Patent Application
20080213036
The present invention pertains to a ball and socket joint, for example, for an axle system of a wheel suspension of a motor vehicle
Ball and socket joints of the type mentioned in the introduction are used, for example, but by no means exclusively, in the area of the wheel suspension, or also as sleeve joints, for example, for articulating roll stabilizers in motor vehicles.
The requirements imposed on such ball and socket joints include especially a high specific load-carrying ability and low bearing clearance under both static and dynamic loads, as well as low maintenance requirement or absence of need for maintenance possibly over the service life of the motor vehicle, the lowest possible weight and small space requirement. In addition, low-cost manufacture shall be possible.
A ball and socket joint usually has an essentially ring-shaped or pot-shaped joint housing, in the interior space of which the bearing shell or ball shell of the ball and socket joint is arranged. The bearing surface of the ball shell, which is in contact with the surface of the ball of the ball and socket joint, essentially corresponding to the shape of the joint ball in ball and socket joints known from the state of the art. In other words, the bearing surface of the ball shell corresponds to the shape of a spherical segment.
Based on manufacturing tolerances of the ball shell as well as due to forces occurring during the mounting of the ball and socket joint, which occur especially when the ball shell is pressed into the joint housing, the shape of the bearing surface of the bearing shell on the completely mounted ball and socket joint does not usually correspond to the intended spherical shape or the shape of the ball of the ball pivot any longer. The shape of the ball shell rather often corresponds approximately to a ellipsoid of revolution, whose greater semiaxis coincides with the longitudinal axis of the ball pivot or the ball sleeve. In other words, this means that the contact between the ball and the ball shell often does not take place over the entire surface, as is actually desirable, but rather only in the middle area of the ball shell in the form of a linear contact or strip-like contact.
In addition, the ball shells of most ball and socket joints have an especially small wall thickness for geometric reasons in the middle area, i.e., in the approximate area in which their diameter is greatest, relative to a longitudinal section through the ball and socket joint. In addition to the shape deviations described, this leads to an especially high spring rate or especially low elasticity of the ball shell in this middle section.
These two factors together cause that the surface pressure between the ball and the ball shell does not usually correspond any longer to the ideal, uniform load distribution on the entire available bearing surface in the ball and socket joints known from the state of the art. The majority of the load is rather concentrated on the mentioned middle section of the ball shell in the area of the greatest diameter thereof relative to the longitudinal section through the ball and socket joint. This leads to non-uniform, increased bearing wear, prematurely increased bearing clearance, lower bearing capacity and reduced service life in the prior-art ball and socket joints.
Against this background, the object of the present invention is to provide a ball and socket joint with which the drawbacks of the state of the art are overcome. In particular, the distribution of the surface pressure between the ball and the ball shell shall be as flat and uniform as possible, the specific load-carrying ability and bearing capacity of the ball and socket joint shall be improved, the bearing clearance shall be reduced, and prolonged service life shall be achieved.
The ball and socket joint according to the present invention comprises at first, in the manner known per se, an essentially ring-shaped or pot-shaped joint housing. The joint housing has an essentially cylindrical inner space, in which the ball shell of the ball and socket joint can be arranged. The ball of the ball pivot or of the ball sleeve of the ball and socket joint can be accommodated in the ball shell in a slidingly movable manner.
However, the ball and socket joint is characterized according to the present invention in that the bearing surface of the ball shell essentially corresponds to the surface of an ellipsoid of revolution. The smaller semiaxis of the ellipsoid of revolution coincides with the axial direction of the ball and socket joint, i.e., with the long axis of the ball pivot or with the longitudinal axis of the ball sleeve.
The fact that the bearing surface of the ball shell has the shape of an ellipsoid of revolution causes that the contact between the ball surface and the ball shell is markedly more two-dimensional and is characterized by a more uniform surface pressure distribution in the mounted ball and socket joint, unlike in the state of the art, because the deformation effects and tolerances described in the introduction, which often lead only to a linear or strip-like contact between the ball and the ball shell in the state of the art, are countercompensated because of the fact that the shape of the bearing surface essentially corresponds to an ellipsoid of revolution. In addition, different elasticities of the ball shell, which occur in different areas of the ball shell because of the different wall thicknesses of the ball shell, are counteracted.
The terms “ellipsoid of revolution” and “having the shape of an ellipsoid of revolution” shall not, of course, be interpreted in the strict mathematical sense. A ball shell that has a non-constant radius of curvature in the longitudinal section through the ball and socket joint such that the radius of curvature is slightly smaller in the middle area of the ball shell than in the axial edge areas of the ball shell shall rather also be considered ellipsoidically shaped, an “ellipsoid of revolution” and “having the shape of an ellipsoid of revolution” according to the present invention.
The greater semiaxis of the ball shell having the shape of an ellipsoid of revolution preferably corresponds to the radius of the ball of the ball and socket joint. In other words, this means that the bearing surfaces of the ball shell and the joint ball geometrically intersect or mutually interpenetrate each other in the mathematical sense in the axial edge areas of the ball shell, i.e., in the area of the front sides of the joint housing. Since an actual mutual interpenetration of the bearing surfaces is not possible in the assembled ball and socket joint, this ultimately leads to a certain, defined prestress between the ball shell and the joint ball. Based on the shape of the ball shell according to the present invention, the interpenetration of the bearing surfaces or the defined prestress is concentrated especially on the axial edge areas of the ball shell and progressively decreases with increasing proximity to the middle area of the ball shell.
A clearance-free, full-surface contact will thus become established between the ball shell and the joint ball in the assembled ball and socket joint, and this makes, moreover, possible a uniform distribution of the surface pressure over the entire contact surfaces between the ball shell and the joint ball under load. However, the specific load-carrying ability and bearing capacity of the ball and socket joint also increases as a result, the service life can be prolonged at equal dimensioning, and the ball and socket joint remains in the clearance-free state considerably longer because of the more uniform stress on the bearing surfaces.
The manner in which the ball shell obtains its bearing surface having the shape of an ellipsoid of revolution according to the present invention is irrelevant for the embodiment of the present invention. It is conceivable, for example, that a ball shell manufactured initially with a spherical bearing surface is subjected later to plastic or elastic deformation, for example, even during the operation in which it is pressed into the bearing housing, i.e., order to thus obtain the intended bearing surface having the shape of an ellipsoid of revolution. According to a preferred embodiment of the present invention, the ball shell has, however, the bearing surface having the shape of an ellipsoid of revolution already from the manufacture or the tool. In other words, this means that the desired ellipsoid of revolution shape of the ball shell is produced already at the time of the original shaping of the ball shell. The desired extent of geometric overlap between the joint ball and the ball shell can thus be set especially precisely and reliably.
According to another embodiment of the present invention, the surface of the ball shell is at least slightly concave on the outer circumference of the ball shell. This also leads, similarly to what was described before concerning the joint ball and the joint shell, to a geometric overlap in some areas between the ball shell and the joint housing, in this case between the outer surface of the ball shell and the inner surface of the joint housing. The contact between the ball shell and the joint housing can thus additionally also be improved and the distribution of the surface pressure between the outer surface of the ball shell and the inner surface of the joint housing can likewise be made more uniform and hence with increased bearing capacity.
According to another embodiment of the present invention, the bearing shell has a slot relative to the ball and socket joint in the axial direction. In other words, this means that the ball shell does not represent a closed ring any longer, but is in the form of a “C” (with very small slot width) from the manufacture. This is advantageous because the assembly of the ball and socket joint is simplified hereby. In addition, the slot in the ball shell can be used to absorb thermal expansions in the area of the ball and socket joint, which is of particular significance in the ball and socket joint according to the present invention, because there is an especially intimate and clearance-free contact between the ball shell, on the one hand, and the joint ball and the joint housing, on the other hand, essentially without the possibility of absorbing thermal expansions, in the ball and socket joint according to the present invention due to the ellipsoid of revolution shape of the ball shell.
According to an especially preferred embodiment of the present invention, the ball shell is manufactured from an injection-moldable thermoplastic material, especially polyoxymethylene. Polyoxymethylene has a very low frictional resistance along with high abrasion resistance and high elasticity. In addition, polyoxymethylene has a high chemical resistance, which is significant because of the presence of lubricants and other fuels in motor vehicles.
The manner in which the ball shell is fastened or anchored in the joint housing is at first irrelevant for the embodiment of the present invention. However, according to a preferred embodiment of the present invention, the ball shell is supported in a non-positive manner in the axial direction on both sides in the joint housing. The non-positive axial supporting especially favorably comprises at least 80% of the front surface of the ball shell. The ball shell, which always tends to undergo a certain amount of elastic or plastic creep, is thus supported especially effectively and is enclosed nearly completely all around.
The present invention will be explained in more detail below on the basis of embodiments presented as exemplary embodiments only. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.
In the drawings:
Referring to the drawings in particular,
It can be clearly recognized that the wall thickness of the ball shell 2 is especially small in the middle area 5 thereof relative to the axial direction of the ball pivot 4, while the wall thickness increases greatly towards the front-side end areas 6 of the ball shell 2 and finally amounts to a multiple of that in the middle area 5 of the ball shell 2 there.
However, as was described in the introduction, this causes that the ball shell 2 has a several times higher spring rate or a much lower elasticity in its middle area 5 than in the areas 6 that are near the end in the axial direction. However, a non-uniform surface pressure distribution inevitably becomes established due to these non-uniform elasticities, as soon as the ball and socket joint is loaded, even in the hypothetical case in which the bearing surface of the ball shell 2 would have the ideal spherical shape. Such a non-uniform surface pressure distribution, plotted over the surface 10 of the bearing 2, is shown as an example in
The surface pressure distribution X strongly concentrated on the middle section 5 of the ball shell 2 arises even for the hypothetical case of the ideal spherical shape of the bearing surface from the fact that the near-edge areas 6 of the ball shell 2 yield more strongly without essential absorption of force because of their greater elasticity under load compared to the middle area 5, which is why the middle area 5 of the ball shell 2 must inherently account for the greatest percentage of carrying due to its much lower elasticity.
In addition to this, the ball shell 2 is usually also deformed elastically during mounting in the joint housing 1, so that the bearing surface of the ball shell 2 will have an essentially high ellipsoidal shaped surface after the installation, cf. the schematic view of an ellipsoid of revolution 7 with the principal axes a, b in
The greater principal axis a of the ellipsoid 7 formed by the bearing surface of the ball shell 2 coincides with the longitudinal axis of the ball pivot 3. However, this means that there is no surface contact between the joint ball 3 and the slightly high ellipsoidically deformed ball shell 2 any longer even in the load-neutral state of the ball and socket joint. The surface contact, which is actually desirable, is rather transformed essentially into a linear contact in the middle area 5 of the ball shell 2, i.e., in the area of its smallest wall thickness, because of the deformation of the ball shell 2.
All this leads as a result to a poor contact pattern between the joint ball 3 and the ball shell 2, with the corresponding disadvantageous consequences concerning service life, wear, absence of clearance and load-bearing capacity of the ball and socket joint.
The shape of the ball shell areas appears from the schematic view of an example of a transverse ellipsoid according to
Together with the slightly concave outer contour 111 of the ball shell 2, the ellipsoid of revolution shape of the bearing surface 10 of the ball shell 2 causes the effect of the load concentration in the middle area 5 of the ball shell 2, which was described above, to be counteracted by the areas 6 of the ball shell 2 that are nearer to the edge acquiring a defined extent of radial prestress. Thus, a full-surface contact develops between the ball shell 2 and the joint ball 3 in the assembled state of the ball and socket joint, which leads to the surface pressure distribution between the joint ball 3 and the ball shell 2 becoming more uniform together with the above-mentioned radial prestress.
This is shown as an example in
However, since only a single slot 14 is present here, which is, moreover, extremely narrow in the assembled state, the carrying surface of the ball shell 2 is reduced by the slot 14 to an insignificant extent only, so that the maximum bearing capacity of the ball and socket joint is guaranteed practically without any restriction.
The embodiment of the ball and socket joint according to the present invention which is being considered here with the ball shell according to
The use of slotted ball shells 2 according to
It becomes clear as a result that thanks to the present invention, a ball and socket joint is created which offers advantages over ball and socket joints known from the state of the art especially in terms of service life, fail-safe operation and load-bearing capacity. The more uniform surface pressure distribution in the bearing gap between the ball shell and the joint ball guarantees especially low wear and lower moments of friction. Thus, the present invention makes a contribution to the improvement of the reliability of ball and socket joints and the elimination of the need for maintenance of ball and socket joints, especially in respect to use in the area of the axle systems and wheel suspensions of motor vehicles.
While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 018 663.7 | Apr 2005 | DE | national |
This application is a United States National Phase application of International Application PCT/DE2006/000702 and claims the benefit of priority under 35 U.S.C. § 119 of German Patent Application DE 10 2005 018 663.7 filed Apr. 21, 2005, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE06/00702 | 4/20/2006 | WO | 00 | 10/17/2007 |
