METHOD FOR MANUFACTURING A HOLLOW VALVE FOR INTERNAL COMBUSTION ENGINES

Description

Field of invention

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

[0002] Intake and exhaust valves in internal combustion engines are subject to high thermal and mechanical stress. Adequate cooling is therefore essential to ensure the long-term functionality of the valves. Hollow disc valves offer advantages over solid stem valves and hollow stem valves (i.e., a hollow valve where only the stem has a cavity) because a cavity is present in both the stem and the valve head, allowing for improved internal cooling using a cooling medium such as sodium. Further advantages include reduced weight, the avoidance of hot spots (in the internal combustion engine), and CO2 reduction.

[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. Welding points on the valve head or at other operationally critical locations should also be avoided. Another disadvantage of known methods is the often large number of process steps required, as described in EP 2325446 A1. However, rapid forming processes are advantageous for the cost-effective production of large quantities.

[0004] For example, EP 0898055 A1 and US 006006713 A describe a hollow disc valve manufactured by closing a hollow blank by welding (friction welding, laser welding) or armor plating. Other publications dealing with the manufacture of hollow valves are CN 104791040 A and JP 1995102917.

[0005] However, this manufacturing process is subject to significant wear problems due to the high-alloy valve steels. The production of the valve preform results in very high tool wear, particularly on the die. This translates to a short tool life and high tooling costs.

[0006] Known methods for manufacturing cavity valves with non-uniform stem internal geometry are shown in US 20090020082 by using inserts over the drilled disc surface and in DE 102010051871 A1 by production using an ECM process.

[0007] DE 102015118495 A1, which forms the basis for the preamble of claim 1, discloses a method for manufacturing a valve body of a hollow valve.

[0008] One object of the present invention is therefore to provide a manufacturing process for hollow valves or for a valve body for hollow valves which does not have the aforementioned disadvantages and at the same time has high productivity, good material utilization and fast forming processes.

[0009] Another object of the present invention is to adapt the manufacturing process in such a way as to reduce wear on the tool. Summary of the invention

[0010] According to the invention, the problem is solved by a method for manufacturing a valve body of a hollow valve according to the features of attached claim 1.

[0011] The process for manufacturing a valve body of a hollow valve includes the steps of providing a workpiece, i.e. blank or semi-finished product, and drilling the workpiece to produce a preform which has a cup with a hollow shape formed by the cup wall.

[0012] According to one aspect of the present invention, the drilling process can be carried out by means of a forming die which is pressed against the workpiece with an axial force and at least three pressure rollers which are pressed onto the workpiece with a radial force.

[0013] According to one aspect of the present invention, the preform can have a valve head / disc and a valve stem with a reduced stem diameter compared to the valve disc.

[0014] According to one aspect of the present invention, the at least three pressure rollers can be arranged opposite the forming die such that their radial force acts between the tip and a thickest diameter or in between.

[0015] According to one aspect of the present invention, the workpiece can be held in a workpiece holder on a spindle and rotate about its longitudinal axis, and the forming die can rotate synchronously with the spindle.

[0016] According to one aspect of the present invention, the at least three pressure rollers and the forming die can perform an axial movement synchronously.

[0017] According to one aspect of the present invention, the process can be carried out as cold, semi-warm or hot forming.

[0018] According to one aspect of the present invention, the cross-section of the hollow shape can be circular or have a driver profile such as uniform thickness, ellipse, polygon or axially directed multi-slot and tooth profiles.

[0019] According to the invention, the problem is further solved by a hollow valve comprising a valve body manufactured using the above method. Brief description of the drawing

[0020] Exemplary embodiments of the invention are described in more detail below with reference to the figures, wherein Figures 1a - 1f various intermediate steps in the manufacture of a valve body of a hollow valve (shown in Fig. 1d or Fig. 1f ) from a blank (shown in Fig. 1a ); and Figures 2a, 2b, 2b Show a process step of drilling in a 3D view.

[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, which is valid for all figures. The representations in the figures are purely schematic and do not necessarily depict the actual dimensions. Detailed description of the invention

[0022] In the Figures 1A to 1F The sectional views show various intermediate stages of the manufacturing process, including optional or preferred manufacturing steps / intermediate stages.

[0023] Preferably used 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 a shape that is at least partially cylindrical, preferably a circular cylindrical shape, corresponding to the circular shape of the valve body or valve to be manufactured.

[0024] Blank 2 is placed in a Fig. 1b The depicted cup-shaped semi-finished product (or workpiece) 4 is formed. The cup-shaped semi-finished product 4 comprises a bottom section 10, from which a valve head (or valve disc) 12 is later formed, and a tubular wall (or annular wall) 14, which surrounds a cylindrical, preferably circular, 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 flow between the bottom section 10 and the tubular wall 14.

[0025] Instead, the cup-shaped semi-finished product 4 can be provided directly; the process then starts with the provision of the Fig. 1b illustrated cup-shaped semi-finished product 4.

[0026] In a subsequent forming step, the valve head 12 is formed from the bottom section 10. A preform 6 of the valve body obtained in this way is in Fig. 1cdepicted.

[0027] 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. Extrusion or forging is preferably used. In extrusion, a punch is pressed into the blank 2 or the semi-finished product 4 to form the cavity 8 or the valve head 12, i.e., it is essentially a reverse (cup) extrusion or a transverse extrusion. The preform 6 can also be formed directly from the blank 2 in a single forming step, e.g., forging or extrusion.

[0028] In the next processing step, from Fig. 1c after Fig. 1dThe 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., the (central) axis of the tubular wall; 'radial' is correspondingly a direction orthogonal to the axial direction. Thus, the length of the tubular wall 6 is measured in the axial direction.

[0029] For this purpose, for example, pressure rolling or cylinder pressure rolling can be carried out over a pressure rolling mandrel 22. During pressure rolling, the preform rotates, and at least three pressure rollers 24, 26, rotating with it by friction, are pressed against the outside of the tubular wall and moved in the axial direction, resulting in a plastic deformation. The associated incremental deformation leads to advantageous work hardening of the processed steel. Overall, the wall thickness of the tubular wall decreases while the axial length of the tubular wall simultaneously increases. The at least three pressure rollers are moved axially several times, if necessary, until the desired increase in length or reduction in wall thickness is achieved. During successive passes, the radial distances of the at least three pressure rollers from the axis of the tubular wall are successively reduced.

[0030] The flow forming process, due to the flow forming mandrel 22 used, essentially leads to an elongation of the tubular wall 14, whereby its outer diameter decreases slightly (corresponding to the decrease in wall thickness). If a greater decrease in the outer diameter is desired, flow forming can also be carried out with several flow forming rollers without a flow forming mandrel.

[0031] If the dimensions of the preform 6 and the parameters of the flow rolling are selected such that the length of the tubular wall 14 achieved by the flow rolling, the outer diameter achieved by the flow rolling and an inner diameter of the tubular wall 14 of the preform (which corresponds to a diameter of the flow rolling 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 (vg1. Fig. 1dIt should be noted that the relative dimensions shown in the figures do not necessarily correspond to the actual relative dimensions; in particular, in Fig. 1d The diameter of the valve disc / head is smaller in relation to the stem diameter than shown for a typical actual valve, and the stem diameter is also larger in relation to the length of the stem (20) than is typically shown.

[0032] In conclusion (by Fig. 1d above Fig. 1e after Fig. 1f Optionally, the outer diameter of the tubular wall 14 is reduced to obtain a finished valve body 18 for a hollow disc valve whose valve stem 20 has a predetermined outer diameter, i.e., a desired target diameter; cf. Fig. 1fThis forming step is preferably carried out without a mandrel so that the diameter can be effectively reduced. Besides reducing the outer diameter, this step also leads to further elongation of the tubular wall 14 and, if performed without a mandrel, to an increase in the wall thickness of the tubular wall 14. The wall thickness would therefore need to be adjusted slightly smaller in the preceding flow forming step to achieve a specific wall thickness, and thus a specific inner diameter for a given outer diameter D, while taking the thickness increase into account in the final step.

[0033] To complete the manufacturing process of the hollow valve, a cooling medium, e.g. sodium, can be poured 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 friction welding or another welding process (not shown in the figures).

[0034] Reducing the outer diameter can be done in several steps (an intermediate step is, for example, in Fig. 1e (as shown), whereby the individual sub-steps can optionally be carried out 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); also, the diameter of the mandrels can be reduced in successive sub-steps.

[0035] Fig. 2a, 2b and 2crepresent the process step of drilling pressure, which takes place between Fig. 1a or Fig. 1b and Fig. 1c takes place, presented in a 3D view.

[0036] In an optional first step, a cavity is created in the blank 2 at the location where the subsequent cavity 8 will be formed. This serves to position and center the die 22 against the blank 2 or to simplify the following production step. This results in a workpiece as a semi-finished product 4 with a cavity, as shown in Fig. 1b The workpiece for drilling can therefore be an unmachined blank 2 or a semi-finished product 4.

[0037] Workpiece 2, 4 is processed as in Fig. 2a shown inserted into a workpiece holder 32 and clamped in a spindle of a lathe or automatic lathe.

[0038] The actual drilling step begins with the placement of the forming die 22 (and the drilling rollers 24, 25, 26) centrally on the end face 3 of the workpiece 2, 4, as shown in Figs. 2a and 2b (Shown enlarged). A preform 6 can be manufactured directly from the bar stock 2 in a single process step. This preform can then have a valve disc 12 and a valve stem 20 with a stem diameter reduced compared to the valve disc. The stem diameter of this preform 6 can be larger than that of the finished valve.

[0039] To produce the preform 6, the spindle rotates the workpiece 2, 4 about its longitudinal axis. The forming die 22, which can also be called a die or die-rolling mandrel, can rotate with the workpiece 2, 4, e.g., by friction or by a drive. Alternatively, the forming die 22 can also remain stationary but move only axially. In the latter case, significant heat generation would be expected. When the three equidistant die rollers 24, 25, 26 (co-rotating die rollers) are pressed against the side wall 14 of the workpiece 2, 4 by an applied radial force 23, they rotate 27 about their axes (not about the axis of rotation of the spindle 33) due to friction. The direction of rotation 33 of the workpiece 2, 4, together with workpiece holder 32 and (optional) forming die 22 and the direction of rotation 27 of the pressure rollers 24, 25, 26 are indicated in the figure by curved arrows.

[0040] The assembly consisting of the forming punch 22 and the pressure rollers 24, 25, 26 is moved uniformly and synchronously in the axial direction towards the spindle. Alternatively, the workpiece 2, 4 can be moved against the tool assembly. This results in plastic deformation of the workpiece 2, 4. The pressure rollers and the axially acting forming punch operate simultaneously. The forming punch 22 penetrates the workpiece centrally and forms a tubular wall 14 of a cup, with an inner diameter corresponding to the outer diameter of the forming punch 22. The outer diameter of the tubular wall 14 is limited by the pressure rollers 24, 25, 26. These rollers also simultaneously perform a stretching / rolling step. The excess, displaced material of the workpiece 2, 4 flows away, so that the length of the tubular wall 14 increases in the axial direction (backward extrusion of a cup – cup extrusion).The translational direction of movement 21 of the forming die 22 and the pressure rollers 24, 25, 26 is indicated by arrows in the figure. The flow direction of the material in the tubular wall 14 is opposite.

[0041] Advantageously, the presented bore forming process combines the benefits of cup extrusion and roll forming in a single step. In other words, the steps of extrusion and drawing of the preform 6 are eliminated and instead combined into one step of the bore forming process.

[0042] In the Figures 2Three pressure rollers 24, 25, 26 are shown as examples. If several pressure rollers are used, they are preferably distributed regularly around the circumference; i.e., with two pressure rollers, the angle (in the circumferential direction) between the pressure rollers is approximately 180°, with three pressure rollers approximately 120°, etc. This ensures that the preform 6 is supported in all directions and that lateral forces on the workpiece 2, 4 are avoided.

[0043] In an arrangement according to the invention (not shown in the figures), there is a radial and an axial offset between the pressure rollers 24, 25, 26. Radial offset means that the radial distance of the pressure rollers 24, 25, 26 from the central axis differs. The axial offset of the pressure rollers 24, 25, 26 causes the pressure roller 24 closer to the workpiece to first contact and machine the workpiece 2, 4, while pressure rollers 25, 26 located further away machine the workpiece later, i.e., the areas already machined by the previous pressure roller 24. In this way, the thickness of the tubular wall 14 can be stretched in stages. Therefore, the pressure roller 24 closest to the workpiece must have the greatest radial distance from the central axis for the first stretching step, followed by the one with the second greatest radial distance, and so on. In this way, the process can be accelerated, since several radius orWall thickness reduction steps can be carried out in one pass.

[0044] Multiple sets (not shown) of pressure rollers can also be arranged. The pressure rollers of each set 24, 25, 26 are arranged without offset. The sets are spaced axially apart, and each set causes a partial stretching of the workpiece 2, 4. This reduces / avoids transverse and torsional forces on the workpiece compared to pressure rolling with radial / axial offset, while still achieving the advantage of stepwise stretching and lower flow forces in the workpiece material.

[0045] The drilling process can result in a semi-finished product 4 with a cup that forms a cavity 8 (see Fig. 1bHowever, the valve head 12 can also be produced during this machining step. For this purpose, the distance of the axes of rotation 27 of the pressure rollers 24, 25, 26 from the axis of rotation 33 of the workpiece 2, 4 must be adjustable so that, during the axial displacement of the tool arrangement, the resulting thickness of the tubular wall 14 is variable and a contour with valve head 12 (as in Fig. 1c (shown) can be produced. A special consideration here is that the valve base 10 is produced either by parting off the workpiece with another tool (chisel) or, if necessary, by clamping the workpiece snugly so that the valve base 10 forms from the base of the workpiece. Furthermore, it should be noted that the forming die 22 should not move synchronously with the pressure rollers 24, 25, 26 during the final section, in order to avoid creating a continuous cavity 8 instead of a blind hole.

[0046] Advantageously, bore pressing allows for high productivity, good material utilization, short manufacturing times, and a continuous forming process. Material savings of up to 90% compared to deep drilling are achievable. At the same time, an undesirable weld seam on the surface of the valve disc 12 is avoided.

[0047] Partial bulk forming processes, such as bore pressing, are characterized by the fact that the material is not plasticized throughout the entire forming volume, but rather in temporally and spatially limited increments. Compared to cup reverse extrusion, this allows for a reduction in the punch force, while even achieving a length-to-diameter ratio approximately four times greater.

[0048] Due to the high hydrostatic pressure component, the process is particularly suitable for high-strength materials. Tools with low shape storage capacity are used in the drilling process. Reference symbol list

[0049] 2 Blank, bar stock, slug 3 End face of the blank 4 (Cup-shaped) semi-finished product 6 Preform 8 Cavity 10 Bottom section 12 Valve head / valve disc 14 Tubular wall 16 Finished valve body for hollow stem valve 18 Finished valve body for hollow disc valve 20 Valve stem 21 Axial force 22 Die rolling mandrel, forming die, die 23 Radial force 24 Die rolling roller 25 Die rolling roller 26 Die rolling roller 27 Direction of rotation of the die rolling rollers 32 Workpiece holder 33 Direction of rotation for machining 40 Powder 41 Protective layer 42 Cooling layer

Claims

1. A method for producing a valve body (16, 18) of a hollow valve, comprising the following steps: providing a workpiece blank (2, 4); spin extrusion of the workpiece (2, 4) to create a preform (6), which has a cup comprising a hollow mold (8) formed by the cup wall (14); wherein the shaft diameter is further reduced in a further process step by means of flow forming of the preform (6) after the spin extrusion, characterized in that there is a radial and an axial offset between at least three flow forming rollers (24, 25, 26) during the flow forming, wherein the radial distance of the flow forming rollers (24, 25, 26) from the central axis is different, wherein a flow forming roller (24) having a largest radial distance from the central axis first machines the workpiece, followed by the one having a second-largest radial distance from the central axis.

2. The method according to claim 1, characterized in that the spin extrusion is carried out by means of a molding die (22), which is pushed against the workpiece (2, 4) with an axial force (21), and the at least three flow forming rollers (24, 25, 26), which are pushed onto the workpiece (2, 4) with a radial force (23).

3. The method according to claim 1 or 2, characterized in that the preform (6) has a valve head / plate (12) and a valve shaft (20) comprising a shaft diameter, which is reduced compared to the valve plate.

4. The method according to one of claims 2 to 3, characterized in that the workpiece (2, 4) is received in a workpiece holder (32) at a spindle and rotates around its longitudinal axis, and the molding die (22) rotates synchronously with the spindle.

5. The method according to one of claims 2 to 4, characterized in that the at least three flow forming rollers (24, 25, 26) and the molding die (22) synchronously carry out an axial movement (21).

6. The method according to one of the preceding claims, characterized in that the method is carried out as cold, warm, or hot forming.

7. The method according to one of the preceding claims, characterized in that the cross section of the hollow mold (8) is circular or has a follower profile, such as ellipse, polygon, or axially oriented multiple spline and toothing profiles.

8. The method according to one of the preceding claims, characterized in that the molding of the valve head (12) takes place by means of extrusion or forging of the valve head (12) after the spin extrusion in a further process step.