Abstract:
A switch assembly that includes the coaxial assembly of a cylindrical core extending from a base to a distal end, an outer knob rotatable about the core, a switching means generating an electrical signal dependent on the rotations of the knob, and a guiding means for guiding the knob in its motions relative to the core. The guiding means includes an upper-guide in the vicinity of the core-end and the knob-top, and a lower-guide in the vicinity of the core-base and the knob-base. The upper-guide includes elements rolling between an upper-guide inner race integral to the core and an upper-guide outer race integral to the knob.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §371 of published PCT Patent Application Number PCT/EP 2012/051331, filed Jan. 27, 2012, claiming priority to European patent application number EP 11154571.1 filed on Feb. 15, 2011, and published as WO2012/110297 on Aug. 23, 2012, the entire contents of which is hereby incorporated by reference herein. 
     TECHNICAL FIELD OF INVENTION 
     The present invention relates to a multifunctional switch with an indicator, in which a plurality of functions can be selected and validated through a rotation and a push of a dial knob. 
     BACKGROUND OF INVENTION 
     Rotary switch of the aforementioned type exists and are commonly implemented in automotive environment for instance to control the air conditioning or a Hi-Fi system. 
     Said switches are typically built on an electronic printed circuit board (PCB). A tubular cylindrical core is fixed on the PCB, and serves as a primary guide for a bushing that is placed over said cylindrical core. An external rotary knob, accessible to an operator, is placed over the bushing. The switch is in mechanical and electrical connection with the PCB and the rotation of the knob selects various functions. Furthermore, similarly to a key on a computer key board, a function chosen by rotation can be validated by a push on the knob, which then axially slides on the bushing toward the PCB and commutes an electrical switch. An elastic mean, such as a coil spring, biases the knob away from the pushed position, where a function has been validated, back to an extended position, when not pressed by the operator. To help the operator in the function selection, a liquid crystal display (LCD) may be fixed on the core while the knob remains open or provided with a transparent window in order to leave a visual direct access to the LCD. A back illumination of the LCD is made possible as the hollow center of the cylindrical core is a light channel for a light beam generated by a light source, typically a light-emitting diode (LED) fixed on the PCB. 
     In EP1555684, Kikuya et al. disclose such a rotary switch. This and other switches of the same type have been successfully implemented in diverse environments including inside many vehicles. In US2004/0154910, Hayashi discloses a rotary switch having a knob guided on a fixed cylindrical core between the lower portion of the core on which the knob slides and, the upper portion of the core where a set of balls maintained in individual radial cavities are radially pressed by springs against the knob&#39;s inner cylindrical surface. Consequently to this arrangement, each ball slides between the surfaces of the knob, of the cavity and the final turn of the spring. 
     Unfortunately all these switches suffer from characteristics inherent to their structure. Indeed, the sliding of the knob and of other elements result in an unpleasant friction feeling perceived by the operator. Furthermore, said friction goes against a desired accuracy of the positioning of the knob. Even though the switch may be provided with an indexing feature, the friction generates a need to manually slightly adjust the angular position of the knob. Also, after being pushed to validate a function, the friction acts against a self-return of the knob in the extended rest position. In an attempt to minimize the friction, the functional gap between the knob and the bushing has to be increased above the mandatory minimum required to accommodate the manufacturing tolerances. Consequently, under small lateral forces, the knob is subject to a very unpleasant little wobble perpendicular to the rest-pushed direction. This increases the perceived feeling of inaccuracy. 
     It is important to propose to the market a rotary switch having a push function that is solving aforementioned problems in having the desired wobble-free precise and accurate motion of the knob. 
     SUMMARY OF THE INVENTION 
     In carrying out the above object and other objects, features of the present invention provide a switch assembly according to the characteristics&#39; of claim  1 . 
     The switch assembly comprises the coaxial assembly along a longitudinal axis of a cylindrical core, axially extending from a base to a distal end, the core-base being fixed to a base plate, an outer knob, extending from a knob-base to a distal knob-top, the knob being axially rotatable about the core, a switching mean generating an electrical signal dependent on the rotations of the knob, and a mean for guiding the knob in its motions relative to the core. Said mean comprises an upper-guide, in the vicinity of the core-end and the knob-top, and a lower-guide in the vicinity of the core-base and the knob-base. The upper-guide comprises rolling elements, said elements rolling between an upper-guide inner race integral to the core, and an upper-guide outer race integral to the knob. Thanks to this upper bearing arrangement, the rotations of the knob are advantageously friction-free. 
     The outer race is truncated with upward apex and the switch assembly further comprises a mean for generating an upward axial force biasing the rolling elements onto the upper-guide outer race. This advantageously eliminates any free play that would be detrimental to the tactile feeling when operating the knob. 
     The lower-guide may as well comprise rolling elements, said elements rolling between a lower-guide inner race integral to the core, and a lower-guide outer race integral to the knob. The lower-guide outer race is truncated with downward apex and the switch assembly further comprises a mean for generating a downward axial force biasing the rolling elements onto the lower-guide outer race. Thanks to this bearing-like arrangement for the lower guide symmetrical to the upper guide, undesirable friction is eliminated in the motion of the knob. 
     The means for biasing the rolling elements are placed between the upper-guide and the lower-guide and are equally pushing apart said guides in opposite axial directions. This advantageously reduces the number of components by combining the means for biasing, in using a single mean that serves both purposes for the upper guide and for the lower guide. 
     To avoid misalignment of the upper and the lower guide, a mean maintaining them in coaxial alignment is provided. 
     The switch assembly further comprises a mean for indexing the rotation of the knob. This mean comprises an indexing member biased by an elastic member against an indented path integral to the knob. The indexing member is linked to the base. A symmetrical mounting with the indented path integral to the base and the indexing member is linked the knob is possible. This advantageously keeps the knob in position when not operated. 
     The knob is further able to axially translate relative to the core between a first position and a second position. The mean for guiding the knob in its motions relative to the core guides the knob when it translates. Another switching mean generating another electrical signal dependent on the translation of the knob is provided. This, for instance, enables to validate functions. 
     The switch assembly further comprises another mean generating a unidirectional axial force onto the knob forcing said knob to return into the first position after being displaced from said first position. This keeps the knob in the first position when it is not operated. Said mean for generating a unidirectional axial force comprises the indented path and the indexing member. The indents of the indented path are operated in a groove having two symmetrical sides. In the first position, the indexing member is biased in the bottom of the groove generating on the knob symmetrical and balanced upward and downward forces. The knob is at equilibrium. When away from the first position, the indexing member travels on one side of the groove generating on the knob the unidirectional force that forces the knob to return to the first position. 
     Furthermore, the knob-top is open or provided with a transparent mean leaving visual access to a display, fixed on the core-end. Also, to improve the visibility of the display, the core is tubular and its the hollow center is a light channel for a light beam generated by a source. The light beam back illuminates the display. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention is now described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a switch assembly as per the invention; 
         FIG. 2  is a section of the switch of  FIG. 1 , the section being in a vertical plan passing through rolling elements, the switch being in rest position; 
         FIG. 3  is the same section as in  FIG. 2 , the switch assembly being in pushed position; 
         FIG. 4  is a section of the switch of  FIG. 1 , the section being in a vertical plan passing through guiding elements, the switch being in rest position; 
         FIG. 5  is the same section as in  FIG. 4 , the switch assembly being in pushed position; 
         FIG. 6  is a section enabling to place the apex of truncated sectors; 
         FIG. 7  is a detail view of a ball and of the guiding element as per a first embodiment of the present invention; and 
         FIG. 8  is a detail in perspective of the guiding elements as per a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, similar elements could be designated with the same reference numbers. 
     In a motor vehicle an operator can control a function by manipulating the knob of a switch assembly  10 . Thanks to a bearing-like arrangement, the knob  12  of the switch assembly  10  of the present invention has rotary and push capabilities. It thus enables function selection and function validation. The rotation may be limited to a certain angular sector or may be end-less. The translation is typically limited to a commutation between an extended rest position P 1  and a pushed active position P 2 . Other choices are of course possible such as three or more translation positions with intermediates between a full extended position and a full pushed position. 
     The description focuses on a particular embodiment of the invention and alternatives are briefly mentioned without any intention to limit the scope of the invention to these specific embodiments. 
     The description will use a tri-orthogonal direct coordinate system (X, Y, Z) as shown in  FIG. 1 . For clarity and concision purposes, and to ease the understanding, a bottom-up orientation as shown on the Figs. will also be used. Therefore, the terms low, high, over, under, superior, inferior, above, below, top, bottom, horizontal, vertical, downward and upward may be utilized without any intention to limit the scope of the invention, especially in regards of the numerous possibilities of installation of the switch assembly in a vehicle. The plan (X, Y) is then described as the horizontal plan and the axis Z is the vertical axis normal to the horizontal plan. 
     The perspective and semi-transparent  FIG. 1  enables to identify, to position and to understand the function of the key constituents that will be described afterward in greater details. 
     On a base plate  14  that may eventually be a printed circuit board (PCB) is fixed a support  16  wherein a cylindrical core  18  is received and fixed. Said core  18  vertically extends as a cylinder— FIG. 2 . Over the core  18  are placed a lower bearing  20  and an upper bearing  22 , both having balls  24  in contact with the core&#39;s cylindrical surface  25 , thus constituting the inner race for the bearings  20 ,  22 . The knob  12  coaxially covers the bearings  20 ,  22 . The knob  12  has an internal surface  26 —FIG.  2 —comprising two truncated sectors  28 ,  30 , oppositely oriented and constituting the respective outer race of the bearings  20 ,  22 . To eliminate any internal free play that would be detrimental to the tactile feeling, the bearings  20 ,  22 , are aligned and pre-loaded thanks to a plurality of springs  32  and pins  34  interposed between the respective cages  36 ,  38 , of the bearings  20 ,  22 . The balls  24  are thus biased against their respective outer race  28 ,  30 . 
     The switch assembly  10  further comprises a liquid crystal display (LCD)  40 , or any other type of display, fixed on the top of the core  18 . A transparent window  42  is fixed on the top of the knob  12  enabling the operator to see the information displayed on the LCD  40 . An alternative to a transparent window may be to leave open the top of the knob  12 . The core  18  is tubular and its hollow center  43  is a light channel for the back illumination of the display  40 . 
     Furthermore, an indexing device  44  is provided. It comprises an index  46 —FIG.  4 —horizontally biased onto an indented peripheral sector  50  of the knob  12 . While the index  46  is maintained in a recess of the support  16 , the indented sector  50  moves with the knob  12 . Additionally to providing rotational indexing, the indexing device  44  automatically generates an upwardly oriented force F when the knob  12  is pushed down. The force F biases the knob  12  back up and maintains it in the rest position P 1 . 
     The PCB  14  and the switch assembly  10  are also provided with all necessary electrical equipment, for instance in order to wire the LCD  40  or to capture the motions, rotation and push, of the knob  12 . When an operator pushes the knob  12  an electrical switch  52  is commuted. To enhance the tactile feeling and generate a pleasant more sudden vertical force felt by the operator, one or more deformable silicone domes  54  are typically placed over, or next by, the electrical switch  52  and are pressed when the electrical switch  52  commutes. 
       FIGS. 2 to 6  detail the structural embodiment of the switch assembly  10 . On the horizontal PCB  14  is fixed the support  16  that is provided with a hole having a peripheral wall fitted to receive and to fix the cylindrical core  18  that upwardly extends from its base, the “core-base”  56  to its distant end, the “core-end”  58 . The core  18  further comprises the inner light channel  43  that has a cylindrical bottom continued by a truncated portion integral to the core-end  58 . Other shapes of the light channel  43  are possible such as a continuous cone or a continuous cylinder. In a preferred embodiment represented on the Figures, the light channel and the cylindrical core are molded in one piece. A multi-piece process is possible, said pieces being fixed together afterward. The LCD  40  is horizontally fixed to the core-end  58  and is electrically connected to the PCB  14  via wires (not shown) preferably arranged between the light-channel  43  and the cylinder. The LCD  40  is back lighted thanks to a light-emitting diode (LED)  59 , or any other light-emitting device, connected on the PCB  14  substantially in the center of the support&#39;s hole. The light-channel  43  upwardly conducts the light beam to the LCD  40  providing backlightion. Alternatively to the LCD  40 , other type of display may be chosen, such a simple window where an icon would be drawn. 
     The preferred way for fixing the core  18  to the PCB  14  is, as shown, via the core-base  56  in the support&#39;s hole where it may be glued or fixed using any known process. Alternatively, the fixation may be operated otherwise, for instance, directly from the core-base  56  to the PCB  14  or even via the bottom of the light-channel  43  to the PCB  14 . At the other extremity of the core  18 , the fixing of the LCD  40  on the core-end  58  is presented on the  FIGS. 2-5  using a mechanical flange  60 . It may alternatively be done using any other known fixing process such as gluing or crimping. 
     The knob  12  is coaxially assembled over the core  18  and it extends from its base, the “knob-base”  62  that is slightly above the PCB  14 , to its top, the “knob-top”  64  that is over the core-end  60 . While the knob&#39;s external surface is shown cylindrical, it may take any other shape and may be covered with a layer of material  68  easing the handling and fine manipulation for tuning. Inside of the knob  12 , the truncated section  28  has its apex A 1 —FIG.  6 —downwardly oriented and the upper truncated  30  section has its apex A 2  upwardly oriented. In between the truncated sections  28 ,  30 , the knob&#39;s inner surface  26  is cylindrical. 
     The upper and lower bearings  22 ,  20 , are placed between the core  18  and the knob  12 . As can be seen on the  FIGS. 2 and 3 , the balls  24  of the bearings  20 ,  22 , contact the external cylindrical surface  25  of the core  18  and the truncated sectors  28 ,  30 , of the inner surface  26  of the knob  12 . The set of balls  24  of any of the two bearings  20 ,  22 , is maintained in a horizontal plan spatially positioned on a circle thanks to the cages  36 ,  38 , that are provided with recesses  70 — FIGS. 7 and 8 . Within each recess  70  is placed one ball  24 . Preferably but not mandatory, and in order to ease the manufacturing process, the upper and lower bearings  20 ,  22 , are made identical then are mounted head to tail in the switch assembly  10 . Each bearing  20 ,  22 , comprises six balls  24  equally distributed every 60 degree. The balls  24  of the bearings  20 ,  22 , are vertically aligned by pair. Within their respective recesses  70 , the balls  24  are maintained preferably free, the width and height of the recess  70  being very slightly larger than the diameter of the ball  24 . Alternatively the ball  24  can be maintained with a very little press fit. 
     In a first embodiment, presented on the  FIGS. 1 to 7 , each recess  70  consists in a four walls window within which a ball  24  is placed by push it in a radial direction. 
     In a second embodiment, presented on  FIG. 8 , each recess  70  has only three walls and is open in the vertical direction Z. The ball  24  is placed between the two lateral walls  70   a ,  70   b . On both side of the recess  70  is operated a vertical slot  71   a ,  71   b , so that the side walls  70   a ,  70   b , are indeed lugs extending in the vertical direction Z from a base to a distal extremity. Thanks to this, the side walls  70   a ,  70   b , are provided with a little elasticity enabling a little angular motion about a radial axis passing through the base of the lug. At their distal extremity, the side walls  70   a ,  70   b , are provided with a snap-on feature  70   c  that brings the walls  70   a ,  70   b , closer to each other than they are by the main part of the recess  70 . The ball  24  is put in place by pressing the ball  24  in the vertical Z direction between the distal extremities of the walls  70   a ,  70   b , over the snap-on feature  70   c . Thus, the walls  70   a ,  70   b , give way thanks to the elasticity provided by the slots  71   a ,  71   b . When the ball  24  has passed the snap-on feature  70   c , it gets into the main part of the recess  70  and consequently the walls  70   a ,  70   b , get back in their vertical position capturing the ball  24  in the recess  70 . The ball  24  is free between the three walls of said recess  70 . A feature prevents the ball  24  to fall off the recess  70  in the radial direction. Such a feature can easily be arranged thanks, for instance, to a non-straight cross section of the window observed in a horizontal plane. A cylindrical cross section is one of the multiple possibilities that would prevent the ball  24  to fall of the window  70 , while still being free when in place. 
     In further alternative embodiments, differences between the bearings could be arranged, such as in the size or number of the balls or the making of the cages. Furthermore, the bearings are represented and described as ball bearings. Alternatively, the rolling elements may be rollers which axis would intersect by the apex of the outer races. Rollers may provide larger contact area with their inner and outer races and, considering that most of the time the knob does not move, this may help in avoiding local indents in the races. 
     The functioning of the switch assembly  10  is optimized as the bearings  20 ,  22 , are maintained coaxially aligned and are pre-loaded. 
     In the  FIGS. 1 to 7  a first embodiment is presented. It eases the process and minimizes packaging. Each of the bearing&#39;s cage is provided with a plurality of vertical slots. Each slot in the upper bearing  22  faces a slot in the lower bearing  20  thus creating pairs of slots. In each pair is placed a vertically acting spring  32  biasing a vertical pin  34 . As detailed on the  FIG. 7 , the spring  32  placed and maintained in a slot  72  of the lower cage  36 , the “lower-slot”  72 , is associated to a pin  34  placed, in the slot  74  of the upper cage  38 , the “upper-slot”  74 . The pin  34  downwardly extends from the upper-slot  74  into the lower-slot  72  where it compresses the spring  32  that generates a counter force. In the lower-slot  72 , the pin  34  is not fixed. This enables vertical motion of the lower-cage  36  relative to the upper-cage  38 . Consequently, on one side the pins  34  align the cages  36 ,  38 , by linking them to each other and enabling vertical relative motion and, on the other side the pre-loading is operated as the springs  32  bias the pins  34  and therefore push the bearings  20 ,  22 , apart in opposite directions. Also, other alternative embodiments can be imagined and are not fully described. For instance the pins  34  represented as separate parts, could be molded integral with the upper cage. 
     As shown on  FIG. 7 , a clipping device  76  keeping the bearings  20 ,  22 , together eases the assembly. The clipping device  76  comprises a lug  78  upwardly extending from the lower cage  36  and engaging and clipping into a window  80  of the upper cage  38 . Multiple other arrangements easing the assembly may be developed. What is important is that the device  76  does not prevent the motion of the cages  36 ,  38 , relative to each other. 
     In a second embodiment detailed in  FIG. 8 , the coaxial alignment and the pre-load are provided separately in non-combined devices. 
     The coaxial alignment is ensured by having each cage  36 ,  38 , provided with three undercuts  35   a  alternating with three extensions  35   b  regularly distributed about the vertical Z axis. Assembled head to tail, each cage  36 ,  38 , presents each of its undercuts  35   a  to an extension  35   b  of the other cage  38 ,  36 , for complementary engagement. As shown on  FIG. 8 , the undercuts  35   a  are female undercuts partially and locally reducing the wall thickness of the cages  36 ,  38 , on a horizontal angle and a vertical height. The extensions  35   b  are male arcuate lugs vertically extending and having dimensions set for complementary engagement with the slots  35   a.    
     Dimensionally,  FIG. 8  represents an arrangement alternating undercuts and extensions having on an angle of approximately 45 degrees followed by a complementary horizontal portion of 15 degrees that joins an undercuts  35   a  to an extension  35   b . The vertical height of the undercuts  35   a  and extensions  35   b  is preferably, but not mandatory, limited by the recess  70 . 
     In the second embodiment represented on  FIG. 8 , the pre-loading is ensured by three coil springs  32  vertically pushing apart the cages  36 ,  38 , in opposite directions. Each cage is provided by vertical slots  72  operated in the horizontal portions that join the undercuts  35   a  to the extensions  35   b . To have the cages  36 ,  38 , manufactured identical, which is not mandatory, the slots  72  should be in the middle of the horizontal portions so that, when presented head to tail for complementary engagement of undercuts and extensions, the slots  72  would constitute aligned pairs wherein the springs  32  would be placed. 
     The second embodiment of  FIG. 8  does not have clipping device  76  similar to what is represented in  FIG. 7  for the first embodiment. Nevertheless, a clipping device  76  can easily be adapted to the second embodiment. For instance by having a similar arrangement of lug  78  and window  80  placed on the horizontal portion joining the undercuts and the extension where there is no spring  32 . Another possibility is to provide the undercuts  35   a  with portion that would be deeper, or even would constitute a through window, in which would clip the complementary extension  35   b . In anyway, said clipping should not prevent the vertical relative motion of the cages  36 ,  38 . 
       FIG. 8  shows the three undercuts and the three complementary extensions on the inner side of the cages. Obvious alternatives are possible, such as a similar device on the outer side of the cages. Also, another angular arrangement for the undercuts and extensions or another number of undercuts and extensions is possible. 
     Whatever the embodiment is, when assembled, the balls  24  of the upper bearing  22  are biased upward in contact against the upper truncated surface  30  of the knob  12  and, symmetrically, the balls  24  of the lower bearing  20  are biased downward in contact against the lower truncated surface  28  of the knob  12 , said knob  12  being able to rotate and to translate about the axis Z. 
     When the knob  12  is rotated, the ball  24  rotates between the inner race  25  and the truncated outer race  28 ,  30 . The balls  24  push the cages  36 ,  38 , in rotation about the vertical Z axis at half the rotation speed of the knob  12 . In this motion, in order to push the cages in rotation, the balls  24  are in sliding contact with a side wall  70   a ,  70   b , of the recess  70 . 
     An optimal functioning of the switch assembly  10  is ensured by an optimum dimensioning of all components where, under nominal conditions, the cages are able to axially move relative to each other and also, the balls  24  are in contact with their respective outer races  28 ,  30 , in the middle of the truncated surface and also, that the load is evenly distributed over all the balls  24 . Also, the rotation of the ball  24  between the inner race  25  and its outer race  28 ,  30 , should in no way be prevented by the minor sliding against a side wall  70   a ,  70   b.    
     As shown on the  FIGS. 2-5 , to ease the manufacturing and the assembly of the switch  10  the knob  12  may be manufactured in distinct upper  12 U and lower parts  12 L, thereafter integrally fixed. The lower part  12 L comprises the lower truncated surface  28  and the upper part  12 U comprises the upper truncated surface  30 . 
     The indexing device  44  present in the lower part of the knob  12  is particularly detailed on  FIGS. 4 and 5 . The support  16  receives in a horizontal recess  82  a coil spring  48  that biases the index  46  provided with a spherical end. Balls or other devices are known in the art to be used for similar purposes. The knob  12  is integrally provided on its periphery with the indented sector  50 . Said sector  50  has indents comprised in vertical plans including the axis Z. Said indents also have a V-shape cross section, the axis of which is horizontal. 
     In the rest position P 1 —FIG.  4 —the spherical end of the index  46  is biased against the bottom of the V. The force of the spring  48  is equally divided at the index  46  contacts in upward and downward vertical forces resulting in a vertical equilibrium having no influence on the knob&#39;s position. When moved away from the rest position P 1 , for instance when moved to the pushed position P 2 —FIG.  5 —the knob  12  translates downward and so does the indented path  50 . Consequently, the contact point of the index  46  moves upward to the upper branch of the V. This destroys the balance of vertical forces. Only upward forces F are generated pushing the knob  12  back up to the rest position P 1 . 
     The forces F applied are relatively low in the magnitude of few Newton&#39;s. An operator will have no difficulty to push the knob  12  commuting from the rest position P 1  to the pushed position P 2 . Should the operator want; he would have no further difficulty to maintain the knob  12  in the pushed position P 2 . It is only when the knob  12  will be relieved that the upward forces F will return and maintain the knob  12  into the rest position P 1 . 
     Now are described some functional aspects of the switch assembly  10  as per the invention. 
     To achieve the above mentioned optimal functioning conditions, and considering the unavoidable variations of dimensions due for instance to manufacturing tolerances, humidity changes, expansion and contraction of material due to temperature variations, roundness imperfections, material composition, etc. . . . the optimum dimensioning of all components should accommodate proper functional gaps between the alignment features. For instance, the undercuts  35   a  should be slightly larger than the extensions  35   b . Furthermore, these functional gaps should allow for a very slight tilt of the cages relative to each other thus compensating for all dimensional variations that will occur during the product life. 
     When assembled and not operated, the knob  12  remains in the rest position P 1 . The balls  24  are biased in the middle of their respective truncated outer races  28 ,  30 , and consequently against their cylindrical inner races  25 . The system is at equilibrium. 
     When the knob  12  is rotated, the balls  24  rotate on the inner race  25  and on the outer races  28 ,  30 . 
     When the knob  12  is pushed, the upper and the lower bearings  20 ,  22 , travel downward together with the knob  12 . The distance between the cages  36 ,  38 , does not change and the pre-load generated by the springs  32  does not change either. The travel distance being of very few millimeters, any friction of the balls does not affect the motion of the knob  12 .