METHOD FOR PRODUCING A HOLLOW VALVE FOR INTERNAL COMBUSTION ENGINES

Description

Field of the invention

[0001] The present invention relates to a method for producing hollow valves, or cavity valves, for internal combustion engines and hollow valves produced thereby. State of the art

[0002] Intake and exhaust valves in combustion engines are components subject to high thermal and mechanical stress. Adequate cooling is therefore necessary to ensure long-term functionality of the valves. Hollow disc valves have advantages over solid-stem valves and hollow-stem valves (i.e., a hollow valve with a cavity only in the stem) because a cavity is present in both the stem and the valve head, allowing for improved internal cooling using a cooling medium, e.g., sodium. Other advantages include lower weight, the avoidance of hot spots, and a reduction in CO2 emissions.

[0003] Hollow valves are typically manufactured using a combination of different processes, such as forging, turning, and welding. Turning or milling the cavity is particularly costly. Weld spots on the plate surface or other operationally critical locations should also be avoided. A further disadvantage of known processes is that a large number of process steps are often necessary. For example, EP 0898055 A1 relates to a hollow plate valve manufactured by closing a hollow blank by welding. Furthermore, DE 102015220891 A1 discloses the production of an internally cooled valve by forming a cup-shaped preform using rolling. Further relevant prior art can be found in EP 2 690 262 A1 and EP 0 151 976 A.

[0004] The object of the present invention is therefore to provide a manufacturing method for hollow valves or for a valve body for hollow valves, which does not have the disadvantages mentioned and at the same time has high productivity and good material utilization. Summary of the invention

[0005] According to the invention, the problem is solved by a method for producing a valve body of a hollow valve according to the features of the appended claim 1.

[0006] The method for manufacturing a valve body of a hollow valve comprises the steps of providing a preform having a valve head and a tubular wall surrounding a cylindrical cavity, and flow forming the tubular wall over a flow forming mandrel inserted into the cavity to increase a length of the tubular wall.

[0007] According to one aspect of the present invention, providing the preform may comprise: providing a cup-shaped semi-finished product, the semi-finished product having the tubular wall surrounding the cylindrical cavity of the semi-finished product and a bottom portion; and forming the valve head from the bottom portion.

[0008] According to a further aspect, providing the cup-shaped semi-finished product may comprise: providing an at least partially cylindrical blank; and forming the cup-shaped semi-finished product from the blank.

[0009] According to a further aspect, the forming of the cup-shaped semi-finished product can be carried out by extrusion or forging.

[0010] According to a further aspect, the valve head can be formed by extrusion or forging.

[0011] According to the invention, several spinning rollers are used in the spinning process, with three spinning rollers preferably being used.

[0012] According to the invention, the plurality of spinning rollers are radially and axially offset from one another during the spinning process.

[0013] According to a further aspect, the method may further comprise: further flow forming the tubular wall without a flow forming mandrel.

[0014] According to another aspect, the method may further comprise: reducing an outer diameter of the tubular wall after the flow forming.

[0015] According to a further aspect, the reduction of the outer diameter of the tubular wall can be carried out by swaging or drawing in.

[0016] According to a further aspect, the reduction of the outer diameter of the tubular wall can be carried out without a mandrel.

[0017] According to a further aspect, the reduction of the outer diameter of the tubular wall can be carried out with a mandrel inserted into the cavity.

[0018] According to a further aspect, the method may further comprise: filling a cooling medium, in particular sodium, into the cavity; and closing the cavity.

[0019] The problem is further solved by a hollow valve comprising a valve body manufactured using the above method. Short description of the drawing

[0020] In the following, exemplary embodiments of the invention are described in more detail with reference to the figures, wherein Figures 1A - 1F various intermediate steps of the inventive production of a valve body of a hollow valve (shown in Fig. 1D or Fig. 1F ) from a blank (shown in Fig. 1A ) show; and Fig. 2 shows a sectional view during flow forming.

[0021] In the following, the same reference symbols are used for identical or similar elements or components in both the description and the drawing. A list of reference symbols is also provided that applies to all figures. The designs shown in the figures are merely schematic and do not necessarily represent the actual size ratios. Detailed description of the invention

[0022] In the Figures 1A to 1F Various intermediate stages of the manufacturing process according to the invention are shown in sectional views, with optional or preferred manufacturing steps / intermediate stages also being shown.

[0023] Preferably serves as a starting point, see Fig. 1A , a blank 2 made of a valve steel known to those skilled in the art. The blank has an at least partially cylindrical shape, preferably a circular cylindrical shape, corresponding to the circular shape of the valve body or valve to be produced.

[0024] The blank 2 is placed in a Fig. 1B The cup-shaped semi-finished product (or workpiece) 4 shown is formed. The semi-finished product 4 in the form of a cup comprises a base section 10, from which a valve head (or valve disk) 12 is later formed, and a tubular wall (or annular wall) 14, which surrounds a cylindrical, preferably circular-cylindrical, cavity 8 of the cup-shaped semi-finished product 4 and from which a valve stem 20 is later formed. During the subsequent forming steps, material may possibly flow between the base section 10 and the tubular wall 14. More generally, according to the invention, the cup-shaped semi-finished product 4 is provided directly; the method then starts with the provision of the Fig. 1B shown cup-shaped semi-finished product 4.

[0025] In a subsequent forming step, the valve head 12 is formed from the base section 10. A preform 6 of the valve body thus obtained is Fig. 1C shown.

[0026] Both the forming of the blank 2 into a cup-shaped workpiece 4 and the forming of the valve head 12 from the base section 10 can be carried out, for example, by a hot or cold forming process. Preference is given to extrusion or forging. During extrusion, a punch is pressed into the blank 2 or the semi-finished product 4 in order to form the cavity 8 or the valve head 12, i.e., it is essentially (cup) backward extrusion or transverse extrusion. The preform 6 can also be formed directly from the blank 2 in a single forming step, e.g., forging or extrusion.

[0027] In the next processing step, from Fig. 1C after Fig. 1D, an axial length of the tubular wall 14 is increased. 'Axial' here refers to the direction defined by the tubular wall 14 (i.e., the future shaft), i.e., to the (center) axis of the tubular wall; 'radial' is a direction orthogonal to the axial direction. A length of the tubular wall 6 is therefore measured in the axial direction.

[0028] For this purpose, according to the invention, flow-forming or cylindrical flow-forming is carried out over a flow-forming mandrel 22; cf. Fig. 2During flow forming, the preform rotates and at least one flow forming roller 24, 26, which rotates due to frictional engagement, is pressed against the outside of the tubular wall and moved in the axial direction, resulting in a plastic deformation. The associated incremental forming leads to advantageous work hardening of the machined steel. Overall, the wall thickness of the tubular wall decreases while at the same time the axial length of the tubular wall increases. The at least one flow forming roller is moved several times in the axial direction if necessary until the desired increase in length or reduction in wall thickness is achieved. The radial distance of the at least one flow forming roller from the axis of the tubular wall is successively reduced in successive passes.

[0029] Due to the use of a flow-forming mandrel, flow-forming essentially results in an elongation of the tubular wall 14, with its outer diameter decreasing slightly (corresponding to the reduction in wall thickness). If a greater reduction in the outer diameter is desired, flow-forming can also be performed with multiple flow-forming rollers without a flow-forming mandrel.

[0030] If the dimensions of the preform 6 and the parameters of the flow-forming are selected such that the length of the tubular wall 14 achieved by the flow-forming, the outer diameter achieved by the flow-forming and an inner diameter of the tubular wall 14 of the preform (which corresponds to a diameter of the flow-forming mandrel) correspond to the desired dimensions of the hollow valve to be produced, a valve body 16 for a hollow stem valve can be obtained in this way (cf. Fig. 1D, it being noted that the relative dimensions shown in the figures do not necessarily correspond to the actual relative dimensions, in particular Fig. 1D the diameter of the valve disc / head in relation to the stem diameter is shown smaller than in a typical actual valve, and the stem diameter in relation to the length of the stem 20 is also shown larger than usual).

[0031] Finally (from Fig. 1D above Fig. 1E after Fig. 1F ), optionally, the outer diameter of the tubular wall 14 is reduced in order to obtain a finished valve body 18 for a hollow disc valve, the valve stem 20 of which has a predetermined outer diameter, ie a desired target diameter; cf. Fig. 1FThis forming step is preferably carried out without a mandrel inserted so that the diameter can be effectively reduced. This step leads, in addition to a reduction in the outer diameter, to a further elongation of the tubular wall 14 and, if carried out without a mandrel, to an increase in the wall thickness of the tubular wall 14. The wall thickness would therefore have to be set somewhat smaller in the preceding flow-forming step if necessary in order to obtain a specific wall thickness, and thus a specific inner diameter for a given outer diameter D, taking into account the increase in thickness in the final step.

[0032] The outer diameter of the tubular wall 14 can be reduced by swaging or necking, with swaging being preferred. When swaging, it is important that no further forming step of the valve body 18 for a hollow disc valve takes place after swaging to reduce the outer diameter of the tubular wall 14, as this would impair the positive material properties obtained by swaging. Therefore, swaging is the final forming step in this case.

[0033] Rotary swaging is an incremental pressure forming process in which the workpiece to be machined is hammered in rapid succession from different sides in a radial direction. The resulting pressure causes the material to 'flow', so to speak, and the material structure is not distorted by tensile stresses. Rotary swaging is preferably carried out as a cold forming process, i.e. below the recrystallization temperature of the material being machined. The key advantage of using rotary swaging as the final forming step is that compressive stresses are induced by the radial force application during rotary swaging, preventing the occurrence of tensile stresses that increase the susceptibility to cracks; this is particularly true for the surface layers of the hollow shaft.The rotary swaging thus interacts advantageously with the preceding, also incremental forming process of flow-forming, so that optimal material properties, e.g. strength, are achieved.

[0034] Further advantages of rotary swaging as a final forming step – compared to drawing or necking – include the better achievable surface quality and the relatively greater reduction in the diameter of the stem per step. Due to the high achievable surface quality and the very tight tolerances that can be maintained with rotary swaging, post-processing of the valve stem is usually unnecessary. Free-forming or upsetting processes – such as necking – generally only achieve poorer surface quality and tolerance compliance. Accordingly, after rotary swaging to reduce the outer diameter of the tubular wall, no further process step involving a drawing process or necking should be carried out.

[0035] In order to complete the manufacturing process of the hollow valve, a cooling medium, e.g. sodium, can be filled into the cavity of the valve body via the outwardly open end of the valve stem and then this end of the valve stem can be closed, e.g. by a valve stem end piece, which is attached, for example, by means of friction welding or another welding process (not shown in the figures).

[0036] The reduction of the outer diameter can be carried out in several sub-steps (an intermediate step is, for example, in Fig. 1E shown), whereby the individual sub-steps can be carried out either with or without a mandrel (at the beginning of a sub-step, the diameter of a mandrel can be smaller than the diameter of the cavity); the diameter of the mandrels of successive sub-steps can also be reduced.

[0037] Fig. 2 represents the process step of flow forming, which takes place between Fig. 1C and Fig. 1Dtakes place, in a sectional view. Here, a flow-forming mandrel 22 is inserted into the cavity of the preform 6. The flow-forming mandrel rotates together with the preform 6 and a tailstock 28, which supports the preform on the valve base. Two opposing flow-forming rollers 24, 26, which also rotate by friction, are pressed against the tubular wall 14. The flow-forming rollers 24, 26 are moved relative to the preform in the axial direction, resulting in plastic deformation of the tubular wall 14, whereby the outer radius of the tubular wall 14 decreases and, at the same time, the length of the tubular wall 14 increases (in the axial direction). The material of the tubular wall 14 "flows" in the direction of movement of the flow-forming rollers 24, 26 (synchronized flow-forming rollers).The directions of rotation of the preform (together with the flow-forming mandrel and tailstock) and the flow rolls, the direction of movement of the flow rolls 24, 26 and the flow direction of the material of the tubular wall 14 are indicated by arrows in the figure.

[0038] In Fig. 2 Two spinning rollers 24, 26 are (partially) shown as examples; the use of more than two spinning rollers is also possible, with the use of two or three spinning rollers being preferred. The plurality of spinning rollers are preferably distributed evenly around the circumference; ie, with two spinning rollers, the angle (in the circumferential direction) between the spinning rollers is approximately 180°, with three spinning rollers, approximately 120°, etc. This ensures that the preform is supported in all directions.

[0039] There is a radial and an axial offset between the spinning rollers, as shown in Fig. 2shown. Radial offset means that the radial distance of the spinning rollers 24, 26 from the center axis is different. The axial offset is achieved by moving the spinning rollers 24, 26 at different times, whereby (obviously) the sequence is such that the spinning roller with the greatest radial distance from the center axis is moved first, followed by the one with the second greatest radial distance, and so on. In this way, the process can be accelerated, since several radius or wall thickness reduction steps can be performed in a single pass. Instead of a radial offset of spinning rollers of the same diameter, spinning rollers with different diameters can also be used. List of reference symbols

[0040] 2Blank 4Cup-shaped semi-finished product 6Preform 8Cavity 10Bottom section 12Valve head / valve plate 14Tubular wall 16Finished valve body for hollow stem valve 18Finished valve body for hollow disc valve 20Valve stem 22Spinning mandrel 24Spinning roll 26Spinning roll 28Tailstock

Claims

1. A method for producing a valve body (16, 18) of a hollow valve, comprising the following steps: providing a preform comprising a valve head (12) and a tubular wall (14), which surrounds a cylindrical cavity (8); flow forming the tubular wall (14) above a flow forming mandrel (22), which is inserted into the cavity (8), in order to increase a length of the tubular wall (14), wherein several flow forming rollers (24, 26) are used during flow forming, wherein the several flow forming rollers (24, 26) are radially and axially offset to one another during the flow forming, characterized in that the flow forming mandrel rotates together with the preform (6) and a tailstock (28), which supports the preform at the valve bottom.

2. The method according to claim 1, wherein the provision of the preform comprises: providing a bowl-shaped semi-finished product (4), wherein the semi-finished product has the tubular wall (14), which surrounds the cylindrical cavity (8) of the semi-finished product, and a bottom section (10); and molding the valve head (12) from the bottom section (10).

3. The method according to claim 2, wherein the provision of the bowl-shaped semi-finished product (4) comprises: providing an at least partially cylindrical blank (2); and molding the bowl-shaped semi-finished product (4) from the blank (2).

4. The method according to claim 3, wherein the molding of the bowl-shaped semi-finished product (4) takes place by means of extrusion or forging.

5. The method according to claim 2, wherein the molding of the valve head (12) takes place by means of extrusion or forging.

6. The method according to one of the preceding claims, wherein three flow forming rollers are used.

7. The method according to one of the preceding claims, further comprising: a further flow forming of the tubular wall (14) without flow forming mandrel.

8. The method according to one of the preceding claims, further comprising: reducing an outer diameter of the tubular wall (14) after the flow forming.

9. The method according to claim 8, wherein the reducing of the outer diameter of the tubular wall (14) takes place by means of rotary swaging or feeding.

10. The method according to one of claims 8 or 9, wherein the reducing of the outer diameter of the tubular wall (14) takes place without mandrel.

11. The method according to one of claims 8 or 9, wherein the reducing of the outer diameter of the tubular wall (14) takes place with a mandrel, which is inserted into the cavity.

12. The method according to one of the preceding claims, further comprising: filling a cooling medium, in particular sodium, into the cavity; and closing the cavity.