SHOCK ABSORBER WITH TWO PRESSURIZED CHAMBERS AND A METHOD FOR ADJUSTING PRESSURE BALANCE BETWEEN SAID

SHOCK ABSORBER'S TWO CHAMBERS

Technical field

The invention relates to a method for controlling the pressure balance between the two chambers of a shock absorber, in which the damping medium pressure in the return chamber does not fall below a preselected minimum pressure, whereby cavitation is avoided. As a result of the method, energy is stored in the shock absorber, so that a pressure is built up in a resilient device, preferably in the form of an accumulator, disposed in the return chamber. In the resilient device a resilient function is created, which initially during the compression stroke increases the pressure in the return chamber. The invention also relates to a pressurized shock absorber, in which a resilient device comprising a pressurizing member or pressurizing medium, which acts in an inner volume delimited from the return chamber, is disposed directly adjacent to the return chamber.

Background to the invention

A shock absorber operates on an adjustment of the pressure ratio between the pressure upon the damping medium in the compression and the return chamber respectively. A high pressure gives a higher rigidity, that is to say that a greater force is required to compress the damping medium. The piston which separates the compression and the return chamber can be provided with flow-adjusting valves, or it is fully leak-tight where externally mounted valves adjust the flow between the chambers. The pressure drop over the piston determines the pressure ratio and this can be altered dynamically by having a system pressure act upon the damping medium. The system pressure is determined by a pressurizing member mounted in or on the shock absorber

body. The pressurizing member is connected to and pressurizes either just the compression chamber or both the compression and the return chamber. The pressurizing member is designed to take charge of the pressure medium which is pushed aside by the piston rod, to absorb the changes in damping medium volume caused by temperature differences, and to generate a certain basic pressure (the system pressure) in the shock absorber. The damping medium flow between the pressurizing member and both or one of the damping chambers can be adjustable with one or more adjustable valves, hereinafter referred to as cylinder valves.

In a compression stroke in an ideal shock absorber with a cylinder valve, the pressure in the return chamber is constantly equal to the system pressure. The counterpressure which is created with the aid of the cylinder valve therefore compensates for the reduction in pressure in the return chamber which is brought about by the pressure drop over the piston.

In a real shock absorber, an ideal compression stroke is an impossibility, since the rigidity in the return chamber is higher than the rigidity in the compression chamber when the shock absorber approaches the rebounded state. The pressure in the compression chamber is therefore not built . up as fast as the pressure in the return chamber falls, with the result that it is not possible to use the cylinder valve to increase the pressure in the return chamber. With too low a pressure in the return chamber, the risk of cavitation increases, resulting in a loss of damping forces .

Examples of previously known solutions of this problem can be found in US2004134730 or in the Applicant's own patent EP0601982. EP0322608 further shows an embodiment in which the damping medium is conducted both through the piston and in a duct outside the damping chamber,

depending on the stroke rate. In the case of certain rapid compression movements of the stroke, it is difficult, however, for the damping medium to manage to pass through this duct, which means that the pressure in the return chamber nonetheless falls below that in the compression chamber.

In document DE10052789, a shock absorber is shown which solves another problem, namely the adjustment of the damping flow between the return chamber and a space

(19) designed to absorb the piston rod displacement and any differences in damping medium volume due, for example, to temperature changes. The flow of damping medium from the return chamber into the space (19) is adjusted with an adjustable damping valve (11) . The valve plate (13) of the adjustable damping valve is pretensioned with a resilient pressurizing device (31) disposed in a sleeve-shaped part (25) around the outer strut of the shock absorber. By pressurizing the valve plate (13) in varying measure, the damping medium flows through the valve only once certain damping movement speeds are attained.

Summary of the invention When a resilient device, for example of the kind described in DE10052789, is directly disposed in the return chamber without being made to act upon the valves of the shock absorber, an unexpected solution emerges to the above-described problem. Initially during the compression stroke, a resilience is then created, which acts upon the damping medium volume in the return chamber such that the pressure in the chamber does not fall below a predetermined minimum pressure which gives rise to cavitation, i.e. this resilient device compensates for the reduction in pressure in the return chamber due to the pressure drop over the main piston which occurs under rapid damping movements. This minimum pressure can be, for example, the system pressure. The resilient device can be

- A - compared with an accumulator in which energy is stored through a build-up of pressure in the device and in which the stored energy is used to create the resilient function. The resilient device can be disposed in both a shock absorber with external oil ducts to the valves, such as the shock absorbers in US2004134730, EP0601982 and EP0322608, or in a simpler variant of a shock absorber in which the oil flow between the chambers only takes places through the piston.

Once the pressure in the return chamber has risen above the minimum level, the effect of the resilient device and the build-up of force during the remaining part of the compression stroke are purely dependent on the basic damping character of the shock absorber. The pressure which is required to start an expansion of the resilient device is therefore chosen such that it is not resilient under normal system pressure, but when the pressure is lowered, the resilient function sets in. Expediently, this pressure is chosen such that a sufficient margin against cavitation is reached.

During a return stroke, the pressure in the return chamber is always greater than or equal to the system pressure. The resilient device then assumes a bottomed state and the spring function ceases. A bottomed state is shown, for example, in figures Ia and Ib.

The resilient device can be variously configured. In all embodiments, it comprises an elastic member which can be fixed to the piston rod directly adjacent to the main piston, adjacent to the outer end of the damping cylinder or in a space in the piston rod. The resilient device works as an accumulator which is designed to store the energy of liquids and gases. The energy is stored by pressure being built up in the device either via a mechanical elastic member, such as a spring or an 0-ring, or by pressurization with a compressible

medium, for example a gas. These embodiments are set out in greater detail below.

In a first embodiment, the elastic function of the member is created by pretensioning of a seal placed between two mutually adjustable parts. Different pretensionings of the seal and different choices of seal size and material produce different magnitudes of the resilience which is brought about by the member. The resilient device can also be made up of two mutually adjustable parts, in which a spring with a certain defined spring constant is disposed between the parts. At a certain force created by the pressure in the return chamber, which force exceeds the force from the seal or the spring and is preferably determined by the system pressure, the parts bottom one against the other against, for example, a lug disposed on the second part. Once the parts have bottomed, pressure and force are built up without the effect of the elasticity of the seal and the force of the spring respectively.

The resilient device can also be made up of two mutually displaceable and sealed parts, in which the volume which is formed between the parts is filled with pressurizing medium, for example gas, so that a certain pressure prevails. The pressure which acts in the inner volume of the device interacts with the pressure from the damping medium in the return chamber. As long as the pressure in the damping medium is less than the pressure in the space between the sealed parts, the two parts move toward each other. When the pressure ratios are altered and the pressure in the damping medium becomes greater than the pressure in the space between the sealed-off parts, the two parts bottom against each other and the device becomes inflexible. That is to say, at normal system pressure or above, it is not resilient, but when the pressure is lowered, the resilient function sets in.

Since the force and pressure ratios between the compression and the return chamber are now controllable, a further advantage with the above invention is that the valve disposed between the compression chamber and the pressurizing member can always be used to counteract a fall in pressure on the return side of the piston and to adjust for the desired damping character.

In one embodiment of the invention, the shock absorber is constructed such that the function involving the resilient device is included in a shock absorber in which the pressure side of the pressurization vessel is always connected to both the compression and the return chamber to ensure that a positive basic pressure always acts upon the low-pressure side of the shock absorber piston.

List of figures The invention is described in greater detail below with references to the accompanying drawings, in which:

fig. 1 shows a sectional view of a shock absorber according to a first embodiment with pressure equalization only over the piston, fig. Ia shows a detail view of the first embodiment of the resilient device, in which the device is bottomed, fig. Ib shows a detail view of the second embodiment of the resilient device, in which the device is bottomed, fig. 2 shows a sectional view of a shock absorber according to a third embodiment with pressure equalization only over the piston, fig. 2a shows a detail view of the third embodiment of the resilient device, in which the device is bottomed, fig. 2b shows a detail view of the fourth embodiment of the resilient device, fig. 2c shows a detail view of the fifth embodiment of the resilient device,

fig. 3 shows a detail view of a sixth embodiment of the resilient device, fig. 4 shows a sectional view of a shock absorber according to a seventh embodiment with pressure equalization both over the piston and in a duct outside the piston.

Detailed description of the invention

Figure 1 shows a sectional view of a shock absorber 1 according to a first embodiment. The cylindrical body 2 of the shock absorber 1 is divided by a main piston 3 disposed on a piston rod 4 in a compression 2a and a return chamber 2b. The main piston 3 is preferably provided with valves 3a in the form of, for example, washers/shims, which contribute to a certain flow between the compression and the return chamber 2a, 2b via flow passages 3b disposed in the main piston 3. The main piston 3 can also be solid (see fig. 4), in which case the damping medium flow between the chambers 2a, 2b is adjusted via externally disposed valves 41a, 41b. A variable first pressure pi prevails in the compression chamber 2a and a variable second pressure p2 prevails in the return chamber 2b. Connected to the compression chamber 2a, via a duct 5 and an adjustable valve β disposed therein, is a pressurizing vessel 7. The inner volume of the pressurizing vessel 7 can preferably be divided by a floating piston 7a, which is acted upon by a third pressure or system pressure p3 created, for example, by gas or a mechanical pressure member such as a spring, which then instead creates a pressurizing force. The floating piston 7a can also be replaced by a pressurized rubber bladder or corresponding device for pressurizing a medium. The valve 6 which adjusts the flow between the pressurization vessel 7 and the compression chamber 2a is designed according to the prior art and is not explained in greater detail in this publication.

In the return chamber 2b there is disposed a resilient device 8, which comprises an elastic member 9.

An enlarged view of the resilient member 8 according to this embodiment is shown in figure Ia. The resilient device 8 is fixed to the piston rod 4 directly adjacent to the main piston 3, but released from the valves 3a of the main piston 3, and the resilient function is achieved by the enclosure of an elastic member 9 by two mutually movable and sealed first and second spring parts 10, 11. The first spring part 10 is cup-shaped with an outer collar part 10b, and within it moves the second spring part 11.

When the variable second pressure p2 in the return chamber 2b falls below the system pressure p3 or other predetermined minimum pressure level due to, for example, a fast stroke, the force which is created by the second pressure p2 upon the resilient member 8 is less than the counterholding force created by the elastic member 9, so that the movable spring parts 10, 11 spring onto the elastic member 9. Once a certain predetermined second pressure p2, preferably greater than or equal to the system pressure p3, acts upon the resilient member 8, the effect of the latter is terminated, i.e. the resilient function disappears and the build-up of force during the compression stroke takes place without the effect of the member 8. This is made possible by the fact that the two mutually movable spring parts 10, 11 hit a mechanical stop 12 in the form of a lug created in the outer collar part 10b of the first spring member 10 when the elastic member has been compressed by a certain distance d. The mechanical stop 12 is preferably placed at the distance d at which equilibrium prevails between the force which is produced by a compression of the elastic member 9 and the force which acts upon the pressure zone of the movable second spring part 11, which pressure zone is caused by the second pressure p2 in the return chamber

2b. The stop 12 can also be placed at such a distance that a sufficient margin against cavitation is reached, preferably the counterpressure created by the resilient device 8 being between 1 and 3 bar lower than the system pressure p3 in the pressurization vessel 7.

Also in the embodiment according to figure Ib, the resilient device 8 is made up of two mutually adjustable first and second spring parts 10, 11 sealed with seals 13a, 13b. The first spring part 10 is fixed between a prominent part of the piston rod 4 and the main piston 3. For the formation of the space 8a, the first spring part 10 has an inner 10a and an outer collar part 10b, in which the inner collar part directly surrounds the piston rod 4 and the outer collar part 10b is disposed at a radial distance from the inner collar part. The resilience is created by the fact that the second spring part 11 is disposed in the first spring part 10, between the inner and the outer collar part. In this embodiment, the stop 12 is a locking ring fixed to the inner collar part.

Here, the space 8a formed between the spring parts 10, 11 is filled with a pressurizing medium such as gas, so that a certain fourth pressure p4 prevails in the space 8a. If the fourth pressure p4 is less than the second pressure p2, which is the case throughout the return stroke, the first and second spring parts 10, 11 are pressed together so that the volume in the space 8a becomes as small as possible and the pressure balance in the shock absorber is created between the pressure p2 in the return chamber and the pressure pi in the compression chamber. On the other hand, if the fourth pressure p4 is greater than the second pressure p2 in the return chamber 2b, which can happen initially during a compression stroke, the first and second spring parts 10, 11 are mutually displaced so that the volume in the space 8a increases and a pressure balance is created between the pressure in the space 8a and the

pressure in the return chamber 2b. The fourth pressure p4 which acts in the space 8a is therefore chosen such that the second pressure p2 in the return chamber 2b is always kept higher than the preselected minimum pressure, which can be the system pressure p3 or some other chosen pressure. The device otherwise functions in the same way as the device described in figure Ia.

The resilient device 8 forms a unit which is easily removable from the piston rod 4. The fact that the entire unit can be removed also makes it easy to alter the inner fourth pressure p4 in the unit. This alteration can be made, for example, by filling gas through the filling member 14 before the device 8 is mounted on the piston rod 4.

Figure 2 shows a further embodiment of the invention, in which the resilient device 8 is mounted in the return chamber 2b adjacent to a closing cap 15 fixed to that end of the damping cylinder 2 through which the piston rod 4 extends. The inventive concept in this embodiment is the same as in previously described embodiments, with a number of different pressurizing members 8 described below.

In figure 2a, the simplest form of resilient device 8 is shown, in which a floating spring piston 16 is disposed next to the closing cap 15. The floating spring piston 16 rests on an elastic member 17, in this case a spring, which in turn rests on the closing cap 15 fixed to the damping cylinder 2. The floating spring piston 16 is sealed against the piston rod 5 with an inner seal 18a and against the damping cylinder 2 with an outer seal 18b. In order to facilitate the to and fro movement of the floating spring piston 16 along the piston rod 4, a bushing 19 is disposed between the floating spring piston 16 and the piston rod 4. At a certain force created by the pressure in the return chamber and exceeding the force from the elastic member

17, the floating spring piston 16 is designed such that it bottoms against a mechanical stop 12 disposed in the piston 16. Once the spring piston 16 has bottomed, pressure and force are built up without influence from the resilient device 8.

In figure 2b, a resilient device 8 is shown, in which the device is composed of two first and second spring parts, which are mutually adjustable and are sealed with seals 13a, 13b and are here also denoted by 10, 11. In this embodiment, the first spring part 10 is arranged as a seal head and is fixed with a thread, for example, in the damping cylinder 2. The first spring part 10 also has an inner and an outer collar part 10a, 10b, and in the space between these collar parts 10a, 10b there is disposed the second spring part 11. The space 8a formed between the spring parts 10, 11 is filled with a pressurizing medium, for example gas, so that a certain fourth pressure p4 prevails. The fourth pressure p4 is chosen such that it initially deters the second pressure p2 from acquiring a value lower than the system pressure p3. A stop 12 prevents the spring parts 10 and 11 from being moved apart and in this embodiment has the form of a locking ring in a groove in the first spring part 10, which locking ring interacts with a lug in the second spring part 11.

By a valve or similar pressurization member 14, the fourth pressure p4 in the inner space 8a of the resilient device 8 can be adjusted on the basis of and adapted to the desired level, for example in order to compensate for a possible change in the system pressure p3 in the pressurizing vessel 7. It is possible to achieve the desired pressure in the member when the component parts are mounted, i.e. the geometry of the component parts is chosen such that a suitable compression of the enclosed volume is achieved.

In the embodiment according to figure 2c, the device is resilient by virtue of the pretensioning of two seals 21a, 21b disposed between three mutually adjustable third, fourth and fifth spring parts 22, 23, 24. Different pretensionings of the lower seal 21b and different choices of seal size and material produce different magnitudes of the resilience which is brought about by the member. The pretensioning is adjusted by the outer fifth spring part 24 being screwed in or out on a corresponding thread on the third spring part 22.

The third spring part 22 acts both as a sealing seal head on the damping cylinder 2 and as a working part in the resilient device 8. The third spring part 22 is sealed against the damping cylinder with the seal 25 and against the piston rod 5 with a seal 26. Between the piston rod 5 and the third spring part 22 there is also disposed a bushing 28 for reducing the friction between the parts 22, 28. In order to prevent the whole of the third spring part 22 from being moved in too far into the return chamber, a locking ring 29 is disposed in the damping cylinder 2. The lower part of the third spring part 22 bears against a seal 21a resting on the centermost fourth spring part 23. The centermost fourth spring part 23 is held tight inside the damping cylinder 2 with a locking ring 27.

At a certain force created by the pressure in the return chamber and exceeding the force from the seals 21a, 21, the third, fourth and fifth spring parts 22, 23, 24 bottom one against the other. Once the spring parts 22, 23, 24 have bottomed, pressure and force build up without the effect of the elasticity of the seal.

In figure 3, a shock absorber is shown in which the resilient device 8 is disposed inside a piston rod volume 30 in the piston rod 5. The piston rod volume 30 is connected to the return chamber 2b via one or more

ducts or holes 31. The resilient device consists of a piston 32, which is movable in the longitudinal direction of the piston rod 5 and delimits the volume 30 in an inner volume 33. The piston 32 rests on an elastic member 34, for example a spring, a gas volume or an O-ring, and the distance by which the piston can be compressed is determined by a mechanical stop 12. The inventive concept in this embodiment is the same as in previously described embodiments, what differs is the positioning of the device.

Figure 4 shows a spring strut for a vehicle consisting of a shock absorber 1 telescopically introduced into an outer strut 35. The shock absorber 1 comprises a damping cylinder 2, a main piston 3, a piston rod 4, an upper valve housing 36 and a pressurization vessel 7. An upper part 37 of the shock absorber 1 is connected to a part of a vehicle chassis (not shown) , and the lower part of the spring strut 35 is connected to a wheel via a fastening member 38. In the lower part of the outer strut 35, the piston rod 6 is fixedly mounted, which means that the shock absorber 1 moves in and out in the outer strut 35 upon relative movements between the chassis and the wheel.

Around the damping cylinder 2, there is additionally disposed a cylindrical tube 39. Damping medium flows between the damping cylinder 2 and the cylindrical tube 39 during both the return and the compression stroke. Both the return and the compression chamber are therefore connected to a common volume 40 in the valve housing 36, and by connecting the common volume 40 to that space in the pressurization vessel 7 which is pressurized by the gas pressure p3, a shock absorber is created which always operates on a positive pressure during both the compression and the return stroke. The arrangement of two separate valves 41a, 41b in the valve housing 36 allows the character of the damping

force in the two stroke directions to also be adjusted quite separately and independently of each other.

In the return chamber 2b, the resilient device 8 according to the invention is placed adjacent to the main piston 3. All previously described resilient devices 8 and their respective positioning can also be used, of course, in this embodiment of the shock absorber.

The invention is not limited to the embodiment shown above by way of example, but may be modified within the scope of the following patent claims and the inventive concept .