Title of Invention

SPHERICAL ELEMENT FOR TWO-PART BALL PIVOT AND CORRESPONDING METHOD OF PRODUCTION

Abstract

The invention relates to a method of manufacturing balls, in particular for ball joints, and a spherical element for two-part ball journals. The balls proposed by the invention are manufactured by cold flow pressing and then grinding, and a microalloyed carbon-manganese steel is used to make the balls. Using micro-alloyed carbon-manganese steel imparts outstanding strength and hardness to the balls as early as the cold forming stage. The method step of heat treating which has to be carried out when manufacturing balls of the generic type known from the prior art can thereof be dispensed with and need not be replaced, which means that cheaper material can be used. thereby significantly reducing manufacturing costs. The invention enables balls to be manufactured more simply and inexpensively, in particular for two-part ball journals, whilst preserving or increasing the surface and material quality as well as strength and wear resistance at the same time. As a result, the cost of manufacturing the balls is reduced and the problem of impact points on the ball surfaces which often occures during the heat treatment is eliminated.

Full Text

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 ELEMENT FOR TWO-PART BALL PIVOT AND CORRESPONDING METHOD OF PRODUCTION 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 : - be drawn in a drawing unit to obtain the desired material structure and annealed onto spherical cementite (GKZ annealing). The balls of heat-treated steel resulting from the cold pressing process naturally also have to be subjected to the appropriate heat treatment process to impart the desired hardness value and strength properties of the heat-treated steel to the finished balls of heat-treated steel. However, all of this is costly and thus results in high production costs for the balls. Against this background, the objective of the present invention is to propose balls, in particular for two-part ball journals, and a method of producing balls, by means of which said disadvantages of the prior art can be overcome. This being the case, the balls should specifically be simple and inexpensive to manufacture. In particular, the problems caused by the occurrence of impact points on the ball surface should be overcome, thereby obviating the need to remove the impact points subsequently. At the same time, however, it should also possible to obtain or preserve the high material and surface quality of the balls obtained by the known methods as well as the desired high strength of the balls. This objective is achieved on the basis of a method incorporating the characterising features defined in claim 1 and a spherical element incorporating the characterising features defined in claim 7. Preferred embodiments are respectively the subject matter of the dependent claims. The method of producing the balls proposed by the invention comprises the following method steps. In a first method step, a bar portion or wire portion is firstly produced from a semifinished product in a known manner. This being the case, however, a semi-finished -3- product should be used which is made from micro-alloyed carbon-manganese steel. Accordingly, any carbon-manganese steel with micro-alloyed elements produced by hot rolling after melting and resulting in a finely grained, ferritic-pearlitic structure is primarily suitable. The portion is then pickled, in particular to remove oxidation coatings and obtain a metallically pure surface of the portion for subsequent work operations. In another method step, the bar portion or wire portion is then shaped by cold flow processing to obtain the desired spherical shape. In another method step, the ball surface is then ground to obtain the pre-defined mass and pre-defined shape. The method proposed by the invention has some extremely good advantages in many respects. Firstly, instead of using the heat-treated steel known from the prior art, a micro-alloyed carbon-manganese steel can be used to produce the balls. In particular, the micro-alloyed carbon-manganese steel does not have to be heat treated but, as it transpires, already results in outstanding strength and hardness at the cold forming stage which takes place in the method step of cold flow pressing the ball from the bar or wire portion. The method step involving heat treatment which is always necessary in the prior art when producing the balls can therefore be fully dispensed with and need not be replaced, which likewise then eliminates the work and associated costs involved in running the heat treatment. In particular, however, the problem of undesirable impact points on the surfaces of the balls which arises in the methods of the prior art as the hot and soft balls are emptied from the heat-treatment furnace into the quenching medium is also completely eliminated. -4- In other words, this also means that the balls can be dimensioned much more closely to the ultimate dimensions at the cold flow pressing stage already, because it is no longer necessary to make allowance for the considerable amount of material which has to be removed at the stage of grinding the balls in order to remove the impact points, as has been necessary with the prior art methods. Firstly, this means that full use is made of the semi-finished product used, thereby saving on material costs. Secondly, the time needed for subsequent grinding is significantly reduced because much less material has to be removed. Not least, this significantly reduces wear of the grinding tools and the resultant quantity of grinding sludge, which likewise brings further cost savings and makes the manufacturing method more environmentally friendly. It has been found that the cold-pressed balls of micro-alloyed carbon-manganese steel have a significantly higher hardness than the heat-treated balls known from the prior art after pressing due to the cold forming process and due to the specific properties of the micro-alloyed steel described. This higher hardness firstly makes it easier to grind the balls and shortens the requisite grinding time. Secondly, fewer impact points occur on the ball surfaces during handling of the balls throughout the entire production process as a result, especially including after grinding. This is of advantage because a spherical shape which is as close as possible to the ideal spherical surface without impact points results in particularly smooth and low-wear ball joints which exhibit the least possible slip-stick effect during operation as the ball moves in the bearing shell. In preferred embodiments of the invention, the portions are subjected to a drawing process in another method step after pickling and, after pickling, the portions are annealed and drawn onto spherical cementite (GKZ annealing). This results in a cold solidification of the material already prior to the final cold flow pressing process, further increasing the strength of the balls finally obtained. -5- In another, likewise preferred embodiment of the invention, the wire or bar portions are coated with phosphate and/or coated with a dry lubricant prior to drawing or GKZ annealing. Since high pressure stresses occur between workpiece and tool during cold flow pressing, it is usually necessary to take steps to prevent cold welding between the tool and workpiece. For this purpose, a base or phosphate coating is applied to the wire or bar portions. A layer of dry lubricant can in turn be provided on the base layer, which resists pressure to a sufficient degree during the cold flow pressing process and thus prevents any metal contact between the workpiece and tool. The pressure-resistant solid lubricant used might be graphite, molybdenum sulphide, special soaps or waxes, for example. In one preferred embodiment of the invention, the balls are nitrocarburised in another method step after grinding the ball surfaces. Nitrocarburisation improves resistance to corrosion and wear resistance, particularly in the event of surface adhesion between the ball and bearing shell. A nitrocarburised surface also has a reduced coefficient of friction. The reason for this is the so-called binding layer produced on the ball surface during nitrocarburisation with a thickness of only a few hundredths of millimetres which exhibits a high resistance. Furthermore, nitrocarburisation is a relatively environmentally friendly process and offers an advantageous alternative to galvanically deposited coatings. The nitrocarburisation preferably takes place in a salt bath. In another preferred embodiment of the invention, the balls are polished or ground again and then polished in another method step after grinding and nitrocarburisation. This further increases resistance to corrosion and the wear resistance of the ball surface and further reduces the coefficient of friction. In another, likewise preferred embodiment of the invention, the carbon-manganese steel contains a micro-alloying element to accelerate the absorption of nitrogen -6- during nitration or during nitrocarburisation. More particularly preferably, the micro-alloying element is vanadium. Specifically using vanadium as the micro-alloying element accelerates the absorption of nitrogen during nitration. This enables higher hardness values and greater hardening depths of the binding layer to be achieved for the same nitration times, which also further improves corrosion behaviour. Alternatively, the same advantageous properties of the binding layer can be achieved as those obtained with a heat-treated steel but with shorter processing and nitration times. For example, tests have shown that this enables the salt bath process time to be reduced by 33% from 90 minutes to 60 minutes. Overall, the method proposed by the invention secures a further cost advantage over the methods known from the prior art which use heat-treated steel as a means of producing balls, due to the optimised nitration process and the shorter nitration times. The invention further relates to a spherical element, in particular for two-part ball journals. In a manner known per se, a two-part ball journal essentially comprises a stem element and a spherical element incorporating a hole. However, the spherical element proposed by the invention is distinctive because it is made from carbon-manganese steel with micro-alloying elements, obviating the need for heat treatment. The micro-alloyed carbon-manganese steel does not require a heating process but results in outstanding strength and hardness as early the cold forming stage based on flow pressing. Consequently, as mentioned above, the heat treatment needed to produce balls with the methods known from the prior art can be dispensed with, thereby eliminating the corresponding work and associated costs. Furthermore, the problem of undesired impact points on the ball surfaces is solved because the -7- process of emptying the hot and soft balls out of the heat treatment furnace into the quenching medium which causes the problem is dispensed with and is not replaced by some other process. Accordingly, in preferred embodiments of the invention, the micro-alloyed carbon-manganese steel is drawn, subjected to GKZ annealing and coated, in particular with a coating of phosphate. In a preferred embodiment of the invention, the spherical element is nitrocarburised. This improves resistance to corrosion and wear resistance as well as the friction behaviour of the spherical element, particularly in the case of ball joints, in particular as regards adhesion occurring between the ball and bearing shell of ball joints due to low angular speeds. In other, preferred embodiments of the invention, the spherical element is ground and/or polished, thereby resulting in low-friction ball joints of a particularly high quality and long service life. In another, likewise preferred embodiment of the invention, the micro-alloying elements include vanadium. As a result, a particularly hard and thick binding layer is applied to the nitrated or nitrocarburised balls, which specifically improves corrosion behaviour. The invention will be explained in more detail below with reference to only one example of an embodiment illustrated in the appended drawings. Of these: Fig. 1 is a micrograph showing the structure of a heat-treated steel for balls of the type known from the prior art; Fig. 2 is a diagram corresponding to that of Fig. 1, showing the structure of a micro-alloyed carbon-manganese steel for balls of the type proposed by the present invention; -8- Fig. 3 is a graph logarithmically plotting the cumulative fracture probability P as a function of tensile strength o in Mpa based on Weibull; Fig. 4 is a linear bar graph showing a comparison of the strength properties of balls made as proposed by the invention with balls heat-treated in the manner known from the prior art; Fig. 5 is a graph plotting the properties of the binding layer of the balls produced as proposed by the invention due to the nitrocarburisation compared with heat-treated balls of the type known from the prior art; and Fig. 6 shows two different views of a ball produced as proposed by the invention for a two-part ball journal. Fig. 1 is a micrograph on a very much enlarged scale showing the ferritic-pearliric structure of a heat-treated steel for balls of the type known from the prior art. Specifically, this is the structure of a heat-rolled, standard heat-treated steel known under the name of 41Cr4. Fig. 2 is a micrograph showing what is also the ferritic-pearlitic structure of a micro-alloyed carbon-manganese steel for balls of the type proposed by the present invention, on the same scale of magnification as the micrograph showing the heat-treated steel of Fig. 1. It is a micro-alloyed steel which is also heat-rolled during production, known as 35V1 or C-Mn-V. This steel contains the following alloying elements (all figures are given as a percentage by weight): 0.35 % C 0.20 % Si 0.75 % Mn -9- 0.02 % P 0.02 % S 0.20 % Cr 0.15 % Ni 0.20 % Cu 0.10 % V 0.02 % Al 0.01 % N As may be seen from looking at both Figs. 1 and 2, the micro-alloyed steel illustrated in Fig. 2 has a very much finer structure compared with the standard heat-treated steel illustrated in Fig. 1. The fine structure of the micro-alloyed steel illustrated in Fig. 2 specifically makes the micro-alloyed steel particularly easy to cold form, which is of advantage during the process of producing the balls proposed by the invention by cold pressing. Fig. 3 sets out the strength of different cold-pressed balls calculated from hardness measurements. The diagram shows two sets of logarithmically plotted cumulative fracture probability vales P in the form of a Weibull distribution on the vertical axis as a function of tensile strength o in MPa plotted on the horizontal axis. In this connection, the tensile strength was calculated in accordance with DIN 50150 from measured hardness values and the hardness values were measured at different points of the balls. The graph of Fig. 3 contains measurement values for three different ball types. The lozenge-shaped measurement points denoted by letter A in the legend relate to the balls proposed by the invention made from micro-alloyed carbon-manganese steel produced by cold pressing. The square measurement points denoted by letter B in the legend of Fig. 3 relate to balls made from a heat-treated steel of the type known from the prior art. Specifically, this is a standard heat-treated steel known under the name of 38MnB5. The triangular measurement points denoted by letter C in the -10- legend of Fig. 3 again relate to the balls proposed by the invention made from micro-alloyed carbon-manganese steel and the triangular measurement points relate to the balls proposed by the invention after nitrocarburisation As may be seen from Fig. 3, the strength values of the balls proposed by the invention made from micro-alloyed carbon-manganese steel (lozenges) are significantly higher than the strength values of the heat-treated steel known from the prior art (squares). This higher hardness is of advantage when subjecting the balls to a grinding treatment amongst other things, because the grinding time can be significantly reduced as a result, thereby saving on costs. Secondly, because of the higher hardness, particularly few impact points are formed in the ball surfaces during handling of the balls throughout and after the manufacturing process. Balls without impact points are of particular advantage in the case of ball joints because this makes it possible to produce particularly smooth and low-friction ball joints with a long service life, which exhibit a particularly low tendency to stick-slip effects during operation as the ball moves in the bearing shell. The higher hardness of the balls of micro-alloyed carbon-manganese steel proposed by the invention is finally also of advantage because it likewise improves resistance to corrosion and friction behaviour if the balls are used for ball joints. In Fig. 3, the triangular measurement points denote the strength values of the balls proposed by the invention made from micro-alloyed carbon-manganese steel after the balls proposed by the invention have been nitrocarburised. Where imaginary Weibull straight lines (the straight lines defined respectively by a group of measurement points) intersect the y-axis at zero, it may be seen that the balls proposed by the invention made from micro-alloyed carbon-manganese steel still have strength values (triangular measurement points) after nitrocarburisation that -11 - are as high as those of the balls made from heat-treated steel (square measurement points). Although one might have expected to see a recovery of the cold-solidified structure of the balls at the ball surface due to the temperatures of up to around 600°C prevailing during nitrocarburisation and hence an associated high decrease in the high strength values obtained by cold flow pressing, it has surprisingly been found that the high strength of the balls proposed by the invention is advantageously almost fully preserved even after nitrocarburisation. The reason for this is that, because of the micro-alloying elements contained in the material of the balls proposed by the invention, the cold-solidified structure does not totally recover under the conditions of the nitrocarburisation process. The Weibull straight lines of the balls proposed by the invention made from micro-alloyed carbon-manganese steel (triangles and squares) shown in Fig. 3, which have less steep gradients than the heat-treated steel, merely indicate that the rate of cold solidification of the material varies due to the different degrees of forming at different points of the balls because the plotted measurement values were taken over the entire ball cross-section. As tests have shown, this does not have any negative effects with regard to the outstanding suitability of the balls proposed by the invention for use in ball joints. Fig. 4 shows the tensile strength of different balls proposed by the invention made from a different micro-alloyed carbon-manganese steel known as 10MnSi7, also determined on the basis of hardness in accordance with DIN 50150 (vertical bar on the right containing dots in each case), and the tensile strength of the wires from which the respective balls were made (vertical bar on the left containing hatching in each case). By way of comparison, the diagram of Fig. 4 also shows the strength values of a heat-treated steel of the type known from the prior art (horizontal bar). The percentage figures on the horizontal axis indicate the degree to which the wire -12- from which the balls were respectively pressed was drawn prior to pressing. The wire was drawn after hot rolling and prior to pressing the balls. As may be seen, the balls made from micro-alloyed carbon-manganese steel without any heat treatment (right-hand bar containing dots in each case) have a higher strength than the balls of heat-treated steel (horizontal bar) without exception, and do so largely irrespective of the degree of wire drawing and the associated strength of the wire or initial material (left-hand bars with hatching in each case). Fig. 5 illustrates the hardness profile of a ball proposed by the invention made from a micro-alloyed carbon-manganese steel without heat treatment (35V1) after nitrocarburisation, for which purpose hardness measurement values are plotted across the depth, below the ball surface. In the legend of Fig. 5, letter C again stands for the measurement values for the carbon-manganese steel (triangular measurement points). By way of comparison, the corresponding hardness measurement values of a ball made from standard heat-treated steel of the type known from the prior art are also plotted in the diagram of Fig. 5, as indicated by letter B in the legend of Fig. 5 again (square measurement points). As may be seen, the balls proposed by the invention made from micro-alloyed carbon-manganese steel (triangular measurement points) even have a still higher hardness after nitrocarburisation than the corresponding balls of heat-treated steel known from the prior art (square measurement points). As explained above, the higher hardness is of advantage amongst other things for rendering the balls proposed by the invention particularly resistant to wear and making the balls easier to process during grinding, saving time and costs. - 13- By way of comparison, Fig. 5 also shows structurally pre-defined desired values for the hardness at the surface and at a depth of 0.2 mm for balls intended for ball joints, shown in the two horizontal bars of the diagram shown in Fig. 5. As may be seen, the binding layer of the balls proposed by the invention (triangular measurement points) have the required desired values for hardness and even exceed them. Fig. 6, finally, illustrates two different views of a ball produced as proposed by the invention from a micro-alloyed carbon-manganese steel without heat treatment, for a two-part ball journal, incorporating a hole to accommodate the stem element. As may be seen, the balls can be produced by the method proposed by the invention without causing any problems, in particular without cracks and with a perfect surface quality. As a result, it is therefore clear that, as a result of the invention, it is now possible to manufacture balls specifically for two-part ball journals more easily and inexpensively than in the past, whilst simultaneously enabling the surface and material quality as well as the required strength and wear resistance of the balls to be preserved or even increased. Amongst other things, the fact that the heat treatment required in the past can be dispensed with results firstly in a considerable saving on costs and secondly, eliminates the problem of impact points on the ball surfaces which often occurs during the heat treatment. The invention therefore makes a significant contribution to particularly economic production of high quality balls, in particular for ball joints, wheel suspensions, stabilisers and other similar applications. -14- WE CLAIM : 1. Method of manufacturing balls or spherical elements, in particular for ball joints, comprising the following method steps: a) creating a bar portion or wire portion starting from a hot rolled semifinished product of micro-alloyed carbon-manganese steel; b) pickling; c) cold flow processing the portion to form a ball or spherical element; and d) grinding the ball surface. 2. Method as claimed in claim 1, characterised in that at least one drawing operation takes place in another method step b' after method step b (pickling). 3. Method as claimed in claim 1, characterised in that the portion is annealed and drawn onto spherical cementite (GKZ annealing) in another method step b" after method step b (pickling). 4. Method as claimed in claim 2 or 3, characterised in that the portion is coated with phosphate and/or with a dry lubricant during method step b" (GKZ annealing) prior to method step b' (drawing). 5. Method as claimed in one of claims 1 to 4, characterised in that the balls or spherical elements are nitrocarburised in another method step e after method step d (grinding). -15- 6. Method as claimed in claim 5, characterised in that the nitrocarburisation process in method step e takes place in a salt bath. 7. Method as claimed in one of claims 1 to 6, characterised in that the balls or spherical elements are ground and/or polished in another method step f after method step d (grinding) or e (nitrocarburisation). 8. Method as claimed in one of claims 1 to 7, characterised in that the carbon-manganese steel contains a micro-alloying element to accelerate nitrogen absorption during the nitration or nitrocarburisation process. 9. Method as claimed in claim 8, characterised in that the other micro-alloying element is vanadium. 10. Spherical element, in particular for two-part ball journals, which journal comprises the spherical element and a stem element, characterised in that the spherical element is made from carbon-manganese steel containing micro-alloying elements without a heat treatment. 11. Spherical element as claimed in claim 10, characterised in that the spherical element is manufactured from a drawn wire. 12. Spherical element as claimed in claim 10 or 11, characterised in that the spherical element comprises a wire annealed onto spherical cementite (GKZ annealing). -16- 13. Spherical element as claimed in one of claims 10 to 12, characterised in that the spherical element is made from a coated wire, in particular with a phosphate coating. 14. Spherical element as claimed in one of claims 10 to 13, characterised in that the spherical element is nitrocarburised. 15. Spherical element as claimed in one of claims 10 to 14, characterised in that the spherical element is ground. 16. Spherical element as claimed in one of claims 10 to 15, characterised in that the spherical element is polished. 17. Spherical element as claimed in one of claims 10 to 16, characterised in that the micro-alloying element contains vanadium. Dated this 7th day of November, 2006. HIRAL CHANDRAKANT JOSHI AGENT FOR ZF FRIEDRICHSHAFEN AG -17- -17- Abstract The invention relates to a method of manufacturing balls, in particular for ball joints, and a spherical element for two-part ball journals. The balls proposed by the invention are manufactured by cold flow pressing and then grinding, and a micro-alloyed carbon-manganese steel is used to make the balls. Using micro-alloyed carbon-manganese steel imparts outstanding strength and hardness to the balls as early as the cold forming stage. The method step of heat treating which has to be carried out when manufacturing balls of the generic type known from the prior art can therefore be dispensed with and need not be replaced, which means that cheaper material can be used, thereby significantly reducing manufacturing costs. The invention enables balls to be manufactured more simply and inexpensively, in particular for two-part ball journals, whilst preserving or increasing the surface and material quality as well as strength and wear resistance at the same time. As a result, the cost of manufacturing the balls is reduced and the problem of impact points on the ball surfaces which often occurs during the heat treatment is eliminated. To The Controller of Patents The Patent Office Mumbai Fig. 4 -18- Figure 4

Documents:

1312-mumnp-2006-abstract(31-12-2007).doc

1312-mumnp-2006-abstract(31-12-2007).pdf

1312-mumnp-2006-abstract.doc

1312-mumnp-2006-abstract.pdf

1312-mumnp-2006-cancelled pages(31-12-2007).pdf

1312-mumnp-2006-claims(granted)-(31-12-2007).doc

1312-mumnp-2006-claims(granted)-(31-12-2007).pdf

1312-mumnp-2006-claims.doc

1312-mumnp-2006-claims.pdf

1312-mumnp-2006-correspondance-received.pdf

1312-mumnp-2006-correspondence(31-12-2007).pdf

1312-mumnp-2006-correspondence(ipo)-(27-1-2009).pdf

1312-mumnp-2006-description (complete).pdf

1312-mumnp-2006-drawing(31-12-2007).pdf

1312-mumnp-2006-drawings.pdf

1312-mumnp-2006-form 1(31-12-2007).pdf

1312-mumnp-2006-form 18(7-11-2006).pdf

1312-mumnp-2006-form 2(granted)-(31-12-2007).doc

1312-mumnp-2006-form 2(granted)-(31-12-2007).pdf

1312-mumnp-2006-form 26(30-11-2006).pdf

1312-mumnp-2006-form 3(31-12-2007).pdf

1312-mumnp-2006-form 5(7-11-2006).pdf

1312-mumnp-2006-form-1.pdf

1312-mumnp-2006-form-18.pdf

1312-mumnp-2006-form-2.doc

1312-mumnp-2006-form-2.pdf

1312-mumnp-2006-form-3.pdf

1312-mumnp-2006-form-5.pdf

1312-mumnp-2006-form-pct-ib-304.pdf

abstract1.jpg