Patent Publication Number: US-6209409-B1

Title: Housing structure for power steering system

Description:
FIELD OF THE INVENTION 
     The present invention relates to a housing structure for power steering system and more particularly, to a housing structure for power steering system reduced in weight and subjected to a corrosion resisting treatment. 
     DESCRIPTION OF THE PRIOR ART 
     Heretofore, there have been proposed power steering systems which have accomplished weight reduction by, for example, replacing iron with aluminum as a material for a pinion housing (see, for example, Japanese Unexamined Patent Publications Nos. 55(1980)-79756 and 63(1988)-235168). Additionally, studies have been made on the adoption of magnesium alloys in pursuit of further weight reduction of the system. 
     Unfortunately, however, the pinion housing and the like of the power steering system are parts included in a so-called automotive foot portion which are exposed to rain blowing therein and water splashed up from the road. In most cases, such water is aqueous electrolytic solution containing impurities. For instance, in a vehicle traveling on a road near the sea, the pinion housing and the like may be exposed to water containing salt or the aqueous electrolytic solution. Hereupon, it is to be noted that the housing, which is formed of magnesium alloy, is in contact with a bolt, a cylinder tube or the like which is formed of a different type of metal (e.g., iron or the like) from the magnesium alloy. When a contact portion of the housing with the bolt etc. is exposed to the aqueous electrolytic solution, a local cell is established at the contact portion. Magnesium is thereby ionized and dissolved, so that the corrosion of the housing occurs. 
     That is, the magnesium alloy is useful for forming a lightweight housing, but on the other hand, it causes the corrosion of the housing. 
     OBJECT AND SUMMARY OF THE INVENTION 
     An object of the present invention is to reduce the weight of the housing and to prevent the corrosion of the housing. 
     A steering gear housing included in a housing structure for power steering system of the present invention, comprises: a valve housing formed of one of metal and resin material; a pinion housing formed of magnesium alloy; a metal bolt for fastening the valve housing and the pinion housing to each other; and an insulating portion interposed in a joint portion between the pinion housing and a metal member of any member adjacent thereto including the valve housing and the metal bolt, the metal member being formed of a different type of metal from which the pinion housing is formed of. 
     The housing of such a structure is reduced in weight by the use of the magnesium alloy and protected against the corrosion thereof by virtue of presence of the insulating portion. 
     In the above housing structure, the insulating portion may be formed on a surface of the metal member by applying thereto at least one of abase treatment layer formed by base treatment and a paint coating layer as a top coat layer formed by painting. In this case, presence of at least one of the base treatment layer and the paint coating layer prevents the housing from being corroded. Also, presence of both the base treatment layer and the paint coating layer positively prevents the housing from being corroded. Thus, the housing can be protected against the corrosion thereof over an extended period of time. 
     Further, in the above housing structure, the valve housing may be formed of the resin material. The pinion housing may be provided with a sleeve-like projection which extends from a female-screw portion of the pinion housing and which is fitted in the valve housing to define an insert for the valve housing for insertion of the metal bolt. In this case, the insert is integrally formed with the pinion housing, thereby eliminating fear for the corrosion due to contact of the insert with the pinion housing. 
     Further, in the above housing structure, the valve housing formed of the resin material may include the insert formed of iron metal for insertion of the metal bolt, while a sealing member of a rubber or synthetic resin material may be interposed between the insertion port and the pinion housing. In this case, the sealing member eliminates direct contact between the insert and the pinion housing, thereby preventing occurrence of corrosion. 
     Further, in the above housing structure, the valve housing may be formed of the magnesium alloy. Resin coated sealing members may be interposed between the valve housing and the metal bolt. In this case, the valve housing and pinion housing are formed of the same type of metal so that contact therebetween causes no corrosion. Also, direct contact of the valve housing with the metal bolt is prevented by the sealing member. Therefore, these elements eliminate occurrence of corrosion. Further, in this housing structure, the pinion housing may include the female-screw portion, in which sealing agent is filled, to have the metal bolt insert therein and having a hole extending downwardly through the pinion housing from a bottom of the female-screw portion. In this case, the sealing agent is extruded from the hole by inserting the metal bolt in the female-screw portion, thereby preventing salt water or the like from entering the female-screw portion. Occurrence of corrosion is thus prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary sectional view showing an essential portion of a valve housing structure for use in a power steering system in a first embodiment of the present invention; 
     FIG. 2 is a fragmentary sectional view showing an essential portion of a valve housing structure for use in the power steering system in a second embodiment of the present invention; 
     FIG. 3 is a fragmentary sectional view showing an essential portion of a valve housing structure for use in the power steering system in a third embodiment of the present invention; 
     FIG. 4 is a fragmentary sectional view showing an essential portion of a valve housing structure for use in the power steering system in a fourth embodiment of the present invention; 
     FIG. 5 is a fragmentary sectional view showing an essential portion of a valve housing structure for use in the power steering system in a fifth embodiment of the present invention; and 
     FIG. 6 is a sectional view showing an overall construction of the power steering system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereafter, preferred embodiments of housing structure for a power steering system will be described with reference to the accompanying drawings. FIG. 1 to FIG. 5 are fragmentary sectional views showing essential portions of housing structures according to the preferred embodiments. FIG. 6 is a sectional view showing an overall construction of the power steering system that is common to the respective embodiments. 
     Referring to FIG. 6, the overall construction of the power steering system will be described. The power steering system  1  is a rack and pinion type hydraulic power steering system in which a valve assembly  1 A, a rack and pinion assembly  1 B and a hydraulic cylinder assembly  1 C are interconnected. The power steering system is arranged such that a hydraulic assisting steering force generated in response to operation of a steering wheel is added to a steering force applied from an output shaft  4  to a rack  8 . A valve housing  6  as a casing of the valve assembly  1 A, a pinion housing  11  as a casing of the rack and pinion assembly  1 B and a cylinder tube  19  as a casing of the hydraulic cylinder assembly  1 C are discretely formed beforehand and coupled to each other. 
     The valve assembly  1 A includes a rotary valve mechanism, through a central portion of which extended are an input shaft  2  coupled to the steering wheel (not shown) and a torsion bar  3 . The input shaft  2  and the torsion bar  3  are secured to each other only at one of axial ends thereof (upper ends thereof (not shown)). A lower end of the torsion bar  3  is connected to the output shaft  4  such that the steering force (rotation torque) applied to the input shaft  2  is transmitted to the output shaft  4  via the torsion bar  3 . Therefore, the input shaft  2  (as a rotary member) is adapted for rotation relative to the output shaft  4  (similarly as the rotary member) by a quantity of torsion of the torsion bar  3  that depends on a steering resistance. 
     The valve assembly  1 A is provided on its outer circumference with an intake port  36 , a discharge port  40 , a first connection port  32  (for connection of pipe) and a second connection port  34  (for connection of pipe). 
     The intake port  36  is connected to a pump  37  via a pipe  73 . The pump  37  serves to suck up oil from a tank  41  for feeding the oil to the valve assembly  1 A at a predetermined pressure. The discharge port  40  communicates with the tank  41  via a pipe  74  so that the oil discharged from the valve assembly  1 A is returned to the tank  41 . 
     The output shaft  4  is formed with a pinion  7  at a lower end portion thereof, and the pinion  7  is engaged with the rack  8  which is held in the rack and pinion assembly  1 B. The pinion housing  11  holds the output shaft  4  by means of a ball bearing  12  and a needle bearing  13 . 
     Laterally opposite ends of the rack  8  are coupled to steered wheels (not shown) which are steered in accordance with lateral movement of the rack  8 . The hydraulic cylinder assembly  1 C continuous to the rack and pinion assembly  1 B is an actuator for providing the assisting steering force, and piston action of the actuator assists the lateral movement of the rack  8 . 
     The hydraulic cylinder assembly  1 C is essentially composed of the cylinder tube  19  and a piston  20  integrally formed with the rack  8 . The piston  20  partitions an interior of the cylinder tube  19 , thus defining a pair of oil chambers  21  and  22 . At an end of the oil chamber  21 , an oil seal  45  is interposed between the rack  8  and the cylinder tube  19 . On the other hand, a rack bushing  46  is interposed between the rack  8  and the cylinder tube  19  at an end of the oil chamber  22 . An oil seal  47  is interposed between the rack bushing  46  and the rack  8 , and also an  0 -ring  48  is interposed between the rack bushing  46  and the cylinder tube  19 . The rack bushing  46  is secured to the cylinder tube  19  by means of a snap ring  49 . 
     On an outer circumference of the cylinder tube  19 , connection ports  75  and  76  are disposed in corresponding relation with the oil chambers  21  and  22 , respectively. The connection ports  75  and  76  are respectively connected to the connection ports  34  and  32  of the valve assembly  1 A via pipes  72  and  71 . 
     Next, the structures of the valve housing  6  and the pinion housing  11  are described in detail with reference to FIG.  1 . It is to be noted here that FIG. 1 is a fragmentary sectional view showing in detail a construction of a portion indicated by “A” in FIG. 6 which illustrates the housing structure according to a first embodiment. 
     Now referring to FIG. 1, the valve housing  6  is formed of iron metal, whereas the pinion housing  11  is formed of magnesium alloy for weight reduction. The valve housing  6  and the pinion housing  11  are fastened to each other by engaging a bolt  51  (of iron metal) with a female-screw portion  11   a  in the pinion housing  11 , via a spring washer  52  (of iron metal) and a washer  53  (of iron metal). The valve housing  6  and the pinion housing  11  thus combined constitute a steering gear housing as a whole. 
     An outer surface of the valve housing  6  is subjected to an ordinary coating processing using an epoxy-base paint. An entire surface of the pinion housing  11  is subjected to a chromate treatment for magnesium alloy, as a base treatment. A base treatment layer formed by this chromate treatment has properties of insulation in addition to corrosion resistance. 
     On the other hand, the bolt  51 , the spring washer  52  and the washer  53  are subjected to a chromate treatment for iron metal, as the base treatment, thereby to form thereon a base treatment layer L 1 . This base treatment layer has also properties of insulation in addition to corrosion resistance. Further, the bolt  51 , the spring washer  52  and the washer  53  are subjected to a coating processing by applying epoxy-base paint on each base treatment layer L 2 , thereby to form a paint coating layer. 
     The above-mentioned base treatment layers and the paint coating layer define an insulating portion interposed between the pinion housing  11  and the bolt  51 . 
     The chromate treatment for the pinion housing  11  is typically a shielding chromic acid treatment process equivalent to JIS Type-II MX-2 or a dichromate treatment process equivalent to JIS Type-III MX-3. Ordinary, zinc plating precedes the chromate treatment for iron metal. A direct contact is thus prevented between the pinion housing  11  formed of the magnesium alloy and the valve housing  6 , the bolt  51 , the spring washer  52  and the washer  53  which are formed of a different type of metal from which the pinion housing is formed of. 
     In addition, an insulating layer  54  is formed on the valve housing  6  and/or the pinion housing  11  by applying a paint to at least each place where the valve housing  6  or the pinion housing  11  will be joined with or may be in contact with another member. A rubber-base paint or an epoxy-base paint is used for forming the insulating layer  54 , which has a thickness of 15 to 20 μm. 
     The above-mentioned base treatment layer on the pinion housing  11 , the epoxy-base paint on the valve housing  6  and particularly the insulation layer  54  define an insulating portion interposed between the pinion housing  11  and the valve housing  6 . 
     The insulating portions serve to suppress transfer of electrons between the members, thereby preventing the occurrence of corrosion of the housings. 
     Incidentally, it is desirable for enhancement of insulation between the pinion housing  11  and the bolt  51  to form the insulating layer  54  on an internal surface of the female-screw portion  11   a  too. 
     To examine an effect of the aforementioned corrosion resisting treatment, a salt water spray test according to JIS-Z2371 was conducted on an untreated article A without any corrosion resisting treatment, a chromate-treated article B, and a chromate-treated and paint-coated article C obtained by further forming the insulating layer  54  on the article B. According to the observation in the test, rust was developed on the untreated article A after a lapse of about 4 hours from the start of the test. On the other hand, rust was developed on the chromate-treated article B after a lapse of about 24 hours from the rust development on the untreated article A (after a lapse of 28 hours from the start of the test). Further, rust was developed on a paint-coated portion of the chromate-treated and paint-coated article C after a lapse of about 452 hours from the rust development on the chromate-treated article B (after a lapse of 480 hours from the start of the test). 
     This corroborates that the chromate-treated article B presents corrosion resistance about 7 times (28/4) greater than the untreated article A, whereas the chromate-treated and paint-coated article C presents corrosion resistance about 120 times (480/4) greater than the untreated article A. 
     It was further corroborated that the chromate-treated article B and the chromate-treated and paint-coated article C present greater effect of preventing damage from physical impact such as of a stone piece hitting thereupon, as compared with the untreated article A. That is, these articles offer increased pitting resistance as a spillover effect. 
     FIG. 2 is a fragmentary sectional view showing a housing structure according to a second embodiment. Referring to the figure, the valve housing  6  is formed of resin material, whereas the pinion housing  11  is formed of magnesium alloy. The female-screw portion  11   a  of the pinion housing  11  is projected into the valve housing  6  to define a sleeve-like projection  11   b . The projection  11   b  is fitted in the valve housing  6  so as to serve as an insert (inserted metal) mounted in the resin valve housing  6 . The bolt  51  and the spring washer  52  are formed of iron metal and subjected to the chromate treatment and paint coating processing similarly to the first embodiment hereof. The washer  55  is formed of resin material, serving to hold the valve housing  6  on the pinion housing  11 . 
     In the above construction, the valve housing  6  and the pinion housing  11  are in contact with each other, but the contact does not cause the corrosion of the pinion housing  11  because the valve housing  6  is formed of the resin material. As to the bolt  51  and the pinion housing  11 , the bolt  51  is subjected to the chromate treatment and paint coating processing for insulation, thereby forming the base treatment layer L 1  and the paint coating layer L 2  for preventing occurrence of corrosion. The spring washer  52  is kept out of direct contact with the pinion housing  11  by the washer  55  as an insulating portion interposed therebetween. 
     FIG. 3 is a fragmentary sectional view showing a housing structure according to a third embodiment. Referring to the figure, the valve housing  6  is formed of resin material, whereas the pinion housing  11  is formed of magnesium alloy. The entire surface of the pinion housing  11  is subjected to the chromate treatment or a combination of the chromate treatment and paint coating processing. An insert  56  formed of iron metal is fitted in the valve housing  6 , and the insert  56  allows the bolt  51  to insert therein. The bolt  51 , the spring washer  52  and the washer  55  are the same with those of the second embodiment hereof. The bolt  51  is engaged with the female-screw portion  11   a  of the pinion housing  11 . Interposed between the insert  56  and the pinion housing  11  is a sealing member  57  which is formed of rubber or synthetic resin material or composed of a metal piece having a surface thereof coated with a fluorocarbon paint such as composed of polytetrafluoroethylene or the like. 
     In the above construction, the sealing member  57  is interposed between the insert  56  and the pinion housing  11 , thereby keeping the both out of direct contact with each other and hence, there occurs no corrosion. Since the valve housing  6  is formed of the resin material, contact of the valve housing  6  with the pinion housing  11  does not cause the corrosion of the pinion housing  11 . As to the bolt  51  and the pinion housing  11 , the bolt  51  is subjected to the aforementioned combination of the chromate treatment and paint coating processing for forming the layers L 1  and L 2  for insulation. This is effective to prevent the occurrence of corrosion. 
     FIG. 4 is a fragmentary sectional view showing a housing structure according to a fourth embodiment. Referring to the figure, the valve housing  6  and the pinion housing  11  are both formed of magnesium alloy. Subsequent to the chromate treatment, the valve housing  6  and the pinion housing  11  each have at least an outside surface thereof coated with a rubber-base or an epoxy resin-base paint. The bolt  51  and the spring washer  52  are the same with those of the second embodiment hereof. The bolt  51  is engaged with the female-screw portion  11   a  of the pinion housing  11 . Interposed between the spring washer  52  and the valve housing  6  is a gasket-like seal  58 , which is composed of a metal piece coated with a fluorocarbon resin material. Additionally, a seal  59  (a member similar to the seal  58 ) is interposed between the valve housing  6  and the pinion housing  11 . 
     In the above construction, the valve housing  6  and the bolt  51  formed of metals different from each other are kept out of contact with each other by means of the seal  58  interposed therebetween and hence, there occurs no corrosion. As to the bolt  51  and the pinion housing  11 , the bolt  51  is subjected to the aforementioned combination of the chromate treatment and paint coating processing for forming the layers L 1  and L 2  for insulation, thereby preventing occurrence of corrosion. 
     FIG. 5 is a fragmentary sectional view showing a housing structure according to a fifth embodiment. The structure of this embodiment is the same as that of the fourth embodiment except for that a hole  11   c  extends from a bottom of the female-screw portion  11   a  of the pinion housing  11 . 
     In assembly of this structure, an adhesive as a sealing agent is filled in the female-screw portion  11   a . The pinion housing  11  is chromate treated as aforementioned. In a dipping process for treating the housing with a chromate liquid, the liquid or air may remain within the female-screw portion  11   a . It is preferred to remove the remaining liquid or air which may cause unevenness in the chromate layer formed on the housing. The hole  11   c  vertically extends from the bottom of the female-screw portion la through the pinion housing to discharge the aforementioned liquid, air and a part of adhesive from the female-screw portion. A usable adhesive is of an anaerobic curing type, such as an acrylic resin. Accordingly, inserting the bolt  51  extrudes uncured adhesive from the hole, while the adhesive  60  remaining in the female-screw portion  11 a is anaerobically cured to serve for the prevention of corrosion of the thread portion. Additionally, the hole  11   c  is so small in size that the adhesive  60  is also allowed to cure in the proximity of an exit of the hole  11   c . This closes the hole  11   c , thereby preventing salt water or the like from entering the female-screw portion  11   a  through the hole  11   c.