Abstract:
A coupling element, for example a coupling sleeve ( 1 ), having coupling teeth ( 2 ) which, over a part ( 2   a ) of their length, have a gradually diminishing cross-sectional profile, a so-called back rake. Over the part, the coupling teeth have a portion ( 7   b ) nearest the tooth root having a first pressure angle, which portion connects to a portion ( 7   b ) extending to the tooth tip and having a greater pressure angle than the former portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation patent application of International Application No. PCT/SE2004/000588 filed 15 Apr. 2004 which is published in English pursuant to Article 21(2) of the Patent Cooperation Treaty and which claims priority to Swedish Application No. 0301839-7 filed 24 Jun. 2003. Said applications are expressly incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to a coupling element in a motor vehicle gearbox, which coupling element has coupling teeth distributed in a circle and that are orientated at least substantially in the axial direction of the gearbox. The coupling teeth are arranged, upon engagement with coupling teeth on an interacting coupling element, to lock an element mounted rotatably in the gearbox, for example a gearwheel or a shaft, which coupling teeth, when viewed from one end, have a gradually diminishing cross-sectional profile over at least a part of their extent in the axial direction. 
         [0003]    The invention also relates to a method of manufacturing a coupling element having coupling teeth that are distributed in a circle and which when, viewed from one end, at least over a part of their length, have a gradually diminishing cross-sectional profile. In a first stage of the manufacturing method, machining of the coupling element takes place so that coupling teeth, so-called original teeth, of constant cross-sectional profile over their length are formed. In a second stage, machining of the original coupling teeth takes place so that these, at least over a part of their length, are lent a cross-sectional profile which gradually diminishes from one end. 
       BACKGROUND OF INVENTION 
       [0004]    It is common practice to, in gearboxes, configure the coupling sleeves and the coupling rings of the disengagable gearwheels with coupling teeth which are tapered so that mutually contacting V-shaped ends of the teeth on the coupling sleeves and the coupling rings are wider than the teeth at a distance from said ends. It is customary to say that the flanks of the coupling teeth are configured with a “back rake”. The rotational force upon mutually contacting surfaces with back rake will consequently produce a resultant force which acts in the direction of engagement of the coupling sleeve in order to prevent an engaged gear from being unintentionally disengaged. 
         [0005]    In the manufacture of the aforementioned coupling elements, efforts have previously been made to configure the back-raking toothed portion with flanks having the same pressure angle as the original tooth flank. In this way, the least possible material quantity is pressed aside or removed from the original flank. 
         [0006]    Following manufacture of a coupling element having constant cross-sectional profile of the coupling teeth, various machining methods have been utilized in order thereafter to form toothed portions having back-raking flanks. The two most common methods are press-rolling and machine-cutting. Both have advantages and disadvantages. 
         [0007]    In press-rolling, the material is pressed aside from the portion with back rake. This method has the advantage that the tooth can be configured with back rake right down to the tooth root without the need for undercutting, which otherwise weakens the tooth. The disadvantage is that the wear on the tool is very high, resulting in relatively short service life, so that the tool costs become relatively high. Pressed-aside material is left behind as a bead, which can also be disadvantageous. 
         [0008]    In the other method, material is removed by machine-cutting using a cutting tool to which motion along a cycloid curve is imparted. The advantage is longer service life of the tool but, in return, it is not possible to form, for example, internal teeth on a coupling sleeve with back rake right down to the tooth root without any undercutting in the root of the profile of the original tooth or, in the absence of undercutting, without a sharp shelf being formed in the tooth at a distance from the bottom of the tooth space of the original tooth. In both cases, this can mean that the strength is reduced in the coupling teeth with back rake. Undercutting can mean, moreover, that the strength is reduced in the profile of the original tooth, which can be a major disadvantage should the continuation thereof, substantially axially in connection with the back-raking coupling teeth, be used, for example, in a splined joint or as a rolling tooth against an opposing gearwheel. 
       SUMMARY OF INVENTION 
       [0009]    The purpose of the present invention is to produce a coupling element of the type introduced above and which is configured such that its back-raking tooth portions can be manufactured by machine-cutting almost right down to or even down into the tooth root of the profile of the original tooth without the need for undercutting thereof and without the formation of a sharp shelf. 
         [0010]    This is achieved according to the teachings of the invention by the opposing flanks of the coupling teeth over the back-raking part having a first portion nearest the tooth root having a first pressure angle, which first portion connects to a second part extending to the tooth tip and having a greater pressure angle than the first portion. 
         [0011]    In the method of manufacturing the coupling element which has been described in the introduction, this is quite simply achieved by the teeth, in the second stage, being machined so that the pressure angle of the machined flank surfaces is greater than the pressure angle following the machining in the first stage. 
         [0012]    In the machining, the tool is directed so that the flank on the back rake bevels into the shape of the original tooth in the lower part of its flank or even some of the way down into its tooth root; i.e., the tool is directed so that the flank surfaces with back rake form a transition at an obtuse angle to the original flank surfaces. This avoids machining along a profile between back rake and the profile of the original tooth, which creates a shelf; i.e., a sharp transition between the flank surfaces of the back rake and the original flank surfaces. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention is described in greater detail with reference to the illustrative embodiments shown in the accompanying drawings, in which: 
           [0014]      FIG. 1  shows a diagrammatic plan projection of a part of a coupling sleeve, synchronizing ring and coupling ring in a motor vehicle gearbox; 
           [0015]      FIG. 2  shows a diagrammatic side view of a part of a coupling sleeve and the motional path of a cutting tool in a known machining process; 
           [0016]      FIG. 3  shows a view corresponding to  FIG. 2 , but with the motional path of the tool in the machining process according to the present invention; and 
           [0017]      FIG. 4  shows a diagrammatic perspective view of a coupling tooth of a coupling element according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In  FIG. 1 , a coupling sleeve  1  is shown that is intended for non-rotatable, yet axially displaceable, mounting on a hub of a shaft mounted rotatably in a gearbox (not shown). 
         [0019]    The coupling sleeve  1  is configured with coupling teeth  2  intended to be brought into engagement with coupling teeth  3  on a coupling ring  4  intended to be connected non-rotatably to a gearwheel (not shown) mounted rotatably on the shaft. A synchronizing ring with locking teeth  6 , disposed between the coupling sleeve  1  and the coupling ring  4 , is denoted by  5 . 
         [0020]      FIG. 2  shows a part of a coupling sleeve  1  having teeth  2  formed in an initial machining so that opposing flanks  7  are parallel; i.e., the thickness of the teeth  2  does not vary along the length of the teeth. In the figure, the dashed line “a” illustrates the motional path of two different cutting tools “b” and “c” respectively in subsequent machining of the teeth  2  according to a previously known method for forming the portion with back rake. The motion along the shown path “a” can be produced with a tool which replaces a tooth on a gearwheel rolling against the teeth  2  of the coupling sleeve. 
         [0021]    With the tool “b”, the cutting of the material is commenced at the tooth tip and concluded directly above the tooth root. With the tool “c”, the cutting takes place in the reverse direction. Common to both tools is, however, that they move along a path “a” which, within the cutting region of the tools, lies substantially parallel with the original tooth flank  7 . The result is that a “shelf”  8  having a sharp edge  9  is formed directly above the tooth root. In order to escape the formation of such a shelf, the teeth can be configured with an undercut, but the tooth root is then weakened, in return. 
         [0022]    As in  FIG. 2 ,  FIG. 3  shows a part of a coupling sleeve  1  having teeth  2  formed in an initial machining so that opposing flanks  7  are parallel; i.e., the thickness of the teeth does not vary along the length of the teeth. 
         [0023]    According to the present invention, the tools “b” and “c” for machining the portion with back rake, here directed along a path “d” which, within the cutting region of the tool, is not parallel with the original tooth flank  8 . The coupling teeth  2  thereby acquire a flank portion  7   a  with greater a pressure angle than the remaining flank portion  7   b , which has the pressure angle of the original tooth (see  FIG. 4 ). 
         [0024]    By choosing a larger pressure angle for the flank portion  7   a , the formation of a shelf  8  is avoided. Instead, a transition  10  with an obtuse angle, preferably selected to be greater than 150 degrees, is formed between the flank portions  7   a  and  7   b  when the tool is directed along a path “d” whose turning point lies in a tooth space  11 , in the shown illustrative embodiment approximately in the middle of a tooth space  11 , and not, as in the previously known case shown in  FIG. 2 , within the cutting region of the tooth. 
         [0025]    In  FIG. 4 , a coupling tooth is shown having a front portion  2   a  with back rake and a rear portion  2   b  with no back rake. The difference in the pressure angle between the flank portions  7   a  and  7   b  is here clearly visible at the transition surface  13  between the front portion  2   a  and the rear portion  2   b.    
         [0026]    The invention has been described above with reference to a coupling sleeve  1  having internal teeth  2 , but opposite-running coupling elements with external cogs, for example the coupling rings, must, of course, be given a tooth form matched to the tooth form of the coupling sleeve. 
         [0027]    Within the scope of the invention, the coupling element can constitute part of a gearwheel having involute teeth, which engage with teeth on an opposite-running gearwheel, in which case the coupling teeth with back rake are configured in one piece with and form an extension of the involute teeth of the gearwheel. Such a gearwheel with integrated coupling sleeve might, for example, be the ring gear in a planetary gearing in a range-change gearbox, in which the ring gear is axially displaceable and in which its coupling teeth are brought into engagement with a coupling ring, fixedly connected to the gearbox casing, for locking of the ring gear.