FORM 2
THE PATENT ACT 1970 (39 of 1970)
&
The Patents Rules, 2003 COMPLETE SPECIFICATION
(See Section 10, and rule 13)
1. TITLE OF INVENTION BALL-AND-SOCKET JOINT

2. APPLICANT(S)
a) Name
b) Nationality
c) Address

ZF FRIEDRICHSHAFEN AG GERMAN Company 88038 FRIEDRICHSHAFEN GERMANY

3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed : -

Description
The invention relates to a ball and socket joint, for example for an axle system or a wheel suspension of a motor vehicle, of the type outlined in the introductory part of claim 1.
Ball and socket joints of the type mentioned above are employed in wheel suspensions or alternatively as a sleeve joint, for example for linking transverse stabilisers in motor vehicles, although not exclusively.
The requirements placed on such ball and socket joints specifically include a high specific ability to withstand stress and low bearing play, both in static and dynamic load situations, as well as low maintenance requirements or no maintenance as far as possible during the service life of the motor vehicle, as low as possible a weight and a low space requirement. Furthermore, the manufacturing process should be inexpensive as far as possible.
A ball and socket joint usually has an essentially annular or pot-shaped joint housing, in the interior of which the bearing shell or ball shell of the ball and socket joint is disposed. In the case of ball and socket joints known from the prior art, the bearing surface of the ball shell sitting in contact with the surface of the ball of the ball and socket joint essentially corresponds to the shape of the joint ball, at least in the ideal situation. In other words, the bearing surface of the ball shell conforms to the shape of a spherical portion.
However, due to tolerances occurring during production of the ball shell and as a result of forces occurring during assembly of the ball and socket joint, especially when the ball shell is being pressed into the joint housing, the shape of the bearing surface of the bearing shell usually no longer corresponds to the intended spherical shape or the shape of the ball of the ball journal when the ball joint is in the final assembled state. Instead, the shape of the ball shell often corresponds to an ellipsoid
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in revolution, the bigger half-axis of which coincides with the longitudinal axis of the ball journal or ball sleeve. In other words, this means that there is often no full-surface contact between the ball and ball shell as actually desired, and instead, there is only a linear contact or contact along a strip in the middle region of the ball shell.
For geometric reasons, the ball shell of most ball and socket joints also has a particularly slim wall thickness in this middle region - in other words approximately in the region of its biggest diameter by reference to a longitudinal section through the ball and socket joint. In addition to the deviations in shape described above, this also results in a particularly high spring constant or particularly low elasticity of the ball shell in this middle portion.
As a result of both of these aspects, the surface contact between the ball and ball shell of ball and socket joints known from the prior art does not usually correspond to the ideal uniform load distribution across all of the available bearing surface. Instead, a major part of the load is concentrated on the above-mentioned middle portion of the ball shell in the region of its biggest diameter by reference to the longitudinal section through the ball and socket joint. This leads to uneven, increased bearing wear, prematurely high bearing play, lower load-bearing capacity and a reduced service life in known ball and socket joints.
Against this background, the objective of this invention is to propose a ball and socket joint by means of which these disadvantages of the prior art can be overcome. In particular, the intention is to obtain a distribution of the surface contact between the ball and ball shell that results in as uniform and even a contour as possible, improves the specific ability to withstand stress and load-bearing capacity of the ball and socket joint, reduces the joint play and obtains a higher service life.
This objective is achieved on the basis of a ball and socket joint incorporating the characterising features defined in claim 1. Preferred embodiments are defined in the dependent claims.
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In a manner known per se, the ball and socket joint proposed by the invention firstly comprises an essentially annular or pot-shaped joint housing. The joint housing has an essentially cylindrical interior, in which the ball shell of the ball and socket joint can be disposed. Accommodated in the ball shell so that it can effect a sliding motion is the ball of the ball journal or the ball sleeve of the ball and socket joint.
However, the ball and socket joint based on the invention is distinctive due to the fact that the bearing surface of the ball shell essentially conforms to the surface of an ellipsoid in revolution. The smaller half-axis of the ellipsoid in revolution therefore coincides with the axial direction of the ball and socket joint, namely with the longitudinal axis of the ball journal or the longitudinal axis of the ball sleeve.
The rotationally ellipsoid shape of the bearing surface of the ball shell means that when the ball and socket joint is assembled, the contact between ball surface and ball shell is significantly flatter than is the case with the prior art and is produced with a more uniform distribution of surface contact. The reason for this is that the shape of the bearing surface conforming to an ellipsoid in revolution counteracts the deformation effects and tolerances described above, which often lead to only a linear or strip-shaped contact between the ball and ball shell according to the prior art. Furthermore, differing degrees of elasticity of the ball shell which occur due to the differing wall thickness of the ball shell in the various regions of the ball shell are counteracted.
In this respect, the expressions "ellipsoid in revolution" or "rotationally ellipsoid" should naturally not be interpreted in the strict mathematical sense. Rather, what is claimed by the invention is a ball shell which has a non-constant radius of curvature in the longitudinal section through the ball joint and the radius of curvature in the middle region of the ball shell is slightly smaller than in the axial peripheral regions of the ball shell.
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This being the case, the bigger half-axis of the rotationally ellipsoid ball shell conforms to the radius of the ball of the ball and socket joint. In other words, what this means is that the bearing surfaces of the ball shell and joint ball geometrically cross or mutually intersect one another in the mathematical sense in the axial peripheral regions of the ball shell, namely in the region of the end faces of the joint housing. Since an actual mutual intersection of the bearing surfaces is not possible in the assembled ball and socket joint, this results in a certain defined pre-tensioning between the ball shell and joint ball. Due to the shape of the ball shell proposed by the invention, the intersection of the bearing surfaces or the defined pre-tensioning is specifically concentrated at the axial peripheral regions of the ball shell and becomes increasingly wide the closer it is to the middle region of the ball shell.
As a result, when the ball and socket joint is assembled, a clearance-free, full-surface contact is obtained between the ball shell and joint ball, thereby permitting a uniform distribution of the surface contact across the entire contact surfaces between the ball shell and joint ball in the load situation. However, this also increases the specific load-resistance and load-bearing capacity of the ball and socket joint and the service life can be increased using the same dimensions, and the ball and socket joint remains in the desired clearance-free state for a significantly longer time due to the fact that stress is distributed more uniformly on the bearing surfaces.
In order to implement the invention, it is firstly not important how the ellipsoid shape proposed by the invention is imparted to the bearing surface of the ball shell. For example, it would be conceivable to use a ball shell produced with an initially spherical bearing surface which is subsequently plastically or elastically deformed, including during the operation of pressing it into the bearing housing for example, in order to obtain the intended rotationally ellipsoid bearing surface. In a preferred embodiment of the invention, however, the ball shell already has the ellipsoid bearing surface imparted during production or by the mould. In other words, this means that the desired ellipsoid shape is already imparted to the ball shell during the process of re-shaping the ball shell. This makes it possible to set the desired
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degree of geometric overlap between the joint ball and ball shell so that it is particularly precise and reproducible.
In another embodiment of the invention, the surface of the ball shell is at least slightly concave at the outer periphery of the ball shell. As with the joint ball and ball shell described above, this also results in a geometric overlap between certain regions of the ball shell and joint housing, in this case between the external surface of the ball shell and the internal surface of the joint housing. This also enables the contact between the ball shell and joint housing to be improved and the distribution of the surface contact between the external surface of the ball shell and the internal surface of the joint housing also to be rendered more uniform and hence more capable of supporting load.
In another embodiment of the invention, the ball shell has a slot disposed in the axial direction by reference to the ball and socket joint. In other words, this means that the ball shell is no longer a closed ring but is made in the shape of a "C" during production (with a very small slot width). This is of advantage because it simplifies assembly of the ball and socket joint. The slot in the ball shell can also be used to accommodate heat expansion which occurs during operation of the ball and socket joint, which is of particular importance with respect to the ball and socket joint proposed by the invention because the ellipsoid shape of the ball shell results in a particularly intimate and clearance-free contact of the ball shell on the joint ball and joint housing, essentially leaving no option for accommodating heat expansion.
In one particularly preferred embodiment of the invention, the ball shell is made from a thermoplastic material which can be injection moulded, in particular polyoxymethylene. Polyoxymethylene has a very low friction resistance whilst simultaneously exhibiting good abrasion resistance and high elastic spring capacity. Polyoxymethylene also has a high resistance to chemicals which is important due to the presence of lubricants and other operating substances on the motor vehicle.
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The way in which the ball shell is secured or anchored in the joint housing basically has no relevance in terms of implementing the invention. In a preferred embodiment of the invention, however, the ball shell is positively supported in the joint housing at both sides in the axial direction. This being the case, the positive axial support occupies at least 80% of the end face of the ball shell by particular preference. In this way, the ball shell, which has a tendency to exhibit a certain degree of elastic or plastic creep, is supported particularly effectively and is enclosed almost completely all the way round.
The invention will be explained in more detail below with reference to examples of embodiments illustrated in the appended drawings. Of these:
Fig. 1 is a schematic diagram showing a longitudinal section of a ball and socket joint based on the prior art;
Fig. 2 is a schematic diagram of an ellipsoid in revolution;
Fig. 3 is a schematic diagram showing an embodiment of a sleeve joint based on this invention in a half-longitudinal section;
Fig. 4 is a schematic diagram showing another ellipsoid in revolution;
Fig. 5 is a schematic diagram on a larger scale showing a ball shell and the contour of the joint ball of a sleeve joint illustrated in Fig. 3;
Fig. 6 is a schematic diagram showing the surface contact distribution of a ball and socket joint known from the prior art compared with a ball and socket joint proposed by the invention;
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Fig. 7 is a schematic, isometric diagram showing the ball shell of one embodiment of a ball and socket joint proposed by the invention after the original shaping process;
Fig. 8 is a diagram corresponding to Fig. 7 showing the ball shell illustrated in Fig. 7 in the fitted state; and
Fig. 9 shows the ball shell of a ball and socket joint based on the prior art.
Fig. 1 is a schematic diagram in longitudinal section showing a ball and socket joint based on the prior art. The essentially pot-shaped joint housing 1 may be seen with the bearing shell or ball shell 2 disposed in it. Disposed in the interior of the ball shell 2, in turn, is the ball 3 of a ball journal 4.
As clearly illustrated, the wall thickness of the ball shell 2 is particularly slim in its middle region 5 - by reference to the axial direction of the ball journal 4 - whilst the wall thickness becomes significantly thicker towards the terminal end regions 6 of the ball shell 2, where it finally assumes a multiple of the wall thickness in the middle region 5 of the ball shell 2.
As already described above, however, this means that the ball shell 2 has a higher spring constant by a multiple and much lower flexibility in its middle region 5 than in the regions 6 axially close to the end. This non-uniform flexibility unavoidably causes a non-uniform distribution of surface contact as soon as the ball and socket joint is subjected to stress, and would do so even in the hypothetical situation in which the bearing surface of the ball shell 2 had the ideal spherical shape. Such a non-uniform distribution of surface contact is transmitted via the surface 10 of the bearing shell 2, illustrated by way of example in Fig. 6, and the curve denoted by X reflects the distribution of surface contact of the ball and socket joint based on the prior art and illustrated in Fig. 1.
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The distribution of surface contact X concentrated largely towards the middle portion 5 of the ball shell 2 occurs even in the hypothetical situation where the spherical shape of the bearing surface is ideal, because the regions 6 of the ball shell 2 close to the end flex to a more pronounced degree, due to their higher flexibility, compared with the middle region 5 but without essentially absorbing force, for which reason the middle region 5 of the ball shell 2 necessarily has to absorb the major part on the basis of its much lower flexibility.
Added to this, the ball shell 2 is also elastically deformed during fitting in the joint housing 1 so that, starting form its original spherical shape, the bearing surface of the ball shell 2 assumes an essentially vertically ellipsoid-shaped surface after fitting, as may be seen from a comparison of the schematic diagram of a rotational ellipsoid 7 with the main axes a, b illustrated in Fig. 2.
In this instance, the bigger main axis a of the ellipsoid 7 formed by the bearing surface of the ball shell 2 coincides with the longitudinal axis of the ball journal 3. However, this means that there is no longer any surface contact between the joint ball 3 and the ball shell 2 deformed to a slightly vertically ellipsoid shape, even when the ball and socket joint is in the load-neutral state. Instead, the actual desired surface contact is converted into an essentially linear contact in the middle region 5 of the ball shell 2, in other words in the region of its slimmest wall thickness, due to the deformation of the ball shell 2.
All of this results in a poor bearing pattern between the joint ball 3 and ball shell 2, in turn leading to the disadvantageous consequences in terms of service life, wear, absence of play and load strength of the ball and socket joint.
Fig. 3 illustrates an embodiment of a ball and socket joint based on the invention, in this instance in the form of a sleeve joint. An annular or cylindrical joint housing 1 may be seen first of all, with a ball shell 2 disposed in the interior of the joint housing 1. Disposed in the interior of the ball shell 2 in this instance is the joint ball 3 of a ball
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sleeve 8. The ball shell 2 is therefore almost totally enclosed by the locking rings 9 disposed in the region of the terminal ends of the joint housing 1 or ball shell 2 -together with the flat contact on the joint housing 1 and joint ball 3. This largely prevents any creep of the material of the ball shell 2, which also improves the service life and absence of play of the ball and socket joint.
The shape of the ball shell is based on a transverse ellipsoid shown by way of example in the schematic diagram of Fig. 4 or the diagram of the ball shell 2 shown in Fig. 5 on a larger scale. As may be seen from Fig. 5, the ball shell 2 of the embodiment illustrated has an increasingly larger extra thickness in the direction towards its axial end faces compared with the surface 12 of the joint ball 3 and compared with the cylindrical external shape 13 both in the region of the inwardly lying bearing surface 10 and in the region of its essentially cylindrical external surface 11. In other words, this means that the bearing surface 10 of the ball shell 2 illustrated in Figs. 3 and 5 are no longer spherical (or actually often vertically ellipsoid, see Figs. 1 and 2), as is the case with the prior art but instead essentially assumes the shape of a transverse ellipsoid. In this case, however, the shorter main axis b of the rotational ellipsoid coincides with the longitudinal axis of the ball and socket joint or ball sleeve, as may be seen from the diagram of the transverse ellipsoid schematically illustrated in Fig. 4.
Together with the slightly concave external contour 11 of the ball shell 2, the rotationally ellipsoid shape of the bearing surface 10 of the ball shell 2 means that the effect of the load concentration in the middle region 5 of the ball shell 2 described above is counteracted because the regions 6 of the ball shell 2 closer to the edge contain a defined degree of radial pre-tensioning. When the ball and socket joint is in the assembled state, therefore, there is a full-surface contact between the ball shell 2 and joint ball 3, which in conjunction with said radial pre-tensioning renders the distribution of surface contact between the joint ball 3 and ball shell 2 more uniform.
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This is represented by way of example by the curve Y shown in Fig. 4. By comparing this with the curve X corresponding to the prior art, it is immediately evident that the load F is distributed uniformly across the entire available surface of the ball shell 2. This leads to the higher load-bearing capacity, improved resistance to stress and longer service life of the ball and socket joint described above, whilst simultaneously producing a minimum bearing clearance for a long period.
Figs. 7 and 8 illustrate the ball shell 2 of an embodiment of a ball and socket joint or sleeve joint proposed by the invention, where the ball shell 2 in Fig. 7 is shown in the state immediately after the original shaping process, for example after injection moulding in the case of a ball shell made from thermoplastic material. Fig. 8 illustrates the same ball shell 2 in the assembled state. The ball shell illustrated in Fig. 7 and 8 is split in the axial longitudinal direction by means of a slot 14, which firstly facilitates assembly of the ball and socket joint. The joint housing 1, ball shell 2 and width of the slot 14 may be dimensioned so that the slot 14 of the ball shell 2 is not completely closed when the ball and socket joint is in the assembled state. The slight opening width of the slot 14 which remains in this instance when the ball and socket joint is in the assembled state is available as a means of accommodating the heat expansion of the material of the ball shell 2 in order to prevent mechanical strain and hence associated sluggishness of the ball and socket joint.
However, since only a single and what, in the assembled state, is also an extremely narrow slot 14 exists, the bearing surface of the ball shell 2 is reduced to only a negligible degree by the slot 14 so that the maximum load-bearing capacity of the ball and socket joint remains practically unrestricted.
The embodiment of the ball and socket joint proposed by the invention with the ball shell illustrated in Figs. 7 and 8 also differs from the prior art in this respect. In the prior art, it is standard practice to use ball shells 15 with several wide slots, such as illustrated by way of example in Fig. 9. Such balls shells 15 are also easy to assemble. Due to the numerous wide slots of such ball shells 15, however, the proportion of
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bearing surface of the ball shells 15 is reduced to a not inconsiderable degree, first of all. Secondly, the material of the ball shell 15 is able to creep into the space left free due to the slots when under load, thereby reducing the wall thickness of the ball shell 15, which can result in a significant but undesired bearing play.
The use of slotted ball shells 2 of the type illustrated in Figs. 7 and 8, therefore, also improves the reliability, load-bearing capacity and service life of ball and socket joints compared with the prior art.
Accordingly, it is clear that, due to the invention, a ball and socket joint is proposed which has advantages over ball and socket joints known from the prior art in particular in terms of service life, fault tolerance and ability to withstand stress. The more uniform distribution of surface contact in the bearing gap between the ball shell and joint ball guarantees particularly low wear and low friction torques. The invention therefore helps to improve the reliability and need for maintenance for ball and socket joints, especially with regard to applications involving axle systems and wheel suspensions on a motor vehicle.
List of reference numbers
1 Joint housing
2 Ball shell
3 Joint ball
4 Ball journal
5 Middle region of the ball shell
6 Region of the ball shell close to the periphery
7 Vertical ellipsoid
8 Ball sleeve
9 Locking ring
10 Bearing surface
11 Concave external contour
12 Spherical surface
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13 Cylindrical external contour
14 Slot •
15 Ball shell

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WE CLAIM :
1. Ball and socket joint, for example for an axle system of a motor vehicle, with
an essentially annular or pot-shaped joint housing (1), in the essentially
cylindrical interior of which a ball shell (2) can be disposed, and the ball (3) of
a ball journal (4) or a ball sleeve (8) can be accommodated in the ball shell (2)
so that it can move in a sliding motion,
characterised in that
the bearing surface (10) of the ball shell (2) essentially matches the surface of an ellipsoid in revolution, the smaller half-axis (b) of which coincides with the axial direction of the ball and socket joint.
2. Ball and socket joint as claimed in claim 1,
characterised in that
the bigger half-axis (a) of the ball shell conforms to the length of the radius of the joint ball.
3. Ball and socket joint as claimed in claim 1 or 2,
characterised in that
the ellipsoid-shaped bearing surface (10) is already imparted to the ball shell (2) in the mould.
4. Ball and socket joint as claimed in one of claims 1 to 3,
characterised in that
the surface (11) of the ball shell (2) is concave at the external periphery of the ball shell (2).
5. Ball and socket joint as claimed in one of claims 1 to 4,
characterised in that
the ball shell (2) has a slot (14) in the axial direction.
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6. Ball and socket joint as claimed in one of claims 1 to 5,
characterised in that
the ball shell (2) is made from a thermoplastic material which can be injection moulded, in particular polyoxymethylene.
7. Ball and socket joint as claimed in one of claims 1 to 6,
characterised in that
the ball shell (2) is positively supported in the joint housing (1) on both sides in the axial direction.
8. Ball and socket joint as claimed in claim 7,
characterised in that
the positive axial support respectively comprises at least 80% of the end face of the ball shell (2).
Dated this 10th day of August, 2007

ABSTRACT
The invention relates to a ball and socket joint, for example for an axle system of a motor vehicle. The ball and socket joint has an essentially annular or pot-shaped joint housing 1, in the essentially cylindrical interior of which a ball shell 2 can be disposed. The ball 3 of a ball journal 4 or a ball sleeve 8 can be accommodated in the ball shell 2 so as to move in a sliding motion.
The ball and socket joint proposed by the invention is distinctive due to the fact that the bearing surface 10 of the ball shell 2 essentially conforms to the surface of an ellipsoid in revolution, the smaller half-axis b of which coincides with the axial direction of the ball and socket joint.
The ball and socket joint proposed by the invention offers advantages in terms of
service life, fault tolerance and ability to withstand stress. The more uniform
distribution of surface contact in the bearing gap between the ball shell and joint ball
guarantees low wear and low friction torques.
To,
The Controller of Patents,
The Patent Office,
Mumbai