Abstract:
A shock absorber comprises a damping cylinder pressurized by a system pressure and divided by a piston into a compression chamber and a return chamber. A resilient device is disposed in the return chamber. The resilient device comprises a pressurizing member or pressurizing medium disposed in an inner volume that is delimited from the return chamber. The resilient device acts upon the damping medium volume in the return chamber such that the pressure initially during a compression stroke does not fall below a predetermined minimum pressure. As long as the pressure in the return chamber is less than the pressure created by the resilient device, the device is able to absorb energy. When the pressure in the return chamber is greater than the pressure created by the resilient device, the device becomes inflexible. The resilient device compensates for pressure reduction in the return chamber that occurs under rapid damping movements and that can cause cavitation during a compression stroke.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase of International Application No. PCT/SE2007/000831, filed Sep. 21, 2007, which is based upon Swedish Patent Application No. 0601962-4, filed Sep. 21, 2006, each of which is hereby incorporated by reference in its entirety and priority is claimed to each of these applications. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Certain features, aspects and advantages of the present invention relate to a method for controlling a pressure balance between two chambers of a shock absorber, in which a damping medium pressure in a return chamber preferably does not fall below a preselected minimum pressure, whereby the likelihood of cavitation is greatly reduced. 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, that is disposed in the return chamber. Thus, a resilient function is created in the resilient device, which initially during the compression stroke increases the pressure in the return chamber. 
     Certain features, aspects and advantages of the present invention also relate to a pressurized shock absorber in which a resilient device is positioned within or directly adjacent to a return chamber, the resilient device comprising a pressurizing member or pressurizing medium that acts in an inner volume delimited from the return chamber. 
     2. Description of the Related Art 
     A shock absorber operates by adjusting the pressure ratio between the pressure exerted upon the damping medium in the compression chamber and the pressure exerted upon the damping medium in the return chamber respectively. A high pressure gives a higher rigidity. In other words, a greater force is required to compress the damping medium when operating under a higher pressure. The piston that separates the compression and the return chamber can be provided with flow-adjusting valves or can be leak-tight when used in combination with externally mounted valves that adjust the flow between the chambers. 
     The pressure drop over the piston determines the pressure ratio and the pressure drop can be altered dynamically by having a system pressure act upon the damping medium. The system pressure can be 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 receive the pressure medium that is displaced by the piston rod, to absorb the changes in damping medium volume caused by temperature differences, and to generate a certain basic pressure (i.e., 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 that 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 impossible because 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 does not build 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 and cavitation causes a loss of damping forces. 
     Examples of previously known attempts to solve this problem can be found in US2004134730 or in the Applicant&#39;s own patent EP0601982. EP0322608 further shows an embodiment in which the damping medium is conducted both through the piston and through a duct outside the damping chamber depending on the stroke rate. In the case of certain rapid compression movements, however, it is difficult for the damping medium 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 that solves another problem, namely the adjustment of the damping flow between the return chamber and a space designed to absorb the piston rod displacement and any differences in damping medium volume due to temperature changes, for example. The flow of damping medium from the return chamber into the space is adjusted with an adjustable damping valve. The valve plate of the adjustable damping valve is pretensioned with a resilient pressurizing device disposed in a sleeve-shaped part around the outer strut of the shock absorber. By pressurizing the valve plate in varying amounts, the damping medium flows through the valve only once certain damping movement speeds are attained. 
     SUMMARY OF THE INVENTION 
     When a resilient device of the kind described in DE10052789, for example, 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 is less likely to fall below a predetermined minimum pressure which gives rise to cavitation (i.e. the resilient device compensates for the reduction in pressure in the return chamber caused by the pressure drop over the main piston that occurs during rapid damping movements). The minimum pressure can be the system pressure, for example. 
     The resilient device can be 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 place 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 that is required to start an expansion of the resilient device therefore is chosen such that it is not resilient under normal system pressure but, when the pressure is lowered, the resilient function starts. Expediently, the pressure preferably is chosen such that a sufficient margin against cavitation is provided. 
     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 resilient function ceases. A bottomed state is shown, for example, in  FIGS. 1   a  and  1   b.    
     The resilient device can be variously configured. In most embodiments, the resilient device comprises an elastic member that 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 that 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 O-ring, or by pressurization with a compressible medium, for example a gas. These embodiments are set out in greater detail below. 
     In an embodiment, the resilient 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 brought about by the member. 
     The resilient device also can 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 further 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 formed between the parts is filled with pressurizing medium, for example gas, so that a certain pressure prevails. The pressure that 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 return chamber damping medium is less than the pressure in the space between the sealed parts, the two parts can move toward each other or away from 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 move together and 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. 
     Because the force and pressure ratios between the compression and the return chamber are now controllable, a further advantage with system 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 some embodiments 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 increase the likelihood that a positive basic pressure always acts upon the low-pressure side of the shock absorber piston. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain features, aspects and advantages of the present invention are described in greater detail below with references to the accompanying drawings, in which: 
         FIG. 1  shows a sectional view of a shock absorber comprising a resilient device according to a first embodiment with pressure equalization only over a piston; 
         FIG. 1   a  shows a detail view of the first embodiment, in which the device is bottomed; 
         FIG. 1   b  shows a detail view of a second embodiment comprising a resilient device, in which the device is bottomed; 
         FIG. 2  shows a sectional view of a shock absorber comprising a resilient device according to a third embodiment with pressure equalization only over the piston; 
         FIG. 2   a  shows a detail view of the third embodiment, in which the device is bottomed; 
         FIG. 2   b  shows a detail view of the fourth embodiment; 
         FIG. 2   c  shows a detail view of the fifth embodiment; 
         FIG. 3  shows a detail view of a sixth embodiment; 
         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 PREFERRED EMBODIMENT 
       FIG. 1  shows a sectional view of a shock absorber  1  that is arranged and configured according to a first embodiment. A cylindrical body  2  of the shock absorber  1  is divided by a main piston  3  disposed on a piston rod  4  into a compression chamber  2   a  and a return chamber  2   b . The main piston  3  preferably is provided with valves  3   a  in the form of washers/shims, for example, which contribute to a certain flow between the compression chamber  2   a  and the return chamber  2   b  via flow passages  3   b  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  2   a ,  2   b  can be adjusted via externally disposed valves  41   a ,  41   b.    
     A variable first pressure p 1  prevails in the compression chamber  2   a  and a variable second pressure p 2  prevails in the return chamber  2   b . Connected to the compression chamber  2   a , via a duct  5  and an adjustable valve  6  disposed therein, is a pressurizing vessel  7 . 
     The inner volume of the pressurizing vessel  7  preferably can be divided by a floating piston  7   a  that is acted upon by a third pressure p 3 , or system pressure, that is created, for example, by gas or a mechanical pressure member such as a spring or the like, which then creates a pressurizing force. The floating piston  7   a  also can be replaced by a pressurized rubber bladder or corresponding device for pressurizing a medium. The valve  6  preferably adjusts the flow between the pressurization vessel  7  and the compression chamber  2   a . Any suitable valve configuration can be used. 
     In the return chamber  2   b  there is disposed a resilient device  8 , which preferably comprises an elastic member  9 . 
     An enlarged view of an embodiment the resilient device  8  is shown in  FIG. 1   a . The resilient device  8  can be fixed to the piston rod  4  directly adjacent to the main piston  3 , but spaced from the valves  3   a  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  or by a first spring part  10  and second spring part  11  in which one of the parts  10 ,  11  moves relative to the other. The first spring part  10  is cup-shaped with an outer collar part  10   b , and within it moves the second spring part  11 . 
     When the variable second pressure p 2  in the return chamber  2   b  falls below the system pressure p 3  or other predetermined minimum pressure level due to, for example, a fast stroke in which the volume of the return chamber  2   b  increases, the force that is created by the second pressure p 2  upon the resilient device  8  is less than the counterholding force created by the elastic member  9 , so that the movable spring parts  10 ,  11  are moved apart by the elastic member  9 . Once a certain predetermined second pressure p 2 , preferably greater than or equal to the system pressure p 3 , acts upon the resilient device  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 device  8 ). This is made possible by the fact that the two mutually movable spring parts  10 ,  11  hit a mechanical stop  12 , which can be in the form of a lug created in the outer collar part  10   b  of the first spring member  10 , when the elastic member has been compressed by a certain distance d. The mechanical stop  12  preferably is placed at the distance d at which equilibrium generally prevails between the force that 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 p 2  in the return chamber  2   b . The stop  12  also can be placed at such a distance that a sufficient margin against cavitation is reached. Preferably the counterpressure created by the resilient device  8  is between about 1 and about 3 bar lower than the system pressure p 3  in the pressurization vessel  7 . 
     Also in the embodiment shown in  FIG. 1   b , the resilient device  8  comprises two mutually adjustable first and second spring parts  10 ,  11  sealed with seals  13   a ,  13   b . 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  8   a , the first spring part  10  has an inner collar part  10   a  and an outer collar part  10   b , in which the inner collar part  10   a  directly surrounds the piston rod  4  and the outer collar part  10   b  is disposed at a radial distance from the inner collar part. The resilience is created because the second spring part  11  is disposed in the first spring part  10 , between the inner collar part  10   a  and the outer collar part  10   b . In this embodiment, the stop  12  is a locking ring fixed to the inner collar part  10   a.    
     The space  8   a  formed between the spring parts  10 ,  11  preferably is filled with a pressurizing medium, such as a gas or the like, so that a certain fourth pressure p 4  prevails in the space  8   a . If the fourth pressure p 4  is less than the second pressure p 2 , 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  8   a  becomes as small as possible and the pressure balance in the shock absorber is created between the pressure p 2  in the return chamber and the pressure p 1  in the compression chamber. On the other hand, if the fourth pressure p 4  is greater than the second pressure p 2  in the return chamber  2   b , 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  8   a  increases and a pressure balance is created between the pressure in the space  8   a  and the pressure in the return chamber  2   b . The fourth pressure p 4  that acts in the space  8   a  is therefore chosen such that the second pressure p 2  in the return chamber  2   b  is always kept higher than the preselected minimum pressure, which can be the system pressure p 3  or some other chosen pressure. The device otherwise functions in the same way as the device described in  FIG. 1   a.    
     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 p 4  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 . 
       FIG. 2  shows an embodiment in which the resilient device  8  is mounted in the return chamber  2   b  adjacent to a closing cap  15  fixed to that end of the damping cylinder  2  through which the piston rod  4  extends. A number of different pressurizing members  8 , such as those described below, can be used. 
     In  FIG. 2   a , 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  that is fixed to the damping cylinder  2 . The floating spring piston  16  is sealed against the piston rod  5  with an inner seal  18   a  and against the damping cylinder  2  with an outer seal  18   b . In order to facilitate the to and from 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 further influence from the resilient device  8 . 
     In  FIG. 2   b , a resilient device  8  is shown, in which the device comprises two first and second spring parts  10 ,  11 , which are adjustable relative to each other (i.e., at least one is adjustable relative to the other) and are sealed with seals  13   a ,  13   b . In the illustrated 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 collar part  10   a  and an outer collar part  10   b , and in the space between these collar parts  10   a ,  10   b  there is disposed the second spring part  11 . The space  8   a  formed between the spring parts  10 ,  11  is filled with a pressurizing medium, such as a gas, for example, so that a certain fourth pressure p 4  prevails. The fourth pressure p 4  is chosen such that it initially deters the second pressure p 2  from acquiring a value lower than the system pressure p 3 . A stop  12  reduces the likelihood of the spring parts  10  and  11  from being moved apart and, in this embodiment, comprises a locking ring positioned 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 p 4  in the inner space  8   a  of the resilient device  8  can be adjusted on the basis of, and adapted to, the desired level, in order to compensate for a possible change in the system pressure p 3  in the pressurizing vessel  7 , for example. It is possible to achieve the desired pressure in the device 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 shown in  FIG. 2   c , the device is resilient by virtue of the pretensioning of two seals  21   a ,  21   b  disposed between three mutually adjustable third, fourth and fifth spring parts  22 ,  23 ,  24 . 
     Different pretensionings of the lower seal  21   b  and different choices of seal size and material produce different magnitudes of resilience which can be brought about by the device. The pretensioning can be 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 a seal  25  and against the piston rod  5  with a seal  26 . Between the piston rod  5  and the third spring part  22  there also is 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 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  21   a  resting on the centermost fourth spring part  23 . The centermost fourth spring part  23  preferably 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  21   a ,  21   b , 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 any further effect of the elasticity of the seal. 
     In  FIG. 3 , a shock absorber is shown in which the resilient device  8  is disposed inside a recess formed in a piston rod (i.e., an internal volume  30 ) in the piston rod  5 . The piston rod volume  30  is connected to the return chamber  2   b  via one or more ducts or holes  31 . The resilient device consists of a piston  32 , which is movable in the longitudinal (i.e., axial) direction of the piston rod  5  and delimits the volume  30  from another 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 embodiment shown in  FIG. 3  differs primarily from the embodiments described above in the positioning of the moving member (i.e., the piston) but the embodiment of  FIG. 3  is believed to operate in a very similar manner as the embodiments described above. 
       FIG. 4  shows a spring strut for a vehicle that comprises a shock absorber  1  that is 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 a 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 p 3 , a shock absorber is created that operates substantially always under a positive pressure during both the compression stroke and the return stroke. The arrangement of two separate valves  41   a ,  41   b  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  2   b , the resilient device  8  preferably 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. In addition, it is possible to combine various aspects of the various embodiments described above.