Patent Publication Number: US-2020276766-A1

Title: Method for producing units with axially movable components

Description:
The present invention relates to a method having the features of the preamble of claim  1  and a motor vehicle steering system having the features of the preamble of claim  8 . 
     Sliding connections for mutually movable components, such as coaxial tubes or shafts which are telescopic with respect to on another, which are inserted telescopically and rotationally firmly in one another, are used in various areas of technology. The sliding connection should generally be low-friction, free of play, and mechanically tough. Such telescopic connections are used in various places especially in motor vehicle steering systems. On the one hand, there is a telescopic combination of an inner and an outer casing tube in the steering column itself, which surround a steering shaft and are telescopic to allow the axial displacement of the steering column. Moreover, the steering shaft itself, which transmits the steering torque of the driver to the steering gearing, is generally telescopic. A roughly cloverleaf shaped cross section is used here, which is suitable for the transmitting of the torque, so that the inner shaft piece and the outer shaft piece engage with each other by form fit in the rotary direction. In both instances, a plastic sleeve is often installed between the two mutually sliding components made of metal. The problem and the expense in the fabrication of these connections is that the plastic piece cannot be installed directly as a component and can meet the requirements of freedom from play and a defined friction during the axial displacement movement without further processing steps. 
     In DE 26 35 120 A1 a method is proposed for producing a sliding connection of a shaft, in which an outer shaft and an inner shaft are preassembled with a sleeve in between. The outer shaft is then deformed inwardly, and the components are calibrated to each other with a displacement operation. A heating is also done. The heating may be done with an induction heating, a gas flame, or a contact heating, so that the plastic is placed in a plastic flow movement. The drawback here is the necessary time expense and the required thermal energy for the heating of the components. The end result also is not satisfactory in all cases, since the shaft pieces are heated and the geometry in the joint region changes due to the cooling down of the s once more after the above described process. 
     In U.S. Pat. No. 9,452,444 B2 a method is proposed for the production of a telescopic shaft, in which the two shaft pieces, one of which is coated with a plastic, are at first put together and then a displacement movement is initiated. The displacement force in this process is measured. The displacement is then continued for as long as it takes to reach a desired displacement force. 
     EP 2281731 B1 discloses a similar method. The drawbacks here are the relatively large forces which must be exerted to calibrate the plastic layer. 
     The long process time of the oscillating displacement movement and the costly equipment requirements are also a drawback. 
     Therefore, the problem which the present invention proposes to solve is to provide a method with which the process time can be shortened, less energy expense is required, and a better result can be achieved. This problem is solved by a method having the features of claim  1 . Another problem of the present invention is to create a device with mutually sliding components in which a better freedom from play and a more precise maintaining of given frictional forces or sliding forces are present. This problem is solved by a device having the features of claim  8 . 
     The problem is solved specifically in that the following features are provided in the method for producing an axially movable connection between two components, between which a plastic is arranged as a sliding material: 
     a) providing the two components to be joined, wherein either at least one of the two components has a plastic coating on the surface facing toward the other component or a plastic sleeve is provided between the components, 
     b) joining the components to form a unit, optionally with the plastic sleeve, by means of a pressing force in the axial direction, 
     c) clamping the unit in a device in which the two shaft pieces can be clamped and subjected to a displacement force in the axial direction, 
     d) pressing a sonotrode from one side against the respectively outer component and bracing the component against a counter-holder, 
     e) injecting an ultrasound signal into the sonotrode and moving the shaft pieces back and forth in the axial direction until the displacement force or the displacement velocity reaches a desired target value, 
     f) ending the ultrasound signal and removing the unit from the device. 
     This makes possible a faster and ultimately a more precise calibrating of the plastic sleeve or the sliding sleeve or the plastic coating in the displacement region of the components. 
     The problem of such an axially movable connection between two components, especially two cylindrical components, is to produce a displacement capability with the least possible displacement force and at the same time slight play between the components. In the case when the cylindrical components being shafts are supposed to transmit a torque, furthermore the largest possible torque should be transmitted securely. 
     Especially in this case, when torques are to be transmitted, the cross section areas of the cylindrical components have a configuration deviating from a circular shape. Corresponding slots or teeth or grooves, for example, may then be provided. In the designing of such a connection, a maximum permissible force needed to produce a displacement of the two components relative to each other is required. This force then constitutes the target value for the desired displacement force. It may also be provided to establish the target value for the desired displacement force at a value corresponding to 5%, preferably 10%, below the maximum permissible value for the displacement force that is established in the design. 
     The desired target value for the displacement velocity is determined by ascertaining in experiments the speed at which the desired target value for the displacement force is reached for a given displacement force lying above the maximum permissible displacement force. The target value for the speed is then established accordingly. Advantageously, an end stop can be provided at the target value of 5%, and more preferably at 10%. 
     Preferably in the method an ultrasound signal is injected into the sonotrode with a frequency in the range of 20 kHz to 35 kHz. More preferably, a frequency range of 25 kHz to 30 kHz is used. 
     It may be advantageous to vary the frequency of the ultrasound signal injected into the sonotrode during the course of the process of the method. 
     Preferably, the components are an inner shaft piece and an outer shaft piece, especially an inner steering shaft and an outer steering shaft of a motor vehicle steering system. It may also be provided that the components are an inner casing tube and an outer casing tube of an axially telescopic motor vehicle steering system. 
     If two sonotrodes are pressed against the outer component in step d), a more intensive or otherwise parametrized energy injection is possible. In particular, the two sonotrodes can be injected with ultrasound signals of different frequencies. 
     Furthermore, it has been found that more than two sonotrodes may also be used with advantage in order to further increase the energy injection. A different frequency or a different frequency variation over the process time may be employed at each sonotrode. 
     The ultrasound power may also be set separately for each sonotrode. Variations may also be provided. Thus, for a short starting time of up to 3 s, a high power can be provided, and then a low power for the rest of the process time. The low power is advantageously ⅓ lower than the high power. 
     To implement the method of producing the axially movable connection, the mutual displacement of the two components can be accomplished preferably with a pneumatic cylinder, optionally with a hydraulic cylinder. A force-guided movement of the clamped unit can be accomplished by the injected pressure. The force can also be adjusted for different speeds of movement and the displacement velocity can be measured. 
     In a motor vehicle steering systems having a telescopic steering shaft and/or a telescopic casing tube unit which is produced according to one of the methods described above, shorter possible cycle times and less energy expenditure are achieved in the fabrication process. Furthermore, the motor vehicle steering system has better qualities in regard to robustness, freedom from play, and freedom from noise. 
    
    
     
       Exemplary embodiments of the invention shall be described below with the aid of the drawing. There are shown: 
         FIG. 1 : a schematically represented motor vehicle steering system; 
         FIGS. 2-4 : a lower steering shaft; 
         FIGS. 5-6 : the steering shaft of  FIGS. 2-4  in a cross section; 
         FIG. 7 : the steering shaft of  FIGS. 5-6  during the calibrating of the plastic sleeve in a longitudinal section; 
         FIGS. 8-9 : another embodiment of a steering shaft; 
         FIG. 10 : another embodiment of a steering shaft, an upper steering shaft; 
         FIG. 11 : a casing tube unit; 
         FIGS. 12-13 : the casing tube in perspective representation; 
         FIG. 14 : the casing tube of  FIGS. 12-13  in a cross section; 
         FIG. 15 : the casing tube of  FIGS. 11-14  during the calibrating of the plastic sleeve; and 
         FIG. 16 : the steering shaft of  FIG. 5-6  during the calibrating of the plastic sleeve in a longitudinal section. 
     
    
    
       FIG. 1  shows in a schematic representation a motor vehicle steering system  1  having a steering wheel  2 , which is rotationally fixed to an upper steering shaft  3 . The upper steering shaft  3  is mounted in a bracket  4  in height adjustable and axially movable manner. By a Cardan joint  5 , the upper steering shaft can swivel, but it is rotationally fixed to a lower steering shaft  6 . The lower steering shaft  6 , finally, is connected by a second Cardan joint  7  to a pinion  8 , which engages with a rack segment  9  of a rack  10 . 
     A rotary movement of the steering wheel  2  thus results in a displacement of the rack  10  and in known manner to a swiveling of the steered wheels  11  of the motor vehicle, thereby producing a steering movement and a changing of the direction of travel. 
     The lower steering shaft  6  is shown in further detail in  FIG. 2 . The same or functionally equal components have the same reference numbers in the following figures. 
     The lower steering shaft  6  is provided with the first Cardan joint  5  and the second Cardan joint  7 . The upper, first Cardan joint  5  is rotationally fixed to an inner shaft piece  15 , while the second Cardan joint  7  is rotationally fixed to an outer shaft piece  17 . The outer shaft piece  17  has a rotationally symmetrical circumferential structure  18 , which extends as far as a free end  19  of the outer shaft piece  17 . The structure  18  consists of straight slots  20 , embossed in the shaft piece  17  from the outside. The slots  20  run axially parallel and give the shaft piece  17  somewhat of a star-shaped structure in cross section. 
       FIG. 3  shows the lower steering shaft  6  of  FIGS. 1 and 2  in a perspective view, in which the inner shaft piece  15  and the outer shaft piece  17  are pulled apart. It can be seen here that the inner shaft piece  15  has a region  26  at its free end  25  having a shape deviating from a round circular cross section. The cross section of this region  26  is likewise characterized by slots or grooves, producing a star-shaped cross sectional shape, fitted to the free inner cross section of the outer shaft piece  17 . This shall be described more closely below. In the representation of  FIG. 3 , the inner shaft piece  15  carries a plastic sleeve  30  in the region  26 , which is adapted in its cross sectional shape to the region  26 . 
     This becomes more clear in  FIG. 4 .  FIG. 4  shows the lower steering shaft  6  in a representation corresponding to  FIG. 3 , but the plastic sleeve  30  has been removed from the region  26  of the inner shaft piece  15 . The longitudinal axis and the axis of symmetry  31  also constitutes the axial direction here, in which the lower steering shaft  6  is telescopically formed. 
     The profiling of the lower steering shaft  6  in the regions in which the inner shaft piece  15  and the outer shaft piece  17  overlap and in which the plastic sleeve  30  is arranged between these two shaft pieces produces, when suitably designed, a connection which is fixed in rotation, yet telescopic in the direction of the longitudinal axis  31 . In the case of the lower steering shaft  6 , such a telescopic connection is advantageous, since the steering gearing is installed with the rack  10  in the region of the front axle of the motor vehicle, while the steering column  1  is secured roughly in the region of the dashboard support on the chassis. Relative movements of these fastening points are unavoidable during driving operation of the motor vehicle. These relative movements are absorbed by the design shown for the lower steering shaft  6 . It is important for function and driving comfort that the connection between the two shaft pieces functions permanently free of play, yet low in friction. For this, a precise adapting of the plastic sleeve  30  to the two profilings on its inner side and outer side is required. The method according to the invention, making possible this adapting in especially advantageous manner, shall be described more closely in the following. 
       FIG. 5  shows the lower steering shaft  6  in a cross section in the profiled region, schematized in an installation situation. The inner shaft piece  15  and the outer shaft piece  17  are pushed together in their mutually fitted and profiled region. Between the inner surface of the outer shaft piece  17  and the outer surface of the inner shaft piece  15  the plastic sleeve  30  is situated, which sits there free of play. In order to guarantee freedom from play, the plastic sleeve  30  is provided oversized, so that the seating of the two shaft pieces in each other has a large friction in the beginning. During operation, this high friction would become perceptible and annoying due to unwanted forces and also due to noise production on account of the stick slip effect. For the adapting or calibrating of the plastic sleeve  30  to the exact dimensions of the two shaft pieces, it is provided during the manufacturing process to place a sonotrode  35  in a radial direction against the outer shaft piece  17 . An anvil  36  is placed on the opposite side of the outer shaft piece  17 . In this way, the outer shaft piece  17  is firmly clamped between the sonotrode  35  and the anvil  36 . 
       FIG. 6  shows a similar arrangement to  FIG. 5 . In  FIG. 6 , unlike  FIG. 5 , the sonotrode  35  and the anvil  36  have not been installed in the slots  20  of the outer shaft piece  17 , but instead are mounted on outer faces  37  formed between the slots  20 . 
       FIG. 7  now shows the embodiment of  FIG. 5  in a longitudinal section, once again the sonotrode  35  and the anvil  36  being placed respectively in a slot  20  of the outer shaft piece  17 . The inner shaft piece  15  is inserted in the plastic sleeve  30  and the outer shaft piece  17 . The two shaft pieces are now grasped by clamping jaws  40  and  41 . A control and evaluation unit  68  undertakes the process control. The sonotrode  35  is actuated by a control unit  66  in order to transmit an ultrasound vibration to the outer shaft piece  17 . In this way, the outer shaft piece  17  is placed in a mechanical vibration. Since the outer shaft piece  17  itself vibrates relatively freely, the vibrational energy is transmitted in large measure to the plastic sleeve  30 , which is thereby deformed with high frequency. The plastic sleeve  30  becomes heated in this process. At the same time, the inner shaft piece  15  is moved back and forth in the axial direction by a relative movement in the direction of the double arrow  42  by means of the clamping jaws  40  and  41 . The heated plastic sleeve  30  is thereby adapted to the two mutually facing surfaces of the inner shaft piece  15  and the outer shaft piece  17 . The adapting process can be monitored by detecting the displacement force applied by the clamping jaws  40  and  41  during the reciprocating movement. For this purpose, a force sensor  65  can be provided. Preferably, the heating of the plastic sleeve  30  by means of ultrasound and the movement in the direction of the double arrow  42  is continued until such time as a given maximum displacement force is passed. The adapting process is then finished. After switching off the excitation of the sonotrode  35 , the plastic sleeve  30  cools down quickly, since the two shaft pieces  15  and  17  themselves were essentially not heated by the ultrasound excitation and hence they are cold compared to the plastic sleeve  30 . This promotes the dimensional stability of the plastic sleeve  30  thus calibrated. What is more, the outer shaft piece  17  and the inner shaft piece  15  undergo practically no thermal changes in their dimensions during this process. By contrast, in conventional methods the outer shaft piece was heated and after the calibrating process it cools down once more, so that the achievable precision of the calibrating process of the plastic sleeve  30  is limited. 
     The heating and cooling times of the described process are short, on account of the slight mass of the plastic sleeve  30  to be heated, so that a short cycle time can be achieved. Furthermore, it is enough to heat the plastic sleeve only at the surface, so that it can be easily molded. 
     The above described method can be used not only for profiled shafts, such as the lower steering shaft  6 , in which both the inner shaft piece  15  and the outer shaft piece  17  are profiled. Thus,  FIGS. 8 and 9  show another embodiment of a telescopic steering shaft  50  with an inner shaft piece  51  and an outer shaft piece  52 , which are fitted together telescopically in the direction of a longitudinal axis  53 . The inner shaft piece  51  has an end face region  54  which is surface-coated. For the calibrating of this surface coating in the aforementioned sense, the method described in connection with  FIGS. 5 to 7  can be applied to such a shaft. 
     However, it should be further noted that such a telescopic connection in length can also be advisable in the case of the upper steering shaft  3 , as is shown in  FIG. 10 . In the case when the steering wheel  2  is displaceable to the bracket  4 , such telescopic connections are also used for the upper steering shaft. All of the variants and embodiments represented with the aid of the lower steering shaft  6  are equally applicable to the upper steering shaft  3  as well. Accordingly, the upper steering shaft  3  may comprise an inner steering shaft  71  and an outer steering shaft  72  with a sleeve  74  between them, these being telescopically fitted together in the direction of a longitudinal axis  73 . For the calibrating of this connection, the method described with the aid of  FIGS. 5 to 7  can also be used for the upper steering shaft  3 . 
       FIGS. 11 to 15  show an example of a telescoping casing tube unit  60 . The telescoping casing tube unit  60  comprises the upper steering shaft  3 , which has been described above in  FIG. 1  and  FIG. 10 . The upper steering shaft  3  is mounted in an inner casing tube  61  and an outer casing tube  62 . For the axial displacement of the steering column, the inner shaft piece  71  and the outer shaft piece  72  of the upper steering shaft  3  are displaceable in their longitudinal axis  73 , as was explained with the aid of  FIG. 10 . 
     Moreover, the inner casing tube  61  is displaceable with respect to the outer casing tube  62  in the axial direction, corresponding to the longitudinal direction of the longitudinal axis  73 . Between the inner casing tube  61  and the outer casing tube  62  there is provided a sliding sleeve  63 , which is shown separately in  FIG. 12 . The sliding sleeve  63  sits between the inner casing tube  61  and the outer casing tube  62 , as can be seen in  FIG. 13 . The inner casing tube  61  and the outer casing tube  62  are not shafts in the technical sense, since they have a round circular cross section and cannot transmit any torques. Even so, it is advantageous for the above mentioned reasons when the seating of the two casing tube parts in the region of the sliding sleeve  63  is free of play, yet smooth in movement. For this purpose, the method described with  FIG. 7  is also used for calibrating the sliding sleeve  63  between the inner casing tube  61  and the outer casing tube  62 . This is illustrated in  FIG. 14 . The sonotrode  35  is placed on the outside of the outer casing tube  62 , which is braced against the oppositely placed anvil  36 . The sonotrode  35  is then actuated again by a control unit  66  with electrical voltage of a given frequency or frequency variation. The vibrational energy, in turn, results in a heating of the sliding sleeve  63 . 
       FIG. 15  illustrates how the inner casing tube  61  is clamped between clamping jaws  40  during the process, while the outer casing tube  62  is clamped between clamping jaws  41 . While the sliding sleeve  63  is being heated, the inner casing tube  61  is moved back and forth in the direction of the double arrow  42 . The displacement force F required for this is detected with a force sensor  65 . The displacement force F decreases with the number of stroke movements in the direction of the double arrow  42 . As soon as a given threshold value is reached or passed, the calibrating of the sliding sleeve  63  is terminated. The process control occurs in turn by the control and evaluation unit  68 . The casing tube unit  60  thus prepared is then removed from the clamping jaws  40  and  41 , the anvil  36  and the sonotrode  35  are removed, and the casing tube can be installed in a bracket  4  according to  FIG. 1 . As represented in this exemplary embodiment in  FIG. 10 , the upper steering shaft  3 , which is mounted in the telescopic casing tube, likewise comprises a displacement connection, the displacement connection being able to correspond to the exemplary embodiments of  FIGS. 2 to 9  and being adapted to transmit torques. 
     Alternatively or in combination with the use of a force sensor  65 , a speed sensor  67  may be provided, which can also be designed as a displacement sensor, the speed being determined in a control and evaluation device  68 . This is illustrated in  FIG. 16 .  FIG. 16  shows the embodiment of  FIG. 5  in a longitudinal section, similar to  FIG. 7 , where once again the sonotrode  35  and the anvil  36  are each placed in a slot  20  of the outer shaft piece  17 . The inner shaft piece  15  is installed in the plastic sleeve  30  and the outer shaft piece  17 . The two shaft pieces are now grasped by clamping jaws  40  and  41 . The clamping jaw  41  is held stationary, while the clamping jaw  40  can be subjected to movement by a pneumatic cylinder. A control and evaluation unit  68  undertakes the process control. The sonotrode  35  is actuated by a control unit  66  in order to transmit an ultrasound vibration to the outer shaft piece  17 . In this way, the outer shaft piece  17  is placed in a mechanical vibration. Since the outer shaft piece  17  itself vibrates relatively freely, the vibrational energy is transmitted in large measure to the plastic sleeve  30 , which is thereby deformed with high frequency. The plastic sleeve  30  becomes heated in this process. At the same time, the inner shaft piece  15  is moved back and forth in the axial direction by a relative movement in the direction of the double arrow  42  by means of the clamping jaws  40  and  41 . For this, the pressures p 1  and p 2  are alternately increased and decreased, so that the piston  69  is moved back and forth. The piston  69  is accordingly coupled mechanically to the clamping jaw  40 . The heated plastic sleeve  30  becomes adapted to the two mutually facing surfaces of the inner shaft piece  15  and the outer shaft piece  17 . The adapting process can be monitored by detecting the speed with which the clamping jaw  40  is moved by means of a distance sensor or a speed sensor  67 . Preferably, the heating of the plastic sleeve  30  by means of ultrasound and the movement in the direction of the double arrow  42  is continued until such time as the maximum value of the displacement velocity exceeds a given minimum target value. The adapting process is then finished. This process sequence likewise offers the already mentioned benefits. 
     The sequence of the above described processes thus provides the following partly optional process steps as an exemplary embodiment:
         providing the two shaft pieces to be joined, wherein   either at least one of the two shaft pieces has a plastic coating on the surface facing toward the other shaft piece,   or a plastic sleeve is provided abutting between the shaft pieces,   joining the shaft pieces, optionally with the plastic sleeve in between,   wherein the shaft pieces and optionally the plastic sleeve are configured such that the joining can occur only by overcoming a pressing force, since the sliding fit is designed with an oversize,   clamping the unit in a device in which the two shaft pieces can be clamped and subjected to a displacement force in the axial direction. The device is preferably outfitted such that a displacement force can be measured.   pressing a sonotrode from one side against the respectively outer shaft pieces and bracing the inner piece against a counter-holder (anvil),   injecting an ultrasound signal into the sonotrode and moving the shaft pieces back and forth in the axial direction until the displacement force reaches a desired target value. Alternatively, the method can be carried out such that the shaft pieces are moved relative to each other with a constant force and the displacement velocity is measured. The process is then ended when a particular displacement velocity is achieved.   after the end of the process, the shaft is removed from the device as a finished component and is installed elsewhere.