Method for manufacturing a threaded drive rod

Abstract

Method for manufacturing a threaded rod (15) comprising the steps of: - providing a rod or tube as a blank for chipless thread forming, - chipless thread forming to a pre-dimension at least sectionally on the rod or tube to form a threaded spindle (17), wherein the pre-dimension has an allowance compared to an actual dimension and the main feature of the threaded spindle (17) is produced by forming the unhardened base body, - hardening at least the surface of the threaded spindle (17) formed to a pre-dimension, - grinding the hardened surface to an actual dimension of the threaded spindle (17), wherein the threaded spindle (17) is post-processed by plunge grinding in such a way that the process time during plunge grinding is reduced to one revolution of the threaded spindle (17).

Description

The invention relates to a method for producing a threaded drive rod.Screw drives are general state of the art and are widely used. As the frequency of use and load of such a transmission increase, the requirements for the tolerances and surface hardness of the thread increase. A further development of the screw drives are the ball screws. Ball screw spindles can be produced by rolling, whirling or grinding the ball track in a single- or multiple-start design.In the shapes known for ball screw spindles by rolling, whirling or grinding the ball track, rolling, which is in itself the cheapest machining, leads to results with greater tolerances. In addition, rolling (or rolling) can be carried out only on the uncured material. Therefore, in the known methods, after the spindle raceway has been introduced by forming without machining, the threaded region is subsequently hardened and polished for residual scale removal. Such a method is favorable and fast and with high precision in the soft forming by rolling. However, the subsequent heat treatment requires slight distortions.To improve the tolerances, DE 10 2004 049 365 A1 describes a method for producing a rack with ball screw, in which both the ball screw and the toothed segment are produced in a single clamping of the workpiece in a machine tool. This achieves accuracy for a low-stress multipoint mounting of the rack and ball spindle. The machining is effected by swirling (eccentric turning). Such machining requires a high machining investment, since the machining has a long cycle time.DE 10 2004 049 365 A1 also mentions a method for producing a threaded drive rod, in which the thread is produced by rolling, whirling or eccentric turning.Furthermore, DE 10 2009 029 407 A1 for a steering pinion as an example of a motion-transmitting steering component in a steering system discloses that, in a first step, the steering component is produced from a semi-finished product by machining and, in a second step, a circumferential toothing is produced on the steering component by rolling and the steering component is subjected to at least one further deformation by rolling in the same step. The steering component is hardened subsequent to rolling and then ground.It is an object of the invention to produce a threaded drive rod with high wear resistance at low cost, in particular for use in a steering system.This object is achieved by a manufacturing method according to claim 1.Advantageous refinements are reproduced in the dependent claims.The method according to the invention thus has at least three production steps, namely a non-cutting forming, followed by hardening and then grinding. Further production steps are possible before (e.g. straightening) intermediate (e.g. tempering) and also subsequently (e.g. polishing).The blank for the non-cutting shaping of the thread is advantageously a rod, wherein usually only a portion of the rod is formed into a threaded spindle. A tube can also be used as a blank.By dividing the entire forming wire into a non-cutting and a cutting machining step with intermediate hardening, a short machining time, i.e. a low machining investment, is achieved overall with a simultaneously high precision of the end product.The successive method steps according to the invention make it possible to produce the threaded drive rod particularly easily, quickly and cost-effectively. The main characteristic of the threaded spindle is thereby produced by forming the soft, i.e. uncured, base body. This manufacture of the main embodiment allows a short process time. The non-cutting shaping forms the thread to a preliminary dimension, i.e. with an allowance for machining or a machining allowance to the later actual dimension, which is the dimension actually present on a finished workpiece. This machining allowance is advantageously kept as small as possible, i.e. the chipless preforming is carried out in such a way that after hardening a machining removal is still possible, which should, however, advantageously be small.The grinding to the actual dimension is preferably carried out by grooving over the functional region of the threaded spindle. In this case, a multi-profile disc is used, as a result of which only a few more than one revolution of the preformed threaded spindle has to be processed. Very short process times are thereby achieved. In particular, if the chipless preforming and the hardening already lead to a product close to the actual dimension, continuous grinding with a disk can also be carried out instead of with a multi-profile disk.The hardening of the threaded spindle manufactured to the preliminary dimension is advantageously carried out as a surface hardening. Only the outer layers of the spindle are hardened in this case. Typical curing methods for this are flame curing, induction curing, laser beam curing and electron beam curing. Case hardening (for example nitriding) is also possible. In the hardening processes by mere heating (e.g. flame hardening), quenching is subsequently carried out in water or oil, wherein surface layer austenitization takes place. The advantage of surface hardening is not only the simple process technique, but also the combination of a high toughness and elasticity in the interior of the workpiece with the hard and thus low-wear surface.Particularly advantageously, the surface hardening of the threaded spindle, if desired also of the rack region of a possibly adjoining rack, takes place inductively. Induction curing achieves partial curing of the surface, as a result of which a very hard edge layer is formed, but the core remains tough.With a heat treatment of the threaded spindle, apart from the surface hardening or overall hardening, a recrystallization, a low-stress annealing, an annealing and / or a thermochemical treatment can take place.The threaded drive rod produced in this way is particularly suitable for ball screws in which the threaded spindle is operatively connected to a recirculating ball nut via a multiplicity of balls. The advantage of the ball screw is the low friction of the balls with simultaneously associated low wear. In this way, in particular a very high positioning accuracy between the ball nut and the threaded drive rod is also achieved.Such a ball screw system meets the high requirements for a steering gear. Particularly preferably, the ball screw is driven by an electric motor. Such an electromechanical steering system can function as a steering aid and / or as an independent steering system. The latter allows, for example, automated parking of a motor vehicle.For this purpose, the threaded drive rod is designed as a track rod arrangement which is connected terminally via joints to wheel carriers. The electromechanical drive acts directly or indirectly on the recirculating ball nut in order to set it in rotation about the threaded spindle in accordance with the desired steering assistance, so that the threaded drive rod and also the joints acting on the wheel carriers are displaced longitudinally relative to the recirculating ball nut.The non-cutting shaping of the thread is advantageously effected by rolling or rolling, wherein rolling leads to shorter cycle times and is therefore preferred. The soft rolling of the spindle region can preferably be carried out with low precision, since only the grinding carried out after the hardening is to be carried out with the tolerances predetermined for the nominal dimension of the workpiece.The steering knuckle steering system which is usually used in passenger motor vehicles and light commercial vehicles is actuated via a tie rod arrangement (also referred to as a steering rod arrangement), wherein a central region of the tie rod arrangement usually has a toothed rack which is usually accommodated in a steering gear fixed to the body. The rack is axially displaceable relative to the housing of the steering gear in order to set a steering angle at the wheels of the vehicle. For this purpose, the toothed rack is pivotably coupled at each of its ends to a track rod which, in turn, is pivotably connected at each of the other ends to a respective one of the vehicle wheel suspensions. The gear portion of the rack meshes with a steering pinion of a steering shaft to transmit a driver-side steering command to the vehicle wheels. In electromechanical steering systems, an electric motor assists the steering movement.Advantageously, the electric motor engages in the steering torque via a threaded spindle which is integrated in the track rod arrangement.The threaded spindle can be arranged as an extension of the tooth section of the toothed rack; in further embodiments, the toothed rack section can also be dispensed with, wherein the steering pinion then engages on the (ball) circumferential nut of the threaded spindle and thus transmits the steering torque exerted by a driver on the steering wheel to the track rod arrangement. In the latter embodiment, the electric motor engages directly or indirectly with the (ball) nut. Such embodiments are described in DE 10 2005 019 435 A1 and DE 10 2008 041 335 A1.In a further embodiment, as described in WO 03 / 016122 A1, the electric motor is mounted coaxially with the threaded spindle of the push rod arrangement. The rotor of the electric motor is arranged around the recirculating ball nut and connected to the latter in a rotationally fixed manner directly or via elastic coupling elements.In a further embodiment, the electromechanical steering aid is arranged axially parallel to the threaded spindle, wherein the ball nut is driven via a gear drive or, preferably, by means of a toothed belt.Conventional steels are used for the threaded drive rod.The invention is explained in more detail below with reference to an exemplary embodiment shown in the drawings. They show FIG. 1 shows an overall steering arrangement as can be used in smaller and middle motor vehicles; FIG. 2 shows a detailed view of the steering gear with electromechanical steering aid; and FIG. 3 shows the steering rod with toothed rack and threaded spindle.The electromechanical steering system 1 shown in FIG. 1 can be adjusted by means of an intervention on a steering wheel 13 and / or by means of the ball screw 8. A rotation of the steering wheel 13 is recorded by a steering angle sensor 25 and is passed on directly to a steering column 5. The steering column 5 couples to a universal joint shaft 6, which is connected via a steering torque transmitter 26 to a steering pinion 4, which meshes as part of a steering gear 2 on a toothing 16 of a toothed rack 20. The steering column 5 has a steering wheel adjustment 14, which in the released state allows a longitudinal and / or height adjustment of the steering wheel 3.The rack 20 is part of a steering rod 40 in a track rod arrangement 28, which has track rod ends 29 and 30 at the end and a threaded spindle 17 centrally between the rack 20 and one of the track rod ends (here: 30) on the steering rod 40. The steering wheel engagement thus reaches a first section 11 of the track rod arrangement 28 via the steering pinion 4. There the steering engagement is transformed from a rotational movement into a longitudinal movement of the track rod arrangement 28, which is passed on via joints 21 and 22 to steering levers (also called track levers) of wheel carriers 23 and 24. The steerable wheels 33 and 34 are in turn seated on the wheel carriers 23 and 24.The steering gear 2 is accommodated in a steering gear housing 3 which is fixedly connected to the vehicle body (not shown). In the steering gear housing 3, a ball screw 8 is furthermore located on a second section 12 of the track stage arrangement 28, which ball screw has, in addition to the threaded spindle 17, a recirculating ball nut 18 (FIG. 2 ), which is in constant engagement with the threaded spindle 17 via balls 19.The recirculating ball nut 18 together with a toothed belt 9 forms a gear stage 7 which can be driven on the input side by an electric motor 10.This second section 12 of the track rod arrangement 28 offers the possibility, via the threaded drive rod 15, of a self-contained steering engagement coupled to the first section. Thus, the steering of the wheels 33, 34 can take place solely via the steering wheel 13 or the electric motor 10 or both.For a suitable interaction of the two steering options, a control unit 27 is provided, via which a steering aid (assistance of the manual steering; reduction of the manual steering force on the steering wheel 13) or an independent steering (parking aid; parking steering assistant) can take place. For this purpose, the control device receives signals from the steering angle sensor 25 and the steering torque sensor 26 (when engaging the steering wheel 13) and calculates a desired assistance force for the electric motor 10 therefrom. During automatic parking, the electric motor 10 operates independently, i.e. no manual engagement takes place at the steering wheel 13. For this purpose, the control device receives signals from a control unit (not shown), which detects a parking situation (adjacent motor vehicles, curb edge... ) with the aid of its sensor system (e.g. ultrasound) and presets the required steering angle.For track compensation and for compensating changed angular positions of the joints 21 and 22, longitudinal adjustments (with respect to the track rod arrangement) and joints 31, 32 are also provided in the track rod arrangement 28.In FIG. 2, the electromechanical steering system is shown in detail with its two sections 11 and 12. In the first section 11 there is the toothed rack 20 which meshes with its toothing 16 on the steering pinion 4. The steering pinion 4 itself is connected in a rotationally fixed manner to the universal joint shaft 6, by means of which a manual steering angle can be effected. The rack 20 is integrally connected as part of the steering rod 40 to the threaded drive rod 15, which is part of the second section 12. The threaded spindle 17 of the threaded drive rod 15 is in turn part of the ball screw 8, which can be driven via the ball nut 18, which is in engagement with the threaded spindle 17 via the balls 19. For this purpose, the electric motor 10 is provided, the output of which acts via the toothed belt 9 on the outer periphery of the recirculating ball nut 18 and together with the latter forms the threaded step 7.In FIG. 3, the combination of the threaded drive rod 15 and the toothed rack 20 is shown in detail, which form the central region of the toothed rack 40 in a one-piece construction.The two steering gears of sections 11 and 12 are accommodated in a common steering gear housing 3.As the base body for the production of the steering rod 40, a blank made of a heat-treatable steel is preferably used. In a first step, the toothing 16 of the toothed rack 20 for the manual steering gear 11 is introduced, for example, by machining such as reaming, whirling or milling. In a second step, the main embossing of the thread of the threaded spindle 17 for the electromechanical steering gear 2 is formed by rolling or rolling in a continuous or piercing process. The sequence of these first two steps can be interchanged; both processing operations can also be carried out in one clamping operation.To increase the strength, the threaded spindle 17 and preferably also the toothed rack 20 are subjected to a heat treatment. Since the edge layers of the toothing 16 and of the threaded spindle 17 are to be very wear-resistant and therefore hard, but the core of the steering rod 40 is to remain tough, edge layer hardening or surface hardening is preferably carried out as the heat treatment. This is advantageously carried out inductively here, wherein the edge layer of the workpiece is austenitized by the heating and this austenite formation is maintained by quenching with water or oil in the edge layer. The steering rod 40 can be hardened over its entire length or partially in the region of the threaded spindle 17 and / or the toothing 16.The surface hardening in the region of the toothing 16 and the threaded spindle 17 achieves a surface hardness in the range between 50 and 65 HRC (Rockwell hardness according to scale C) and thus very good protection against wear.The scaling associated with the surface hardening or the formation of oxide layers and distortions can be compensated in the region of the toothing solely by polishing or also by re-grinding, in particular grooving. The threaded spindle 17, which has been formed with an allowance, is finished by grinding, in particular grooving. The process times during grooving are very low; in the shortest case, the grinding time can be reduced to one revolution of the threaded spindle 17, i.e. a pitch width (or a little more than safety allowance). This minimum grinding time is dependent on the allowance, which in turn is dependent on the possible distortion and the scale. The best values for the chipless preforming to a preliminary dimension and the subsequent grinding are to be adjusted by tests and regular quality controls.By grinding, possible variations in the geometry and the surface of the threaded spindle 17 after the heat treatment are equalized and the required surface quality and dimensional accuracy of the thread are achieved. By keeping the oversize of the main soft-formed embossing of the threaded spindle 17 small, only a small amount of material removal and, associated therewith, a short process time are required in the later grinding process. Also, during surface layer hardening, only a part of the radial extension of the same is removed.In principle, it is possible to temper the steering rod 40 in an intermediate step, in particular before or with the curing. The tempering is a combination of hardening and tempering the steel at higher temperatures in order to provide with this heat treatment the material with high strength and at the same time high toughness properties. This tempering can be carried out partially or over the entire length of the steering rod 40.List of reference characters1 Electromechanical steering system 2 Steering gear 3 Steering gear housing 4 Steering pinion 5 Steering column 6 Universal joint shaft 7 Gear stage 8 Ball screw drive 9 Belt 10 Electric motor 11 First section 12 Second section 13 Steering wheel 14 Steering wheel adjustment system 15 Threaded drive rod 16 Toothing system 17 Threaded spindle 18 Recirculating ball nut 19 Balls 20 Toothed rack 21 Joint 22 Joint 23 Wheel carrier with steering lever 24 Wheel carrier with steering lever 25 Steering angle transmitter 26 Steering torque transmitter 27 Steering device for the steering aid 28 Track rod arrangement 29 Track rod end 30 Track rod end 31 Longitudinal adjustment, joint 32 Longitudinal adjustment system, joint 33 Wheel 34 Wheel 40 Steering rod

Claims

Method for producing a threaded drive rod (15), having the steps - providing a rod or a tube as a blank for the chipless forming of a thread, - chipless forming of the thread to an initial dimension at least in sections on the rod or the tube, forming a threaded spindle (17), wherein the initial dimension has an initial dimension compared to an actual dimension and the main protrusion of the threaded spindle (17) is produced by forming the uncured main body, - curing at least the surface of the threaded spindle (17) formed to an initial dimension, - grinding the cured surface to an actual dimension of the threaded spindle (17), wherein the threaded spindle (17) is remachined by grooving in such a way that the process time during grooving is reduced to one rotation of the threaded spindle (17).Method according to claim 1, characterised in that the non-cutting shaping is effected by rolling or rolling.Method according to Claim 1 or 2, characterized in that the hardening of the threaded spindle (17) produced to the preliminary dimension is carried out as surface hardening.Method according to claim 3, characterized in that the surface hardness is in the range between 50 and 65 HRC.Method according to one of Claims 1 to 4, characterized in that a heat treatment of the threaded spindle (17) is used to carry out recrystallization, stress relief annealing, annealing and / or thermochemical treatment.

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