Abstract:
Flexible thermoplastic rack bar for meshing with a gear wheel, particularly for the drive of displaceable vehicle elements, like roof elements, sliding windows, and the like, having a series of supported teeth and a pull-resistant reinforcement embedded essentially parallel to a pitch line of the rack bar. The rack bar teeth are supported on at each lateral side and the reinforcement is disposed only in a cross-sectional area of the rack bar which lies heightwise between the head and the root of the rack bar teeth, preferably, substantially along a central transverse plane of the rack bar.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a flexible thermoplastic rack bar for meshing with a gear wheel, particularly intended for the drive of displaceable motor vehicle elements, like roof parts, sliding windows, and the like, having a series of supported teeth and a pull-resistant reinforcement embedded substantially parallel to a separation line of the rack bar. 
     It is a known rack bar of this kind (German Patent 31 23 554), the rack bar teeth are supported in their root area on one side. As a reinforcement, a steel band is located below the teeth, which has lateral clips which extend into the individual teeth of the rack bar. The manufacture of such a reinforcement is relatively costly, and furthermore, the obtainable tooth height for a preselected height dimension of the rack bar is limited, which has a detrimental effect on the transfer of force, in spite of the clips reaching into the individual teeth. 
     Similar problems regarding the ratio of cross-sectional surface and transferable force are encountered in connection with another known rack bar (German Gebranchsmuster 85 31 110), in which a series of individual, short rack bar segments made of synthetic material have two wire ropes embedded in the root area of the rack bar teeth, the wire ropes extending parallel to the longitudinal direction of the rack bar, interconnecting the individual segments. 
     SUMMARY OF THE INVENTION 
     It is the primary object of the present invention to provide a flexible thermoplastic rack bar, which is easy to manufacture, and has preselected cross-sectional dimensions which will facilitate an increased transfer of force. 
     Based on a flexible thermoplastic rack bar of the kind described above, the above-noted object of the invention is achieved by unilaterally supporting the rack bar teeth at both flanks, and by arranging the reinforcement only in a cross-sectional area of the rack bar which is between the head and the root of the rack bar teeth. 
     In the rack bar according to the invention, the pull-resistant reinforcement is disposed directly at, or in closest proximity to the force transfer point of the gearing. This, and the bilateral support of the teeth, ensure that a particularly high degree of tooth stability is obtained by simple means. Moreover, the inventive design facilitates the maintaining of an especially large ratio of tooth height to the outer dimension of the rack bar in the direction of the tooth height. Accordingly, the transferable force, relative to the rack bar cross section, is substantially increased over that of the prior art. 
     Thus, the reinforcement, appropriately, extends substantially along the center plane of the rack bar. The reinforcement may simply consist of at least one grid band in whose longitudinal direction there is a series of through openings with a division corresponding to the division of the rack bar teeth, whereby the through openings, with respect to the rack bar teeth, are aligned and dimensioned such that the grid band leaves openings between the rack bar teeth. In this manner, grid band webs separating the through openings from each other are embedded into the teeth, a fact, which, in connection with the bilateral support of the teeth, results in a particularly high degree of tooth stability. 
     Appropriately, the through openings of the grid band are at least approximately centered with respect to the rack bar teeth. The through openings of the grid band may be enclosed on all sides, when viewed from the top. In accordance with another embodiment, the design also can be such that the through openings of the grid band in top view are alternatingly open at one or the other longitudinal side of the grid band. With all other conditions being equal, the first variation results in a higher degree of pull resistance of the rack bar, while the second variation facilitates an increased lateral flexibility of the rack bar. In either case, the grid band advantageously can be made of spring steel or similar material. However, the reinforcement may also consist of at least one fiber strand, made of steel rope, glass fibers, carbon fibers, aramid fibers, or the like, at each side of the rack bar teeth. 
     To ensure a particularly favorable meshing of the gearing with the rack bar, the rack bar teeth, preferably, but not necessarily have a conventional involute gear profile. The tooth height of the rack bar teeth is, advantageously, in a range of 0.5 to 1.0 and preferably in a range of 0.6 to 0.8 times the outer dimension of the rack bar in the direction of the tooth height. 
     In instances where a particularly high degree of tooth stability is essential, the rack bar teeth can be additionally supported in the root area. Conversely, if the rack is provided with a series of through openings in the root area of the rack bar teeth, an increased flexibility of the rack bar is ensured. 
     Consecutive rack bar teeth can be connected at both sides via a web whose height is in a range of 0.6 to 1.0 times the tooth height. 
     The outer surface of the rack bar, further, can be formed such that it abuts only part of the outer surface of the guide element which at least partially surrounds it. Such a construction reduces sliding friction between the rack bar and the guide element. 
     These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a portion of a rack bar, made of synthetic material, in accordance with the invention; 
     FIG. 2 is a partial top view onto the rack bar shown in FIG. 1; 
     FIG. 3 is a longitudinal section taken along line 3--3 of FIG. 2; 
     FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 3; 
     FIG. 5 is a cross-sectional view taken along line 5--5 of FIG. 3; 
     FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 3; 
     FIG. 7 is a partial top view onto the grid band, used as reinforcement for the synthetic material rack bar of FIGS. 1-6; 
     FIG. 8 is a partial top view onto a modified grid band embodiment; 
     FIG. 9 a cross-sectional view of a synthetic material rack bar, corresponding to that of FIG. 5, showing a different form of reinforcement; 
     FIG. 10 is a longitudinal sectional view, corresponding to that of FIG. 3, showing a variation of a synthetic material rack bar in which a series of through openings are formed in the root area of the rack bar teeth; 
     FIG. 11 is a cross-sectional view taken along line 11--11 of FIG. 10; and 
     FIGS. 12 and 13 are cross-sectional views, corresponding to those of FIGS. 4 and 5, for a rack bar with a different outer contour. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The rack bar, designated 15 as a whole, and depicted in FIGS. 1-7, has a flexible cable-like body 16, made of synthetic material, which, for example, can be made of thermoplastic polyester, elastomer or polyamide material. The tooth rack 15 is provided with a series of teeth 17 along its length. The pitch of the teeth is designated T z  in FIG. 3, and the teeth have preferably, but not necessarily a conventional involute profile, even though not depicted as such. In the example depicted, adjacent teeth 17 are connected at both sides 18 and 19, as well as in their root area 20. 
     A grid band 21 is embedded into thermoplastic body 16 as a pull-resistance reinforcement. The grid band 21 consists of spring steel or another material capable of imparting the necessary pull resistance to the thermoplastic body. As can be seen particularly from FIG. 7, a series of openings 22 are formed in the longitudinal direction of the grid band 21. The interval of these recesses is designated with T a  in FIG. 7, whereby T a  equals pitch T z  of teeth 17. The openings 22 are aligned and dimensioned with respect to teeth 17 in such manner that grid band 21 leaves the openings 23 between the teeth 17 unobstructed. The grid band 21 extends parallel to the pitch line of rack bar 15 in a cross-sectional area of the rack bar disposed, in the direction of the tooth height, between head 24 and root 25 of teeth 17, so that one of the cross webs 26 of grid band 21 transversely extends through each tooth 17, as specially illustrated in FIGS. 2, 3 and 4. Preferably, grid band 21 extends along the center plane of rack bar 15, whereby its center axis 27 is somewhat below the profile center axis 28 of teeth 17 (FIG. 5), at or in close proximity to the force transference point of the gearing. 
     The rack bar 15 illustrated has a circular base cross section. Alternatively, however, other cross-sectional shapes can easily be employed, e.g. elliptical or oval base cross sections or a uniform or non-uniform polygonal base cross section. Independent of the base cross section chosen in a specific case, a tooth height a can be selected which, with respect to the outer dimension of the rack bar 15 in the direction of the tooth height, i.e. the overall height b of the rack bar (FIG. 5), is relatively large. Accordingly, in the FIG. 5 example, ratio a/b has a value of about 0.72. Generally, for the ratio a/b, a range of 0.5 to 1.0, and, preferably, of 0.6 to 0.8 is appropriate. 
     In the rack bar 15 depicted, adjacent teeth 17 are interconnected by a respective web 29 of thermoplastic body 16 at each of sides 18, 19, and the longitudinal webs 30 of grid band 21 embedded therein. In the example depicted, the height of webs 29 is essentially equal to the tooth height a. If, however, an increased flexibility of the rack bar is of importance, the height of webs 29 can be selected to be smaller than the tooth height a, and preferably within a range of 0.6 to 1.0 times of the tooth height. 
     While in the embodiment of the grid band 21 in accordance with FIGS. 2 to 7, the grid band openings 22, respectively, are enclosed on all sides by the cross webs 26 and the longitudinal webs 30 of the grid band, FIG. 8 shows a modified grid band 31, which, in top view, has a meander shape, so that the openings 22 of the grid band in top view alternatingly are laterally closed at only one or the other longitudinal side of the grid band 31. Such a construction enhances the lateral flexibility of the rack bar, which can be a significant benefit when the rack bar is used as a drive cable. 
     A further modified embodiment of the rack bar in accordance with FIG. 9, differs from the previously described embodiments in that, in place of the grid band 21, 31, a continuous fiber strand 34 is used as a reinforcement at each side of the rack teeth 17, respectively. Each such fiber strand 34 extends in the longitudinal direction of the rack bar, and may consist, in particular, of a steel rope, glass fibers, carbon fibers, aramid fibers, or similar pull-resistant fibers. 
     FIGS. 10 and 11 show a modified rack bar 15&#39; which differs from the rack bar 15 according to FIGS. 1-7 only in that its thermoplastic body 16&#39; is provided with a series of through openings 35 in the root area 20 of teeth 17. If required, this design, likewise, facilitates increased flexibility of the rack bar. 
     FIGS. 12 and 13, finally, depict cross-sections in accordance with FIGS. 4 and 5, in which the thermoplastic body 16&#34; of the rack bar, in the area of its outer surface, is formed such that the rack bar only abuts with part of the facing surface of a guide element 37, which at least partially surrounds its circumference, for example at the inside wall of a guide tube or a cylindrical guide channel. For example, as shown in FIG. 13, the outer surface of the rack bar has a cross-sectional shape forming lobes 38, the apex of which slide on the inner surface of the guide tube. In this manner, the sliding friction of the rack bar can be minimized in the associated guide element. 
     The flexible rack bars described are especially suitable for the drive of displaceable motor vehicle elements, like automatic windows or roof parts, e.g. the covers of sliding roofs, lifting roofs, sliding lifting roofs, and the like. In each case, the rack bar, appropriately, is made as a molded or continuous casting part, whereby, guide elements and the like, e.g. slide members, can be molded to the rack bar, as required. For example, the cable-like nature of the rack bar enables it to be utilizable for the drive cables of motor vehicle roofs of the type shown in U.S. Pat. Nos. 4,978,165; 4,919,005; 4,969,681, etc. 
     While we have shown and described various embodiments in accordance with the present invention, it is understood that the same is not limited thereto, but is susceptible of numerous changes and modifications as known to those skilled in the art, and we, therefore, do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.