Spring devices – Vehicle – Torsion
Reexamination Certificate
2000-10-02
2002-08-20
Butler, Douglas C. (Department: 3683)
Spring devices
Vehicle
Torsion
C280S124130, C280S124177
Reexamination Certificate
active
06435491
ABSTRACT:
BACKGROUND OF INVENTION
The invention relates to vehicle suspension devices. More particularly, the invention relates to vehicle suspension devices in which the main element having the “suspension spring” function is an elastomeric flexible joint. Patent Application WO 97/47486 describes an axle of this type.
One of the advantages of using elastomeric flexible joints which act as suspension springs at the level of the joint bearings of vehicle suspension devices is to permit greater integration of the spring, shock absorber and vibration- or shock-filtering functions. According to the specifications which have to be met for the different vehicles, an axle of this type may make it possible to reduce the total number of parts used, in particular because separate filtering blocks, such as are currently used for linking the axles or the suspension arms to the body of the vehicle, are not used.
In the suspension joints, elastomeric blocks are now universally used and have supplanted the use of roller bearings or plain bearings because they are capable of imparting the required degree of freedom while ensuring good filtering which is beneficial to comfort, in particular from an acoustic point of view. Furthermore, it is possible to impart to these joints and filtering blocks relatively elaborate guiding functions by controlling their deformations under operating stresses. This is done in order to produce, for example, self-steering axles, that is to say, axles which slightly turn the plane of the wheels merely under the action of the load transfers. Another example of the possibilities provided by these elastomeric joints is to make it possible to integrate, in addition to the degree of freedom necessary for the main function of suspension, a degree of horizontal freedom which substantially improves the comfort of the vehicle.
In the case of the elastomeric joints as used to link the suspension arms (lower wishbone, trailing arms, etc.) to the body, their contribution to taking up the load is negligible (of the order of one percent). In fact, their contribution to taking up the load, which is low and inherent (since the elastomer is bound to the metal parts which surround it), does not enter into the dimensioning at all.
If it is desired to impart to such joints a real contribution to taking up the load, by therefore making them into a true suspension spring under torsional stress, a change in the trim height of the vehicle is noted over time. It is known that creep of the elastomer is one cause of this. However, it is also known that the main part of the creep takes place very quickly, that is to say, in a few days, so that it is sufficient to make allowance for this when mounting the spring so that the vehicle trim height after a few days is the height intended when the vehicle was designed. This, in practice, consists of selecting the mounting azimuth while taking into account both the rated load of the vehicle and the creep.
Unfortunately, there are still variations in vehicle trim height which cannot be explained by these phenomena. These variations appear to be erratic. Shifts in vehicle trim height which may be of as much as several centimeters have been noted. This problem is a great nuisance in particular for final adjustment operations for the geometric parameters which depend on the vehicle trim height or trim (that is to say, the angular position of the body relative to the horizontal ground) of the vehicle. This may be the case, for example, for the orientation of the beam of the headlights or of the brake power distributor.
It has been discovered that the hysteresis of the elastomer is one of the causes of these variations. The hysteresis, which is a phenomenon which is well-known per se, means that the apparent stiffness of the material is different during the increase in the stress and during the reduction in this stress. The characteristic “outward” and “return” stress/strain curves are not superposed as, for example, for a conventional metal spring. It has been discovered that, as long as the elastomeric spring has not been stressed as far as the bump stop, the vehicle trim height (under equal load) is not stabilized. It would appear that the extent of this phenomenon is due to the fact that the starting part of the operation of the spring, that is to say, the state which is free of any stress, is very far from the zone of the stress/strain graph which is covered by the elastomeric spring during operation on a vehicle. Said zone of the graph is displaced substantially each time the spring reaches a “new” maximum deformation, greater than the maximum deformations previously achieved. In fact, it is noted that the mechanical characteristics of the elastomeric spring become substantially constant over time only once the spring has reached (at least once) its state of maximum deformation. It is this observation which forms the basis of the principle of the invention, according to which the joint bearing a load, in order to stabilize its point of operation, must undergo deformation comparable to the maximum deformation provided for during operation.
SUMMARY OF THE INVENTION
The invention therefore relates to a process for stabilizing the mechanical operating characteristics of an elastomeric flexible joint, said joint being intended to act as a spring in a suspension device for a vehicle, said vehicle having a body and wheels, said flexible joint being intended to take up a given rated load, said suspension permitting movement as far as a maximum compression limit, said process consisting, before the putting into normal service of the vehicle, of subjecting said joint to a deformation R comparable to a maximum deformation M corresponding to the deformation undergone by said joint when said suspension reaches said maximum compression limit.
Preferably, this process is characterized in that said deformation R is greater than or equal to said maximum deformation M.
Preferably, this process is furthermore characterized in that said flexible joint is subjected to said deformation R, then said deformation R is relaxed until a deformation S is achieved which corresponds substantially to said rated load.
Preferably, the process of the invention is also characterized in that, said vehicle having a vehicle trim height relative to the ground, said flexible joint is subjected to said deformation R during the final assembly of the vehicle and before adjustments linked to said vehicle trim height are carried out.
Preferably, the process of the invention is characterized in that said deformation R is achieved, after mounting said suspension device on the vehicle, by making said suspension device undergo a movement which extends as far as said maximum compression limit.
Preferably, this process is furthermore characterized in that, the vehicle being on the ground, said deformation R is obtained, after mounting said suspension device on the vehicle, by exerting an appropriate force on said body of said vehicle.
Alternatively, the process of the invention is characterized in that said deformation R is obtained, after mounting said device on the vehicle, by exerting a force on the wheels of said vehicle, the body of which is kept immobilized.
Alternatively, the process of the invention is characterized in that said flexible joint is subjected to said deformation R before the mounting of said suspension device on said vehicle.
Preferably, this process is furthermore characterized in that said flexible joint is subjected to said deformation R before said flexible joint is mounted within said suspension device.
The invention also consists of an elastomeric flexible joint intended to act as a spring for a vehicle suspension device, said joint bearing a substantial part of the load, said flexible joint being characterized in that it comprises means suitable for keeping it in a temporary state of preselected deformation, said preselected deformation being of the same type as a deformation caused by said load.
Preferably, the flexible joint of the invention is characterized in that it comprises
Blondelet Michel
Coue Jean
Pennequin Patrick
Vantal Marie-Héle{grave over (n)}e
Baker & Botts L.L.P.
Butler Douglas C.
Compagnie Generale des Etablissements Michelin--Michelin & Cie
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