JP2016060417A - Rack bush for rack pinion type steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rack bush for a rack pinion type steering device less likely to deteriorate or deform even when used for a long period of time in severe environment.SOLUTION: A rack bush 1 for a rack pinion type steering device slidably supports a rack shaft 2 of the rack pinion type steering device in an axial direction in a rack housing 3, and is made of a synthetic resin material satisfying both of requirements that a strain amount is to be 5% or less after a creep test and that an abrasion depth is to be 40 μm or less after a reciprocating dynamic friction wear test.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a rack bush used for attaching a rack shaft of a rack and pinion type steering apparatus to a rack housing.

Conventionally, rack and pinion type steering devices have been widely used as steering devices for vehicles. A rack and pinion type steering device is a device that converts the rotational motion of a steering shaft that rotates with steering of a steering wheel into axial motion of a rack shaft by a rack and pinion mechanism.
In such a rack and pinion type steering device, the rack shaft is accommodated in a rack housing, and the rack housing is attached to the vehicle body frame. A substantially cylindrical rack bush is used as a sliding bearing for slidably supporting the rack shaft in the axial direction within the rack housing.

  Patent Document 1 describes a bush bearing which is an example of a rack bush of a rack and pinion type steering device. This bush bearing is composed of a bush main body and an endless annular elastic member, the bush main body has a slit extending in the axial direction, and an annular groove is formed on the outer peripheral surface of the bush main body. An endless annular elastic member is mounted in the annular groove of the bushing body. Patent Document 1 discloses a polyamide resin, a polyolefin resin, and a fluororesin as synthetic resins that form the bush body.

JP 2003-322165 A

In recent years, the demand for rack and pinion type steering devices has become stricter, and it has been required to maintain operational stability for a longer period of time. Specifically, due to grease lubrication, the rack bush supports the rack shaft at an appropriate position for 10 years in a severe usage environment where the temperature range is −40 ° C. to 120 ° C., and the rack shaft moves up and down. It is to prevent vibration and sound from hitting (keeping silence and slidability). The vertical movement of the rack shaft occurs when the rack bush is deformed by the influence of deterioration with time and a gap is generated between the rack housing and the rack bush.
An object of the present invention is to provide a rack bushing for a rack and pinion type steering device that does not easily deteriorate or deform even when used for a long time in a harsh environment (for example, at -40 ° C to 120 ° C for 10 years). Is to provide.

In order to solve the above-described problems, a rack bush of each aspect of the present invention is a rack bush that supports a rack shaft of a rack and pinion type steering device so as to be slidable in an axial direction within a rack housing. It has a structure (1), (2), or (3), respectively.
(1) Synthetic resin material satisfying both the strain amount after the creep test conducted by the following method is 5% or less and the wear depth after the reciprocating frictional wear test conducted by the following method is 40 μm or less. It is formed with.

<Creep test method>
A test piece that satisfies the requirements of JIS K7162 and has a total length of 150 mm, a narrow parallel portion length of 80 mm, an end width of 20 mm, and a thickness of 4 mm is prepared, and the temperature is measured according to JIS K7115. : 60 ° C., load stress: 20 MPa, test time: 100 hours.

<Reciprocating friction and wear test method>
A test piece of 40 mm × 20 mm × thickness 4 mm was prepared, and a SUJ2 ball having a diameter of 9.525 mm was placed on the test piece, temperature: 80 ° C., load stress: 8.5 MPa, speed: 20 mm / s, The ball is reciprocated under the conditions of a reciprocating distance of 40 mm and a reciprocating frequency of 5000 times.

(2) After the thermal shock test performed by the following method, the gap between the following cylinder and the torus is 0.4 mm or less, and is formed of a synthetic resin material that forms the following cylinder.
<Thermal shock test method>
A cylinder formed of a synthetic resin material and having the same inner diameter and outer diameter as the rack bush, a shaft body in which the rack shaft is cut longer than the cylinder in the longitudinal direction, and a circle made of an aluminum alloy having the same inner diameter as the rack housing Use a ring. The shaft body is inserted into the cylinder, and the annular body is fitted to the outer peripheral surface of the cylinder and put into a thermal shock test apparatus, and held at −40 ° C. for 1 hour and held at 120 ° C. for 1 hour. This is repeated 500 cycles.

(3) It is formed of a synthetic resin material that satisfies all of the following configurations (a) to (c).
(a) The amount of strain after the creep test performed by the above method is 4.3% or less.
(b) The wear depth after the reciprocating friction wear test performed by the above method should be 32 μm or less.
(c) A synthetic resin material for forming the cylinder in which a gap between the cylinder and the torus is 0.32 mm or less after the thermal shock test performed by the above method.

  According to the present invention, there is provided a rack bush of a rack and pinion type steering device that is unlikely to deteriorate or deform even when used for a long time in a harsh environment. By using this rack bush, it is possible to obtain a rack and pinion type steering device in which the rack shaft is supported at an appropriate position by the rack bush and the occurrence of vibrations and hitting sounds due to the vertical movement of the rack shaft is suppressed. .

It is a partial fracture front view showing a rack and pinion type steering device having a rack bush of an embodiment. It is sectional drawing of the A section of FIG. It is sectional drawing which shows A part of the rack housing of FIG. It is a perspective view which shows the rack bush of embodiment. It is a front view of the rack bush of FIG. It is AA sectional drawing of FIG. FIG. 5 is a view taken in the direction of arrow X in FIG. 4. FIG. 5 is a view as seen from the direction of arrow Y in FIG. 4. It is a perspective view which shows the rack bush from which a shape differs from FIG. FIG. 10 is a front view of the rack bush of FIG. 9. It is AA sectional drawing of FIG. FIG. 10 is a view on arrow X in FIG. 9. FIG. 10 is a view on arrow Y in FIG. 9. It is a perspective view which shows the rack bush from which a shape differs from FIG. It is a front view of the rack bush of FIG. It is AA sectional drawing of FIG. FIG. 15 is a view on arrow X in FIG. 14. It is a Y arrow line view of FIG. It is sectional drawing explaining an example of a column bush. It is a figure which shows the column bush of FIG. 19, Comprising: The upper half is A arrow directional view of FIG. 19, and a lower half is BB sectional drawing of FIG.

Embodiments of the present invention will be described below, but the present invention is not limited to these embodiments.
As shown in FIG. 1, a rack bush 1 of this embodiment supports a rack shaft 2 of a rack and pinion type steering device 100 so as to be slidable in an axial direction within a rack housing 3.

As shown in FIGS. 2 and 3, a recess 31 for attaching the rack bush 1 is formed on the inner peripheral surface of the rack housing 3. The recess 31 includes a small-diameter recess 31a and a large-diameter recess 31b.
As shown in FIGS. 4 to 8, the rack bush 1 has a substantially cylindrical shape and has a cylindrical main body portion 11 having a constant outer diameter and inner diameter, and the main body portion 11 is provided at both ends in the axial direction of the main body portion 11. A large diameter portion 12 having a larger outer diameter and a small diameter portion 13 having a smaller outer diameter than the main body portion 11 are formed. The inner peripheral surfaces 1a and 1b at both ends in the axial direction of the rack bush 1 are formed in a tapered shape that increases in diameter from the central side in the axial direction toward each end in the axial direction.

An annular groove 14 for fitting the elastic ring 4 is formed on the outer peripheral surface of the main body 11 at two axial positions (the large diameter portion 12 side and the small diameter portion 13 side from the axial center).
The rack bush 1 is formed with three slits 15 extending in the axial direction from the large diameter portion 12 to the middle of the small diameter portion 13. The slits 15 are formed at 120 ° intervals in the circumferential direction of the cylinder forming the rack bush 1.

The rack bush 1 satisfies both the strain amount after the creep test performed by the above method is 5% or less and the wear depth after the reciprocating frictional wear test performed by the above method is 40 μm or less. After the thermal shock test performed by the above-described method, the gap between the cylinder and the torus is 0.4 mm or less.
In the rack bush 1, the main body 11 is arranged in the small-diameter recess 31 a of the rack housing 3 shown in FIG. 3 with the elastic ring 4 fitted in the annular groove 14 of the main body 11, and the large-diameter portion 12 in the large-diameter recess 31 b. Is placed. When the rack shaft 2 is inserted into the rack bush 1 in this state and the state shown in FIG. 2 is reached, the diameter of the main body 11 is reduced by the action of the slit 15 and the elastic ring 4. Thereby, the rack bush 1 is disposed at an appropriate position of the rack housing 3 and supports the rack shaft 2 slidably in the axial direction with a tightening margin.

  Further, when the rack shaft 2 is inserted into the rack bush 1, the insertion of the rack shaft 2 can be easily performed because the main body 11 is elastically deformed by the slit 15 and the elastic ring 4. Further, since the inner peripheral surfaces 1 a and 1 b at both ends in the axial direction of the rack bush 1 are formed in a taper shape, when the rack shaft 2 is inserted into the rack bush 1, Damage is prevented.

  Since the rack bush 1 of this embodiment is formed of the above-mentioned synthetic resin material, it is deteriorated or deformed even if it is used for 10 years in a harsh operating environment where the temperature range is -40 ° C to 120 ° C due to grease lubrication. It is hard to occur. Therefore, by using the rack bush 1 of this embodiment, the rack shaft 2 is supported at an appropriate position by the rack bush 1, and the rack and the sound generated by the vertical movement of the rack shaft 2 are suppressed. The pinion type steering device 100 can be obtained.

[Rack bush shape]
The present invention is characterized by the material forming the rack bush, and the rack bush may have any shape. An example of a rack bush having a shape different from that of the rack bush 1 of FIG. 4 is shown below.
The rack bush 5 shown in FIGS. 9 to 13 is substantially cylindrical and includes divided bodies 5A to 5C that are divided into three in the radial direction. The divided bodies 5 </ b> A to 5 </ b> C form a substantially cylindrical body including a main body portion 51 and a large diameter portion 52 with a gap 55 corresponding to the slit 15 of the rack bush 1 being opened and arranged in a cylindrical shape. The gaps 55 exist at intervals of 120 ° in the circumferential direction of the substantially cylindrical body. An annular groove 54 for fitting the elastic ring 4 is formed at two positions in the axial direction (both ends from the center in the axial direction) on the outer peripheral surface of the substantially cylindrical main body 51. In each of the divided bodies 5A to 5C, grooves 54A to 54C constituting the annular groove 54 are formed.

The inner peripheral surfaces 5a and 5b at both ends in the axial direction of the rack bush 5 are formed in a taper shape with a diameter increasing from the central side in the axial direction toward each end in the axial direction.
The divided bodies 5A to 5C are integrated by fitting the elastic ring 4 into the annular groove 54 including the grooves 54A to 54C in a state where the divided bodies 5A to 5C are arranged in a cylindrical shape with a gap 55 therebetween.
The rack bush 5 is disposed in the recess 31 of the rack housing 3 shown in FIG. 3 with the elastic ring 4 fitted in the annular groove 54 of the main body 51. When the rack shaft 2 is inserted into the rack bush 5 in this state, the gap 55 is opened and the rack bush 5 is expanded in diameter, and the rack shaft 2 is completely inserted into the rack bush 5 in the elastic ring. 4 and the clearance 55 reduce the diameter of the rack bush 5. Thereby, the rack bush 5 is disposed at an appropriate position of the rack housing 3 and supports the rack shaft 2 slidably in the axial direction with a tightening margin.

The rack bush 6 shown in FIGS. 14 to 18 has a substantially cylindrical shape, and has a cylindrical main body portion 61 having a constant outer diameter and inner diameter. A large-diameter portion 62 having a large diameter is formed. The inner peripheral surfaces 6a and 6b at both ends in the axial direction of the rack bush 6 are formed in a taper shape with a diameter increasing from the central side in the axial direction toward each end in the axial direction.
On the outer peripheral surface of the main body 61, annular grooves 64 for fitting the elastic ring 4 are formed at two locations in the axial direction (both ends from the center in the axial direction). The rack bush 6 is formed with a spiral cut 65 extending in the axial direction of the cylinder.

  The rack bush 6 is disposed in the recess 31 of the rack housing 3 shown in FIG. 3 with the elastic ring 4 fitted in the annular groove 64. In a state where the rack shaft 2 is disposed in the rack bush 6 in this state, the rack bush 6 is elastically deformed and reduced in diameter by the action of the elastic ring 4. Thereby, the rack bush 6 is disposed at an appropriate position of the rack housing 3, and supports the rack shaft 2 slidably in the axial direction with a tightening margin.

[About synthetic resin materials]
The present inventors examined how much clearance is generated between the rack shaft and the rack bush, and between the rack bush and the rack housing, and vibration and hammering are generated. However, we examined the range where no vibration or sound hits. As a result, as an accelerated test corresponding to 10 years of use at -40 ° C to 120 ° C, the above-described thermal shock test is performed, and if the gap between the cylinder and the torus is 0.4 mm or less, the rack bush is deformed. As a result, it was found that no vibration or sound was generated even when a gap was generated.

Further, when the strain amount after the creep test is 5% or less and the wear depth after the reciprocating frictional wear test is 40 μm or less, vibration and sound are not generated in the gap between the rack bush and the rack housing. It was found that the necessary functions as a rack bush can be maintained for a long time.
In order to obtain a rack bush that satisfies these conditions, it is necessary to appropriately select the constituent components (synthetic resin, filler, additive) of the synthetic resin material (resin composition) forming the rack bush. This is described below.

The base synthetic resin is preferably a thermoplastic resin.
Specifically, a polyacetal resin and a polyamide-type resin are mentioned. Polyamide resins include polyamide 6, polyamide 66, polyamide 46, polyamide 6T / 6-6, polyamide 6T / 6I, polyamide 6T / 6I / 6-6, polyamide 6T / M-5T, which are semi-aromatic polyamides, Polyamide 9T and the like are preferable.

Polyester resins (polyethylene terephthalate, polybutylene terephthalate), polyphenylene sulfide resins, and polyether ether ketone resins can also be used.
A thermoplastic elastomer can also be suitably used. Specific examples include polyester-based, polyurethane-based, and polyamide-based, and engineering plastic-based thermoplastic elastomers such as polyester-based and polyamide-based are preferable.

  The rubber component of the thermoplastic elastomer to be used, so-called soft segment component, is not particularly limited. Soft segment candidates include many polyesters, polyethers, polybutadienes, polyisoprenes, polycarbonates, and polycaprolactams, but polyesters or polyethers are readily available from the market. In particular, in consideration of hydrolysis resistance, a polyether segment is preferable. Specific examples of the polyether segment include polytetramethylene oxide, polypropylene oxide, polyethylene oxide, and copolymers thereof.

  As the thermoplastic elastomer, a dynamically crosslinked thermoplastic elastomer may be used. The dynamically crosslinked thermoplastic elastomer is a polymer alloy composed of a thermoplastic resin and rubber, and has higher oil resistance and heat resistance than conventional thermoplastic elastomers. Since the dynamic cross-linking thermoplastic elastomer can be designed by arbitrarily combining a thermoplastic resin and rubber, various combinations are being studied.

  Specifically, dynamically crosslinked thermoplastic elastomers such as polyolefin, styrene, and polyamide are commercially available. For example, “Hyper Alloy Actimer (registered trademark)” manufactured by Riken Technos Co., Ltd. (compatible with styrene-based dynamically cross-linked elastomer, polyester elastomer, polyurethane elastomer, polyamide elastomer), nylon resin and silicone rubber A dynamically crosslinked thermoplastic elastomer (commercially available from Multibase) can be used.

  As the filler, reinforcing fillers such as glass fiber, carbon fiber, aramid fiber, silicon carbide fiber, wollastonite, zinc oxide whisker, potassium titanate whisker, and aluminum borate whisker can be added. Non-fibrous fillers such as zeolite, kaolin, mica, clay, talc, alumina and silica can also be added. In addition, compounding agents such as pigments, dyes, antistatic agents, and solid lubricants such as fluororesins may be added in a very small amount as required.

  In order to prevent deterioration due to heat during molding and use, iodide-based heat stabilizers, phenol-based or amine-based antioxidants are added alone or in combination as additives. May be.

[Column bush]
The above-mentioned synthetic resin material can be used as a material for a column bush that rotatably supports a steering shaft within a steering column. The column bush formed of the above-described synthetic resin material is less likely to be deteriorated or deformed even when used for a long time in a harsh environment as compared with a conventional product.

  As shown in FIGS. 19 and 20, the column bush 10 is a sliding bearing that rotatably supports the steering shaft 20 within the steering column 30. The column bush 10 is a cylindrical body having a missing portion (slit) 150 extending in the axial direction, and has an annular groove 140 in which the elastic ring 4 is fitted on the outer peripheral surface. The outer diameter of the column bush 10 is the same as or slightly larger than the inner diameter of the steering column 30 in a free state. The column bush 10 is reduced in diameter by fitting the elastic ring 4 in the annular groove 140 so that the space between the facing surfaces 150a facing each other at the defect 150 becomes narrow.

19, the elastic ring 4 is pressed against the inner peripheral surface of the steering column 30, and there is a gap between the column bush 10 and the inner peripheral surface of the steering column 30, and the column bush 10 and the steering shaft 20 The gap is zero. That is, the column bush 10 is elastically supported with respect to the steering column 30 by the elastic ring 4. Thereby, an increase in steering torque and occurrence of abnormal noise are suppressed.
There is also a column bush having a structure in which the width of the annular groove 140 is increased and a leaf spring is fitted into the annular groove 140.

The following tests were carried out by preparing the following synthetic resin materials.
<Synthetic resin material>
Test No. 1: “Delrin (registered trademark) 100P NC010” manufactured by DuPont Co., Ltd. (high viscosity acetal homopolymer)
Test No. 2: “Delrin (registered trademark) 111P NC010” manufactured by DuPont Co., Ltd. (high viscosity acetal homopolymer, improved crystallinity)
Test No. 3: “Duracon (registered trademark) M90-44” (polyacetal) manufactured by Polyplastics Co., Ltd.
Test No. 4: “Duracon (registered trademark) M25-44” manufactured by Polyplastics Co., Ltd. (high viscosity polyacetal)
Test No. 5: “Duracon (registered trademark) OL-10” (oil-containing polyacetal) manufactured by Polyplastics Co., Ltd.
Test No. 6: “Duracon (registered trademark) SW-01” (polyacetal with improved sliding property) manufactured by Polyplastics Co., Ltd.

<Creep test method>
With each of the synthetic resin materials described above, a test piece that satisfies the requirements of JIS K7162 and has a total length of 150 mm, a narrow parallel portion length of 80 mm, an end width of 20 mm, and a thickness of 4 mm is manufactured. Each test piece is used according to JIS K7115, under the conditions of temperature: 60 ° C., load stress: 20 MPa, test time: 100 hours.

<Reciprocating friction and wear test method>
A test piece of 40 mm × 20 mm × thickness 4 mm is produced from each of the synthetic resin materials described above. A ball made of SUJ2 having a diameter of 9.525 mm was placed on each test piece, under conditions of temperature: 80 ° C., load stress: 8.5 MPa, speed: 20 mm / s, reciprocation distance: 40 mm, and reciprocation times: 5000 times. Move the ball back and forth.

<Thermal shock test method>
A cylinder having the same inner diameter and outer diameter as the rack bush 1 is formed of the above-described synthetic resin materials. A shaft body in which the rack shaft 2 is cut longer than the cylinder in the longitudinal direction and an annular body made of an aluminum alloy having the same inner diameter as the rack housing 3 are used. Insert the shaft into the cylinder, fit the torus on the outer peripheral surface of the cylinder, put it in the thermal shock tester, and hold at -40 ° C for 1 hour and hold at 120 ° C for 1 hour alternately 500 cycles are performed.

These tests were conducted, and the amount of strain after the creep test, the wear depth after the reciprocating frictional wear test, and the gap between the cylinder and the torus after the thermal shock test were examined. The results are shown in Table 1 below.
Moreover, the endurance test was done with the following method.
<Durability test method>
The rack bush 1 shown in FIG. 4 was formed from each of the above-described synthetic resin materials and incorporated in the rack and pinion type steering apparatus shown in FIG. 1, and the steering was repeated at an ambient temperature of 120 ° C. The time when a sudden abnormal noise occurs during steering or a significant deterioration in steering feeling (such as a significant increase in sliding torque) is judged as the life, and the steering is stopped at that point. (Cycle number) was examined. The results are also shown in Table 1 below.

As can be seen from the results in Table 1, No.1 to No.4 synthetic resin materials satisfying all of “strain amount of 4.3% or less”, “wear depth of 32 μm or less”, and “gap amount of 0.32 mm or less” are used. In the steering device incorporating the rack bush formed in this way, a durability of over 170,000 cycles is obtained, and the rack is formed using No. 5 and No. 6 synthetic resin materials that do not satisfy any of the above ranges. The durability of the steering device incorporating the bush was 130,000 cycles or less. If durability exceeding 150,000 cycles is obtained, it can be said that durability is higher than the current product.
From the above results, by using the synthetic resin materials No.1 to No.4 corresponding to the embodiments of the present invention, the rack bushing of the rack and pinion type steering device has been used for a long time in a harsh environment. It can also be seen that those that are less prone to deterioration and deformation are obtained.

DESCRIPTION OF SYMBOLS 1 Rack bush 11 Main body part 12 Large diameter part 13 Small diameter part 14 Annular groove 15 Slit 2 Rack shaft 3 Rack housing 31 Recessed part 31a Small diameter recessed part 31b Large diameter recessed part 4 Elastic ring 5 Rack bushing 5A-5C Divided body 51 Main body part 52 Large diameter Portion 54 Annular groove 55 Clearance 6 Rack bush 61 Body portion 62 Large diameter portion 64 Annular groove 65 Spiral cut 10 Column bush 20 Steering shaft 30 Steering column 140 Annular groove of the column bush 150 Column bush defect 150a Opposing the defect Surface 100 Rack and pinion steering device

Claims (3)

A rack bush for supporting a rack shaft of a rack and pinion type steering device so as to be slidable in an axial direction within a rack housing,
A test piece that satisfies the requirements of JIS K7162 and has a total length of 150 mm, a narrow parallel portion length of 80 mm, an end width of 20 mm, and a thickness of 4 mm is prepared, and the temperature is measured according to JIS K7115. : 60 ° C, load stress: 20 MPa, test time: strain amount after creep test performed under the conditions of 100 hours is 5% or less,
A test piece of 40 mm × 20 mm × thickness 4 mm was prepared, and a SUJ2 ball having a diameter of 9.525 mm was placed on the test piece, temperature: 80 ° C., load stress: 8.5 MPa, speed: 20 mm / s, The wear depth after a reciprocating friction wear test in which the ball is reciprocated under the conditions of a reciprocating distance of 40 mm and a reciprocating frequency of 5000 times is 40 μm or less,
Rack bush made of synthetic resin material that satisfies both. A rack bush for supporting a rack shaft of a rack and pinion type steering device so as to be slidable in an axial direction within a rack housing,
A cylinder formed of a synthetic resin material and having the same inner diameter and outer diameter as the rack bush, a shaft body in which the rack shaft is cut longer than the cylinder in the longitudinal direction, and a circle made of an aluminum alloy having the same inner diameter as the rack housing Using a ring
The shaft body is inserted into the cylinder, and the annular body is fitted to the outer peripheral surface of the cylinder and put into a thermal shock test apparatus, and held at −40 ° C. for 1 hour and held at 120 ° C. for 1 hour. After the thermal shock test in which 500 cycles are alternately performed,
A rack bush formed of a synthetic resin material forming the cylinder, wherein a gap between the cylinder and the torus is 0.4 mm or less. A rack bush for supporting a rack shaft of a rack and pinion type steering device so as to be slidable in an axial direction within a rack housing,
(a) A test piece that satisfies the requirements of JIS K7162 and has a total length of 150 mm, a narrow parallel portion length of 80 mm, an end width of 20 mm, and a thickness of 4 mm is prepared, and a method according to JIS K7115 And the amount of strain after a creep test performed under the conditions of temperature: 60 ° C., load stress: 20 MPa, test time: 100 hours, is 4.3% or less,
(b) A test piece of 40 mm × 20 mm × thickness 4 mm was prepared, a SUJ2 ball having a diameter of 9.525 mm was placed on the test piece, temperature: 80 ° C., load stress: 8.5 MPa, speed: 20 mm / S, reciprocating distance: 40 mm, reciprocating frequency: 5000 times, the wear depth after the reciprocating friction wear test for reciprocating the ball is 32 μm or less,
(c) a cylinder formed of a synthetic resin material and having the same inner diameter and outer diameter as the rack bush, a shaft body in which the rack shaft is cut longer than the cylinder in the longitudinal direction, and an aluminum alloy having the same inner diameter as the rack housing Using a torus consisting of
The shaft body is inserted into the cylinder, and the annular body is fitted to the outer peripheral surface of the cylinder and put into a thermal shock test apparatus, and held at −40 ° C. for 1 hour and held at 120 ° C. for 1 hour. After the thermal shock test in which 500 cycles are alternately performed,
The gap between the cylinder and the torus is 0.32 mm or less, the synthetic resin material forming the cylinder,
Rack bush made of synthetic resin material that satisfies all of the above.

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