A hydro-pneumatic spring suspension strut for motor vehicles

Abstract

 A hydro-pneumatic spring suspension strut for vehicles provides damping limited proportionally to the load, and comprises an hydraulic telescopic oneway shock absorber (3) and a load bearing pneumatic spring (2). The gas space of the pneumat spnng (2) is bounded by a rotting membrane (6) and rigid walls (4, 7) fixable to the vehicle body and wheels. The shock absorber cylinder (22) is connected to an auxiliary membrane (15) open to the pressure prevailing inside the pneumatic spring (2) to urge the cylinder (22) in the direction of extension of the strut (1, 35, 41) and to bear against a piston column (5) in the strut (1,35,41) the oscillation of which is to be damped. The auxiliary membrane (15) enables the cylinder (22) to move relative to the piston column (5) when its biasing force is balanced by the damping force of the shock absorber (3).

Description

[0001] This invention concerns a hydro-pneumatic spring suspension strut for motor vehicles including an air spring and a one-way hydraulic telescopic shock absorber the damping force of which varies according to the load of the strut or that of the motor vehicle.

[0002] In one known variant of motor road vehicle suspension the vehicle body is suspended by an air spring mounted between the body and the running wheels with an hydraulic telescopic shock absorber connected in parallel with the air spring. The height of the vehicle body is maintained substantially constant, independently of the load on the vehicle,by interposing a height corrector valve, and as a consequence of the characteristics of the air spring, the natural frequency of the vehicle can be adjusted to a favourably low value.

[0003] In the course of relative movement between the vehicle body and the wheels, the hydraulic shock absorber exerts a damping force proportional to the velocity of that relative movement and converts the kinetic energy of the oscillating movements of the vehicle into heat.

[0004] Spring strut constructions are already known in which the shock absorber and the air spring are combined into a single or integrated unit, see for instance the hydro-pneumatic spring struts for vehicles disclosed in Patent Specification Nos. 1 152 316 and 1 184 225 of the Federal Republic of Germany. Both of these known constructions disclose a rolling membrane air spring mounted on an hydraulic telescopic shock absorber in , such a way that one side (face) of the rolling membrane air spring is fixed to that end of the piston rod of the shock absorber which is connected to the vehicle body, while the other side of the rolling membrane is connected to a base cylinder of the shock absorber linked to the wheels, and in use the membrane can roll along the outer surface of the base cylinder during oscillation of the body. The advantages of the above-mentioned constructions compared with separately mounted air springs and shock absorbers are that these single-unit spring suspension struts require less installation space, their mounting on the vehicle is simpler, that the coincident line of action of the spring and of the damping force does not produce any torque, whereby to simplify the construction, and that the piston rod of the shock absorber is better protected against contamination, which is advantageous from the point of view of its useful life. On the other hand, it is unfavourable that the load on the individual elements of the shock absorber increases and a failure of or a fault developing in the shock absorber can also cause damage to the rolling membrane.

[0005] It is commonly a disadvantageous characteristic of both the traditional, separately mounted air spring and shock absorber and of the above-mentioned single-unit or integrated type hydro-pneumatic spring suspension struts that although the shock-absorbing characteristics of the air spring is suitably influenced with the aid of the height corrector valve as a function of the varying load on the vehicle, the hydraulic shock absorber exerts its effect substantially'independently of the load, that is to say the damping force is a function of the velocity of oscillations (bouncing) only. From this it follows that if for instance the shock absorption or oscillation of damping of a given vehicle is adjusted to be optimal at full load, then on partial load it will become over- damped which gives rise to an uncomfortable ride.

[0006] The aim of this invention as defined in the appended claims is to overcome, or at least to reduce, the above-described disadvantage of suspension struts and to provide improved struts wherein the magnitude of the damping force varies with variations in the bad on the spring strut, i.e. on the vehicle, whereby the shortcoming of known struts, namely that the magnitude of the damping is optimal only in one arbitrarily chosen load condition, is obviated.

[0007] The aim of the invention was sought to be achieved in one preferred embodiment thereof by not fixing one of the cylindersor the piston rods of the shock absorber - preferably not fixing the cylinder- to the running gear (wheels) or to the vehicle body, but instead subjecting it to the pressure of the air in the air space of the air spring with the aid of an auxiliary membrane, whereby to load it in the direction of extension so that in its rest position the cylinder (or piston rod) bears against the element the oscillation of which is to be damped, but aan also move away from that element.

[0008] When the relative velocity of oscillation between the running gear and the vehicle body exceeds the limit velocity at which the damping force which is proportional to the velocity, is equal to the supporting force of the auxiliary membrane - which in turn depends on the internal air pressure- the cylinder moves away from its supported rest position and thereafter the shock absorber will_;move only at this limit velocity and provide a damping force _' of a magnitude determined by the auxiliary membrane. On the other hand, when the said relative velocity falls below the limit velocity, the cylinder of the shock absorber will re-assume its supported rest position.

[0009] Since usually the air pressure in the air spring changes according to the load on the body, due to the effect of the height corrector valve, the damping force limited by the auxiliary membrane will also be proportional to the load on the spring strut.

[0010] Accordingly, the hydro-pneumatic spring suspension strut of this invention is characterised in that one of the two relatively movable elements of the shock absorber is connected to an auxiliary membrane that is subjected to the pressure prevailing in the-gas space of the pneumatic spring so as to urge said one element in the direction of extension of the strut to bear against and move with the member of the strut, the oscillation of which is to be damped, the auxiliary membrane enabling said element to move relative to said member when the damping force of the shock absorber equals or exceeds the force exerted by said auxiliary membrane and abutment means for limiting the movement of said one element.

[0011] In a preferred embodiment of this invention both the resilient jacket of the air spring and the auxiliary membrane are constituted as rolling membranes and the piston of the air spring is arranged on a piston column so that it can be rigidly connected to the running gear. The shock absorber cylinder has an end face formed as an abutment surface capable of co-operating with a counter-abutment surface formed in the piston column. The piston rod of the shock absorber is connected to a rigid wall of the air spring, which wall can be fixed to the vehicle body.

[0012] In another advantageous embodiment, the piston rod of the shock absorber is fixed to the running gear and one rigid wall of the load-bearing air or gas spring is formed as a piston of the rolling membrane fixed to the wheels, the piston rod of the shock absorber being connected to the piston of the air spring which latter has an auxiliary piston formed as a rigid wall adapted to be secured to the vehicle body. The outer rim of the auxiliary membrane is sealingly fixed to this auxiliary piston, while the counter-abutment is Disposed in the above-mentioned rigid wall of the air spring; and there is a guide pin in a guide bush fixed to an end face of the shock absorber cylinder which face serves as a supporting surface and is provided with a resilient bearing plate, the end face of the guide bush serving as the counter-abutment of the above-mentioned bearing plate. Further, a stop plateis arranged on the end of the guide pin projecting beyond the guide bush to limit the extent of resilient extension, and the stop plate carries an abutment or impact damping spring bearing against the end face of the guide bush.

[0013] The hydraulic telescopic shock absorber forming part of the spring suspension strut according to the present invention is a so-called one-way shock absorber which is a term of art signifying that its damping force for the same oscillation velocity on extension (in tension) is several times greater than on retraction (in compression). Thus, the load-dependent limitation of the damping force, which in practice is generated only on extension, ensures that the damping of the vehicle remains approximately the same at partial load as at full load. At thesame time, the strut also establishes the limits of the movements in extension and in compression. The stroke of the shock absorber forming part of the spring strut is shorter than the total springing path of movement of the spring strut, the difference being equal to or greater than the amplitude of those oscillations of the wheels on a smooth road surface which are caused by the resilience of the tyres and by any unbalance of the wheels.

[0014] In a suitable construction of the shock absorber, its cylinder may be finned or may have a finned cooling head, to improve its cooling.

[0015] The invention will now be described in greater detail and by way of example with reference to three preferred embodiments illustrated in the accompanying drawings, in which: _.

Figure 1 is a longitudinal cross-section through a first embodiment of a spring strut according to the invention, wherein the piston rod of the shock absorber is fixed to the upper casing of an air spring;

Figure 2 is'a longitudinal cross-section through a second embodiment of a spring strut according to the invention, wherein the piston rod of the shock absorber is fixed into a piston provided in the air spring; and

Figure 3 is a longitudinal cross-section of a third embodiment of a spring strut according to the invention, shown mounted in a motor vehicle.

[0016] Like or functionally equivalent parts have been allotted like reference numbers in the drawings.

[0017] Referring to the embodiment of a spring strut for a motor vehicle shown in Figure 1,. the strut 1 consists' of three man parts, namely an air spring 2, an hydraulic shock absorber 3 and-a support member 5 on which the different elements are supported and mounted; hereafter the member 5 will be referred to as "piston column 5". The air spring 2 of Figure 1 is a rolling membrane type of air spring with a rigid wall portion 7 adapted to be connected to the body of the vehicle, not shown in Figure 1, by way of bolts 9. An air coupling duct 8 extends through the wall 7 and communicates with the internal air space of the air spring 2, the duct 8 being connectable to a source of air in the vehicle.

[0018] A membrane 6 is sealingly secured to the wall 7 and during operation it can roll downwardly, as viewed, on the outer casing of a piston 4 mounted on piston column 5. The membrane 6 has an inner rim 11 sealingly fixed to a rim 12 of the piston 4 by means of a locking ring 14. The locking ring 14 also fixes and seals the outer rim 13 of an auxiliary membrane 15 the inner rim 16 of which is fixed to a collar 17 on the outer surface 23 of a cylinder 22 forming part of the shock absorber 3.

[0019] In use, the auxiliary membrane 15 can roll on the inner surface 19 of the piston 4 of the air spring 2 as well as on the outer surface 23 of the shock absorber cylinder 22...

[0020] The shock absorber 3 is a telescopic, one-way type of shock absorber having an apertured and valved piston 21 connected to one end of a piston rod 20 the other end of which is connected to a block 10 of the wall 7. The magnitude of the forces generated on the piston 21 on extension is a multiple of those in compression, which in practice is considered to be a one-way mode of operation. The cylinder 22 is extended downwardly in the form of a guide pin 26 guided in a guide bush 27, the outer surface of which is part-spherically convex and engages a complementarily concave, part-spherical seat 33 formed in a connecting rim portion 29 of the piston column 5. Planar end faces 32, 34 of the bush 27 serve as supporting and stop faces. The end face 34 is a counter-abutment surface to the bearing rim 24 of the shock-absorber cylinder 22, or more particularly, to a resilient bearing plate 25 disposed on the bearing rim 24. The end face 32 is a counter-abutment surface to an abutment disc 31 serving to limit the extension of the shock absorber. The disc 31 is mounted at the end of the guide pin 26 and is separated from the face 32 by a spring 30 of elastomeric material. The connecting rim 29 has bolts 28 for clamping the strut 1 to the vehicle wheels or to a part moving with the wheels.

[0021] The extension of the spring strut 1 is limited by the disc 31 of the shock absorber 3 which means that the springing path of the spring strut 1 is longer.than the maximum displacement of the shock absorber 3. The surface area of the auxiliary membrane 15 is chosen to be approximately one-third of the effective surface area of the rolling membrane 6 and that its displacement is one-fifth of the total spring path length of the strut 1.

[0022] The operation of the spring strut 1 is as follows:

The air spring 2 of the strut 1 is filled with air via the coupling 8 to the required operational pressure by the air system of the vehicle and the height corrector or control valve, not shown in the drawing, whereby to provide a predetermined body height or clearance independently of the load. The air pressure in the air spring 2 is load-dependent and in general is proportional to the load. In normal use, on compression the wall 7 and the air spring piston 4 approach each other, the volume of the air spring 2 is reduced and the rolling membrane 6 rolls down on the outer rim 18 of the piston 4. The pressure in the spring 2 increases with decreasing volume, and the increasing pressure exerts an increasing spring force to the effective surface of the rolling membrane 6 of the air spring 2. The same air pressure is exerted on the auxiliary membrane 15, thus tending to hold the cylinder 22 of the shock absorber 3 via thec disc 25 to the piston column 5 while the damping force of the shock absorber 3 acts in the opposite direction with a magnitude determined by the air pressure. As long as the force on the shock absorber cylinder 22 exerted by the auxiliary membrane 15 is greater than the damping force the resilient bearing disc 25 continues to be held down on and against the piston column 5. As the velocity of oscillation increases a state of dynamic equilibrium is reached in which the force on the shock absorber cylinder 22 transmitted by the auxiliary membrane 15 is equal to the damping force. In this State, the force holding the shock absorber cylinder 22 down against the piston column 5 ceases, and if the oscillation velocity increases further, the movement of the shock absorber cylinder 22 lags or falls behind relative to the piston column 5 and the wheels and the shock absorber cylinder 22 will only come again to rest against the piston column 5 and move together with it when the oscillation velocity falls below the above-mentioned limit value. The movement of the shock absorber cylinder 22 relative to the piston cylinder 5 is made possible by the auxiliary membrane 15 being a rolling membrane so that it can roll on the inner surface 19 of the air spring piston 4 and on the outer surface 23 of the shock absorber cylinder 22.

[0023] As a result of the above-described mode of cperation, on extension the damping force cannot exceed the value determined by, and proportional to, the air pressure of the auxiliary membrane 15 and the air spring 2. Since the static air pressure is proportional to the load on the spring strut 1, when the latter extends, a load- proportional limitation of the damping force is ensured. The dynamic variation of the air pressure during the oscillations has very little effect on the load- proportional limitation of the damping force because, " on the one hand for the most frequently applied suspensions of motor vehicles, the variation of the pressure of the air in the air springs is only about + 10% of the static pressure in most practical cases and, on the other hand the oscillation lags in phase behind the oscillation path. It follows from that that high oscillation velocities occur around the mid-position i.e. the basic operational position, at which the dynamic air pressure is approximately the same as static air pressure. A limitation of the damping force is needed at higher oscillation velocities which is ensured in the above-described way. On extension, i.e. when the wheels move away from the body of the motor vehicle, the force exerted on the body of the motor vehicle by-the spring strut 1 is equal to the difference between the spring force of the air spring 2 and the load-proportionally limited damping force of the shock absorber 3.

[0024] The extension of the spring strut 1 is reliably limited by the fact that the stroke of the- shock absorber 3 is shorter than the path length of springing movement of the spring strut 1 and in a fully extended position first the piston 21 of the shock absorber cylinder 22 mechanically abuts the end of the cylinder and the guide pin 26 is displaced in the bush 27, which compresses the elastomeric spring 30 and then abuts the stop disc 31.

[0025] Thus the simple construction of the spring strut according to Figure 1 of the present invention makes load- proportional control of the damping force possible, while requiring relatively little space because the air spring and the shock absorber are integrated into a single unit; moreover, the piston rod of the shock absorber operates in a clean and practically uncontaminatable environment, and no torque is generated between the two suspension elements 2, 3 because the air spring 2 and the shock absorber 3 are mounted concentrically.

[0026] The embodiment shown in Figure 2 differs from that of Figure 1 mainly in that it is the cylinder 22 bf,the shock absorber 3 mounted in a spring strut generally designated by 35 that is fixed to the body of the motor vehicle while the piston rod 20 is fixed, directly or indirectly, to the wheels. An auxiliary piston,36 forming part of a rigid wall 7 is secured to the outer rim 13 of the auxiliary membrane 15 at its rim 38 so that the membrane 15 can roll on the inner surface 37 of the piston 36. The guide pin 26 of the cylinder 22 of the shock absorber 3 is guided in the guide bush 27 mounted in the wall 7.

[0027] The air spring piston 4 has a connecting rim 39 by which it can be fixed to the wheels, with the aid of bolts 40.

[0028] The mode of operation of the spring strut 35 is essentially the same as that of the spring strut 1 shown in Figure 1. The space requirements of the two struts 1 and 35 are the same. A consideration in choosing between these two spring struts is whether the space requirements of the guide pin 26 and the damping spring 30 are better accommodated in the vehide body or in the coupling to the wheels.

[0029] Figure 3 shows a further embodiment of a strut mounted in a vehicle and designated generally by 41. The main advantage of the spring strut 41 shown in Figure 3, as compared with the struts 1 and 35 is its reduced longitudinal dimensions, i.e. its reduced size in the direction of the axis of the shock absorber. The difference is equal to the combined length of the guide pin 25 and the stop disc 31 in Figure 2. The wall 7 is fixed to a body 51 by bolts 9, and a connection rim 49 of the air spring piston 4 is fixed to a bridge body 50 by bolts 48.

[0030] The air supply system feeding the air spring consists of a compressor 57, a duct 55b connecting the compressor 57 to a tank 56, a duct 55a connecting the tank 5.6 to a height corrector (height level control) valve 54. The pressure coupling 8 of the air spring 2 is connected through a duct 52 to the control valve 54 which is connected to the bridge body 50 by an articulated link 53.

[0031] The main difference between the spring strut 41 and the spring strut 1 shown in Figure 1 is that there is a stop collar 42 on the cylinder 22 of the shock absorber which is disposed between two resilient support rings 45, 46 for limiting cylinder displacement, and the rings 45, 46 are embedded in a groove 43 in the rim 12 of the air spring piston 4. An elastomeric stop 47 is secured on the air spring piston 4 for limiting the "bounce" of the strut 41. The distance between the two rings 45, 46 is chosen to be equal to or greater than the amplitude of wheel oscillations, on a smooth road surface, due to the resilience of the tyres and any unbalance of the wheels.

[0032] There is a gap 44 in the stop collar 42 to provide permanent communication between the upper and lower air spaces of the air spring 2. The operation of the strut 41 is essentially the same as that of the strut 1 shown in Figure 1.

[0033] When the vehicle is not in operatinn the wall 7 and body part 51 are supported by the stop 47. When the engine of the vehicle is started up the' air compressor 57 fills the tank 56 with air which then enters the air spring 2 via the height control valve 54 until the level reaches the preset value independently of the load of the vehicle. When the level reaches the preset value, the control valve 54 closes and remains closed within the predetermined spring path length. In this way the pressure in the air spring 2 is proportional to the load and the force exerted on the auxiliary membrane 15 is also proportional to the air pressure. At rest, the stop collar 42 of the cylinder 22 of the shock absorber 3 is supported on the ring 45. In this state the pressure between the upper and lower air spaces of the air spring 2, separated from each other by the stop collar 42, can be equalised through the gap 44. When the body 51 and the bridge 50 move relative to each other, the strut 41 operates in essentially the same way as was described above in connection with Figure 1.

Claims

1. A hydro-pneumatic spring suspension strut for vehicles which provides damping limited proportionally to the load, comprising an hydraulic telescopic shock absorber (3) and a load-bearing pneumatic spring (2) connectable to a source of pressurised gaseous fluid, the gas space of the pneumatic spring (2) being bounded by a resilient wall (6) and rigid walls (4, 7) adapted for fixing to. the vehicle body (51) and the. wheels, and the constructional elements of the shock absorber (3) are in part arranged in the gas space of the pneumatic spring (2), characterised in that one (22) of the two relatively movable elements (20, 22) of the shock absorber (3) is connected to an auxiliary membrane (15) that is subjected to the pressure prevailing in the gas space of the pneumatic spring (2) so as to urge said one element (22) in the direction of extension of the strut (1, 35, 41) and to bear against and move with the member (4 - 5) of the strut (1, 35, 41) the oscillation of which is to damped, the auxiliary membrane (15) enabling said element (22) to move relative to said member (4 - 5) when the damping force of the shock absorber (3) equals or exceeds the biasing force of the auxiliary membrane (15) and abutment means (24, 27; 42, 45 and 46) for limiting the movement of said one element (22).

2. A suspension strut according to'claim 1, characterised in that the inner rim (16) of the auxiliary membrane (15) is fixed to the cylinder (22) of the hydraulic telescopic shock absorber (3) while its outer rim (13) is fixed to one of the above-mentioned rigid walls (7) of the pneumatic spring (2) and constitutes one of the boundary walls of the gas space of the pneumatic spring (2); and that said abutment means includes a stop (27,45) fixed to one of said rigid walls (4 or 7) bearing the outer rim (13) of the auxiliary membrane (15), while the co-operating counter-stop (24; 42) of said abutment means is arranged on the cylinder (22) of the shock absorber (3); and that the piston rod (20) of the shock absorber (3) is fixed to the other of said rigid walls (7 or 4) of the pneumatic spring (2).

3. A suspension strut according to claim 1 or claim 2, characterised in that both the resilient wall (6) of the pneumatic spring (2) and the auxiliary membrane (15) are formed as rolling membranes, and that the rolling membrane (6) of the pneumatic spring (2) is connected to a rigid piston-like wall (4) provided with securing elements (28, 29, 40) for securing to the wheels.

4. A suspension strut according to any preceding claim characterised in that a piston-like rigid wall (4) of the pneumatic spring (2) is arranged on a piston column (5) which has fixing elements for connection to the wheels, the end face (24) of the shock absorber cylinder (22) forming one of said stops while the counter-abutment (27) is formed as a supporting surface in the piston column (5), and that the piston rod (20) of the shock absorber (3) is connected to the rigid wall (7) of the pneumatic spring (2) which is connectable to the vehicle body.

5. A suspension strut according to any of claims 1 to 3, characterised in that it has an auxiliary piston (36) which forms a part of the rigid wall,(7) of the pneumatic spring (2) connected to the vehicle body, the outer rim (13) of said auxiliary piston (36) being rigidly and sealingly secured to the auxiliary membrane (15); and that the said counter-abutment$(27) is disposed in the said rigid wall (7) of the pneunatic spring (2) and the piston rod (20) of the shock absorber (3) is connected to the piston-like rigid wall (4) of the pneumatic spring (2).

6. A suspension strut according to any preceding, claim, characterised in that a guide pin (26) is fixed to one end face (24) of the cylinder (22) of the shock absorber (3), the said end face (24) being provided with a resilient supporting plate (25), the guide pin (26) being guided in a guide bush (27) an end face (34) of which forms a counter-abutment to said supporting plate (25), and a stop plate (31) for limiting the extension of the strut is mounted on the end of the guide pin (26) projecting beyond the guide bush (27), there being a spring (30) between said stop plate (31) and the end face (34) of the bush (27).

7. A suspension strut according to any preceding claim, characterised in that the said bush (27) is orientably journalled in a seating (33), co-operating parts of the bush (27) and the seating (33) forming a ball joint.

8. A suspension strut according to claim 1 or claim 2, characterised in that the outer rim (13) of the auxiliary membrane (15) is sealingly fixed as sealing to the rim (12) of the piston-like rigid wall (4) of the pneumatic spring (2), the wall (4) journalling resilient abutment rings (45, 46) which sandwich an abutment collar (42) on the shock absorber cylinder (22) to limit the displacement of the latter, a channel (44) being formed in the collar (42) to connect the gas spaces separated by the rim (12) of the piston-like wall (4); and that the piston rod (20) of the shock absorber (3) is connected to the other rigid wall (7) which can be fixed to the vehicle body (51).

9. A suspension strut according to any preceding claim, characterised in that the stroke of the spring strut (1, 35, 41) is longer than that of the shock absorber (3).

Drawing