Manufacturing method for transmission gear used in an electro-mechanical power steering system

Abstract

The present invention relates to a method for manufacturing a transmission gear, which transmission gear includes a hollow shaft (5), and the hollow shaft is connected to a radially spaced coaxial ring gear (5) by a connecting body (6) injection-molded using plastic injection. In this method, the ring gear (5) and the hollow shaft (4) are positioned in an ejector-side half mold (72) axially closed by a nozzle-side half mold (71) in a cavity (73) of an injection mold (7), and molten plastic is axially injected into an intermediate space (8) between the ring gear (5) and the hollow shaft (4) through at least one injection nozzle (77, 79) in the nozzle-side half mold (71). In order to provide a less complex method with broader application possibilities, it is proposed according to the present invention that when the injection mold (7) is closed, an axial opening (43) at the end side of the hollow shaft (4) is closed by the nozzle-side half mold (71) and sealed relative to the intermediate space (8).

Description

Field of the Invention

[0001] The present invention relates to a method for manufacturing a transmission gear, which includes a hollow shaft. The hollow shaft is connected to a radially spaced coaxial ring gear by a connecting body injection-molded using plastic injection. In this method, the ring gear and the hollow shaft are positioned in an ejector-side half mold axially closed by a nozzle-side half mold in the cavity of the injection mold, and molten plastic is axially injected into the intermediate space between the ring gear and the hollow shaft through at least one injection nozzle in the nozzle-side half mold. A worm gear for an electromechanical power steering system manufactured according to this method is also the subject of the present invention. Background Art

[0002] In an electromechanical power steering system, the assist torque is generated by an electric motor. The assist torque is coupled to the steering shaft through a transmission to assist the manual steering torque. The transmission is formed as a reduction transmission and includes a first transmission element coupled to the motor shaft, such as a worm that is non-rotatably connected to the motor shaft, and a transmission gear, such as a worm gear, that is in transmission engagement with it and is torque-locked to the steering shaft.

[0003] In a general design, the transmission gear has a hub, which is at least sectionally designed as a hollow shaft. The hollow shaft is rotatably mounted about a transmission axis in the transmission and can be torque-locked to the steering shaft. A ring gear, which may have, for example, worm gear teeth, coaxially surrounds the hollow shaft and is non-rotatably connected to the hollow shaft by a connecting body radially arranged therebetween.

[0004] To optimize the operating characteristics, it is known that the hollow shaft and the ring gear are made of different materials adapted to the corresponding stresses. For example, the hollow shaft is made of steel, and the ring gear is made of plastic or non-ferrous metal, and they are non-rotatably connected to each other. For this purpose, for example, it is known from EP 1 777 439 A1 that in a plastic injection molding method, molten thermoplastic plastic is injected between the hollow shaft and the ring gear, and after curing, the plastic forms a connecting body that is preferably material-fit and form-fit connected to the hollow shaft and the ring gear.

[0005] In EP 1 777 439 A1, an injection molding method is proposed, in which a hub and a toothed ring are positioned in a cavity of a two-piece injection mold in an ejector-side half mold closed by a nozzle-side half mold, and a connecting body is produced in a petal gate. Here, an intermediate space is filled with a plastic melt through an annular injection nozzle coaxially surrounding the axis of the transmission device in the nozzle-side half mold. Thereby, a gate projection in the shape of a tapered sleeve is formed, which has to be removed from the fully solidified connecting body by post-treatment, for example by machining, which is technically complex in production. In addition, during the injection molding process, it is necessary to prevent the plastic melt from penetrating into the end-face opening of the hollow shaft. For this purpose, it is proposed to dip the core into the opening from the ejector-side half mold to fill and close it. However, this is only possible if the opening axially passes through the hub smoothly and with a constant cross-section and has no cross-sectional changes or undercuts, which cause the ejector-side half mold to be unable to be demolded. This basically limits the application to simple hub geometries.

[0006] In addition, in order to prevent the plastic from penetrating into the gap between the core and the opening, the core must have a low-tolerance fit, which is also expensive. In addition, the inserted core cannot prevent the hollow shaft from coming into at least partial contact with the plastic on the outside on the nozzle side, and the plastic must be removed subsequently. Summary of the Invention

[0007] In view of the above problems, the object of the present invention is to provide a method for manufacturing a transmission gear with an injection-molded connecting body, which is less complex and provides a wider range of application possibilities.

[0008] According to the present invention, this object is achieved by the method of the present invention.

[0009] In the method for manufacturing a transmission gear, the transmission gear includes a hollow shaft, which is connected to a toothed ring coaxially surrounding it at a radial distance by a connecting body injection-molded using plastic. In this method, the toothed ring and the hollow shaft are positioned in a cavity of an injection mold in an ejector-side half mold axially closed by a nozzle-side half mold, and molten plastic is axially injected into the intermediate space between the toothed ring and the hollow shaft through at least one injection nozzle in the nozzle-side half mold. In this method, according to the present invention, when the injection mold is closed, the axial opening on the end face of the hollow shaft is closed by the nozzle-side half mold and sealed relative to the intermediate space.

[0010] The hub consists of a hollow shaft that axially extends in the direction of the axis of the transmission device and has an axial opening on the end face, which can form an axially penetrating through-hole or can also be a blind hole closed within the hollow shaft.

[0011] The two-piece injection mold has a nozzle-side half mold and an ejector-side half mold, which are assembled axially together based on the axis of the drive unit until they abut against each other on the parting surface in the case of the injection mold being closed.

[0012] Before closing, a hollow shaft, for example made of steel, is inserted axially and centrally into the ejector-side half mold, and a toothed ring, for example made of plastic or non-ferrous metal, is inserted coaxially therewith. Then the injection mold is closed by axially attaching the nozzle-side half mold. In this closed state, not only is the mold cavity formed together with the ejector-side half mold defined and closed towards the outside, but according to the invention, the nozzle-side half mold interacts with the hollow shaft such that the axial opening that is open on the nozzle side is tightly sealed. Thus, the remaining intermediate space is axially defined between the mold surfaces of the nozzle-side and ejector-side half molds and radially defined between the toothed ring and the outer surface of the hollow shaft. Since the nozzle-side opening of the hollow shaft is tightly sealed by the nozzle-side half mold, the internal space of the opening is tightly sealed with respect to the intermediate space.

[0013] In the next step, molten plastic, i.e., plastic melt, is injected axially, i.e., in the direction of the axis of the drive unit, through a suitable injection nozzle device, through the nozzle-side half mold, into the intermediate space. After curing, the plastic forms a connecting piece that can be connected to the hollow shaft and / or the toothed ring in a material-locking manner and additionally in a form-fitting manner by form-fitting elements embedded in the plastic. By sealing the opening according to the invention, the penetration of the plastic melt into the interior of the hollow shaft is prevented. Thereby, gate residues in the opening area are largely avoided, and thus the costs of post-treatment are advantageously reduced.

[0014] Another significant advantage compared to the prior art in which the mentioned core passes through the opening from the ejector side is that regardless of the through cross-section and possible shape changes, cross-section changes, or undercuts, the opening can be reliably closed by the nozzle-side half mold. If the opening is formed by a blind hole, it can also be easily and reliably closed, which cannot be achieved by the prior art. Thereby, a greater scope of application is achieved in terms of the design of the hollow shaft.

[0015] For example, it can be provided that the opening is formed as a through hole having a through cross-section axially passing through the hollow shaft, and this through cross-section is larger on the nozzle side than on the ejector side. For example, the weight can be optimized or the adaptation to the steering shaft connection can be optimized by using an internally stepped hole.

[0016] The nozzle-side half mold and the hollow shaft preferably have corresponding axial sealing surfaces that abut tightly against each other when the injection mold is closed. Since the axially acting closing force of the injection mold acts on the axial sealing surface, a particularly reliable sealing of the opening with respect to the intermediate space can be achieved.

[0017] Additionally or alternatively, the nozzle-side half mold and the hollow shaft may have corresponding radial sealing surfaces that abut against each other with a very small gap when the injection mold is closed.

[0018] Another advantageous possibility is that the corresponding sealing surfaces on the nozzle-side half mold and the hollow shaft are at least sectionally formed as cones. For example, the hollow shaft may have a sealing surface that tapers towards the nozzle side, and the nozzle-side half mold may have a corresponding conical or cylindrical opening for sealing relative thereto. The conical sealing surface can be conical, or it can also be convex spherical or rounded. The advantage of such a conical sealing surface is that the closing force of the injection mold in the axial direction can be utilized for sealing.

[0019] For example, it can be set that when the injection mold is closed, the nozzle-side half mold closely abuts against the hollow shaft at the end face. For example, a coaxially surrounding, axial, radial, and / or conical sealing surface can be formed on the end face edge of the opening, which is in close contact with the corresponding sealing surface on the inner wall of the nozzle-side half mold.

[0020] An advantageous embodiment can set that the hollow shaft section of the hollow shaft that axially protrudes from the connecting body and / or the gear ring at the end side is received in the recess of the nozzle-side half mold. The hollow shaft can have axially protruding sections on one or both sides, which can be designed, for example, with connecting means for torque-locking connection with the steering shaft, such as holes and / or journals with circular or non-circular cross-sections for force-fit and / or form-fit connections. Since when the injection mold is closed, the axially protruding hollow shaft section on the nozzle side dips into the recess of the nozzle-side half mold, it is very easy to match different geometries and dimensions of the transmission gear. The sealing according to the invention can be carried out in the end region of the end face or also in the extension of the protruding hollow shaft section. For this purpose, axial, conical, and / or radial sealing surfaces can be provided as described above, for example.

[0021] When implementing this method, it can be advantageous to distribute a plurality of pin gates peripherally. The pin gate has an injection nozzle that axially converges into the intermediate space. For each pin gate, the plastic melt is injected into the intermediate space through the injection nozzle, thereby enabling a geometrically matched uniform filling. Thus, a uniform, low-stress, and high-load-bearing connecting body can be produced. Three, four, six, eight, twelve, or more pin gates can be distributed - preferably evenly - peripherally. Technologically, the advantage of the pin gate is that the gate cross-section given by the nozzle cross-section of the injection nozzle can be very small, almost point-like, so that during the demolding process, the gate channel can be torn off or cut off with a relatively small lateral force. Since the pin gate is positioned close to the nozzle-side axial surface of the connecting body, mechanical post-treatment of the gate projection is usually not required, which means that the manufacturing workload can be reduced compared to umbrella gates.

[0022] In order to separate one or more gate points by shearing or tearing, a favorable further development of the method can provide that, after the plastic has solidified, the ejector-side half mold and the nozzle-side half mold are separated from each other by an axial translational and a rotational movement, preferably superimposed as a helical movement. For demolding, the two half molds are separated from each other against the axial closing direction, where the mold cavity is opened and the finished transmission gear can be ejected from the ejector-side half mold. Since the two half molds move helically relative to each other during the separation process, a shearing force is exerted circumferentially on the gate bulge formed in the injection nozzle region by the rotational component of the movement, whereby it is cut off or torn off during demolding and post-processing can be omitted. It is also conceivable and possible that the half molds can be separated from each other by a pure axial translational movement.

[0023] In some cases, it is also possible to fill the intermediate space in the umbrella-shaped gate, for example in order to optimize the injection molding of special geometries. Furthermore, it is conceivable and possible that the intermediate space is filled in the annular gate. This ensures that the intermediate space can be made with a uniform wall thickness.

[0024] The invention also includes a worm gear for an electromechanical power steering system, which includes a hub formed as a hollow shaft, which is connected to a tooth ring coaxially surrounded at a radial distance by a connecting body made of a thermoplastic and injection-molded by plastic injection molding. The worm gear is produced according to the method of the invention as described above. The hub and / or the tooth ring can be made of a metallic material or plastic. It is also conceivable and possible that a worm gear for a steer-by-wire system is provided, where the connection between the steering wheel and the steering wheel is established by an electrical signal. Description of the Drawings

[0025] The advantageous embodiments of the invention are explained in more detail below with reference to the drawings. The drawings show in detail:

[0026] Figure 1 A schematic view of the steering system of a motor vehicle is shown,

[0027] Figure 2 It shows according to Figure 1 A schematic view of the power drive of the steering system,

[0028] Figure 3 A perspective view of the transmission gear of the power drive is shown,

[0029] Figure 4 A sectional view of the injection mold according to the invention is shown,

[0030] Figure 5 It shows according to Figure 4 A schematic view of the melt flow in the injection mold,

[0031] Figure 6Shows a cross-sectional view of a second embodiment of an injection mold according to the present invention,

[0032] Figure 7 Shows a cross-sectional view of a third embodiment of an injection mold according to the present invention. Detailed Description

[0033] In the different figures, identical parts always carry the same reference numerals and are thus generally only named or mentioned once.

[0034] Figure 1 Shows a schematic view of a motor vehicle steering system 100. It includes a steering shaft 1 into which a driver can introduce a manual steering torque or steering moment as a steering command via a steering wheel 102. The steering moment is transmitted via the steering shaft 1 to a steering pinion 104 that meshes with a rack 106 in a steering gear. The rotation of the steering pinion 104 is converted therein into a linear displacement of the rack 106, which then causes a steering angle of steerable vehicle wheels 110 on its side via tie rods 108.

[0035] The electric power assistance can be provided in the form of an assistance drive 112 coupled to the steering shaft 1 on the input side, an assistance drive 114 coupled to the pinion 104, and / or an assistance drive 116 coupled to the rack 106. The respective assistance drives 112, 114 or 116 couple an assistance torque into the steering shaft 1 and / or the steering pinion 104 and / or couple an assistance force into the rack 106, whereby the driver is supported in the steering operation. Figure 1 The three different assistance drives 112, 114 and 116 shown therein illustrate possible positions of their arrangement.

[0036] Normally only one of the shown positions is occupied by the assistance drive 112, 114 or 116. The assistance torque or assistance force to be applied by means of the respective assistance drive 112, 114 or 116 to support the driver is determined taking into account the steering moment manually introduced by the driver as determined by a torque sensor 118. Alternatively or in combination with the introduction of the assistance torque, an additional steering angle can be introduced into the steering system 100 by means of the assistance drives 112, 114, 116, which is added to the steering angle applied by the driver via the steering wheel 102.

[0037] The steering shaft 1 has an input shaft 10 connected to the steering wheel 102 on the input side and an output shaft 12 connected to the rack 106 via the steering pinion 104 on the output side. The input shaft 10 and the output shaft 12 are connected by Figure 1The torsion bars that are not visible in the figure are torsionally elastically coupled to each other. Thus, when the output shaft 12 does not rotate in exact synchronism with the input shaft 10, the torque applied by the driver to the input shaft 10 via the steering wheel 102 always causes a relative rotation of the input shaft 10 with respect to the output shaft 12. This relative rotation between the input shaft 10 and the output shaft 12 can be measured using a rotational angle sensor, and the corresponding input torque with respect to the output shaft 12 can accordingly be determined based on the known torsional stiffness of the torsion bar. In this way, the torque sensor 118 is formed by determining the relative rotation between the input shaft 10 and the output shaft 12. Such a torque sensor 118 is known in principle and can be implemented, for example, as an electromagnetic sensor device or by another measurement of the relative rotation.

[0038] Correspondingly, only when the output shaft 12 rotates relative to the input shaft 10 against the rotational resistance of the torsion bar will the steering torque applied by the driver to the steering shaft 1 or the input shaft 10 via the steering wheel 102 cause one of the assist drives 112, 114, 116 to input an assist torque.

[0039] Alternatively, the torque sensor 118 can also be arranged at position 118', where the openings of the steering shaft 1 in the input shaft 10 and the output shaft 12 and the torsional elastic coupling via the torsion bar are accordingly in different positions, so as to be able to determine the relative rotation from the relative rotation of the output shaft 12 connected to the input shaft 10 via the torsion bar and thus accordingly determine the input torque and / or the assist torque to be introduced.

[0040] According to Figure 1 the steering shaft 1 further includes at least one universal joint 120, through which the orientation of the steering shaft 1 in the motor vehicle can be adapted to the spatial conditions.

[0041] Figure 2 An example of an electromechanical assist torque drive 2 is shown, which can be used as the assist drive 112 or 114 to couple an assist torque into the steering shaft 1.

[0042] The assist torque drive 2 has a transmission 21, which has a worm 22 that is in driving engagement with a transmission gear 3 formed as a worm wheel.

[0043] The transmission gear 3 is non-rotatably connected to the steering shaft 1 and is rotatably supported in the transmission 21 together with it about a transmission axis L that is here the same as the longitudinal axis of the steering shaft 1.

[0044] The worm 22 is coupled to the motor shaft 23 of the electric motor 24 and can be driven to rotate by the motor, where the transmission gear 3 rotates about the transmission axis L through driving engagement.

[0045] The transmission gear 3 is in Figure 3It is shown separately in a perspective view. It has a hub formed as a hollow shaft 4, which is surrounded by a ring gear 5 coaxially with the axis L of the transmission. The hollow shaft 4 is firmly connected to the ring gear 5 by a connector 6 made of thermoplastic and injected into the radial intermediate space by plastic injection molding.

[0046] The hollow shaft 4 has a first hollow shaft section 41 protruding axially on the end face and a hollow shaft extension 42 extending axially from the other side. An insert 400 is provided between the hollow shaft sections 41, 42. The insert 400 is integrally formed with the hub 4. An opening 43 open on the end face of the hollow shaft section 41 extends through the entire hollow shaft 4 as a through hole, as Figure 4 shown in the longitudinal section along the axis L of the transmission in. It can be seen therefrom that the opening cross-section of the opening 43 is stepped in length, and the inner diameter D in the hollow shaft section 41 is larger than the inner diameter in the hollow shaft extension 42.

[0047] Figure 4 A longitudinal section of the transmission gear 3 in the two-piece injection mold 7 during the production process is shown. The injection mold has a nozzle-side half mold 71 arranged on the nozzle side - the left side in the figure, and an ejector-side half mold 72 arranged on the ejector side - the right side in the figure. In the closed state shown, the half molds 71 and 72 abut against each other in the parting plane T.

[0048] The half molds 71 and 72 enclose a cavity 73, which is mainly formed in the ejector-side half mold 72 in the example shown.

[0049] For manufacturing according to the method of the present invention, the separately provided ring gear 5 and hollow shaft 4 are positioned in the cavity 73 of the ejector-side half mold 72, especially in the region of the insert 400, as shown in the figure. Then the nozzle-side half mold 71 and the ejector-side half mold 72 are assembled axially in the parting plane T, i.e., in the direction of the axis L of the transmission, where the cavity 73 is closed with a closing force F, as in Figure 4 shown by the arrow in. Here, an intermediate space 8 is radially reserved between the hollow shaft 4 and the ring gear 5, and its cross-section corresponds to the connector 6.

[0050] When the injection mold 7 is closed, the hollow shaft section 41 protruding towards the nozzle side now sinks into a recess 74 in the nozzle-side half mold 71. An axial sealing surface 44 arranged on the end face of the hollow shaft section 41 closely abuts against a corresponding axial sealing surface 75 on the nozzle-side half mold 71, and the axial sealing surface 75 is arranged at the bottom of the recess 74 here. Thus, the intermediate space 8 is sealed relative to the opening 43. The sealing surfaces 44, 75 can also be provided in such a region of the cavity 73 where the insert 400 abuts against the nozzle-side half mold 71 and the ejector-side half mold 72.

[0051] The nozzle-side half mold 71 has a plurality of injection nozzles 76 which axially merge into the intermediate space 8 in a manner preferably evenly distributed on the outer periphery to form a point gate. The injection nozzles 76 can be supplied with plastic melt from a central feed channel 78 through distribution channels 77 arranged in a star shape or over the entire outer periphery.

[0052] During the actual injection molding process, the liquid plastic melt is injected into the intermediate space 8 through the feed channel 78, the distribution channels 77, and the injection nozzles 76, as schematically shown in Figure 5 After curing, the plastic forms a plastic body 6 which substantially fills the intermediate space 8 and is firmly connected to the hollow shaft 4 and the gear ring 5 in a material-locking manner, and preferably also in a form-fitting manner.

[0053] The injection nozzles 76 form point gates with a relatively small cross-section on the nozzle-side axial outer periphery of the connecting body. For demolding, the nozzle-side half mold 71 is axially separated from the ejector-side half mold 72 along the axis L of the drive device against the closing force F, while at the same time rotating it around the axis L of the drive device, thereby generating a helical movement. Thereby, the point gate protrusions in the region of the injection nozzles 76 at the axial end face of the connecting body are cut off. No further post-processing is required.

[0054] Figure 6 and Figure 7 shows a view similar to Figure 4 in which the ejector-side half mold 72 is omitted.

[0055] In Figure 6 the embodiment, instead of Figure 4 the point injection nozzles 76, coaxial annular surrounding injection nozzles 79 are provided. Here, the plastic melt is axially injected into the intermediate space 8 through the surrounding annular gap. This allows adjustment of the filling behavior.

[0056] In Figure 7 the shown embodiment, similar to Figure 4 a plurality of injection nozzles 76 distributed on the outer periphery are provided, but they do not directly merge into the intermediate space 8 as point gates, but into a prechamber 81 formed upstream in their axial direction. The prechamber 81 can be formed as a coaxial annular space which is connected to the intermediate space 8. In this case, the prechamber 81 is first filled with plastic, and then the intermediate space 8 is filled. Finally, the prechamber is removed by machining. Thereby, the injection of the plastic melt can be adjusted and the filling optimized.

[0057] Explanation of reference numerals

[0058] 1 Steering shaft

[0059] 10 Input shaft

[0060] 100 Motor vehicle steering system

[0061] 102 Steering wheel

[0062] 104 Steering pinion

[0063] 106 Rack

[0064] 108 Tie rod

[0065] 110 Wheel

[0066] 112, 114, 116 Boost drive

[0067] 118 Torque sensor

[0068] 12 Output shaft

[0069] 120 Connector

[0070] 2 Auxiliary torque drive

[0071] 21 Transmission

[0072] 22 Worm

[0073] 23 Motor shaft

[0074] 24 Motor

[0075] 4 Hollow shaft

[0076] 41 Hollow shaft section

[0077] 42 Hollow shaft extension

[0078] 43 Opening

[0079] 44 Sealing surface

[0080] 5 Ring gear

[0081] 6 Connecting body

[0082] 7 Injection mold

[0083] 71, 72 Half mold

[0084] 73 Mold cavity

[0085] 74 Recess

[0086] 75 Sealing surface

[0087] 76 Injection nozzle (pinpoint gate)

[0088] 77, 79 Injection nozzle

[0089] 8 Intermediate space

[0090] 81 Front chamber

[0091] L Axis of the transmission

[0092] T separating surface

[0093] F closing force

[0094] D, d diameter

Claims

1. A method for manufacturing a transmission gear, the transmission gear including a hollow shaft (4), the hollow shaft being connected to a radially spaced coaxial ring gear (5) by a connecting body (6) injection-molded using plastic injection. In the method, the ring gear (5) and the hollow shaft (4) are positioned in an ejector-side half mold (72) axially closed by a nozzle-side half mold (71) in a cavity (73) of an injection mold (7), and molten plastic is axially injected into an intermediate space (8) between the ring gear (5) and the hollow shaft (4) through at least one injection nozzle (77, 79) in the nozzle-side half mold (71). It is characterized in that when the injection mold (7) is closed, an axial opening (43) at the end side of the hollow shaft (4) is closed by the nozzle-side half mold (71) and sealed with respect to the intermediate space (8).

2. The method according to claim 1, wherein The nozzle-side half mold (71) and the hollow shaft (4) have corresponding axial, radial, and / or conical sealing surfaces (44), which are in close contact with each other when the injection mold (7) is closed.

3. The method according to claim 2, wherein When the injection mold (7) is closed, the nozzle-side half mold (71) is in close contact with the hollow shaft (4) at the end side.

4. The method according to any one of the preceding claims 1-3, characterized in that, A hollow shaft section (41) of the hollow shaft (4) that axially protrudes from the connecting body (6) and / or the ring gear (5) at the end side is received in a recess (74) of the nozzle-side half mold (71).

5. The method according to any one of the preceding claims 1-3, characterized in that, The opening (43) is formed as a through hole with a through cross-section that is larger on the nozzle side than on the ejector side.

6. The method according to any one of the preceding claims 1-3, characterized in that, A plurality of dot gates (76) are distributed on the outer circumference.

7. The method according to any one of the preceding claims 1-3, characterized in that, After the plastic solidifies, the ejector-side half mold (72) and the nozzle-side half mold (71) are separated from each other by an axial translation and a rotational movement.

8. The method according to claim 7, wherein After the plastic solidifies, the ejector-side half mold (72) and the nozzle-side half mold (71) are separated from each other by an axial translation and a rotational movement, superimposed as a helical movement.

9. The method according to any one of the preceding claims 1-3, characterized in that, The intermediate space (8) is filled with a mushroom gate.

Images