Abstract:
Method and apparatus for eliminating turbulence-induced noise in a pulsation-absorbing flexible hose, as in a hydraulic power steering system containing a pressure fluid-feeding pump and a steering gear operated by the pressure fluid discharged from the pump. The hose has a restrictor positioned in the hose bore, and the restrictor has a flow-through bore of smaller diameter than that of the adjacent wall of the hose bore. The flow-through bore has a venturi tube cross section. The restrictor venturi inlet, throat and outlet are designed so as to efficiently conduct fluid therethrough by matching the characteristics of the fluid, the operational pressures, fluid density and other system parameters such that the venturi operates below the lower critical value of the Reynolds number of fluid flow through the restrictor to thereby minimize or eliminate fluid turbulence in the restrictor outlet and/or immediately downstream thereof.

Description:
FIELD OF THE INVENTION 
   This invention relates to noise and/or vibration attenuation apparatus for a system conveying liquid under pressure and to a method of attenuating noise and/or vibration in such a system, particularly in the hydraulic system of the power steering unit of a vehicle. 
   BACKGROUND AND SUMMARY OF THE INVENTION 
   Fluid-borne noise is commonly present in hydraulic systems powered by pumping apparatus such as gear, vane or piston pumps. Typically, the noise results when pressure waves generated as pump flow ripple encounter system flow impedance. Fluid-borne noise present in a hydraulic system causes mechanical apparatus, such as hydraulic lines, control valves, hydraulic motors, and supporting structural members, to vibrate. In many cases, such vibration is coupled to the atmosphere and is the source of objectionable acoustic noise. It is of course, desirable to attenuate such fluid-borne noise. 
   Fluid-borne noise reduction apparatus in the high pressure side of the prior art power steering system usually comprises a flexible metal or plastic tube, called a tuning cable, placed inside a section of volumetrically compliant hose. One of the earliest such prior art apparatus is that disclosed in Klees U.S. Pat. No. 3,323,305. 
   In the low pressure side of prior art power steering systems, another type of “tuning” 0  device is used, one that does not employ the tube-within-tube tuning cable concept but rather simply a restrictor in the return hose line. Such restrictors usually are of the constant-diameter-passageway type shown as restrictor  10  in Katayama et al. U.S. Pat. No. 4,285,534. Such restrictor elements are inserted in a selected location in the low pressure return line flexible hose to form a flow barrier that helps “balance” 0  pressures in the power steering system and thereby prevent “shudder” 0  under certain operating conditions, as is well understood in this art. 
   In many power steering systems, such a balancing restrictor inserted in the return side of the circuit typically operates under system fluid pressures of approximately 100 to 150 psi, which are much lower pressures than are typical in the pump output side of the power steering circuit where pressures may be in the vicinity of 1500 psi. In accordance with the present invention, it was noted that under certain conditions the return side restrictor produced an audible “hiss” noise that could be heard in the passenger cab. It was speculated that the cause might be the abrupt internal angle (chamfer) at the entrance to the flow-through, constant diameter passage of the restrictor, the abrupt angle at the exit of this flow-through passage, the surface finish of the passage, the possibly sharp edges at the entrance and exit of the flow-through passage, as well as the abrupt diameter differential between the hose I.D. and the restrictor passage I.D. Any or all of these parameters were thought to have contributed to making the fluid flow go turbulent in the vicinity of the exit of the restrictor flow-through passage, thereby producing the “hiss” noise. 
   Another problem, unrelated to the noise problem experienced with the current production low pressure side restrictor (often referred to in the trade as a “dogbone”), was hose pinching when the crimp collar location was not correct relative to the dogbone exterior shape of the restrictor. In such cases, the raised edge of the restrictor sometimes caused hose damage and failure. 
   Assuming that the “hiss” noise problem was indeed due to the creation of a turbulence condition in the fluid flow just before or after the exit of the restrictor, ancillary problems would be excessive pressure drop and heat generated by such turbulence, as well as potential cavitation wear on the wall of the tubing. Moreover, these ancillary problems could be present even in the absence of the turbulence reaching a level sufficient to produce the annoying audible “hiss” sound. 
   Accordingly, and by way of summary description, and not by way of limitation, the present invention is directed to providing a new form of restrictor for use in noise-reduction fluid conduits subject to pressure pulsations. The restrictor has a central flow-through passage open at its opposite axial ends, but this flow-through passage is configured in the form of a classic venturi instead of the conventional constant diameter flow-through passage. In a present preferred but exemplary embodiment of the invention, the restrictor has its flow-through passage configured as a venturi having a shallow convergent (in the direction of fluid flow) tapered inlet, a constant diameter throat and a shallow divergent (in direction of fluid flow) tapered outlet. The restrictor flow-through passage is thus constructed with the configuration of a venturi inlet, throat and outlet arranged and operable, under the pressure and fluid flow conditions in which the restrictor is used, so as to minimize turbulence in the restrictor outlet and/or immediately downstream thereof. 
   Preferably the cross sectional configuration of the interior of the restrictor is symmetrical about all axes, and the taper angle for both the inlet and the outlet is approximately 8°. Although the restrictor can be made of metallic material such as brass, aluminum or steel, it is preferred to injection mold the restrictor out of a suitable plastic material to achieve smoother wall surfaces in the flow-through passage, as well as for economy of system assembly and part costs. 
   In accordance with the method of the invention, the venturi inlet, throat and outlet are designed so as to conduct fluid therethrough in the operating system of the pressure fluid device by matching the characteristics of the fluid, the operational pressures, fluid density and other relevant system parameters, such that the venturi operates below the lower critical value of the Reynolds number of the fluid flow through the restrictor to thereby minimize or eliminate noise by minimizing or eliminating turbulence in the fluid in the restrictor outlet and/or exiting immediately downstream from the venturi restrictor. As indicated hereinabove, the venturi restrictor of the invention is presently intended primarily for use in the power steering return line to the system reservoir that supplies the pump input side of the power steering circuit where fluid pressures of 100 to 150 psi are typical, because this appears to be the area where the turbulence problems are most acute and are manifesting themselves with the annoying “hiss” noise. However, the preferred embodiment of the restrictor, due to its other advantageous features as well as the venturi shape of the flow-through passage, may also be advantageously employed in the high pressure side of the power steering circuit as a central restrictor in the typical tuning cable assembly employed in this side of the system. 
   Other and ancillary novel features of the invention will become apparent from the following detailed description and appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with additional objects, features and advantages thereof, will be best understood from the following detailed description, the appended claims and the accompanying drawings in which: 
       FIG. 1  is a longitudinal center cross sectional view, taken on the section line  1 — 1  of  FIG. 2 , of a presently preferred but exemplary embodiment of a power steering restrictor component constructed in accordance with the present invention; 
       FIG. 2  is an end view of the exit end of the restrictor of  FIG. 1 , i.e., the right hand end of the restrictor as viewed in  FIG. 1 ; 
       FIG. 3  is a side elevational view of a prototype constructed pursuant to  FIGS. 1 and 2 . 
       FIG. 4  is a fragmentary elevational view of a section of the power steering return line hose having a restrictor of the invention inserted therein and held in place by a conventional crimp collar; 
       FIG. 5  is a cross sectional view taken on the line  5 — 5  of  FIG. 4 ; 
       FIG. 6  is a fragmentary elevational view (photocopy) of a prototype restrictor-containing “balancing” section of a power steering system return line constructed in accordance with  FIGS. 4 and 5 ; 
       FIG. 7  is a fragmentary longitudinal center section of a tuning cable embodiment of the invention utilizing a modified restrictor of the invention employed in the high pressure side hose of the power steering system; 
       FIG. 8  is a schematic diagram of a power steering system employing the restrictor and tuning cable assembly of  FIG. 7  in the high pressure side of the system and employing the balancing restrictor described in conjunction with  FIGS. 1–6  in the low pressure return side of the system to provide one embodiment of an improved power steering system in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2  illustrate a power steering system restrictor  20  constructed in accordance with a presently preferred first embodiment of the invention. The direction of fluid flow through restrictor  20  is indicated by the arrow FF in  FIG. 1 . Various dimensional parameters are designated by the dimensional lines and arrows labeled “A through J” and are tabulated hereinafter by way of a working example. 
   Restrictor  20  generally comprises a tubular metallic body having a cylindrical exterior surface  21  with an outside diameter (O.D. dimension) designated by reference character “A” in  FIG. 1 . Resrictor  20  has a central flow-through passage  22  open at its opposite axial ends and constructed in the form of a venturi passage as defined by a shallow tapered inlet passage  24 , a relatively short length constant diameter throat passage  26 , and a shallow tapered outlet passage  28 . Preferably the cross sectional configuration of the interior of the restrictor, as defined by passage sections  24 ,  26  and  28 , is symmetrical about all axes. Preferably the taper angle “B” for both the inlet and outlet is about 8°. It will be seen that the inlet passage  24  is convergent in the direction of fluid flow FF, and outlet passage  28  is divergent in the direction of fluid flow FF. The diameter dimension of throat passage  26  is indicated by the reference character “C”. The axial length of throat  26  is designated by the reference character “D”. The maximum I.D. of inlet passage  24  at its inlet end  30  is the same as that of the outlet passage  28  at its outlet end  32  and is designated by the reference character “E” in  FIG. 1 . 
   Preferably the external surface  21  of restrictor  20  is interrupted in its central region by a series of five shallow grooves  36  that define therebetween a series of four equally spaced and equal axial length lands  38 . The axial length dimension of each grooves  36  is designated by the reference character “F”, whereas the axial length dimension of each land  38  is designated by the reference character “G”. The depth dimension of each groove  36  is indicated by the reference character “H”, and the distance from the endmost of grooves  38  to the associated inlet end  30  or outlet end  32  is designated by the dimension indicated by the reference character “I”. Preferably the end edges of inlet end  30  and outlet end  32  are chamfered to the specification indicated by the reference character “J”. 
   By way of preferred working example, the following values may be employed in constructing the preferred but exemplary embodiment of restrictor  20  shown in  FIGS. 1 and 2 . 
   
     
       
             
             
             
           
         
             
                 
             
             
               Dimensional 
                 
                 
             
             
               Parameter 
               Value (in metric units) 
               Range (in metric units) 
             
             
                 
             
           
           
             
               A 
               9.017–8.763 mm 
               10.033–8.765 mm 
             
             
               B 
               8° 
               4°–15° 
             
             
               C 
               2.64–2.89 mm 
               2.64–4.57 mm 
             
             
               D 
               2.667–2.43 mm 
               .76–12.2 mm 
             
             
               E 
               7.493 Ref. mm 
             
             
               F 
               1.905–1.651 mm 
               .127–1.651 mm 
             
             
               G 
               2.667–2.413 mm 
               .76–2.667 mm 
             
             
               H 
               .203–.406 mm 
               .127–.406 mm 
             
             
               I 
               8.052–9.576 mm 
             
             
               J 
               .457–.203 mm 
             
             
                 
             
           
        
       
     
   
   Preferably the array of five external grooves  36  are formed as sharp edge grooves as disclosed in Cunningham U.S. Pat. No. 6,419,278 issued Jul. 16, 2002 and assigned to the assignee of record herein, namely Dana Corporation of Toledo, Ohio. As so constructed, and as shown in  FIG. 4  herein, the sharp edge grooves  36  and associated lands  38  thereby improve sealing with an encompassing rubber composite hose  50  that may be sealably secured onto restrictor  20  with a suitable conventional crimped dogbone collar  52 , as is customary practice currently. Although restrictor  20  maybe constructed of metallic material, such as aluminum, brass or steel alloy, and machined to the configuration established by the foregoing dimensional parameters, or even die cast of zinc or similar material, it is preferred to injection mold restrictor  20  of a suitable plastic (polymeric) material, such as high melting polyamides, e.g., nylons, teflons, or lower melting polymeric materials, e.g., HDPE, polypropylene, polyesters, polyurethanes, to thereby achieve smoother wall surfaces in the flow through passageways  24 ,  26  and  28 , as well as for economy of system assembly and part costs. 
   Note that due to the symmetry of the internal configuration of restrictor  20  the same may be reversed in assembly without affecting operational performance one iota. By injection molding of restrictor  20 , the problems of surface finish roughness by machining of the passageways, potentially producing sharp edges and/or burrs at the entrance and exit of the passageway, are eliminated. Moreover, by using the sharp edge alternating grooves  34  and lands  36  feature with the hose clamp  52  and encircling rubber hose  50 , good sealing and holding power is obtained while eliminating the problem of hose pinching when the crimp location was not correct relative to the prior restrictor dogbone shape, i.e., the raised edge of the prior restrictor then sometimes causing hose damage and failure. 
   It has been found that the venturi configured flow-through passageway  22  of restrictor  20 , with its narrow taper angles of the inlet and outlet (e.g., 8°), has apparently reduced the Reynolds number of restrictor  20  for the aforementioned low pressure return line power steering system application (also referred in  FIG. 8  hereinafter) to an extent sufficient to eliminate the turbulence that caused the “hiss” noise when using the prior art commercial restrictor. The venturi shape allows flow of the power steering fluid without turbulence in the outlet exit throat  28  and/or immediately downstream therefrom in the associated return line hose  50 . Since turbulent flow region is not reached in operation with restrictor  20 , there is no “hiss” sound emanating from the power steering hose. Moreover, the heat created by the prior turbulence is substantially reduced if not eliminated, thereby enabling the power steering hose system to run cooler. Back pressure losses unavoidably introduced by disposing a restrictor in the return line are also reduced. Making the restrictor  20  by a plastic molding process enhances the surface finish over that of a machined metal counterpart, and renders the flow passage walls smoother, which thereby reduces even more so the Reynolds number of restrictor  20 . 
   In the tuning cable embodiment shown in  FIG. 7 , a system high pressure side hose  60  encompasses a modified restrictor  64  of the invention having the same sharp grooves and lands  36  and  38  as restrictor  20 . Due to these grooves and lands, restrictor  64  can be clamped merely with a suitable hose clamp  60  (such as those ear type clamps made by Oetiker® Company of Livingston, N.J., Marlette, Mich. and Hattiesburg, Miss.) at much less clamping pressure than is the case using the high pressure crimped-collar-type seal  52  conventionally employed in such tuning cable or restrictor-return line hose assemblies. Of course, if the holding power of this type clamp  60  is not sufficient for a given high pressure line application, then another, suitably stronger yet conventional, circumferentially tensioned clamp may be substituted, such as a worm gear clamp. If necessary, even a crimp collar  52  may be used. It is to be understood that restrictor  20  can be likewise clamped in hose  50 . As also shown in  FIG. 7 , the use of plastic material for the modified restrictor  64  also enables a plastic tuning cable  66  to be attached by inserting one end into a suitable modified outlet throat  28  of restrictor  64 , thereby making a plastic-to-plastic connection or telescopic joint that can be ultrasonically welded or solvent welded, as at  68 , to provide a perfect seal at low cost. Using plastic material to construct restrictor  64  (and/or  20 ) also avoids corrosion problems and provides an improved cleanliness level for the power steering system. In some high side applications, restrictor  64  or  20  may be similarly installed, but without use of any tuning cable, and yet certain noise reduction effects achieved. 
   Use of a generally constant diameter outside configuration or surface  21  for restrictor  20 ,  64  avoids the aforementioned problem of pinch points between the encircling flexible hose  50 ,  60  and restrictor  20  under even crimp collar clamping pressure. Making the restrictor symmetrical renders the part reversible in assembly so that there is no chance it can be assembled backwards by error in production. 
   It is also to be understood that various parameters of restrictor  20  may be varied to suit particular applications and tuning cable and/or return line restrictor hook-ups. For example, it has been found that the exit angle B is more important to be held within the range specified herein above, whereas the taper angle of inlet passage  24  may be made much greater and more abrupt without impairing anti-turbulence performance. For example, the entrance taper angle may range up to a value of about 20° if it is desired to foreshorten the overall axial length of restrictor  20 ,  64 . In designing the configuration of the flow-through passage  22 ,  22 ′, the various parameters of operation that determine the Reynolds number for the venturi must be observed to preferably stay below the Reynolds number for the onset of turbulent conditions. 
   As set forth in more detail hereinafter in describing the power steering system of  FIG. 8  of the invention, it should be understood that, although the “hiss” noise problem surfaced in the low pressure return side of the prior art power steering system, either restrictor  20  or  64  as constructed within the parameters of the present disclosure may certainly be employed in a tuning cable assembly, such as assembly  64 – 66  of  FIG. 7 , and connected in the high pressure output side, i.e., the pump-to-gear leg of the improved power steering circuit of the invention as shown in  FIG. 8 , and despite the absence of a “hiss” problem, still used there to advantage due to its other important advantageous features. Of course, the diameter of throat  26 , of restrictor  64  is enlarged (e.g., from 0.109 inches (2.76 mm) to say 0.169 inches (4.29 mm) to accommodate the different system conditions in a high pressure side application. The standard taper angle of approximately 8° of either restrictor  20  or  64  also can be reduced down to say as low as 4°, but the overall length then required for the restrictor for such a shallow angle is usually too much of penalty in most applications. 
   Also, although one standard theory of employing a restrictor in achieving tuning or noise dampening in a tuning cable is its creation of a pressure drop, the restrictor  20 ,  64  of the invention has a substantially lower pressure drop than the conventional constant-diameter-passageway restrictor part, and yet it works well in high pressure side tuning cable noise reduction applications and/or low pressure side anti-shudder system pressure balancing applications. By shaping the restrictor through-passage profile into a venturi configuration, particularly the exit passage  28  in accordance with the foregoing disclosure, the pressure drop experienced with restrictor  20 ,  64  is much less than with the prior commercial restrictor in which turbulence was experienced. In addition to a reduced pressure drop, in one test there was a 30° F. temperature difference, i.e., temperature reduction, reflecting reduction in heat produced in the power steering system return line downstream of the restrictor. Thus, restrictor  20  may be advantageous in return side applications even where an audible “hiss” is not experienced as a problem, and likewise as to restrictor  64  on high side application. Reducing or eliminating turbulence also is believed to eliminate the deleterious effect of cavitation on the tuning cable or rubber hose containing the restrictor. 
   The feature of securing the surrounding hose  50 ,  60  to restrictor  20 ,  64  by an inexpensive hose clamp  62  ( FIG. 7 ) to retain the restrictor allows lower strength plastic material to be used in constructing restrictor  20 ,  64 , which then enables injection molding and material cost advantages versus using a crimped metal collar  52  that entails higher loads being placed on the restrictor. Such loads could cause a low strength plastic restrictor to break or collapse. Therefore restrictor  20  in the  FIGS. 4 and 5  crimped collar embodiment is preferably made of high strength plastic (e.g., glass-filled nylon, etc.) material or metallic material. Use of plastic material for restrictor  20 ,  64  also avoids the problems of rust and machining debris experienced with prior metal restrictors. This leads to an improved cleanliness level for the finished power steering system. 
   Referring in more detail to  FIG. 8 , an improved power steering system  100  is schematically illustrated in one preferred but exemplary system embodiment of the invention. System  100  employs the usual power steering pump  102  coupled in communication via the high pressure side hose line  60  to the input of the power steering gear  104 . The output of gear  104  is coupled through the low pressure side return hose line  50  to the inlet of a power steering reservoir  106  that in turn has its outlet coupled by a feed conduit  108  to the input of pump  102 , thereby completing the hydraulic circuit of system  100 . Note that the high pressure line  60  feeding hydraulic fluid from pump  102  to the power steering gear  104  is equipped with the tuning cable sub-assembly  64 – 66  of  FIG. 7  with restrictor  64  held in place by the aforementioned hose clamp  62 . Although the “hiss” noise problem discussed previously typically is not present in high pressure line  60 , nevertheless the use of the venturi restrictor  64  in an associated tuning cable assembly provides the other advantages discussed above when employed in the high pressure side of the system. For example, the reduction in pressure drop introduced by a restrictor in the high pressure side, regardless of the material of which the restrictor is constructed, provides an improvement in system efficiency. 
   It is also to be understood that other methods of attachment of tuning cable  66  and other forms of tuning cables may be employed in association with variations of restrictor  64  within the scope of the invention. For example, the venturi restrictor could be constructed to receive the open end of a tuning cable, such as cable  66  being fitted telescopically over the outlet end of the restrictor rather than into the end of the restrictor. Likewise, another additional tuning cable (not shown) may be employed upstream of the venturi restrictor, and communicating with the entrance passage  24 ′ of restrictor  64 . In addition, metallic venturi restrictors may be suitably configured to enable attachment by electromagnetic forming (Magnaforming) of an open end of helically wound metal tuning cable onto or into the restrictor, or attachment by telescopic crimp attachment using conventional dogbone restrictor end shell crimping methods. 
   System  100  employs in the low pressure hose line  50  the restrictor-hose assembly  20 - 50 - 52  of  FIGS. 4–6  as described hereinabove, or variations thereof in accordance with the spirit and scope of the invention as described previously. Thus, the problem of audible “hiss” in the low pressure line  50  is at least substantially reduced if not eliminated due to the use of venturi restrictor  20  held in place in hose  50  by crimp collar  52 . Of course, it now will be understood that collar clamp band  62  may be substituted for the crimp collar  52  to hold the low pressure side restrictor  20  in place in hose  50 . At this point it also should be noted that another advantage of constructing restrictor  20  and/or  64  of plastic versus metallic material provides a burr-free exterior surface having a lower coefficient of sliding friction. This renders it easier to assemble restrictor  20 ,  64  along and into the fabric-surfaced bore  54  of hose  50 , or like bore  67  of hose  60 , with less likelihood of damage to the interior wall surface of the hose bores. In this connection, note that the shallow grooves  36  and intervening lands  38  employed to assist in sealing and securing the restrictor within the encircling hose material do not offer as much sliding resistance as a restrictor provided with larger-sized protruding ribs or corrugations found on some prior art restrictors. 
   Alternatively, power steering system  100  may only employ a venturi restrictor of the invention on the low pressure side while employing a conventional tuning cable construction on the high pressure side, or vice versa, although the use of the restrictors  20 ,  64  of the invention in both the high and low pressure sides, as shown in system  100 , is presently preferred to maximize the advantages to be imparted by the invention to a power steering system. Of course, it also will be understood that the principles of the invention can be applied to equivalent return line restrictor constructions and high pressure side tuning cable assemblies in which the restrictor components and associated tuning cable components are varied from the examples disclosed. Additionally, for purposes of disclosure completeness, all of the aforementioned prior art patents cited hereinabove are incorporated herein by reference. Also, with reference to the aforementioned Cunningham U.S. Pat. No. 6,409,278, sealing and securing sub-assembly of the venturi restrictor  64  and tuning cable  66  in  FIG. 7 , and venturi restrictor  20  clamped in hose  50 , can be considered to be literally the same as, or equivalent to, the Cunningham patent nipple tube fitting having a shank fitted within an elastomeric hose, when grooves  36  and lands  38  are constructed in accordance with the teaching the Cunningham &#39;278 patent, and thus also covered by the claims thereof. 
   Although the illustrated embodiments have been discussed in conjunction with conventional hydraulic power-assist vehicle steering systems commonly employed in automotive vehicles of current manufacture, the invention is by no means limited to such applications. Several modifications and variations have been disclosed. Other modifications and variations will readily suggest themselves to persons of ordinary skill in the art. The invention is therefore intended to embrace all such modifications and variations as fall within the spirit and broad scope of the appended claims.