
hand, the piston may be without ports or apertures but in that event the 
communicating ports themselves have valves which open in both directions 
of flow for the absorber fluid. 
In veiw of the fact that only a limited amount of space is usually 
available for shock absorbers in motor-cars for example, especially in the 
longitudinal direction, one advantageous embodiment of the invention 
provides for the compensation chamber to assume the shape of an outer 
sleeve surrounding the cylinder; in this arrangement one of the 
communicating ports is situated immediately beneath the upper seal or 
gland of the cylinder and the other immediately above the lower seal 
whilst ducts issue from both these ports and terminate in the lower region 
of the compensation chamber. Since the compressed gas is contained in the 
upper region of the sleeve-like compensation chamber in this embodiment, 
the duct ends have no communication at all with the compressed gas so that 
only very little of this compressed gas can be absorbed by the operative 
fluid. 
Alternatively, and especially in view of the fact that the present 
invention requires a much smaller compensation chamber than in 
conventional devices, this chamber may be accommodated, in a per se known 
fashion, within the piston rod which would then be of hollow conformation. 
Having due regard to the size of the compensation chamber the piston rod 
may then be readily designed with sufficiently thick walls to meet all 
practical operational needs and requirements. In such a piston rod the 
communicating ports would be situated immediately above and immediately 
below the piston. A division or separation relative to the compressed gas 
volume likewise contained within the piston rod is then effected by a 
dividing piston or plunger. 
Advantageously, and particularly as applied to shock absorbers in motor 
vehicles, the shock absorber may be designed with an upwardly extendable 
piston rod adapted to be connected to the vehicle body by conventional 
securing means. Accordingly, the opposite or lower end of the piston rod 
projects through the lower gland from the lower cylinder end. As a 
protection against dirt, which may constitute a real danger, particularly 
for the lower part of the piston rod, and further for the purpose of 
providing suitable securing means, e.g. an axle-limb bolt, the cylinder is 
conveniently extended in the downward direction beyond the lower gland so 
that the latter will be surrounded by the cylinder, this extended cylinder 
portion conveniently has a base or bottom which is closed except for a 
vent or breather hole to allow the escape of air which in use is displaced 
by the piston rod, and the bottom of the piston rod has a stop or shoulder 
of larger diameter which is engageable with the lower gland to limit 
upward movement of the piston rod relative to the cylinder.

In the arrangement depicted in FIGS. 1 and 2, a compensation or equalizing 
chamber 1 surrounds the cylinder 2 in the manner of a concentric sleeve. A 
piston 4 is secured to a piston rod 3 approximately in the middle thereof, 
the piston being disposed within the cylinder 2. A sealed guide or gland 
for the piston rod 3 is provided in the upper part of the cylinder 2, said 
gland comprising a sealing lip 5, an outer guide ring 6, an inner guide 
ring 7 and a stripper seal 8. A similar sealing guide means comprising 
elements 5 to 7 is also provided in the lower part of the cylinder 2. 
Below the upper seal 5 and above the lower seal 5 there are provided ports 
9 and 10 from which issue respectively the ducts 11 and 12 which both 
terminate in the lower portion of the sleeve-like compensation chamber 1. 
The latter requires no very large volume because its only essential 
function resides in equalizing or compensating for thermal expansion 
variations of the operative fluid. Above the fluid level 13 the 
compensation chamber is filled with compressed gas 14. 
The upper portion of the piston rod 3 projects freely from the cylinder 2 
and is provided with means, not here specifically shown, for attachment to 
a constructional element, e.g. to a vehicle body. The lower portion of the 
piston rod 3 is surrounded by the lower part of cylinder 2 and is provided 
with a stop 15 which limits the upward displacement of the piston rod by 
eventual engagement with the adjacent guides 6, 7. The bottom of the 
cylinder ends in a base or bottom wall 16 which is provided with a vent or 
breather hole 17 for air displaced by the lower portion of the piston rod 
3. 
In the embodiment depicted in FIG. 3, a hollow piston rod 18 accommodates 
the compressed gas volume 14 and the absorber fluid, the latter being 
separated from the former by means of a longitudinally slidable separator 
piston or plunger 19. Compressed gas may be introduced through the opening 
20 which may be seen in the upper part of the piston rod 18 and which is 
adapted to be subsequently closed, whilst the shock absorber may be filled 
as a whole with operative absorber fluid through a corresponding lower 
opening 21 in the hollow piston rod. 
The cylinder 2 is provided with an upper gland or sealed guide means which 
comprises the parts 5, 6, 7 and 8 as in the arrangement shown in FIG. 1. 
The piston rod 18 enters into the cylinder through this gland. Immediately 
above a piston 4 secured to the piston rod 18, there is a communicating 
port 22 whilst another communicating port 23 to the interior of the hollow 
piston rod 18 is arranged below the piston 4. These communication ports 
afford a direct exchange of operative fluid between the hollow interior of 
the piston rod 18 and the interior of the cylinder 2 outside said piston 
rod. 
In this embodiment of the invention the cylinder 2 extends less far 
downwardly than in the other. Instead a smaller diameter cylindrical part 
24 is fixedly fitted therein and this in turn accommodates the lower gland 
comprising the parts 5, 6 and 7. Again the arrangement includes a base or 
bottom 16 provided with a vent hole 17 and a limiting shoulder or stop 
element 15. 
The piston 4 in either of the above-described constructions may be of 
solid, i.e. non perforated design. In that event the communication ports 
9, 10, or 22, 23 will be vital with regard to shock or vibration 
absorption and consequently these ports are then provided with appropriate 
valves which may be arranged to provide different clamping forces in the 
two directions of movement of the piston 4. However, it will be 
appreciated that the piston 4 may also be a conventional damper or 
absorber piston as diagrammatically represented in FIGS. 4 and 5. The plan 
or top view of the piston 4 here reveals several valve bores 25 having 
frusto-conical counter-bores 26 and valve seats 27. One of these valve 
seats 27 is shown in the left side of FIG. 5. It has at one end a face for 
engagement with a conical valve member 28 whilst at the opposite end it 
acts as an abutment for a valve spring 29. The latter loads the valve stem 
30 in such a way that the valve 28 is normally maintained in its closed 
position under a predetermined loading force but opens when the piston 
moves downwardly. Permanently open passageways may also be provided. The 
right side of the piston 4 seen in FIG. 5 has a similar valve which 
operates however in the reverse direction and these two valves may be 
arranged to provide for different damping effects in the two opposite 
directions of movement of the piston.