Spline telescopic shaft, method for manufacturing spline telescopic shaft, and vehicle steering apparatus

A spline telescopic shaft includes an inner shaft having an outer tooth surface, and a cylindrical outer shaft having an inner tooth surface and arranged to fit to the inner shaft slidably in an axial direction of the inner shaft. Either one of the outer tooth surface and the inner tooth surface includes a first resin coating. The other of the outer tooth surface and the inner tooth surface includes a second resin coating. The second resin coating is formed by processing for sliding an intermediate member for manufacturing the inner shaft and an intermediate member for manufacturing the outer shaft in the axial direction, to transfer a part of a resin material provided in the intermediate member for manufacturing the shaft including the one tooth surface for forming the first resin coating to the intermediate member for manufacturing the shaft including the other tooth surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a spline telescopic shaft, a method for manufacturing the spline telescopic shaft, and a vehicle steering apparatus.

2. Description of Related Art

Japanese Unexamined Patent Publication No. 2005-153677 discusses a telescopic shaft for vehicle steering. The telescopic shaft for vehicle steering has a surface hardened layer provided on its tooth surface via shot peening, and a large number of minute recesses are formed on the surface hardened layer. The recesses function as a grease reservoir.

In this type of spline telescopic shaft, resin coatings may be respectively formed on tooth surfaces of an inner shaft and an outer shaft. Frictional resistance in engagement between teeth of the inner shaft and teeth of the outer shaft is reduced by forming the resin coatings. Thus, friction between both the teeth is suppressed. Backlash between both the shafts in the early stages of use of the telescopic shaft is reduced. A sliding load between both the shafts can be reduced. Therefore, work for assembling the telescopic shaft into a vehicle is simplified. Moreover, durability is improved. Further, stick slip vibration between both the shafts is suppressed through reduction of a change in the sliding load between both the shafts. This improves quietness.

SUMMARY OF THE INVENTION

Even if a contact surface between teeth of an inner shaft and teeth of an outer shaft seems to be sufficiently ensured by coating respective tooth surfaces of both the shafts with resin in a stage for manufacturing a telescopic shaft, respective surface roughnesses of both the teeth may be great microscopically. In this case, even if no backlash is produced between both the shafts in the early stages of use of the telescopic shaft, backlash in a rotational direction between both the shafts is rapidly increased when a period of time for the use has elapsed. It is because the resin coating rapidly wears by sliding between both the shafts in the early stages of the use of the telescopic shaft. When the telescopic shaft is used over a long period of time, the total number of times of sliding between both the shafts is increased. As a result, more parts of the resin coating wear. Particularly, the surface roughnesses of the tooth surfaces along the axial direction of both the shafts are increased.

The present invention is directed to providing a spline telescopic shaft including an inner shaft having an outer tooth surface formed on its outer periphery, and a cylindrical outer shaft having an inner tooth surface arranged to engage with the outer tooth surface and arranged to fit to the inner shaft slidably in an axial direction of the inner shaft. Either one of the outer tooth surface and the inner tooth surface includes a first resin coating. The other of the outer tooth surface and the inner tooth surface includes a second resin coating. The second resin coating is provided by processing for sliding an intermediate member for manufacturing the inner shaft and an intermediate member for manufacturing the outer shaft in the axial direction, to transfer a part of a resin material provided in the intermediate member for manufacturing the shaft including the one tooth surface for forming the first resin coating to the intermediate member for manufacturing the shaft including the other tooth surface.

According to the present invention, contact between the inner shaft and the outer shaft can be contact between resins having low frictional resistances. A part of a resin material provided to form the first resin coating is transferred by sliding between the inner shaft manufacturing intermediate member and the outer shaft manufacturing intermediate member. Thus, the second resin coating is formed when the spline telescopic shaft is manufactured. As a result of thus forming the second resin coating, backlash (a space) between the first and second resin coatings can be significantly reduced in a circumferential direction of the inner and outer shafts while significantly reducing respective surface roughnesses of the first resin coating and the second resin coating. Thus, each of the resin coatings can be inhibited from rapidly wearing in the early stages of use of the spline telescopic shaft. Further, the wear of the first and second resin coatings can be suppressed over a long period of time. As a result of thus suppressing the backlash between the first and second resin coatings, a rattle sound (a collision sound between both the tooth surfaces generated in a space in a circumferential direction between the inner and outer shafts) can be suppressed. This can more greatly improve quietness. As a result of suppressing the backlash between the first and second resin coatings, a steering feeling can be improved when the spline telescopic shaft is applied to a vehicle steering apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1illustrates a schematic configuration of a vehicle steering apparatus having an intermediate shaft to which a spline telescopic shaft according to an embodiment of the present invention is applied. Referring toFIG. 1, the vehicle steering apparatus1includes a steering shaft3connected to a steering member2such as a steering wheel, and an intermediate shaft5serving as a spline telescopic shaft connected to the steering shaft3via a universal joint4.

The vehicle steering apparatus1includes a pinion shaft7that is connected to the intermediate shaft5via a universal joint6and a rack shaft8. The rack shaft8has a rack8athat engages with a pinion7aprovided in the vicinity of an end of the pinion shaft7. The intermediate shaft5forms a telescopic shaft for vehicle steering.

A rack-and-pinion mechanism including the pinion shaft7and the rack shaft8constitutes a steering mechanism A1. The rack shaft8is supported movably in an axial direction along a right-and-left direction of a vehicle (a direction perpendicular to paper) by a housing10fixed to a vehicle body-side member9. Each of ends of the rack shaft8is connected to a corresponding wheel via a corresponding tie rod and a corresponding knuckle arm, which is not illustrated. A steering torque generated by the steering member2is transmitted to the steering mechanism A1via the steering shaft3and the intermediate shaft5.

The steering shaft3includes a first steering shaft11and a second steering shaft12that are coaxially connected to each other. The first steering shaft11includes an upper shaft13and a lower shaft14that are spline-coupled to each other. The upper shaft13and the lower shaft14are fitted to each other together rotatably and relatively slidably in an axial direction. Either one of the upper shaft13and the lower shaft14constitutes an inner shaft, and the other shaft constitutes a cylindrical outer shaft. In the present embodiment, the upper shaft13and the lower shaft14respectively constitute an inner shaft and an outer shaft.

The second steering shaft12includes an input shaft15, an output shaft16, and a torsion bar17. The input shaft15is connected to the lower shaft14together rotatably. The output shaft16is connected to the intermediate shaft5via the universal joint4. The torsion bar17connects the input shaft15and the output shaft16to each other relatively rotatably.

The steering shaft3is rotatably supported via a bearing (not illustrated) by a steering column20fixed to vehicle body-side members18and19.

The steering column20includes a cylindrical upper jacket21and a cylindrical lower jacket22that are fitted to each other relatively movably in the axial direction, and a housing23. The housing23is connected to a lower end in the axial direction of the lower jacket22. The housing23houses a speed reduction mechanism25. The speed reduction mechanism25decelerates rotation of an electric motor24for steering assist and transmits the power to the output shaft16. Thus, the electric motor24applies a steering assist force to the steering mechanism A1.

The speed reduction mechanism25includes a driving gear26and a driven gear27. The driving gear26is connected to a rotating shaft (not illustrated) of the electric motor24together rotatably. The driven gear27engages with the driving gear26, and rotates together with the output shaft16. The driving gear26is composed of a worm shaft, for example, and the driven gear27is composed of a worm wheel, for example.

An output of the electric motor24is transmitted to the output shaft16via the speed reduction mechanism25, and is further transmitted to the steering mechanism A1via the intermediate shaft5or the like. Thus, the intermediate shaft5constitutes a part of a power transmission path for transmitting an output of the electric motor24to the steering mechanism A1.

The steering column20is fixed to the vehicle body-side members18and19via an upper bracket28on the back side of the vehicle and a lower bracket29on the front side of the vehicle. The upper bracket28can be fixed to the upper jacket21in the steering column20via a column bracket (not illustrated). The upper bracket28is fixed to the vehicle body-side member18using a fixed bolt (stud bolt)30, a nut31, and a capsule32. The fixed bolt30projects downward from the vehicle body-side member18. The nut31is screwed into the fixed bolt30. The capsule32is detachably held in the upper bracket28.

The lower bracket29is fixed to the housing23in the steering column20. The lower bracket29is fixed to the vehicle body-side member19using a fixed bolt (stud bolt)33projecting from the vehicle body-side member19and a nut34screwed into the fixed bolt33.

Referring toFIGS. 1 and 2, the intermediate shaft5serving as the spline telescopic shaft is formed by spline-fitting an inner shaft35and a cylindrical outer shaft36to each other slidably in the axial direction X1of the inner shaft35(an axial direction of the intermediate shaft5) and torque-transmittably.

Either one of the inner shaft35and the outer shaft36constitutes an upper shaft, and the other shaft constitutes a lower shaft. In the present embodiment, the outer shaft36is connected to the universal joint4as the upper shaft, and the inner shaft35is connected to the universal joint6as the lower shaft. A lubricant such as grease is interposed between the inner shaft35and the outer shaft36. This causes sliding resistance to be reduced when the inner shaft35and the outer shaft36relatively move in the axial direction X1.

In the present embodiment, the spline telescopic shaft is applied to the intermediate shaft5. However, the spline telescopic shaft according to the present invention may be applied to the first steering shaft11so that the first steering shaft11performs a telescopic adjustment function and a shock absorption function. In the present embodiment, the vehicle steering apparatus1is an electric power steering apparatus. The spline telescopic shaft according to the present invention may be applied to a steering apparatus for manual steering vehicle.

Referring toFIGS. 2 and 3, the input shaft35includes a core39, and a first resin coating40formed on a surface of a core tooth portion42serving as a spline portion on the leading end side of the core39.

The core39in the inner shaft35is formed of a metal. A portion, on the leading end side, of the core39is a leading end region41. A plurality of core tooth portions42serving as outer tooth portions are formed on the outer periphery of the leading end region41. The core tooth portions42are equally spaced in a circumferential direction of the inner shaft35. The first resin coating40is formed throughout on the outer periphery of the leading end region41, and covers the whole area of surfaces42aof the core tooth portions42. The thickness of the first resin coating40is approximately 1 mm, for example.

Referring toFIG. 3, the first resin coating40is formed of synthetic resin. Examples of the synthetic resin include thermoplastic resin such as polyamide or polyacetal and thermosetting resin such as epoxy resin. An outer tooth surface35bof the inner shaft35is formed on a surface of the first resin coating40. The whole of the outer tooth surface35bof the inner shaft35is formed of the first resin coating40. The outer tooth surface35bis arranged on the outer periphery of the inner shaft35.

The outer shaft36includes a cylindrical core45and a second resin coating46formed on a part of an inner surface of the core45.

The core45in the outer shaft36is formed of a metal. A plurality of core tooth portions48serving as inner tooth portions are formed on the inner periphery of the core45. The core tooth portions48are equally spaced in a circumferential direction of the outer shaft36. Abase end and a leading end of a surface48aof each of the core tooth portions48is exposed, and the second resin coating46adheres to a side surface (tooth surface) of the surface48a.

The second resin coating46is provided by processing for sliding an intermediate member for manufacturing the inner shaft35and an intermediate member for manufacturing the outer shaft36in the axial direction X1, to transfer a part of a resin coating (resin material) provided on the intermediate member for manufacturing the inner shaft35for forming the first resin coating40to the intermediate member for manufacturing the outer shaft36.

The second resin coating46is formed only in a portion, which abuts on the first resin coating40, of the outer shaft36in a circumferential direction C1of the inner shaft35(hereinafter merely referred to as a circumferential direction C1). The second resin coating46is made thinner than the first resin coating40, and the thickness of the second resin coating46is approximately several micrometers, for example.

An exposed portion, which is not covered with the second resin coating46, of the surface48aof each of the core tooth portions48and a surface of the second resin coating46smoothly communicate with each other, and form an inner tooth surface36bof the outer shaft36as a whole (in corporation). Only the second resin coating46on the inner tooth surface36bof the outer shaft36substantially abuts on the outer tooth surfaces35bof the inner shaft35. An engaging portion between the outer tooth surface35band the inner tooth surface36bin the circumferential direction C1is a torque transmission surface.

Processes for manufacturing the intermediate shaft5will be described with reference to a schematic view ofFIG. 4.

In a forging process illustrated inFIG. 4A, an inner shaft manufacturing intermediate member551serving as a first (one) manufacturing intermediate member is obtained. The inner shaft manufacturing intermediate member551is formed by forging a material (metal material), and has the core tooth portion42on its outer periphery.

In a preprocessing process illustrated inFIG. 4B, a surface of the core tooth portion42on the inner shaft manufacturing intermediate member551illustrated inFIG. 4Ais subjected to preprocessing for coating. More specifically, processing in a stage preceding a stage in which a resin layer56is formed in a coating process illustrated inFIG. 4C, described below.

The processing in the preceding stage is processing for roughening a surface of the core tooth portion42on the inner shaft manufacturing intermediate member551in order to improve adhesive properties between the surface of the core tooth portion42and the resin layer56. Examples of this processing include base processing such as shot blasting and primer coating. Thus, an inner shaft manufacturing intermediate member552(corresponding to the core39in the inner shaft35) having the core tooth portion42, which has been subjected to preprocessing, formed on its surface, as illustrated inFIG. 4B, is obtained.

In the coating process illustrated inFIG. 4C, the resin layer56is then formed on at least a part (the whole in the present embodiment) of the core tooth portion42on the inner shaft manufacturing intermediate member552illustrated inFIG. 4B. Thus, an inner shaft manufacturing intermediate member553having the resin layer56formed on the inner shaft manufacturing intermediate member553, as illustrated inFIG. 4C, is obtained. More specifically, the inner shaft manufacturing intermediate member552that has been subjected to the preprocessing, illustrated inFIG. 4B, is heated. The inner shaft manufacturing intermediate member552is then dipped for a predetermined period of time in a tank including resin powder in a fluidized state. Thus, the resin powder is melted by heat after adhering to the inner shaft manufacturing intermediate member552. As a result, the inner shaft manufacturing intermediate member553having the resin layer56formed on the inner shaft manufacturing intermediate member553is obtained. A cross section on an outer peripheral surface of the resin layer56forms a wave shape. The resin layer56may be formed by injection-molding.

Then, in a spline forming process illustrated inFIG. 4D, a cylindrical surface broach500is prepared. The surface broach500is a broach for giving a required shape to an outer surface of a workpiece. A broach for giving a required shape to an inner surface of the workpiece is referred to as an internal broach. The inner shaft manufacturing intermediate member553having the resin layer56formed thereon is fitted (inserted or pressed) into the surface broach500. The inner shaft manufacturing intermediate member553is reciprocally slid (relatively moved) in its axial direction.

Thus, a resin coating59is formed on a surface42aof the core tooth portion42on an inner shaft manufacturing intermediate member554(the resin coating59is indicated by a broken line), as illustrated inFIG. 5. A surface of the resin coating59is formed in a shape along the shape of the surface42aof the core tooth portion42(a shape substantially similar to the shape of the surface42aof the core tooth portion42). A plurality of broach teeth501are equally spaced in a circumferential direction on an inner peripheral surface of the surface broach500(indicated by a broken line inFIG. 5). A cross-sectional shape of the broach teeth501is substantially matched with a cross-sectional shape of the core tooth portions48in the outer shaft36(indicated by an imaginary line inFIG. 5).

A space501a(the width of a tooth groove) between the broach teeth501is wider than a space48b(the width of a tooth groove) between the core tooth portions48(space501a>space48b). The distance between a central axis of the surface broach500and the bottom of the tooth groove between the broach teeth501is shorter than the distance between a central axis of the outer shaft36and the bottom of the tooth groove between the core tooth portions48by a distance F1.

Therefore, the tooth groove between the broach teeth501is wider and shallow than the tooth groove between the core tooth portions48. By providing the distance F1, the distance between the tooth tip of the inner shaft35and the tooth bottom of the outer shaft36is substantially the distance F1, as illustrated inFIG. 3.

Referring toFIGS. 4D and 5, in the broaching, the surface broach500and the inner shaft manufacturing intermediate member553are coaxially arranged, and are fitted to each other and relatively slid in the axial direction, as described above. By the broaching, the broach tooth501cuts off an excess portion56aof the resin layer56. Thus, the width (equal to the space width501a) of teeth formed by the broach teeth501(corresponding to teeth of the resin coating40) is larger than the space width48bbetween the core tooth portions48. Therefore, the inner shaft35and the outer shaft36are fitted to each other with a negative space therebetween in the circumferential direction C1of the inner shaft35, as illustrated inFIG. 3. Thus, the inner shaft manufacturing intermediate member554is formed.

Then, in a joint connecting process illustrated inFIG. 4E, a universal joint4is connected to an end of the inner shaft manufacturing intermediate member554, to form an inner shaft manufacturing intermediate member555. In the spline forming process illustrated inFIG. 4D, the universal joint4is not attached to the inner shaft manufacturing intermediate member554. This enables the inner shaft manufacturing intermediate member554to be inserted through the surface broach500throughout in its axial direction.

Then, a press fitting process is performed, as illustrated inFIG. 4F, before a sliding process illustrated inFIG. 4G. In the press fitting process, the inner shaft manufacturing intermediate member555and an outer shaft manufacturing intermediate member57serving as a second manufacturing intermediate member are fitted to each other with a negative fitting space therebetween. The outer shaft manufacturing intermediate member57has a similar configuration to that of the outer shaft36except that the second resin coating46is not formed. A pressing load at this time is approximately 2 kN to 5 kN, for example. As illustrated inFIG. 5, the tooth thickness of the inner shaft manufacturing intermediate member555used in the press fitting process (the space width501a, i.e., the thickness in a free state of the inner shaft manufacturing intermediate member555) is larger than the space width of the outer shaft manufacturing intermediate member57(the space width48bbetween the core tooth portions48).

In the sliding process illustrated inFIG. 4G, the inner shaft manufacturing intermediate member555that has pressed into the outer shaft manufacturing intermediate member57is forcibly slid in the axial direction X1relative to the outer shaft manufacturing intermediate member57. At this time, a torsional torque E1is exerted between the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57. More specifically, the outer shaft manufacturing intermediate member57is fixed by a jig65while the inner shaft manufacturing intermediate member555is fixed to an arm66, for example. The inner shaft manufacturing intermediate member555is slid in the axial direction X1relative to the outer shaft manufacturing intermediate member57in the axial direction X1while being twisted with respect to the outer shaft manufacturing intermediate member57by the arm66. The torsional torque E1need not be exerted between the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57.

The sliding between the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57causes the core tooth portion48in the outer shaft manufacturing intermediate member57and the resin coating59on the inner shaft manufacturing intermediate member555to come into frictional contact, to generate frictional heat. As a result, apart of a resin material composing the resin coating59is softened and transferred to the surface48aof the core tooth portion48, to adhere to the surface48a. Thus, the resin coating59forms a first resin coating40, to complete the inner shaft35.

The second resin coating46is formed on the core tooth portion48in the outer shaft manufacturing intermediate member57, to complete the outer shaft36. The sliding between the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57is stopped when a measured value of a sliding load between the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57is a predetermined threshold value or less.

In the sliding process, the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57may be heated to a temperature that is less than a melting temperature (a temperature close to a melting point) of the resin material composing the resin coating59by an external heater.

In a grease coating process illustrated inFIG. 4H, a surface of the first resin coating40in the inner shaft35is coated with grease68. The inner shaft35that has been coated with the grease68is assembled into an outer shaft36, to complete an intermediate shaft5serving as a spline telescopic shaft, as illustrated inFIG. 4I.

As described above, according to the present embodiment, the first resin coating40in the inner shaft35and the second resin coating46in the outer shaft36are brought into contact with each other. This enables contact between the inner shaft35and the outer shaft36to be contact between resins having low frictional resistances. A part of the resin coating59provided to form the first resin coating40is transferred to the outer shaft manufacturing intermediate member57by the sliding between the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57. Thus, a second resin coating46is formed when the intermediate shaft5is manufactured. As a result of thus forming the second resin coating46, respective surface roughnesses of the first resin coating40and the second resin coating46are significantly reduced. Moreover, backlash (a space) between the first and second resin coatings40and46can be significantly reduced in the circumferential direction C1of both the shafts35and35. Thus, the first and second resin coatings40and46can be inhibited from rapidly wearing in the early stages of use of the intermediate shaft5. Further, the wear of the first and second resin coatings40and46can be suppressed over a long period of time. As a result of thus suppressing the backlash between the first and second resin coatings40and46, a rattle sound (a collision sound between both the tooth surfaces35band36bgenerated in a space in the circumferential direction between the inner shaft35and the outer shaft36) can be suppressed. This can improve quietness. As a result of suppressing the backlash between the first and second resin coatings40and46, a steering'feeling can be improved.

In other words, both the manufacturing intermediate members555and57are slid in the sliding process, so that a part of the resin coating59on the inner shaft manufacturing intermediate member555is transferred to the outer shaft manufacturing intermediate member57. As a result, the second resin coating46is formed on the outer shaft manufacturing intermediate member57. As a result of thus forming the second resin coating46on the outer shaft manufacturing intermediate member57, the backlash between the first and second resin coatings40and46can be significantly reduced in the circumferential direction C1while significantly reducing the surface roughnesses of the first and second resin coatings40and46. Thus, the intermediate shaft5using both the shafts35and36can inhibit the first and second resin coatings40and46from rapidly wearing in the early stages of use. Moreover, in the intermediate shaft5, the wear of the first and second resin coatings40and46can be suppressed over a long period of time. As a result of thus suppressing the backlash between the first and second resin coatings40and46, a rattle sound can be suppressed. This can more greatly improve quietness. As a result of suppressing the backlash between the first and second resin coatings40and46, a steering feeling can be improved.

Thus, a highly accurate fitted state between the inner shaft35and the outer shaft36can be realized so that the backlash between the inner shaft35and the outer shaft36can be prevented for a long time. A sliding load between both the shafts35and36can be reduced so that a sliding load in assembling the intermediate shaft5into a vehicle can be reduced and the durability of the intermediate shaft5can be enhanced. A change in the sliding load between both the shafts35and36can be reduced. Thus, so-called stick lip between the inner shaft35and the outer shaft36is prevented so that good quietness can be obtained over a long period of time. Noise generated by the rattle sound between the tooth surfaces35band36bcan be reduced.

The first resin coating40forms the outer tooth surface35b. This enables the outer tooth surface35to be a surface having a low frictional resistance.

Furthermore, the second resin coating46forms the inner tooth surface36bin corporation with the surface48aof the core tooth portion48in the outer shaft36. Thus, the inner tooth surface36bcan be formed of a small amount of resin material.

The thickness of the second resin coating46is smaller than the thickness of the first resin coating40. Thus, the inner tooth surface36bcan be formed of a small amount of resin material.

Furthermore, in the sliding process, the torsional torque E1is applied to the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57. Thus, the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57can be reliably pressure-welded to each other in the circumferential direction C1. Thus, a part of the resin coating59on the inner shaft manufacturing intermediate member555can be reliably transferred to the outer shaft manufacturing intermediate member57.

In the sliding process, frictional heat is applied to the resin coating59on the inner shaft manufacturing intermediate member555by the sliding between the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57. This enables the resin coating59to be softened. Thus, a part of the resin coating59can be more reliably transferred to the outer shaft manufacturing intermediate member57.

Furthermore, the sliding process is completed by setting a load in sliding the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57in the axial direction to a predetermined threshold value or less. Thus, timing at which the sliding process is completed can be easily determined using the load in sliding both the manufacturing intermediate members555and57.

Before the sliding process, the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57are fitted to each other with a negative fitting space therebetween. More specifically, the tooth thickness (the length501a) of the inner shaft manufacturing intermediate member555is made larger than the groove width of the outer shaft manufacturing intermediate member57, to press the inner shaft manufacturing intermediate member555into the outer shaft manufacturing intermediate member57.

This enables the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57to be reliably pressure-welded to each other, thereby enabling the resin coating59on the inner shaft manufacturing intermediate member555to be reliably transferred to the outer shaft manufacturing intermediate member57. Therefore, the second resin coating46can be reliably formed on the outer shaft manufacturing intermediate member57. The first resin coating40on the inner shaft manufacturing intermediate member555and the second resin coating46on the outer shaft manufacturing intermediate member57can be reliably fitted to each other. This enables a contact area between the first and second resin coatings40and46to be reliably increased at a microscopic level (a surface roughness level, i.e., a level of the size of irregularities of a striped processing trace in an axial direction produced on the tooth surfaces35band36bwhen spline processing is performed). As a result, local abrasion can be reliably inhibited from occurring between the inner shaft35and the outer shaft36due to the sliding between both the shafts35and36.

The resin coating59on the inner shaft manufacturing intermediate member554is formed in a spline shape using the surface broach500. As a result, the resin coating59on the inner shaft manufacturing intermediate member554and the core tooth portion48in the outer shaft manufacturing intermediate member57can be fitted to each other with a negative space therebetween in a circumferential direction (torque transmission surfaces contact each other with a negative space therebetween), and a space can be formed between the tooth tip of the resin coating59and the tooth bottom of the outer shaft manufacturing intermediate member57. In the press fitting process for pressing the inner shaft manufacturing intermediate member555having the resin coating59formed thereon into the outer shaft manufacturing intermediate member57, therefore, the manufacturing intermediate members555and57contact each other only on their respective torque transmission surfaces. Thus, the tooth tip of the inner shaft manufacturing intermediate member555and the tooth bottom of the outer shaft manufacturing intermediate member57enter a noncontact state. Therefore, a load required for the press fitting between both the manufacturing intermediate members555and57may be small so that work for the press fitting can be easily performed.

In the sliding process (fitting process), a sliding load between the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57can also be reduced. This enables the sliding process to be performed with a relatively low facility capacity.

If a facility for performing the sliding process has a sufficient capacity, the tooth tip of the inner shaft manufacturing intermediate member555and the tooth bottom of the outer shaft manufacturing intermediate member57may be fitted to each other with a negative space therebetween (in a pressure-welded state).

A surface of the resin coating59on the first manufacturing intermediate member554is formed to a shape along a surface shape of the core tooth portion42in the first manufacturing intermediate member555(a substantially similar shape) by broaching in the spline forming process. Thus, the surface shape of the resin coating59on the first manufacturing intermediate member555can be formed with high accuracy. Therefore, a contact portion between the first manufacturing intermediate member555and the second manufacturing intermediate member57can be set with high accuracy in the sliding process. Thus, a part of the resin coating59can be transferred to a desired portion of the second manufacturing intermediate member57.

As described above, backlash can be prevented from being produced in the intermediate shaft5in the early stages of use of the vehicle steering apparatus1, and the production of backlash can be suppressed over a long period of time. As a result, a good steering feeling can be maintained over a long period of time, and high quietness can be realized by suppressing a rattle sound over a long period of time.

The intermediate shaft5constitutes a part of a power transmission path for transmitting an output of the electric motor24to the steering mechanism A1. The intermediate shaft5receives a great torque (steering assist force) of the electric motor24. However, backlash is prevented from being produced in the intermediate shaft5from the early stages of use of the vehicle steering apparatus1. Even if the great torque is repeatedly exerted on the intermediate shaft5, therefore, the production of backlash between the inner shaft35and the outer shaft36can be reliably suppressed over a long period of time.

As to generation of frictional heat for heating the inner shaft manufacturing intermediate member555in the sliding process illustrated inFIG. 4G, as sliding conditions between the inner shaft manufacturing intermediate shaft555and the outer shaft manufacturing intermediate member57, a sliding stroke between the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57is in a range of ±10 mm to ±50 mm (20 mm to 100 mm), and a sliding frequency is 1.5 Hz to 10 Hz.

The range of the stroke enables the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57to be slid at a sufficient stroke without coming off each other. Therefore, sufficient frictional heat can be applied to the resin coating59.

The sliding frequency enables suitable frictional heat to be applied to the resin coating59.

The necessity of external heating means is eliminated by using frictional heat so that a manufacturing facility can be simplified.

When the heat fitting processing is performed using frictional heat, the inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57can be slid at a long sliding stroke (±30 mm or more) and at high speed (2 Hz or more) to perform the heat fitting process at high efficiency. This is for the following reasons. More specifically, a large amount of heat generation can be obtained, and the temperature of the outer shaft36can be suppressed, therefore, molten resin can be transferred to the outer shaft36in a shorter time. Thus, the thickness of a softened layer of the manufacturing intermediate member57is reduced, generation of roller-shaped abrasion powder can be suppressed, and a tooth surface can stably be formed.

The present invention is not limited to the contents of the above-mentioned embodiment, so, various changes can be made within the scope of claims.

For example, in the spline forming process illustrated inFIG. 4D, the surface broach500may be replaced with an outer shaft manufacturing intermediate member57illustrated inFIG. 6. The outer shaft manufacturing intermediate member57has a similar configuration to that of the outer shaft36except that the second resin coating46is not formed. In this case, an inner shaft manufacturing intermediate member553having a resin layer56formed thereon is fitted to the outer shaft manufacturing intermediate member57, and is slid in an axial direction X1. Thus, a resin coating59in an external spline shape is formed on the inner shaft manufacturing intermediate member553.

The inner shaft manufacturing intermediate member555and the outer shaft manufacturing intermediate member57may be slid in the axial direction X1while being loose-fitted to each other, to form the second resin coating46.

Furthermore, a part of a core tooth portion42in an inner shaft35A need not be covered with a first resin coating40A, as illustrated inFIG. 7. At this time, an outer tooth surface35bA of the inner shaft35A is formed by cooperation between the core tooth portion42and the first resin coating40A. Alternatively, the whole of a core tooth portion48in an outer shaft36B may be covered with a second resin coating46B, as illustrated inFIG. 8. At this time, the second resin coating46B forms an inner tooth surface36bB of the outer shaft36B.

A resin coating59C may be formed on the whole of a surface48aof a core tooth portion48in an outer shaft manufacturing intermediate member57C, and may be transferred to an inner shaft manufacturing intermediate member555C by sliding in an axial direction between the inner shaft manufacturing intermediate member555C and the outer shaft manufacturing intermediate member57C. In this case, the outer shaft manufacturing intermediate member57C is a first manufacturing intermediate member, and the inner shaft manufacturing intermediate member555C is a second manufacturing intermediate member.

In this case, the resin coating59C can be formed by cutting off a resin coating on the outer shaft manufacturing intermediate member57C by the inner shaft manufacturing intermediate member555C actually used or performing broaching with a stick-shaped internal broach having a similar shape to that of a core tooth portion42in an inner shaft35C, to cut off the resin coating on the outer shaft manufacturing intermediate member57C. When the stick-shaped internal broach is used, the broaching is performed so that teeth of the outer shaft manufacturing intermediate member57C and teeth of the inner shaft manufacturing intermediate member555C can be opposed to each other with a negative space therebetween (in a pressure-welded state) in a circumferential direction and with a space therebetween in a radial direction.

The outer shaft manufacturing intermediate member57C and the inner shaft manufacturing intermediate member555C are slid in an axial direction so that a part of the resin coating59C on the outer shaft manufacturing intermediate member57C is transferred to a surface42aof the core tooth portion42on the inner shaft manufacturing intermediate member555C.

Referring toFIGS. 9 and 10, the resin coating59C thus forms the first resin coating40C. A part of the resin coating59C is transferred to the inner shaft manufacturing intermediate member555C so that a second resin coating46C is formed. The outer shaft manufacturing intermediate member57C thus forms an outer shaft36C. The inner shaft35C and the outer shaft36C are fitted to each other, as illustrated inFIG. 10, to form a spline telescopic shaft5C including the inner shaft35C and the outer shaft36C.

An outer tooth surface35bC of the inner shaft35C is formed by cooperation between the surface42aof the core tooth portion42and the second resin coating46C. An inner tooth surface36bC of the outer shaft36C is formed by the first resin coating40C.

In the present embodiment, the vehicle steering apparatus1is a so-called column assist type electric power steering apparatus for applying a steering assist force to the steering shaft3. However, a vehicle steering apparatus to which the spline telescopic shaft according to the present invention is applied may be a so-called pinion assist type electric power steering apparatus for applying a steering assist force to the pinion shaft7. The vehicle steering apparatus may be a so-called rack assist type electric power steering apparatus for applying a steering assist force to the rack shaft8. The present invention may be applied to a steering apparatus for a manual steering vehicle. The present invention is also usable as a telescopic shaft for telescopically adjusting a steering column.

EXAMPLES

The present invention will be described below based on an example and a comparative example.

Example 1 and Comparative Example 1

A spline telescopic shaft in an example 1 in which an inner shaft35and an outer shaft36are fitted to each other was produced.

A spline telescopic shaft in a comparative example 1 was prepared via similar processes to those in the example 1 except that the sliding process illustrated inFIG. 4was omitted. That is, the spline telescopic shaft in the comparative example 1 has a similar configuration to that of the spline telescopic shaft in the example 1 except that a second resin coating46was not formed.

Loading Test 1

As to each of the spline telescopic shafts in the example 1 and the comparative example 1, a load required to start to relatively slide an inner shaft and an outer shaft in an axial direction was measured as a starting load. A load required to continue to relatively slide the inner shaft and the outer shaft in the axial direction was measured as a sliding load.

As to each of the spline telescopic shafts in the example land the comparative example 1, a difference between the starting load and the sliding load was divided by the sliding load. That is, (starting load—sliding load)/sliding load was found.

The results were illustrated inFIG. 11A.

When the result was expressed by percentage, it was approximately 28% in the comparative example 1. This proved that a starting load that was approximately 28% greater than a sliding load was required.

On the other hand, the result was substantially zero % in the example 1. This proved that sliding between the inner shaft and the outer shaft could be started with a substantially similar starting load to the sliding load.

As described above, in the spline telescopic shaft in the example 1, the inner shaft and the outer shaft could be significantly smoothly slid in an axial direction because the load hardly changed depending on whether the sliding was started or was being performed. As a result, a stick slip phenomenon could be reliably suppressed, which proved that quietness could be improved. The sliding load was reliably suppressed, which also proved that the spline telescopic shaft in the example 1 was superior in endurance. Further, the sliding load at the time of assembling into a vehicle could be reduced, which proved that assembling work could also be easily performed. That is, it is proved that both the inner shaft and the outer shaft were easily fitted to each other when assembled.

Loading Test 2

As to each of the spline telescopic shafts in the example 1 and the comparative example 1, a torque of 30 Nm was loaded between the inner shaft and the outer shaft. The sliding load at this time per 1 Nm was calculated.

The results were illustrated inFIG. 11B.

As illustrated inFIG. 11B, the result was approximately 13 N in the comparative example 1. On the other hand, the result was approximately 10 N in the example 1. That is, the result in the example 1 was approximately 23% smaller than that in the comparative examples 1.

As described above, in the spline telescopic shaft in the example 1, a change in the sliding load was significantly smaller than a change in a torque exerted between the inner shaft and the outer shaft. In the spline telescopic shaft in the example 1, the inner shaft and the outer shaft could be thus significantly smoothly slid. As a result, the stick slip phenomenon could be reliably suppressed. This proved that quietness could be improved. The sliding load was reliably suppressed so that a sliding load at the time of assembling into a vehicle could also be reduced. This proved that the spline telescopic shaft in the example 1 could easily be assembled into the vehicle, and was superior in durability.

This application corresponds to Japanese

Patent Application No. 2009-184542 filed with the Japanese Patent Office on Aug. 7, 2009, the disclosure of which is hereinto incorporated by reference in its entirety.