CN115243858A

CN115243858A - Method for manufacturing transmission gear for electromechanical power steering system

Info

Publication number: CN115243858A
Application number: CN202180019331.8A
Authority: CN
Inventors: 菲利普·哥特瓦尔德; 亚历山大·苏尔策
Original assignee: ThyssenKrupp AG; ThyssenKrupp Presta AG
Current assignee: ThyssenKrupp AG; ThyssenKrupp Presta AG
Priority date: 2020-03-06
Filing date: 2021-03-01
Publication date: 2022-10-25
Also published as: WO2021175750A1; DE102020202922A1; EP4114636A1

Abstract

The invention relates to a method for producing a transmission gear comprising a hollow shaft (5) which is connected to a ring gear (5) coaxially surrounding at a radial distance by means of a connecting body (6) which is injection-molded using plastic injection, wherein the ring gear (5) and the hollow shaft (4) are positioned in an ejector-side mold half (72) axially closed by a nozzle-side mold half (71) in a mold 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) by means of at least one injection nozzle (77, 79) in the nozzle-side mold half (71). In order to provide a less complex method and offer the possibility of wider application, according to the invention, the axial opening (43) of the end face of the hollow shaft (4) is closed by the nozzle-side mold half (71) and sealed off from the intermediate space (8) when the injection mold (7) is closed.

Description

Method for manufacturing transmission gear for electromechanical power steering system

Technical Field

The invention relates to a method for producing a transmission gear comprising a hollow shaft which is connected to a ring gear coaxially surrounding at radial distances by means of a connecting body injection-molded using plastic injection, in which method the ring gear and the hollow shaft are positioned in an ejector-side mold half axially closed by a nozzle-side mold half in a mold cavity of an injection mold and molten plastic is injected axially into an intermediate space between the ring gear and the hollow shaft by means of at least one injection nozzle in the nozzle-side mold half. A worm wheel for an electromechanical power steering system manufactured according to the method is also subject matter of the present invention.

Background

In an electromechanical power steering system, an assist torque is generated by an electric motor, which is coupled into a steering shaft through a transmission to assist a manual steering torque. The transmission is formed as a reduction transmission and comprises a first transmission element coupled to the motor shaft, for example a worm which is connected non-rotatably to the motor shaft, and a transmission gear in driving engagement therewith, for example a worm wheel, which is connected in a torque-locked manner to the steering shaft.

In a general embodiment, the transmission gear has a hub which is designed at least in sections as a hollow shaft which is mounted in the transmission in a rotatable manner about a transmission axis and can be connected to the steering shaft in a torque-locked manner. For example, a ring gear, which may have a worm gear toothing, coaxially surrounds the hollow shaft and is connected to the hollow shaft in a rotationally fixed manner by a connecting body arranged radially therebetween.

In order to optimize the operating characteristics, it is known to produce the hollow shaft and the toothed ring from different materials that are adapted to the respective stresses, for example, the hollow shaft from steel and the toothed ring from plastic or non-ferrous metal, which are connected to one another in a non-rotatable manner. For this purpose, it is known, for example, from EP 1 777 439A1 to inject molten thermoplastic between the hollow shaft and the ring gear in a plastic injection molding process, which plastic, after curing, forms a connecting body which is preferably material-and form-fittingly connected to the hollow shaft and the ring gear.

In EP 1 777 439A1 an injection molding method is proposed in which the hub and the ring gear are positioned in an ejector-side half mold closed by a nozzle-side half mold in the mold cavity of a two-piece injection mold and the connections are made in umbrella gates. The intermediate space is filled with the plastic melt by an annular injection nozzle in the nozzle-side mold half, which is coaxially surrounded with respect to the gear axis. The formation of the conical sleeve-shaped gate projection, which has to be removed from the completely solidified connecting body by post-treatment, for example by machining, is therefore complicated in terms of production technology. Furthermore, during the injection molding process, the penetration of the plastic melt into the end-face opening of the hollow shaft must be prevented. For this purpose, it is proposed to dip the core from the ejector-side mold half through the opening to fill and close it. However, this is only possible if the opening passes axially through the hub smoothly and with a constant cross section and without cross section variations or undercuts which render the ejector-side mold half unmouldable. This in principle limits the application to simple hub geometries.

Furthermore, 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. Furthermore, 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, which plastic must be subsequently removed.

Disclosure of Invention

In view of the above, it is an object of the present invention to provide a method for producing a transmission gear having an injection-molded connecting body, which is less complex and offers wider application possibilities.

According to the invention, this object is achieved by a method having the features of claim 1. Advantageous embodiments are given in the dependent claims.

In the method for producing a transmission gear comprising a hollow shaft which is connected to a ring gear coaxially surrounding at a radial distance by a connecting body injection-molded using plastic injection, in which method the ring gear and the hollow shaft are positioned in an ejector-side mold half axially closed by a nozzle-side mold half in a mold cavity of an injection mold and molten plastic is injected axially into an intermediate space between the ring gear and the hollow shaft by means of at least one injection nozzle in the nozzle-side mold half, it is proposed according to the invention that, when the injection mold is closed, an axial opening on an end face of the hollow shaft is closed by the nozzle-side mold half and sealed off from the intermediate space.

The hub is formed by a hollow shaft which extends axially in the direction of the transmission axis and has an end-face axial opening which can form a through-hole which runs axially through it, but can also be a blind hole which is closed in the hollow shaft.

The two-part injection mold has a nozzle-side half and an ejector-side half, which are assembled together in the axial direction on the basis of the gear axis until they abut against one another on the parting plane when the injection mold is closed in order to close the injection mold.

Before closing, a hollow shaft, for example made of steel, is inserted axially centrally into the ejector-side mold half and, coaxially thereto, into a toothed ring, for example made of plastic or non-ferrous metal. The injection mold is then closed by axially attaching the nozzle-side mold halves. In this closed state, the mold cavity formed with the ejector-side mold half is not only delimited to the outside and closed by the nozzle-side mold half, but, according to the invention, the nozzle-side mold half interacts with the hollow shaft such that it tightly closes the axial opening which is open on the nozzle side. The remaining intermediate space is thus defined axially between the mold surfaces of the nozzle-side and ejector-side mold halves and radially between the ring gear and the outer surface of the hollow shaft. Since the nozzle-side opening of the hollow shaft is tightly closed by the nozzle-side mold halves, the interior space of the opening is tightly closed with respect to the intermediate space.

In a subsequent step, molten plastic, i.e. a plastic melt, is injected into the intermediate space by means of a suitable injection nozzle device, axially, i.e. in the direction of the gear axis, through the nozzle-side mold half. After curing, the plastic forms a connection piece which can be connected to the hollow shaft and/or the ring gear in a form-fitting manner and additionally by means of form-fitting elements embedded in the plastic. By sealing the opening according to the invention, the plastic melt is prevented from penetrating into the interior of the hollow shaft. Thereby, gate residues in the opening area are largely avoided, thereby advantageously reducing the costs of the post-processing.

Another significant advantage compared to the mentioned prior art, in which the core passes through the opening from the ejector side, is that the opening can be reliably closed by the nozzle-side mold half, irrespective of the through-cross-section and possible shape changes, cross-section changes or undercuts. If the opening is formed by a blind hole, it can also be closed easily and reliably, which is not possible with the prior art. This allows a wider range of applications in terms of hollow shaft design.

For example, it can be provided that the opening is formed as a through-opening having a through-cross section axially through the hollow shaft, which is larger on the nozzle side than on the ejector side. For example, the weight can be optimized or the adaptation of the connection to the steering shaft can be optimized by using internally stepped bores.

The nozzle-side mold half and the hollow shaft preferably have corresponding axial sealing surfaces which lie tightly against one another when the injection mold is closed. As a result of the axially acting closing force of the injection mold acting on the axial sealing surface, a particularly reliable sealing of the opening with respect to the intermediate space can be achieved.

In addition or alternatively, the nozzle-side mold half and the hollow shaft can have corresponding radial sealing surfaces which bear against one another with little play when the injection mold is closed.

Another advantageous possibility is to form the corresponding sealing surfaces on the nozzle-side mold half and on the hollow shaft at least in sections in a conical shape. For example, the hollow shaft may have a sealing surface tapering toward the nozzle side, and the nozzle-side mold half may have a corresponding conical or cylindrical opening sealed against it. The conical sealing surface may be conical or may also be convex or rounded. The advantage of such a conical sealing surface is that it can be sealed with a closing force of the injection mould in the axial direction.

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

An advantageous embodiment can provide that a hollow shaft section of the hollow shaft which projects axially at the end face from the connecting body and/or the ring gear is received in a recess of the nozzle-side mold half. The hollow shaft may have an axially protruding section on one or both sides, which may be designed, for example, with a connection device for a torque-locking connection with the steering shaft, for example, with a bore and/or a journal having a circular or non-circular cross section for a force-fitting and/or form-fitting connection. Since the hollow shaft section, which projects axially on the nozzle side, dips into the recess of the nozzle-side mold half when the injection mold is closed, it is easy to adapt 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, for example, axial, conical and/or radial sealing surfaces can be provided as described above.

In carrying out the method, it may be advantageous to distribute a plurality of punctiform gates over the periphery. The punctiform gate has an injection nozzle which axially merges into the intermediate space. For each point gate, the plastic melt is injected into the intermediate space via an injection nozzle, whereby a uniform filling matched to the geometry can be achieved. Thereby, a uniform, low stress and high load bearing connection can be produced. Three, four, six, eight, twelve or more spot gates may be distributed-preferably evenly distributed-on the periphery. In terms of manufacturing technology, a point gate has the advantage that the gate cross section given by the nozzle cross section of the injection nozzle can be very small, almost point-like, so that the gate channel can be torn off or cut off with a small transverse force during the demolding process. Since the spot gate is positioned close to the nozzle-side axial surface of the connecting body, mechanical post-processing of the gate projection is not generally required, which means that the manufacturing effort can be reduced compared to a umbrella gate.

In order to separate one or more gate points by shearing or tearing, an advantageous further development of the method can provide that, after the plastic has cured, the ejector-side mold half and the nozzle-side mold half are separated from one another by an axial translational and rotational movement, preferably superimposed as a spiral movement. For demolding, the two mold halves are separated from one another against the axial closing direction, wherein the mold cavity is opened and the finished transmission gear can be ejected from the ejector-side mold half. Since the two mold halves move helically relative to one another during the separation process, shear forces are exerted in the circumferential direction on the gate projections formed in the region of the injection nozzle by the rotational component of the movement, as a result of which they can be cut off or torn off during demolding and post-processing can be omitted. It is also conceivable and possible that the mold halves can be separated from each other by a purely axial translational movement.

In some cases it is also possible to fill intermediate spaces in the umbrella gates, for example in order to optimize injection moulding of a particular geometry. Furthermore, it is conceivable and possible to fill the intermediate space in the ring gate. This ensures that the intermediate space can be produced with a uniform wall thickness.

The invention also comprises a worm gear for an electromechanical power steering system, comprising a hub formed as a hollow shaft, which is connected via a connecting body made of thermoplastic and injection-molded by plastic injection molding to a ring gear coaxially surrounding at radial intervals, which worm gear is produced as described above according to the method of the invention. The hub and/or the ring gear may be made of a metallic material or a plastic. It is also conceivable and possible to provide a worm gear for a steer-by-wire system, wherein the connection between the steering wheel and the steered wheels is established by means of an electrical signal.

Drawings

Advantageous embodiments of the invention are explained in more detail below with reference to the drawings. The figures show in detail:

figure 1 shows a schematic view of a steering system of a motor vehicle,

figure 2 shows a schematic view of a power assist drive of the steering system according to figure 1,

figure 3 shows a perspective view of the drive gear of the booster drive,

figure 4 shows a cross-sectional view of an injection mould according to the invention,

figure 5 shows a schematic view of the melt flow in the injection mould according to figure 4,

figure 6 shows a cross-sectional view of a second embodiment of an injection mould according to the invention,

fig. 7 shows a cross-sectional view of a third embodiment of an injection mould according to the invention.

Detailed Description

In the different figures, identical components always have the same reference numerals and are therefore usually named or referred to only once.

Fig. 1 shows a schematic view of a motor vehicle steering system 100. It comprises a steering shaft 1, into which steering shaft 1 a manual steering torque or steering torque can be introduced as a steering command by the driver via a steering wheel 102. The steering torque is transmitted via the steering shaft 1 to a steering pinion 104 which 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 on its side induces a steering angle of the steerable wheels 110 of the motor vehicle by means of the tie rod 108.

The electric power assistance can be provided in the form of a power assistance drive 112 coupled to the steering shaft 1 on the input side, a power assistance drive 114 coupled to the pinion 104 and/or a power assistance drive 116 coupled to the rack 106. Each respective

power assist drive

112, 114 or 116 couples an assistance torque into the steering shaft 1 and/or the steering pinion 104 and/or an assistance force into the rack 106, as a result of which the driver is supported during the steering operation. Three different booster drives 112, 114 and 116 are shown in fig. 1 to illustrate possible locations for their placement.

Typically only one of the positions shown is occupied by

booster 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 torque determined by the torque sensor 118, which is introduced manually by the driver. Alternatively or in combination with the introduction of the assistance torque, an additional steering angle can be introduced into the steering system 100 by the booster drives 112, 114, 116, which is added to the steering angle applied by the driver via the steering wheel 102.

The steering shaft 1 has an input shaft 10 connected to a steering wheel 102 on the input side and an output shaft 12 connected to a rack 106 via a steering pinion 104 on the output side. The input shaft 10 and the output shaft 12 are torsionally elastically coupled to one another by a torsion bar, which is not visible in fig. 1. Thus, when the output shaft 12 does not rotate in full synchronization with the input shaft 10, the torque applied to the input shaft 10 by the driver via the steering wheel 102 always results in relative rotation of the input shaft 10 with respect to the output shaft 12. Such relative rotation between the input shaft 10 and the output shaft 12 may be measured using a rotational angle sensor, and accordingly a corresponding input torque with respect to the output shaft 12 may 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.

Accordingly, only when the output shaft 12 rotates relative to the input shaft 10 against the rotational resistance of the torsion bar, the steering torque applied to the steering shaft 1 or the input shaft 10 by the driver via the steering wheel 102 causes one of the

assist drivers

112, 114, 116 to input assist torque.

Alternatively, the torque sensor 118 can also be arranged in a position 118', in which the openings of the steering shaft 1 in the input shaft 10 and the output shaft 12 and the torsionally elastic coupling by means of the torsion bar are correspondingly in different positions, in order to be able to determine the relative rotation from the relative rotation of the output shaft 12 connected by means of the torsion bar to the input shaft 10 and thus correspondingly the input torque and/or the auxiliary torque to be introduced.

The steering spindle 1 according to fig. 1 also comprises at least one universal joint 120, by means of which the course of the steering spindle 1 in a motor vehicle can be adapted to the spatial conditions.

Fig. 2 shows an example of an electromechanical auxiliary torque drive 2, which can be used as an

auxiliary drive

112 or 114 for coupling an auxiliary torque into the steering shaft 1.

The auxiliary torque drive 2 has a gear 21 with a worm 22 which is in driving engagement with the transmission gear 3 formed as a worm wheel.

The transmission gear 3 is connected to the steering shaft 1 in a rotationally fixed manner and is mounted together with it in a transmission 21 in a rotatable manner about a transmission axis L, which is identical here to the longitudinal axis of the steering shaft 1.

The worm 22 is coupled to a motor shaft 23 of a motor 24 and can be driven in rotation by the motor, wherein the transmission gear 3 rotates about the transmission axis L by means of a transmission mesh.

The drive gear 3 is shown in fig. 3 in a perspective view in isolation. It has a hub formed as a hollow shaft 4, which is surrounded coaxially to the transmission axis L by a ring gear 5. Hollow shaft 4 is firmly connected to toothed ring 5 by a connecting body 6 made of thermoplastic and injected into the radial intermediate space by plastic injection molding.

The hollow shaft 4 has a first hollow shaft section 41 projecting axially at the end face and a hollow shaft extension 42 extending axially from the other side. An insert 400 is arranged between the

hollow shaft sections

41, 42. The insert 400 is integrally formed with the hub 4. The opening 43, which is open at the end face of the hollow shaft section 41, extends as a through-hole through the entire hollow shaft 4, as shown in fig. 4 in a longitudinal section along the transmission axis L. It can be seen that the opening cross section of the opening 43 is stepped in length, the inner diameter D in the hollow shaft section 41 being greater than the inner diameter in the hollow shaft extension 42.

Fig. 4 shows a longitudinal section of the transmission gear 3 in the production process in a two-part injection mold 7, which has a nozzle-side mold half 71 arranged on the nozzle side, on the left in the drawing, and an ejector-side mold half 72 arranged on the ejector side, on the right in the drawing. In the closed state shown, the mold halves 71 and 72 abut against one another in the parting plane T.

Mold halves 71 and 72 enclose a mold cavity 73, in the example shown, mold cavity 73 is formed primarily in ejector-side mold half 72.

For the production according to the method of the invention, the separately provided ring gear 5 and hollow shaft 4 are positioned in the mold cavity 73 of the ejector-side mold half 72, in particular in the region of the insert 400, as shown in the figures. The nozzle-side mold half 71 is then assembled with the ejector-side mold half 72 in the parting plane T in the axial direction, i.e. in the direction of the transmission axis L, wherein the mold cavity 73 is closed with a closing force F, as is indicated by the arrow in fig. 4. In this case, an intermediate space 8, the cross section of which corresponds to the connecting body 6, remains radially between the hollow shaft 4 and the ring gear 5.

When the injection mold 7 is closed, the hollow shaft section 41, which now protrudes toward the nozzle side, dips into the recess 74 in the nozzle-side mold half 71. The axial sealing surface 44 arranged on the end face of the hollow shaft section 41 bears tightly against a corresponding axial sealing surface 75 on the nozzle-side mold half 71, which axial sealing surface 75 is arranged here on the bottom of the recess 74. Thereby, the intermediate space 8 is sealed from the opening 43. The sealing surfaces 44, 76 may also be provided in the region of the mold cavity 73 where the insert 400 and the nozzle-side mold half 71 and the ejector-side mold half 72 abut.

The nozzle-side mold half 71 has a plurality of injection nozzles 76 which open axially into the intermediate space 8 in a preferably uniformly distributed manner on the outer circumference to form a punctiform gate. The injection nozzle 76 can be supplied with plastic melt from a central feed channel 78 by means of distribution channels 77 arranged in a star-shaped arrangement or over the entire periphery.

During the actual injection molding process, liquid plastic melt is injected into the intermediate space 8 through the feed channel 78, the distribution channel 77 and the injection nozzle 76, as is schematically shown in fig. 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 toothed ring 5 in a material-fitting manner, and preferably also in a form-fitting manner.

The injection nozzle 76 forms a spot gate having a relatively small cross section on the nozzle-side axial outer periphery of the connecting body. For demolding, the nozzle-side mold half 71 is axially separated from the ejector-side mold half 72 along the transmission axis L against the closing force F, wherein it is simultaneously rotated about the transmission axis L, so that a helical movement is produced. Thereby, the dot-like gate projection in the region of the injection nozzle 76 at the axial end face of the connecting body is cut off. No further post-processing is required.

Fig. 6 and 7 show views similar to fig. 4, with ejector-side mold half 72 omitted.

In the embodiment of fig. 6, instead of the punctiform injection nozzles 76 of fig. 4, coaxial annular surrounding injection nozzles 79 are provided. The plastic melt is injected axially into the intermediate space 8 through the surrounding annular gap. This allows the filling behavior to be adjusted.

In the embodiment shown in fig. 7, a plurality of injection nozzles 76 distributed over the circumference are provided, similar to fig. 4, but do not open out directly into the intermediate space 8 as point gates, but into a front chamber 81 formed axially upstream thereof. The antechamber 81 can be formed as a coaxial annular space, which is connected to the intermediate space 8. In this case, the antechamber 81 is first filled with plastic and then the intermediate space 8. Finally, the front chamber is removed by cutting. The injection of the plastic melt can be adjusted and the filling can be optimized.

Description of the reference numerals

1. Steering shaft

10. Input shaft

100. Motor vehicle steering system

102. Steering wheel

104. Steering pinion

106. Rack bar

108. Pull rod

110. Wheel of vehicle

112. 114, 116 assist driver

118. Torque sensor

12. Output shaft

120. Joint

2. Auxiliary torque drive

21. Transmission device

22. Worm screw

23. Motor shaft

24. Electric motor

4. Hollow shaft

41. Hollow shaft section

42. Hollow shaft extension

43. Opening(s)

44. Sealing surface

5. Gear ring

6. Connector body

7. Injection mould

71. 72 half mould

73. Die cavity

74. Concave part

75. Sealing surface

76. Injection nozzle (dot gate)

77. 79 injection nozzle

8. Intermediate space

81. Front chamber

L Transmission Axis

T division surface

F closing force

D. d diameter

Claims

1. Method for producing a transmission gear comprising a hollow shaft (4) which is connected by a connecting body (6) which is injection-molded using plastic injection molding to a ring gear (5) which coaxially surrounds at a radial distance, in which method the ring gear (5) and the hollow shaft (4) are positioned in an ejector-side mold half (72) which is axially closed by a nozzle-side mold half (71) in a mold 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) by means of at least one injection nozzle (77, 79) in the nozzle-side mold half (71),

it is characterized in that the preparation method is characterized in that,

when the injection mold (7) is closed, the axial opening (43) at the end of the hollow shaft (4) is closed by the nozzle-side mold half (71) and sealed off from the intermediate space (8).

2. Method according to claim 1, characterized in that the nozzle-side mold half (71) and the hollow shaft (4) have corresponding axial, radial and/or conical sealing surfaces (44) which bear tightly against one another when the injection mold (7) is closed.

3. Method according to claim 2, characterized in that the nozzle-side mold half (71) bears tightly against the hollow shaft (4) at the end face when the injection mold (7) is closed.

4. Method according to any one of the preceding claims, characterized in that a hollow shaft section (41) of the hollow shaft (4) which projects axially on the end side from the connecting body (6) and/or the ring gear (5) is accommodated in a recess (74) of the nozzle-side mold half (71).

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

6. Method according to any of the preceding claims, characterized in that a plurality of spot gates (76) is distributed over the periphery.

7. Method according to any one of the preceding claims, characterized in that after the plastic has solidified, the ejector-side mold half (72) and the nozzle-side mold half (71) are separated from each other by an axial translational and rotational movement, preferably superimposed as a helical movement.

8. Method according to any of the preceding claims, characterized in that the intermediate space (8) is filled in an umbrella gate.

9. Worm gear (3) for an electromechanical power steering system, comprising a hub formed as a hollow shaft (4) which is connected by a connecting body (6) made of thermoplastic and injected by plastic injection molding with a ring gear (5) coaxially surrounding at a radial distance, characterized in that the worm gear is manufactured by a method according to any one of claims 1 to 8.

10. Worm gear (3) according to claim 9, characterized in that the hub (4) and/or the ring gear (5) are made of a metallic material.

11. Worm gear (3) according to claim 9, characterized in that the hub (4) and/or the ring gear (5) are made of plastic.