DE102008036226B4 - Method for producing a hollow shaft - Google Patents

Abstract

Method for producing a hollow shaft (1) from semi-finished products (9) comprising the following steps: - producing a preform by forging - performing a heat treatment of the preform - forming the preform into an at least partially hollow intermediate form (5) by drilling pressure; and - rotary swaging the intermediate mold (5) to a hollow shaft (1) with a in the course of the hollow shaft (1) substantially the same wall thickness.

Description

The invention relates to a method for producing a hollow shaft of semi-finished products.

In the manufacture of automotive transmissions, it is desirable to keep the overall weight of the transmission as low as possible as the performance of the transmission increases. One way to save weight can be achieved for example by designing a gear shaft as a hollow shaft. The use of one or more shafts in the transmission as hollow shafts increases the power-to-weight ratio of the transmission, i. at the same transmittable torque decreases the overall weight of the transmission. This also reduces the total weight of a motor vehicle, in which the transmission is installed, whereby the fuel consumption is reduced.

The technical need to form one or more shafts in a transmission as a hollow shaft, may also have constructive reasons. An increasing number of shafts in a motor vehicle transmission leads to an increase in the transmission weight, so that measures must be taken to reduce the weight of the individual transmission shafts in the context of resource conservation, such as material and fuel economy and environmental compatibility (emission reduction). In a shaft-in-shaft transmission, for example in a dual-clutch transmission, it is even necessary to make the waves partially or completely hollow.

According to an already known method sequence, the hollow shaft is produced by combining two method steps. In a first step, a solid starting material is formed by a forming process, such as drop forging, to a basic body. In a subsequent machining operation, a bore in the longitudinal direction of the base body is then machined to produce the hollow shaft. By this machining step, a significant portion of the material used is not used. This process, known as deep hole drilling, requires special machinery and tools (deep hole drills) and thus brings with it increased manufacturing costs. Further, with the methods described above it is not possible to have undercuts in the bore, i. along the inner surface of the hollow shaft produced. A disadvantage of this method is from today's point of view that the requirement of resource conservation due to the machining is not met.

From the EP 1745 870 a method sequence described below is known. First, a cylindrical starting material is converted in a first step by cross wedge rolling into a base body. The stock material is positioned between the two profiled rolls or jaws of a cross wedge rolling apparatus. The existing on these rolls or jaws profiles are transferred during the forming of the starting material. After forming by cross wedge rolling, the base body has a desired outer surface profile. That is, by the cross-sectional change by means of cross wedge rolling, the base body has a contoured outer peripheral surface with predetermined continuous diameter transitions, at the transition points radii are formed. This avoids notches, which can adversely affect the strength of the shaft. The introduction of an axial bore in the body takes place during the described cross wedge rolling. In addition to the radial and tangential deformation caused by the cross wedge rolling, an axial force in the longitudinal direction of the starting material is exerted on the workpiece via a mandrel. As a result, a bore with a constant diameter can be generated. The mandrel can be performed either axially movable during cross wedge rolling or be arranged stationary.

Object of the present invention is to develop the known methods and to provide a sequence of process steps, which makes it possible to optimally produce a hollow shaft in a large-scale production in terms of production time, material, and resource use as well as investment.

This object is achieved by a method having the features of claim 1.

• - Herstellen einer Vorform durch Schmieden;
• - Durchführen einer Wärmebehandlung der Vorform;
• - Umformen der Vorform in eine zumindest teilweise hohle Zwischenform durch Bohrungsdrücken; und
• - Rundkneten der Zwischenform zu einer Hohlwelle mit einer im Verlauf der Hohlwelle im Wesentlichen gleichen Wandstärke.

The method sequence according to the invention for producing a hollow shaft, for example for motor vehicle transmissions from semi-finished products, comprises the following steps:

• - making a preform by forging;
• - performing a heat treatment of the preform;
• - Forming the preform in an at least partially hollow intermediate form by drilling pressures; and
• - Rotary kneading of the intermediate form to a hollow shaft with a substantially uniform in the course of the hollow shaft wall thickness.

In a first step, the production of a preform of a shaft by a forging process takes place. For this purpose, a required for the production of the shaft section or blank is separated from a semi-finished and by a forging process in one for the further process steps required preform brought. This preform is then subjected to a heat treatment suitable for the selected material to prepare it for the further manufacturing process. In an advantageous manner, following the heat treatment, the forming of the preform into an at least partially hollow intermediate form takes place by means of bore pressures as the basis for the further production sequence. The Bohrungsdrücken is among others from the DE 103 08 849 A1 known. Bore Pressing is a partial, incremental pressure forming process for producing an axially symmetric hollow shaft made from solid semi-finished products. The preform is added to a spindle in a clamping device and rotated about its longitudinal axis. The material of the preform is displaced by the force applied by the pressing rollers used and a simultaneously acting thrust stamp force and flows counter to the feed direction of the spinning punch. The punch rotates synchronously with the spindle and the preform and performs synchronous axial movement common to the spinning rollers. This creates from the massive preform the at least partially hollow intermediate form.

A significant advantage of the Bohrungsdrückens over the known method of deep hole drilling is the far higher degree of material utilization by the pure transformation of material. The method can therefore be used particularly advantageously for producing a hollow shaft made of cost-intensive materials, since the material loss occurring in a forming process is very low. Another advantageous feature of this method is that the time required to produce a hollow shaft is well below the cost of deep hole drilling. This advantage is particularly important in the context of a production with high quantities of great importance.

After the production of the hollow intermediate form by drilling pressure comes as a further process step for the design of the outer wave contour, a rotary swaging used. The swaging is among others from the DE 10 2005 036 681 A1 known.

Also in rotary swaging is an incremental forming process in which the forming is performed in many small steps. Compared to known continuous processes, the advantage is that a more homogeneous material transformation takes place. This in turn allows high degrees of deformation, since the material's ability to change its shape is utilized equally throughout the entire component cross-section. The rotary swaging allows a uninterrupted flow of material fibers in the workpiece, which leads to better mechanical properties compared to machined shafts. Furthermore, a work hardening introduced by the forming, for example, increases the strength of the component. The method of rotary kneading is further characterized by a high accuracy. This makes it possible to maintain a narrow tolerance range for the dimensions, so that machining of the hollow shaft is usually superfluous. Advantageously, thereby material can be saved and the process chain can be shortened, which has an advantageous effect on the unit cost of the individual component in a large-scale production.

When selecting a semifinished product for producing a hollow shaft in addition to the availability of the corresponding material in the desired starting shape and economic issues play a crucial role. An advantage of the method according to the invention is preferably the use of a cylindrical rod material as a starting semi-finished product for the hollow shaft. This semi-finished product has the advantage over other semi-finished products that it is, for example, cheaper to obtain than a tubular semi-finished product.

In order to prepare the material for the forging process, the semi-finished product is heated to a temperature of preferably 1000 ° C to 1200 ° C. After this heating a material blank is separated from the bar-shaped semifinished product by a shearing process, which is then available for the further process sequence in the required size. Alternatively, it is possible to shear the material blank from a non-heated semi-finished product to the required size and then to heat it for the following forging process.

If the blank is present in the required size, the forging process is carried out in the process sequence according to the invention. This takes place in an advantageous manner as a hot forging process, in which the material has a temperature of about 1000 ° C to 1200 ° C. Particularly advantageous is a drop forging.

Drop forging makes it possible to impress the preform by the contour of a die, the essential geometry parameters of the later hollow shaft. In this process step, the preform already formed the necessary accumulations of material for later processing, to then finish this example, to coils or bearing seats finished

The entire forging process advantageously has the main working steps of upsetting, preforming and finish forming, with which the blank is transferred into the geometry of the preform. If necessary, the forging process can be followed by a burring to the Outer contour of any existing supernatants, such as a Schmiedegrates to liberate.

During the forging process, it may be necessary to apply a cooling lubricant to improve the flow properties of the material used in order to adapt the material properties to the requirements of the forging process.

The forging is carried out, for example, on a horizontal forging press, which can preferably be operated with a stroke rate of up to 80 strokes per minute. This embodiment of the press makes it possible to carry out the forging process under optimum boundary conditions with regard to precision of the product as well as application of components per unit of time.

After the forging process, the preform can be cooled controlled from the forging heat for further processing to adjust certain microstructure and thus certain workpiece properties of the preform depending on the properties of the starting material. As a variant for the controlled cooling and cooling in the air is conceivable. However, this variant has the disadvantage, with regard to the material structure forming during the cooling process, that microstructures possibly arise here which are only of limited suitability for further processing in a subsequent shaping process. The process of forging and subsequent cooling may be followed by a blasting process to free the preform of existing scale, if necessary.

Before the further forming steps, a heat treatment process in the form of an annealing process is performed to improve the material property. In this case, different forms of Normalglühens can be used. Preferably, a BF annealing is used here. This annealing process is annealing to achieve a certain tensile strength. It has a heating of the preform to about 850 - 950 ° C with a subsequent cooling and subsequent tempering at about 500 - 550 ° C. An advantage of this type of heat treatment is a short process time, which makes this process particularly economical. As an alternative annealing method, a BG annealing can also be used. The BG annealing is carried out at about 900-1000 ° C with subsequent controlled cooling to produce a ferrite-pearlite microstructure. Finally, the use of GKZ annealing with the goal of being able to produce a structure of spherical carbides is also possible, since this method improves the cold workability of the material.

After the heat treatment process has been completed, according to the invention, the preform is converted into an at least partially hollow intermediate form by means of bore pressures. Preferably, the bore pressing is performed as a hot forming process with a heated preform. The temperature of the preform is preferably in a range of about 400 ° C to 800 ° C. It is also an application of the Bohrungsdrückens as Kaltumformverfahren conceivable. The hole pressing itself can be performed both from one side and from both sides of the workpiece or preform. For the decision which process variant should be used, parameters such as component size, component shape u.a. Consideration.

To optimize the Bohrdrückprozesses it is advantageous to apply a lubricant to the preform and / or the tool. The lubricant makes it possible to improve the surface quality of the resulting hole in the intermediate mold and leads to an increase in tool life, which improves the process in terms of its efficiency. The order of this lubricant can on the one hand be such that the lubricant is sprayed in liquid form during the Bohrdrückprozesses on the workpiece. On the other hand, an application of the lubricant to the workpiece before the Bohrungsdrückprozess in the form of diving or the like is conceivable. However, it should be noted that the lubricant should dry after application to ensure that it is available at the desired locations during forming.

Before the further forming steps, to improve the material property, it may be advantageous to carry out again a heat treatment process in the form of an annealing process. Again, the above-mentioned annealing process to choose from. The decision as to which and which of the methods is used depends on the material properties and the degree of deformation of the intermediate form and can be redefined for each process.

The finished forming the gear shaft from the intermediate form is then done by rotary swaging. This process step can take place directly after the bore pressure or after the aforementioned additional heat treatment process.

In the process step of rotary kneading creates a hollow shaft with optimum wall thickness, small differences in wall thickness and an optimal fiber resistance for component strength. Preferably, the rotary kneading is performed to create the hollow shaft as a hot forming process with a heated intermediate form. The Temperature of the intermediate form is preferably in a range of about 400 ° C to 800 ° C. Alternatively, the use of rotary kneading is provided as a cold forming process.

If the rotary kneading with heated intermediate form, the finished component is controlled cooled before further processing to adjust depending on the properties of the starting material certain microstructure and thus certain desired properties of the workpiece. As a variant of the method for controlled cooling, cooling in air is conceivable. This variant can, as explained above, have disadvantages with regard to the material structure forming during the cooling process. The resulting structure may only be suitable for further processing in the following manufacturing process. Optionally, the structure also has a disturbing effect on the properties of the finished component.

In the above description, the method steps according to the invention for producing a one-piece gear hollow shaft are described. If, for reasons of geometric requirements, it is necessary to design a shaft in two or more parts, the method sequence according to the invention can be supplemented by a method step joining, for example welding. In this method step, two or more partial wave sections are connected to form a total wave. In this case, the design of the preforms can be such that they are provided in the region of the joint with a depression, for example a bore. These holes form after the joining process an internal cavity in the intermediate form.

Here is preferably used as a joining method, a friction welding. The use of other joining methods, for example pressing, soldering or other welding methods is also conceivable.

• 1: eine schematische Schnittdarstellung einer mit dem erfindungsgemäßen Verfahren hergestellten Hohlwelle zeigt;
• 2: eine schematische Schnittdarstellung einer Zwischenform nach dem Bohrungsdrücken zeigt; und
• 3: eine symbolische Darstellung des Verfahrensablaufes zeigt.

The method according to the invention will be explained in more detail below by way of example with reference to the accompanying drawings, of which:

• 1 : shows a schematic sectional view of a hollow shaft produced by the method according to the invention;
• 2 Fig. 3 shows a schematic sectional view of an intermediate mold after the bore presses; and
• 3 : shows a symbolic representation of the procedure.

The 1 shows a hollow shaft 1 which has undergone the entire process sequence according to the invention. The illustration clearly shows that the strength of the wall 2 over the entire length of the hollow shaft 1 is almost constant. Also in the range of changes in diameter, the strength of the wall changes 2 only insignificant. A material accumulation 3 , For example, a covenant is found in the areas in which, for example, a toothing or a bearing seat is applied in the further manufacturing process in order to maintain the necessary material for the further machining mostly. The illustrated hollow shaft is continuous with a bore 4 Mistake.

In 2 becomes an intermediate form 5 shown. This intermediate form 5 has only gone through the process steps to the hole pressures. Shown is the forging contour produced by drop forging 6 and the holes introduced by the bore press 4 ' and 4 ' , In the illustrated intermediate form 5 were the holes 4 ' and 4 ' introduced from both mutually opposite shaft ends. In about the middle of the intermediate form 5 became for example a first material accumulation 7a in the area between the holes 4 ' and 4 ' and a second accumulation of material 7b in the approximately central outer peripheral region of the forging contour 6 left behind. The material accumulation 7a can be removed in the axial direction after the hole pressing by the introduction of an additional axial force and a feed of the punch.

The 3 shows a symbolic representation of the individual process steps and their sequence. The process starts in a thermal plant 8th for heating a rod-shaped semi-finished product 9 , to which then a shearing device 10 for shearing a blank 11 from the semifinished product 9 followed. Then the blank is 11 a forging press 12 fed where he is in a die 13 forged to preform. After swaging, the preform becomes a first annealing furnace 14a fed, in which a first annealing process takes place. After the annealing process, the preform may be cooled in a controlled manner in a corresponding cooling device (not shown). The heat treatment in the annealing furnace 14a is a machine 15 downstream of the bore press. After drilling press in the appropriate machine 15 becomes an intermediate form thus obtained 5 in a second annealing furnace 14b given and subjected to a second annealing process. At the end of the illustrated process chain is in a suitable plant 16 a rotary swaging of the outer contour carried out to the finished end product, the hollow shaft 1 to obtain. Finally, the hollow shaft 1 be cooled controlled in a corresponding cooling device (not shown).

LIST OF REFERENCE NUMBERS

1

hollow shaft

2

wall

3

material accumulation

4, 4 ', 4 "

drilling

5

intermediate form

6

wrought contour

7a, 7b

material accumulation

8th

heating system

9

Workpiece

10

shearing

11

blank

12

forging press

13

die

14a, 14b

annealing furnace

15

Machine for drilling press

16

Machine for rotary kneading

Claims (21)

Method for producing a hollow shaft (1) from semi-finished products (9) with the following steps: - making a preform by forging; - performing a heat treatment of the preform; - Forming the preform in an at least partially hollow intermediate mold (5) by drilling pressures; and - Rotary kneading of the intermediate mold (5) to a hollow shaft (1) with a in the course of the hollow shaft (1) substantially the same wall thickness. Method according to Claim 1 , characterized in that the semi-finished product (9) is rod-shaped before forging. Method according to Claim 1 or 2 , characterized in that the semi-finished product (9) is heated to about 1000 ° C to 1200 ° C before forging. Method according to one of the preceding claims, characterized in that the forging is a hot forging. Method according to one of the preceding claims, characterized in that the forging is a drop forging. Method according to one of the preceding claims, characterized in that the forged preform has one or more accumulations of material (3). Method according to one of the preceding claims, characterized in that the forging process comprises the steps upsetting, preforming, finish forming and / or trimming. Method according to one of the preceding claims, characterized in that during the forging a cooling lubricant is applied. Method according to one of the preceding claims, characterized in that the forging takes place on a forging press (12). Method according to Claim 9 , characterized in that the stroke rate of the forging press (12) is up to 80 strokes per minute Method according to one of the preceding claims, characterized in that the preform is cooled controlled after forging. Method according to one of the preceding claims, characterized in that the heat treatment is an annealing process. Method according to one of the preceding claims, characterized in that two or more preforms are joined to form an intermediate mold (5). Method according to Claim 13 , characterized in that two or more molds are joined with axial bores such that an intermediate mold (5) is formed with an internal cavity. Method according to one of the preceding claims, characterized in that the bore pressing takes place when the preform is heated. Method according to Claim 15 , characterized in that the temperature of the preform at the hole pressures is about 400 ° C to 800 ° C. Method according to one of the preceding claims, characterized in that the hole pressing a lubricant is used, which is applied before and / or during the Bohrungsdrückens. Method according to one of the preceding claims, characterized in that after the hole pressing a heat treatment process takes place. Method according to one of the preceding claims, characterized in that the rotary kneading takes place with heated intermediate mold (5). Method according to Claim 19 , characterized in that the temperature of the intermediate mold (5) in the rotary swaging is about 400 ° C to 800 ° C. Method according to one of the preceding claims, characterized in that the hollow shaft (1) is cooled controlled after the rotary swaging.

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