Abstract:
An apparatus is shown which comprises first and second mating components with a force limiter located between them. The force limiter includes a band of resilient material, with protrusions therealong, and a slip element. The band and the slip element are secured to the first mating component with the band arranged to apply a radial force against the slip element so as to press it against the second mating component. The frictional force between slip element and the second mating component is insufficient to separate the slip element and the band from the first mating component, so any slipping that occurs between the mating components happens at the boundary of the slip element and the second mating component. A desired slip force can be obtained by choosing a slip element with an appropriate coefficient of friction, rather than by varying the dimensions of the components.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to Great Britain Application No. 0511494.7, filed Jun. 6, 2005, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to force limiting assemblies such as torque limiters or tolerance rings, wherein the force limiter provides an interference fit between parts of an assembly.  
         [0004]     2. Summary of the Prior Art  
         [0005]     Force limiters limit the maximum amount of force than can be transmitted between parts of an assembly. Commonly, a first part of the assembly has a cylindrical portion located in a cylindrical bore of a second part. The invention particularly relates to assemblies having a force limiter that provides an interference fit between a cylindrical component such as a shaft or a bearing and a housing for the shaft.  
         [0006]     Very close tolerances are required where press fits, splines, pins or keyways are employed to transmit forces in applications such as pulleys, flywheels or driveshafts.  
         [0007]     Force limiters may be used to provide an interference fit between parts required to transmit torque or parts required to transmit linear forces. Force limiters such as tolerance rings provide a low cost means of providing an interference fit between parts that may not be machined to exact dimensions. Tolerance rings have a number of other potential advantages, such as compensating for different linear coefficients of expansion between the parts, allowing rapid apparatus assembly, and durability.  
         [0008]     A tolerance ring generally comprises a band of resilient material, for example a metal such as spring steel, the ends of which are brought together to form a ring. A strip of protrusions extend radially outwards from the ring, or radially inwards towards the centre of the ring. Usually, the protrusions are formations, possibly regular formations, such as corrugations, ridges, waves or fingers.  
         [0009]     When the ring is located in the annular space between, for example, a shaft and a bore in a housing in which the shaft is located, the protrusions are compressed. Each protrusion acts as a spring and exerts a radial force against the shaft and the surface of the bore, providing an interference fit between the shaft and the housing. Rotation of the housing or the shaft will produce similar rotation in the other of the shaft or the housing, as torque is transmitted by the ring. Likewise, linear movement of the housing or the shaft will produce similar linear movement in the other of the shaft or the housing, as the linear force is transmitted by the ring. If forces (rotational or linear) are applied to one or both of the shaft and the housing, such that the resultant force between the mating components is higher than a certain threshold value, the shaft or housing will move relative to one another, i.e. they will slip. In this application, this threshold value is referred to as the “slip force” of the shaft, housing and torque limiter apparatus.  
         [0010]     Typically, the band of protrusions is axially flanked by annular regions of the ring that have no formations (known in the art as “unformed regions” of the tolerance ring).  
         [0011]     Although tolerance rings usually comprise a strip of resilient material that is curved to allow the easy formation of a ring by overlapping the ends of the strip, they may also be manufactured as an annular band.  
         [0012]     The term “shaft” as used hereafter includes any assembly component with a generally cylindrical portion such as a shaft or a bearing.  
       SUMMARY OF THE INVENTION  
       [0013]     For most applications, force limiters such as tolerance rings are designed to transmit relatively high levels of torque or linear forces between mating components (so the slip force between the mating components is relatively high). The slip force is proportional to the force applied by the tolerance ring on the mating components, and the friction coefficients of the tolerance ring and the mating components. The force applied by the tolerance ring depends on the strength of the tolerance ring protrusions, and the amount by which the protrusions are compressed. Because of the dimensional tolerances on the mating components and the protrusions, there is a limited range by which the tolerance ring protrusions can be compressed, giving rise to a limited range of forces available. For applications requiring relatively low slip forces, such as those requiring manual adjustment of mating components, even with relatively weak protrusions, the forces applied by the tolerance ring on the mating components may be too high to guarantee controlled low slip forces.  
         [0014]     It is an object of the present invention to provide a force limiter which, in combination with mating components, gives controlled, relatively low, slip forces between the mating components.  
         [0015]     Thus, according to a first aspect of the present invention, there is provided an apparatus comprising:  
         [0016]     first and second mating components, the first and second mating components being, respectively, either:  
         [0017]     a housing having a bore with a shaft therein, and the shaft, or  
         [0018]     a shaft, and a housing having a bore with the shaft therein; and  
         [0019]     a force limiter, the force limiter including a band of resilient material having a strip of protrusions extending therealong, and a slip element;  
         [0020]     wherein the band and the slip element are secured to the first mating component, and the slip element is frictionally engaged with the second mating component, the band applying a radial force against the slip element so as to press the slip element into contact with the second mating component, and  
         [0021]     wherein the securing of the slip element and the band with the first mating component has a securing strength which exceeds the frictional force between the slip element and the second mating component.  
         [0022]     Accordingly, when a resultant force is applied between the first and second mating components that is greater than the slip force, slipping of the apparatus components occurs at the boundary of the slip element and the second mating component.  
         [0023]     Preferably, the slip element is secured to the first mating component by being fixed, e.g. glued, welded or pinned, to the first mating component. In this case, the securing strength of the securing results from the quality and quantity of gluing, welding or pinning, respectively, between the slip element and the first mating component. The fixing must be stronger than the frictional engagement between the slip element and the second mating component, so that the slip element and the band will remain secure to the first mating component whilst any slipping occurs at the boundary between the slip element and the second mating component. In order to provide a surface of the slip element adjacent the first mating component, which is suitable for fixing to the first mating component, the slip element may straddle the band. Alternatively, the securing of the slip element to the first mating component may be provided through frictional engagement between the slip element and the first mating component. With this arrangement, the securing strength of the securing results from the magnitude of the frictional engagement between the slip element and the first mating component, i.e. the value of the coefficient of friction of each of the slip element and the first mating component, the degree of interference between them, and the size of the area over which they are in contact.  
         [0024]     In any case, the band may also be secured to the first mating component by the securing of the slip element to the first mating component.  
         [0025]     Axial movement of the slip element relative to at least one of the first and second mating components may be prevented in at least one direction, for example by the slip element including a lip which projects radially such that it contacts at least part of an axial end of the first or second mating component. With this arrangement, when the slip element is located between the mating components it is prevented by the lip from sliding axially in one direction relative to the first mating component. This is useful during assembly of the apparatus as, for example, when the first mating component is a housing and the second mating component is a shaft, the band and slip element can first be inserted into a bore inside the housing, and then the shaft can be inserted though an opening in the radial centre of the slip element without the slip element sliding further into the bore. Another alternative for the securing of the slip element and the band to the first mating component is for the securing to be provided through frictional engagement between the slip element and the band, and the band and the first mating component. This frictional engagement must be stronger than the frictional engagement between the slip element and the second mating component as explained above.  
         [0026]     Preferably, the slip element extends the entire length of the band. The band and/or the slip element may extend partly or entirely around the perimeter of the shaft. When the slip element extends only partly around the shaft its end are free to approach each other or separate as required, so it is more easily pressed against the second mating component by the radial force exerted by the band.  
         [0027]     The band may be a tolerance ring. The protrusions may extend radially outwards from the ring, or radially inwards towards the centre of the ring. Preferably, the protrusions are formations, e.g. regular formations, such as corrugations, ridges or waves. Alternatively the protrusions may be sprung fingers that project from the strip; this configuration is preferable when relatively low slip forces are required. The sprung fingers may project at an oblique angle, radially outwardly or radially inwardly from the ring.  
         [0028]     The strip of protrusions may be axially flanked by annular regions of the band of resilient material that have no formations.  
         [0029]     The shaft may have a circular cross-section, or a cross-section having one or more flat edges, e.g. a triangular, square, rectangular, or hexagonal cross-section etc.  
         [0030]     When the cross-section of the shaft has one or more flat edges, the band and/or slip element may each have one or more corresponding flat sections which, in combination, extend partly or entirely around the perimeter of the shaft.  
         [0031]     Preferably, when either the band or the slip element engages the housing, they engage, in particular, the wall of the bore of the housing.  
         [0032]     To obtain a desired slip force, the material of the slip element can be chosen to have an appropriate coefficient of friction. Lower values of slip forces can be obtained more reliably by choosing a material with a lower coefficient of friction for the slip element, rather than by varying the dimensions of the band and/or the mating components in order to reduce the press force applied by the strip of protrusions.  
         [0033]     Preferably, the slip element comprises a main ring portion axially flanked on one or both sides by flange portions. The ring portion may be a complete ring, or a split ring. The flange portion(s) project towards the mating component to which the slip element is fixed (i.e. towards the first mating component). Accordingly, the flange portion(s) may project radially inwardly or radially outwardly, depending on whether the first mating component is the housing having the bore, or the shaft.  
         [0034]     The flange portion(s) preferably have a surface, at their respective distal ends, for fixing to the first mating component. The flange portions may be spaced apart by a distance approximately equal to, or larger than, the width of the band of resilient material, such that the slip element may straddle the band.  
         [0035]     Accordingly, the band of resilient material may be fully or partially enclosed by the slip element and the first mating component. Thus, manufacture of the mating components and the tolerance ring apparatus may be straightforward, since both the slip element and the band of resilient material may be essentially fixed in position with respect to the first mating component before both the mating components are brought together for an interference fit.  
         [0036]     The force limiter of the apparatus may further include a second band of resilient material with a strip of protrusions extending therealong. This second band may again be a tolerance ring and may extend partly or entirely around the perimeter of the shaft.  
         [0037]     In this case, the slip element may comprise two main ring portions, the first main ring portion being axially flanked on both sides by first and second flange portions which project towards the first mating component, and the second main ring portion being axially flanked on both sides by said second and a third flange portion, which third flange portion also projects towards the first mating component. The main ring portions of the slip element may be a complete ring or a split ring, as described above.  
         [0038]     Furthermore, in this case, the first and second flange portions may be spaced apart by a distance approximately equal to the width of one of the first or second bands of resilient material, and the second and third flange portions may be spaced apart by a distance approximately equal to the width of the other of the first or second bands of resilient material, such that the slip element straddles each of the bands with the second flange portion located between the bands. Accordingly, each band of resilient material may be at least partially enclosed by the slip element and the first mating component.  
         [0039]     The force limiter of the first aspect of the invention is particularly suitable for apparatuses where an interference fit between mating components has to be relatively low and controlled, for example steering column axial adjustment mechanisms, where the steering column is to be adjustable in length by the driver of the vehicle. However, it will be appreciated by those in the art that there are other possible arrangements to which the invention can be applied.  
         [0040]     According to a second aspect of the present invention, there is provided a method of limiting to a threshold value a force transmitted between parts of an apparatus, said apparatus comprising:  
         [0041]     first and second mating components, the first and second mating components being, respectively, either:  
         [0042]     a housing having a bore with a shaft therein, and the shaft, or  
         [0043]     a shaft, and a housing having a bore with the shaft therein; and  
         [0044]     a force limiter, the force limiter including a band of resilient material having a strip of protrusions extending therealong, and a slip element;  
         [0045]     wherein the band and the slip element are secured to the first mating component, and the slip element is frictionally engaged with the second mating component, the band applying a radial force against the slip element so as to press the slip element into contact with the second mating component, and  
         [0046]     wherein the securing of the slip element and the band with the first mating component has a securing strength which exceeds the frictional force between the slip element and the second mating component,  
         [0047]     the method comprising the step of applying a force to one or both of the first and second mating components,  
         [0048]     wherein, when the resultant force between the first and second mating components exceeds the strength of the frictional engagement between the slip element and the second mating component, the slip element and the second mating component move relative to one another. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0049]     Embodiments of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which:  
         [0050]      FIG. 1  shows an oblique view of a known tolerance ring;  
         [0051]      FIG. 2  shows an oblique view of a slip element of the present invention;  
         [0052]      FIG. 3  shows mating components fitted with a first embodiment of a tolerance ring assembly of the present invention;  
         [0053]      FIG. 4  shows mating components fitted with a second embodiment of a tolerance ring assembly of the present invention.  
         [0054]      FIG. 5  shows mating components fitted with a third embodiment of a tolerance ring assembly of the present invention.  
         [0055]      FIG. 6  shows a cross sectional view through the components of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION  
       [0056]     A known tolerance ring  1  is shown in  FIG. 1 . The tolerance ring  1  comprises a band of resilient material  11 , for example a metal such as spring steel, the ends  12 ,  13  of which are brought together to form a ring. A strip of protrusions  14  extend radially outwards from the ring. The protrusions  14  are regular corrugated formations. The strip of protrusions  14  is axially flanked by annular regions  15 ,  16  of the band of resilient material  11  that have no formations.  
         [0057]      FIG. 2  shows a slip element  2  according to the present invention. The slip element  2  comprises a main ring  21  of flexible material, axially flanked by two circular flanges  22 .  
         [0058]     The slip element  2  is dimensioned such that a tolerance ring, e.g. the tolerance ring of  FIG. 1 , can be fitted around the circumference of the main ring  21  whereupon it fits between the flanges  22 . Ideally, the tolerance ring is fitted tightly between the flanges  22 , whereupon it abuts the main ring  21 .  
         [0059]      FIG. 3  shows a first embodiment of a force limiter according to the present invention that provides an interference fit between two mating components, e.g. mating components of a length adjustable steering wheel column assembly. One of the mating components is e.g. an outer jacket of the steering column, which essentially comprises a housing  3  having a bore  31  therein. The other of the mating component is e.g. an inner tube  4  of the steering column assembly.  
         [0060]     The force limiter comprises a tolerance ring  1 , e.g. as described above with reference to  FIG. 1 , which is fitted around the slip element  2  as described above with reference to  FIG. 2 . Both the slip element  2  and the tolerance ring  1  extend entirely around the perimeter of the shaft  4 .  
         [0061]     The shaft  4  extends through an opening provided through the radial centre of the slip element  2 . The radially inner surface of the main ring  21  of the slip element  2  is frictionally engaged with the surface of the shaft  4 .  
         [0062]      FIG. 3  in fact shows two force limiters according to the present invention, which are spaced apart along the axis of the bore  31 . Using two force limiters in this manner increases the stability of axial alignment between the shaft  4  and the bore  31 .  
         [0063]     The flanges  22  project towards the wall  32  of the bore  31 . The distal ends of the flanges  22  each have a radially outwardly facing surface  23  which is fixed to the wall  32  of the bore  31 . The surfaces  23  are fixed to the wall  32  by glue, pins, welds or by the interaction of engagement features that are e.g. press formed into the surfaces  23  and the wall, or by other appropriate fixing means. However, it will be appreciated that the surfaces  23  may be secured to the wall  32  of the bore  31  through frictional engagement between the surfaces  23  and the wall  32 .  
         [0064]     The tolerance ring  1  is fitted to the radially outer side of the slip element  2  such that it is completely enclosed by the main ring  21 , the flanges  22  and the wall  32  of the bore  31 . Thus, the tolerance ring  1  is secured to the housing  3  by the securing of the slip element  2  to the housing  3 .  
         [0065]     During manufacture, before the slip element is fixed to the wall  32  of the bore  31 , the tolerance ring fitted around the main ring  21  of the slip element  2  protrudes further, radially, than the flanges  22  of the slip element  2 . Accordingly, when the slip element is fixed to the wall  32  of the bore  31 , the tolerance ring is compressed, whereupon it presses against the main ring  21  of the slip element  2 . This pressing force causes the main ring  21  to flex towards the shaft  4 , thus increasing the friction force between the slip element and the shaft  4 .  
         [0066]     In use, if forces (rotational or linear) are applied to one or both of the mating components  3 ,  4 , such that the resultant force between the mating components is lower than the slip force value, the mating components will move in concert with one another, because the force will be transmitted between the mating components through the tolerance ring assembly.  
         [0067]     If, however, the resultant force exceeds the slip force value, the mating components  3 ,  4  will move, i.e. slip, with respect to one another. This slipping occurs at the boundary between the slip element  2  and the surface of the shaft  4 .  
         [0068]     The value of the slip force is therefore dictated by the frictional force between the slip element  2  and the surface of the shaft  4 . This frictional force, and thus the slip force, can be adjusted by e.g. varying the dimensions of the tolerance ring  1 , in order to vary the press force it applies against the slip element  2  and/or by varying the type of material used for the slip element  2 , in order to change the coefficient of friction of the slip element  2 .  
         [0069]     If lower values of slip force are required, a slip element  2  with a lower coefficient of friction is can be used. This is advantageous since the dimensional tolerance of the tolerance ring  1  and mating components  3 , 4  are such that the range of forces that the tolerance ring  1  can apply, under compression, may not be sufficiently low to guarantee controlled relatively low slip forces. Choosing a slip element  2  with a lower coefficient of friction allows the slip force to be lowered without the force applied by the tolerance ring requiring to be lowered.  
         [0070]      FIG. 4  shows a second embodiment of a force limiter according to the present invention used to provide an interference fit between two mating components  3 ,  4 , and works in a similar manner to the assembly of the first embodiment (the same reference numerals are given to identical components of  FIGS. 3 and 4 ); however, the slip element  2   a  is fixed to the shaft  4 , instead of the wall  32  of the bore  31 , and is frictionally engaged with the wall  32  of the bore  31 .  
         [0071]     Accordingly, the flanges  22   a  of the slip element  2   a  project radially inwardly from the flexible main ring  21   a,  and the radially inwardly facing surfaces  23   a,  at the distal ends of the flanges  22   a,  are fixed to the surface of the shaft  4 .  
         [0072]     The tolerance ring  1  is fitted to the radially inner side of the slip element  2   a  such that it is completely enclosed by the main ring  21   a,  the flanges  22   a  and the surface of the shaft  4 .  
         [0073]     With the configuration shown in  FIG. 4 , slipping of the mating components occurs at the boundary between the slip element  2   a  and the wall  32  of the bore  31 .  
         [0074]      FIG. 5  shows a third embodiment of a force limiter according to the present invention that provides an interference fit between two mating components  3 ,  4  and again works in a similar way to the assembly of the first embodiment shown in  FIG. 3 . The same reference numerals are given to identical components.  
         [0075]     However, in this third embodiment, the force limiter includes two axially separated tolerance rings  1  each comprising a band of resilient material  11 ,  12  and each having a strip of protrusions  14  extending therealong. To operate with these two bands  11 ,  12 , the slip element  2   b  comprises two main ring portions  21   b,    21   c.  The first main ring portion  21   b  is axially flanked on both sides by first and second flange portions  22   b,    22   c  which project towards the first mating component (housing  3 ), and the second main ring portion  21   c  is axially flanked on both sides by said second and a third flange portion ( 22   c  and  22   d  respectively). The third flange portion  22   d  also projects towards the housing  3 . The first and second flange portions  22   b,    22   c  are spaced apart by a distance equal to the width of the first band of resilient material  11 , and the second and third flange portions  22   c,    22   d  are spaced apart by a distance equal to the width of the second band of resilient material  12 , such that the bands of resilient material  11 ,  12  are each enclosed by the slip element  2   b  and the housing  3 . In other words, the bands  11 ,  12  are located in grooves between the flange portions  22   b,    22   c  and  22   d.  The tolerance ring  1  (comprising band  11 ) and the slip element  2   b  extend only partly around the perimeter of the shaft  4 , as shown in  FIG. 6 , which is a cross sectional view along line D-D of  FIG. 5 .  
         [0076]     The assembly of the slip element  2   b  and the bands  11 ,  12  is again as described above with reference to  FIGS. 2 and 3 , but the securing of the slip element  2   b  and the bands  11 ,  12  to the housing  3  is provided through frictional engagement between the slip element  2   b  and the bands  11 ,  12 , and the bands  11 ,  12  and the housing  3 .  
         [0077]     The arrangement and operation of components in the third embodiment is then the same as in the first embodiment, with slipping of the mating components  3 ,  4  occurring at the boundary between the slip element  2   b  and the surface of the shaft  4 .  
         [0078]     It will be noted that the embodiment in  FIG. 4  may be similarly modified to have two bands each located in one of two main ring portions of the slip element, with the bands located in grooves between pairs of flange portions such that they are enclosed by the slip element and the shaft  4 . Slipping of the mating components would then occur at the boundary between the slip element and the wall  32  of the bore  31 .  
         [0079]     The third embodiment in  FIG. 5  further shows the slip element  2   b  located at one end of the bore  31  and, aside from the flanges  22   b,    22   c,  and  22   d  which project towards the wall  32  of the bore  31 , the slip element  2   b  further has a lip  24  which projects further radially outwards such that it contacts part of an axial end  3   b  of the housing  3 , i.e. an end of the first mating component to which the slip element  2   b  is secured. With the slip element  2   b  located between the housing  3  and the shaft  4  as shown, it is prevented by the lip  24  from sliding axially in one direction (right as  FIG. 5  is drawn) relative to the housing  3 .  
         [0080]     This arrangement is useful during assembly of the apparatus as, after the bands  11 ,  12  have been fitted to the radially outer side of the slip element  2   b,  they can both be inserted into the bore  31  inside the housing  3  and then the shaft  4  can be inserted though an opening in the radial centre of the slip element  2   b  without the slip element  2   b  sliding further into the bore  31 .  
         [0081]     It will be noted that the embodiment in  FIG. 4  may be similarly modified, such that the slip element is again located at one end of the bore  31  (as in the third embodiment) and, aside from the flanges which project towards the surface of the shaft  4 , the slip element further has a lip which projects further radially inwards such that it contacts part of an axial end of the shaft  4 . Slipping of the mating components would then occur at the boundary between the slip element and the wall  32  of the bore  31 .