Abstract:
A tuning cable assembly comprising a reinforced tube, a tuning cable placed within said reinforced tube, where the tuning cable is comprised of a thermoset material or a cross-linked thermoplastic material, and a narrow annular cavity formed between the inner surface of the reinforced tube and the outer surface of the tuning cable. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. This abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Description:
TECHNICAL FIELD 
   The present invention relates to a tuning cable and in particular to a thermoset tuning cable placed within a reinforced tube for attenuating pressure pulsations in the presence of fluid flow. 
   BACKGROUND OF THE INVENTION 
   Fluid pumps, whether driven by an internal combustion engine, an electric motor or by fluid system valves, have pressure fluctuations or pulses in the associated fluid line in the form of a fluid ripple. The fluid ripple undesirably generates audible and objectionable noise within the fluid pump itself and may also excite components downstream of the fluid pump (e.g., the steering gear in power steering systems), further increasing unwanted noise generation. Apart from the noise itself, the existence of a fluid ripple reduces the operational efficiency of the fluid pump. The pressure fluctuations are generated by pistons, gerotors, gears, vanes or other fluid displacement elements within the fluid pump, and at a frequency that is typically dependent upon the pump speed. 
   Tuning cables are used in the known art to attenuate fluid ripple frequencies in power steering systems. The power steering tuning cables are typically formed from wrapped steel or thermoplastic elements. However, both materials have their drawbacks. For example, in the case of wrapped steel cables, while inexpensive to manufacture, metal particles may cling to the wrapped steel tuning cables, eventually damaging the pump and valves, and reducing the performance of the tuning cable. Thermoplastic tuning cables are costly to manufacture. 
   In determining the optimal material for use in manufacturing power steering tuning cables, three main physical characteristics of the materials are considered. The ultimate or tensile strength, the elongation and the modulus (stress over strain) of materials are analyzed. Based upon these characteristics, the physical characteristics of Teflon® have made it a suitable material for use in the high temperature and extreme chemical environment associated with power steering tuning cables. Other materials, including cross-linked thermoplastic materials, are known in the art and have been used for power steering hoses, but have not been utilized to manufacture tuning cables due to the superior performance of Teflon®. However, as stated above, Teflon® is a costly material. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a tuning cable assembly comprising a reinforced tube, and a tuning cable placed within the reinforced tube. The tuning cable is preferably formed from a thermoset or cross-linked thermoplastic material and includes a narrow annular cavity formed between the inner surface of the reinforced tube and the outer surface of the tuning cable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is an automotive power steering system according to an embodiment of the present invention illustrating one tuning cable. 
       FIG. 2  is an automotive power steering system according to an alternate embodiment of the present invention illustrating the use of two tuning cables in series. 
       FIG. 3  is a cut away of a thermoset power steering tuning cable assembly according to a first embodiment of the present invention. 
       FIG. 4  is a cut away view of a thermoset power steering tuning cable assembly according to an alternate embodiment of the present invention. 
       FIG. 5  is a cut away view of a thermoset power steering tuning cable assembly according to a further alternate embodiment of the present invention. 
       FIG. 6  is an expanded view of a connector in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a simplified representation of a closed-loop automotive power steering system  10  illustrates one environment for use of an inventive tuning cable assembly  20  of the present invention. While a close-looped system is illustrated, the present invention may be used in alternative environments subjected to undesirable pressure fluctuations. In the illustrated embodiment, a power steering pump  11  generates pressure ripples that are transmitted through the closed-loop system  10  by way of a pressure line  12 , a power steering gear  13 , a return line  14 , a reservoir  15 , and through a supply line  16  back to the power steering pump  11 . 
   To reduce the pressure ripples generated from the power steering pump  11  before the pressure ripples reach the power steering gear  13  via the pressure line  12 , a power steering tuning cable assembly, generally shown at  20 , is illustrated as being disposed in the pressure line  12  between the power steering pump  11  and the power steering gear  13 . Although the power steering tuning cable assembly  20  has been illustrated in  FIG. 1  as being disposed between the power steering pump  11  and the power steering gear  13 , the power steering tuning cable  20  can be disposed in alternative locations within system  10  including, for example, within return line  14  between the power steering gear  13  and the reservoir  15  to reduce vibration downstream of power steering gear  13 . In some cases it may be desirable to have multiple tuning cable assemblies  20  such as that illustrated in  FIG. 2  with respect to a system  10 ′ wherein two tuning cable assemblies  20  are disposed in series within pressure line  12  between power steering pump  11  and power steering gear  13 . 
     FIG. 3  illustrates in greater detail a tuning cable assembly  20  according to a first embodiment of the present invention. Cable assembly  20  comprises a first tubing element T 1 , a second tubing element T 2 , and third reinforced tubing element T 3  disposed between tubing elements T 1  and T 2  and including a coupling  40  disposed at opposing ends of tubing element T 3 . Tubing T has an inlet end  22   a  and an outlet end  22   b  in relation to tubing element T 3 . Thus, in the illustrated embodiment, it is assumed that fluid flows downstream from tubing element T 1  through reinforced tubing element T 3  and into tubing element T 2 . In some uses, however, fluid flow may be in the opposite direction. Thus, the nomenclature of “inlet end” and “outlet end” are for illustration only. The tubing T may be made from a polymeric material, as shown in the illustrated embodiment, or may be made from a metal alloy. While not required, tubing T is shown with a generally cylindrical cross-sectional configuration. 
   Disposed within reinforced tubing element T 3  is a hollow tuning cable  24  having an inlet end  24   a  and an outlet end  24   b  downstream of the inlet end  24   a . A connector  30  associated with the tuning cable  24  works in concert with one of the couplings  40  to secure the tuning cable  24  to tubing T. A narrow annular cavity  23  is formed between an outer periphery of tuning cable  24  and an inner periphery of reinforced tubing element T 3 . 
   As illustrated in  FIGS. 3 through 6 , the connector  30  can be made of metal, such as stainless steel, brass or aluminum, or of a suitable plastic or polymeric material. The connector  30  includes a hollow portion  32  that provides a secure connection between tubing T and the tuning cable  24 , and allows fluid to flow from the inlet end  22   a  of tubing T into inlet end  24   a  of the tuning cable  24 . The hollow portion  32  of the connector  30  is secured in the inlet end  22   a  of tubing T by inserting the hollow portion  32  of the connector  30  into the inlet end  22   a . A collar  38  is provided on the outer periphery of the connector  30  and abuts the inlet end  22   a  of tubing T when the connector  30  is fully inserted into the tubing T. In order for the connector  30  to fit within the inlet end  22   a  of the tubing, the inner diameter of the inlet end  22   a  of the tubing T must generally be larger than the outer diameter of the hollow portion  32  of the connector  30  although a friction fit is also possible. At the opposite end of the hollow portion  32  is the end  36  of the connector  30 . The end  36  of the connector  30  can be tapered inwardly to facilitate insertion of the connector  30  into inlet end  24   a  of the tuning cable  24 . Intermediate the hollow portion  32  and the end  36  of the connector  30  is a barbed or ridged portion  35  with barbs or ridges  34  thereon. The tuning cable  24  is secured to the connector  30  by the barbs or ridges  34  of the barb or ridge portion  35 . It can be appreciated that the present invention is not limited to barbs or ridges but may have any configuration that will secure the tuning cable  24  to the connector  30 . Either end of the tuning cable  24  may be connected to the tubing T via the connector  30 ; in the illustrated embodiments of the present invention, the inlet end  24   a  of the tuning cable  24  is connected in fluid communication with the tubing T via the connector  30 . 
   Finally, coupling  40  is used to secure the tubing T and the connector  30 . In the illustrated embodiment, the tubing elements T 1  and T 3  are crimped by compressing coupling  40  from an unloaded orientation having a first inner radial diameter to a permanent loaded orientation having a second inner radial diameter less than the first, the crimping action also acting to firmly secure connector  30  between the inner surface of the coupling  40  and the outer surface of the connector  30 . The coupling  40  can be made of a metal, such as stainless steel, brass or aluminum, or of a suitable hard polymeric material. 
   Referring now specifically to the embodiment of  FIG. 3 , the tuning cable  24  does not extend all the way from the inlet end  22   a  to the outlet end  22   b , but stops short of the outlet end  22   b . The present invention can be practiced with the tuning cable  24  ending at any desired position within the reinforced tubing element T 3 . Thus, the length of tuning cable  24  may be optimized depending on the environment of use to minimize any fluid rippling. Fluid make up, its speed (e.g., the power of pump  11 ), the temperature of the fluid, the speed of sound in the fluid and the frequency response are examples of factors influencing the length of tuning cable  24 . Although either the inlet end  22   a  or the outlet end  22   b  of the tubing T can be connected to the tuning cable  24 , in the illustrated embodiment of the present invention, the inlet end  22   a  is connected to the tuning cable  24  via the connector  30 . 
   In addition to the open end  24   b , or as an alternate embodiment of the present invention, the tuning cable  24  may have one or more apertures or holes  25  that allow the fluid to enter and exit the cavity  23  from the tuning cable  24 . The aperture or hole  25  is shown to have a circular cross-sectional configuration, however, it can be appreciated that the aperture or hole  25  can be of any desired shape. In addition, it can be appreciated that the present invention can be practiced with various numbers and arrangements of apertures or holes  25 . For example, as with the length of tuning cable  24 , the number and size of apertures or holes  25  may vary as a function of the same parameters noted above. Further, the apertures or holes  25  can be provided on one side of the tuning cable  24 , on opposing sides of the tuning cable  24  or about its entire periphery as determined as a result of the operating conditions and noise suppression desired. 
   Referring to  FIG. 4 , an alternate embodiment of the present invention is illustrated. In this embodiment of the present invention, the tuning cable  24 ′ extends continuously from the inlet end  22   a  to the outlet end  22   b  of the tubing T. While not illustrated, a second connector  30  may be disposed between tubing elements T 3  and T 2 . Such an approach may be desirable particularly if the tuning cable  20  has an extended length or the operational conditions may result in vibration or associated movement of tuning cable  24  within reinforced tubing element T 3 . The remaining features of  FIG. 3  are substantially incorporated into the illustrated embodiment of FIG.  4 . 
   Referring to  FIG. 5 , another embodiment of the present invention is illustrated. Power steering tuning cable assemblies  20  can be connected in series, in a manner similar to that described in conjunction with  FIGS. 3 and 4 . 
   Referring to  FIGS. 1-6 , the tuning cable  24  is preferably made from a thermoset material, particularly when used in the environment of a power steering system. The thermoset material offers advantages in creep and melting temperature characteristics over thermoplastic power steering tuning cables. The thermoset material does not have a melting temperature in a region of possible operation. Therefore, concerns associated with extremely high fluid temperatures outside the range of normal operations will not effect the performance of the thermoset power steering tuning cable  24 . In addition, thermoset materials are generally less expensive than thermoplastic materials. 
   In an alternate embodiment of the present invention, the tuning cable  24  is made of a cross-linked thermoplastic material. Cross-linking thermoplastic material with even less expensive material causes the thermoplastic material to undergo a chemical reaction resulting in a cross-linked thermoplastic material. The cross-linked thermoplastic material further reduces the cost of the tuning cable  24  while maintaining substantially similar performance characteristics as thermoplastic or thermoset tuning cables. Additionally, the cross-linked thermoplastic material will still be able to withstand the extreme temperature and chemical environments of fluids associated with steering applications, 
   The thermoplastic material used in the cross-linking process can be any thermoplastic material having reactive sites, such as —NCO, —NH 2 , or —OH, along the polymer chain. Examples of such thermoplastic material include, but are not limited to, nylon 6, nylon 6/6, nylon 4, nylon 11, nylon 12, nylon 6/12, and nylon 6/10. Ideally, a power steering tuning cable  20  in accordance with the present invention is made from the nylon 6 family. However, the thermoplastic material is not limited to the material listed above and may be any thermoplastic material capable of undergoing a cross-linking reaction in the presence of a cross-linking agent. The cross-linking agent of the present invention can be any suitable polyfunctional compound which reacts with the reactive sites on the thermoplastic material to cross-link the thermoplastic polymer. The preferred cross-linking agent is an isocyanate which contains at least 2.1, preferably more, functional groups which react with the reactive sites on the thermoplastic polymer. 
   The cross-linking agent may be cross-linked with the thermoplastic material during the extrusion process. The extruder is maintained at a temperature sufficient to heat the thermoplastic polymer and the cross-linking agent to a temperature where they become very reactive. The thermoplastic polymer and the cross-linking agent begin to cross-link forming an extruded cross-linked thermoplastic polymer which has the physical characteristics necessary for use in a high temperature and harsh chemical environment. 
   Operationally, a thermoset or a cross-linked thermoplastic tuning cable  24  provides a number of additional advantages over the prior art. The amplitude and frequency of the pressure ripples within the tuning cable  24  effects the level of audible noise generated from the fluid; the greater the amplitude or frequency of the pressure ripples, the louder the audible noise. In contrast to the known teachings, the inventor has determined that tuning cables  24  formed from a thermoset material or a cross-linked thermoplastic material are able to lower the frequency of the pressure ripples in the tuning cable, effectively lowering the level of audible noise. It turns out that the speed of sound is much greater for metals than for soft polymers. The metal and thermoplastic materials, such as Teflon®, currently used to manufacture tuning cables have similar sound wave speeds and similar levels of audible noise. Further, tuning cables  24  formed from a thermoset material or a cross-linked thermoplastic material may require a shorter cable length than metal or thermoplastic tuning cables to provide the same level of noise reduction. The result is a tuning cable  24  that has a substantially similar performance of a thermoplastic or metal tuning cable; however, the thermoset or cross-linked thermoplastic tuning cable  24  requires less material, resulting in cost savings, weight savings and space savings. Moreover, the system  10  may be used within a greater confined space and shorter longitudinal extents are necessary. 
   While cross-linked thermoplastics are commercially available and used in the art to manufacture power steering hoses, the physical characteristics of cross-linked thermoplastics were not thought to be ideal for power steering tuning cables. The physical characteristics of a cross-linked thermoplastic are very different from the physical characteristics of Teflon®. For example, nylon 6 has an approximate ultimate strength of 5900 psi and an approximate elongation value of 225%. Teflon®) has an approximate ultimate strength of 3000-5000 psi and an approximate elongation value of 300-500%. The inventor observed that after undergoing the cross-linking process, cross-linked nylon 6 maintains the same physical characteristics as nylon 6. Through experimentation, the inventor has determined that a cross-linked thermoplastic, such as cross-linked nylon 6, tuning cable performs as well as a Teflon® tuning cable, but at a substantially lower cost. 
   It should be understood that the aforementioned and other various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.