阅读说明：本技术 倒立构造的振动衰减装置 (Vibration damping device of inverted structure ) 是由 寒川直辉 小森健太郎 藤元岳洋 于 2019-07-10 设计创作，主要内容包括：本发明提供一种振动衰减装置,其为低摩擦性的倒立构造,在工作缸与外筒之间设有低摩擦性润滑油及低摩擦性套筒。该倒立构造的振动衰减装置的特征在于,包括：工作缸(11)；能够进出所述工作缸内的杆(13)；外筒(12),其与杆(13)连结并插入在工作缸(11)的外周；以及套筒(22),其滑动自如地插入在外筒(12)与工作缸(11)之间,在外筒(12)与工作缸(11)之间填充有润滑油(23),套筒(22)含有聚四氟乙烯和全氟烷氧基烷烃,润滑油(23)含有有机钼添加剂。(The invention provides a vibration damping device which is a low-friction inverted structure, and a low-friction lubricating oil and a low-friction sleeve are arranged between a working cylinder and an outer cylinder. The vibration damping device of the inverted structure is characterized by comprising: a cylinder (11); a rod (13) that can be moved into and out of the cylinder; an outer cylinder (12) that is coupled to the rod (13) and is inserted into the outer periphery of the cylinder (11); and a sleeve (22) slidably inserted between the outer tube (12) and the cylinder (11), wherein a lubricating oil (23) is filled between the outer tube (12) and the cylinder (11), the sleeve (22) contains polytetrafluoroethylene and perfluoroalkoxyalkane, and the lubricating oil (23) contains an organic molybdenum additive.)

1. A vibration damping device of an inverted structure, comprising:

a working cylinder;

a rod capable of entering and exiting the cylinder;

an outer cylinder coupled to the rod and inserted into an outer periphery of the cylinder; and

a sleeve slidably inserted between the outer cylinder and the cylinder,

lubricating oil is filled between the outer cylinder and the working cylinder,

the sleeve contains polytetrafluoroethylene and perfluoroalkoxy alkane,

the lubricating oil contains an organo-molybdenum additive.

2. The vibration damping device of inverted configuration according to claim 1,

the lubricating oil contains 700 to 2000ppm of the organomolybdenum additive.

3. The vibration damping device of inverted configuration according to claim 1,

there is a plurality of said sleeves which are,

the area of the frictional sliding surface of the sleeve located on the lower side in the height direction is smaller than the area of the frictional sliding surface of the sleeve located on the upper side in the height direction.

4. The vibration damping device of inverted configuration according to claim 1,

the sleeve is in the shape of a chamfered cylinder,

the height of the side on which the lateral force is applied is higher than the height of the opposite side of the vibration damping device when the device is in use.

Technical Field

The present invention relates to a vibration damping device of an inverted structure.

Background

A conventional vibration damping device (damping device) having an upright structure has a problem that a lateral force applied to the vibration damping device is received by a rod as a piston in a cylinder thereof, and the rod is easily deformed. However, if the rod diameter is increased in order to increase the rigidity of the rod, the frictional force generated in the vibration damping device increases, and the reaction force of the rod also increases. Therefore, when the vibration damping device having a large rod diameter is used as a shock absorber for a vehicle or the like, ride comfort of the vehicle or the like is deteriorated due to an increase in the shock absorber from the top. It is therefore difficult to achieve both the damping performance and the rigidity of the vibration damping device of the upright configuration.

To cope with such a problem, there is known a vibration damping device of an inverted structure in which an outer cylinder is inserted around a cylinder by inverting the vibration damping device, the outer cylinder is coupled to a rod, and the rod and the outer cylinder together serve as a piston. Since the vibration damping device of the inverted structure can receive a lateral force by the outer cylinder, it is not necessary to increase the rod diameter, and high rigidity can be achieved.

In the vibration damping device of the inverted structure, as compared with the vibration damping device of the upright structure in which only the rod is inserted into and removed from the cylinder, the friction force is easily increased by the bearing portion between the cylinder and the outer cylinder. Further, since the vibration damping device of the inverted structure is limited to the single cylinder type, the repulsive force of the gas chamber is increased as compared with the vibration damping device of the double cylinder type. Such an increase in the repulsive force tends to cause thermal expansion of oil in the vibration damping device of the inverted structure, and when the device is used as a shock absorber for a vehicle or the like, the vehicle height tends to change due to the influence of temperature.

A vibration damping device of an inverted structure is required to solve various factors of an increase in frictional force due to such an inverted structure.

As a technique for reducing the frictional force of the vibration damping device, the following is known.

Patent document 1 discloses a vibration damping device of an inverted structure in which a plurality of oil seals are provided to prevent lubricating oil between a cylinder and an outer cylinder from drying up by falling from the oil seals, thereby achieving smooth relative movement between the cylinder and the outer cylinder.

Patent document 2 discloses a technique for realizing excellent low friction properties by using a resin composition containing a polytetrafluoroethylene resin or the like in a frictional sliding surface of a sleeve used for a bearing portion.

Patent document 3 discloses a lubricating oil composition that achieves excellent low wear and low friction properties by MoDTC having C8-23 alkenyl groups containing 50 to 2000ppm of molybdenum.

Disclosure of Invention

Conventionally, as described above, the reduction in friction of a vibration damping device has been studied, but it has not been said that the reduction in friction of a vibration damping device having an inverted structure, in which the friction force is likely to increase, is sufficiently achieved.

Accordingly, an object of the present invention is to provide a vibration damping device of an inverted structure with low friction.

The inventors of the present invention have intensively studied a technique for reducing the frictional force of a vibration damping device of an inverted structure. As a result, it has been found that, in the vibration device of the inverted structure, half or more of the frictional force is generated in the bearing portion between the cylinder and the outer cylinder, and the frictional force generated by the damping device of the inverted structure can be significantly reduced by reducing the friction in the bearing portion.

The invention according to claim 1 is a vibration damping device of an inverted structure, including: a working cylinder; a rod capable of entering and exiting the cylinder; an outer cylinder coupled to the rod and inserted into an outer periphery of the cylinder; and a sleeve slidably inserted between the outer cylinder and the cylinder, wherein a lubricating oil is filled between the outer cylinder and the cylinder, the sleeve contains polytetrafluoroethylene and perfluoroalkoxyalkane, and the lubricating oil contains an organic molybdenum additive.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a low-friction vibration damping device of an inverted structure can be provided.

Drawings

Fig. 1 is a sectional view showing a schematic structure of a vibration damping device having an inverted structure.

Fig. 2 is an enlarged cross-sectional view of a bearing portion of the vibration damping device having an inverted structure.

Fig. 3 is a graph showing the effect of a change in additives on the frictional force of a lubricating oil.

Fig. 4 is a schematic diagram of an apparatus for measuring the influence of a change in an additive on the frictional force of a lubricating oil.

Fig. 5 is a graph showing the effect of changes in the amount of organic molybdenum additive on the friction of lubricating oil.

Fig. 6 is a graph comparing the load received by the upper sleeve in the height direction with the load received by the lower sleeve in the height direction.

Fig. 7 is a graph showing the relationship between the lateral force and the frictional force applied to the vibration damping device of the example and the comparative example.

Fig. 8 is a schematic view of a chamfered cylindrical sleeve having a height on the side to which a lateral force is applied higher than the height on the opposite side when the vibration damping device is used.

Description of the reference numerals

10 vibration damping device

11 working cylinder

12 outer cylinder

13 bar

14 st liquid chamber

15 nd 2 nd liquid chamber

16 piston

17 high-pressure gas chamber

18 free piston

20 bearing part

21. 21a, 21b oil seal

22. 22a, 22b, 22c sleeve

23 lubricating oil

Detailed Description

Next, a vibration damping device of an inverted structure according to an embodiment of the present invention will be described. The following describes the entire structure of the vibration damping device, and then describes the lubricating oil and the sleeve used in the vibration damping device.

The present invention is not limited to the following embodiments.

< construction of vibration damping device >

Fig. 1 is a sectional view showing a schematic structure of a vibration damping device 10 having an inverted structure according to an embodiment of the present invention. The vibration damping device 10 is a single tube type inverted damper mounted on a vehicle.

The vibration damping device 10 includes a substantially cylindrical cylinder 11 and a substantially cylindrical outer cylinder 12 inserted around the cylinder 11. The cylinder 11 is sealed with cylinder oil, and a rod 13 is inserted therethrough. The outer cylinder 12 is connected to the rod 13, and the outer cylinder 12 is integrated with the rod 13 and is slidable in the axial direction (longitudinal direction) of the cylinder 11.

A piston 16 is attached to the tip end of the rod 13, and divides the inside of the cylinder 11 into a 1 st liquid chamber 14 and a 2 nd liquid chamber 15. The piston 16 has a communication hole (piston valve) through which the cylinder oil passes to damp the vibration received by the vibration damping device 10. The free piston 18 divides the inside of the cylinder 11 into a 2 nd liquid chamber 15 and a high-pressure gas chamber 17.

The bearing portion 20 between the cylinder 11 and the outer cylinder 12 has a substantially annular oil seal 21 (oil seal 21, oil seal 21b) at its upper and lower sides. The oil seal 21a on the upper side in the height direction and the oil seal 21b on the lower side in the height direction are sealed with lubricating oil 23 in the middle. A dust ring 24 is provided above the upper side edge of the cylinder 12 and the oil seal 21a to prevent dust from entering the outer cylinder 12.

The sleeve 22 (sleeve 22a, sleeve 22b) is formed in a substantially cylindrical shape from a material having a low friction coefficient, and slidably holds the cylinder 11 between the oil seal 21a and the oil seal 21 b.

In fig. 1, a plurality of sleeves 22 such as the sleeve 22a and the sleeve 22b are provided as an example, but one or more sleeves 22 may be provided. However, as shown in FIG. 1, a plurality of sleeves 22 are preferably provided. When only one sleeve 22a is provided, it is preferably provided on the upper side in the height direction of the bearing portion 20. The advantages of providing a plurality of sleeves 22 and the advantages of adjusting the positions of the sleeves 22 will be described later in the description of the sleeves.

With such a configuration, the lateral force acting on the vibration damping device 10 can be received by the outer cylinder 12, and the rigidity of the vibration damping device 10 can be improved.

Fig. 2 is an enlarged cross-sectional view of bearing portion 20 of vibration damping device 10 according to the embodiment of the present invention. The scale in the longitudinal direction of fig. 2 is different from that of fig. 1, but the structure is the same.

The bearing portion 20 between the cylinder 11 and the outer cylinder 12 is filled with a lubricant 23 to realize smooth sliding of the cylinder 11 and the outer cylinder 12.

< lubricating oil >

The inventors of the present invention have studied the cause of an increase in the frictional force of a vibration damping device of an inverted structure. As described above, the vibration damping device of the inverted structure is configured such that the vibration damping device of the upright structure is inverted and the outer cylinder is inserted into the periphery thereof. The inventors of the present invention conducted a friction evaluation test under the conditions of a frequency of 0.005Hz and an amplitude of ± 5mm for three types of devices, namely, a device having only an upright structure, a device having an upright structure with an outer cylinder and a structure associated with the installation of the outer cylinder, and a device having an upright structure with an outer cylinder, a dust-proof ring, and a structure associated with the installation of the outer cylinder and the dust-proof ring. As a result, the frictional force generated by the device having only the upright structure was 85.4N, and when the upright structure was provided with the outer cylinder, the frictional force increased by 122.6N, and when the dust ring was further increased, the frictional force increased by 17.8N. That is, it was found that, of the frictional force 225.8N of the entire vibration damping device having the inverted structure including the outer cylinder and the dust-proof ring, 122.6N, which is half or more, is generated by the bearing portion.

Thus, the inventors of the present invention have studied reduction of the frictional force between the sleeve 22 and the lubricating oil 23 constituting the bearing portion 20 in order to reduce the frictional force at the bearing portion 20.

First, the friction reduction of the lubricating oil 23 will be described. The inventors of the present invention have studied to reduce friction of the lubricating oil 23 by adding an additive to the lubricating oil 23.

Fig. 3 shows a graph showing the influence of the change of the additive on the frictional force of the lubricating oil, and fig. 4 shows a schematic diagram of an apparatus for measuring the influence of the change of the additive on the frictional force of the lubricating oil.

Specific evaluation methods are as follows.

The surface of the plate metal was coated with a resin obtained by mixing 85 mass% of Polytetrafluoroethylene (PTFE) and 15 mass% of Perfluoroalkoxyalkane (PFA), thereby forming a guide metal 31. The device for friction evaluation was fabricated by immersing the guide metal 31 in an oil bath of about 25ml of lubricating oil, and placing a chromium-plated cylindrical rod 32 on the guide metal 31. Since the rod 32 is the same as the piston rod of the conventional vibration damping device having an inverted structure, the surface of the guide metal 31 that frictionally slides with the rod 32 is always in the lubricant oil in this device.

As the lubricating oil, three types of conventional oil only for a shock absorber oil, oil in which a conventional amine additive was added to a conventional shock absorber oil, and oil in which molybdenum dithiocarbamate (MoDTC) was added as an organic molybdenum additive to a conventional shock absorber oil were prepared, and three types of devices for friction evaluation were prepared, in which the lubricating oil was different.

The width of the guide metal 31 in each friction force evaluation device was 10. + -. 1.5 mm.

In the device for friction force evaluation, a constant load is applied to the rod 32 from the top downward, i.e., toward the direction indicated by the upper arrow in fig. 4. The load is comparable to the lateral force applied to the vibration damping device.

The loaded state is maintained, and the frictional force generated when the guide metal 31 is continuously reciprocated (slid) in the long dimension direction, i.e., the direction indicated by the lower arrow of fig. 4, is measured once per hour.

The amount of movement of the slide is, for example, ± 10mm centered on the starting point, and the frequency is, for example, 2Hz (120 rpm/min). As an example of the measurement, the measurement was performed under a load of 24N (surface load of 9MPa) and a load of 70N (surface load of 15MPa), respectively.

The vertical axis of fig. 3 represents the friction force, and the horizontal axis represents the slip duration. The solid line represents "data measured under load 24N", and the broken line represents "data measured under load 70N". The dots indicate the data of the lubricating oil only of the conventional damper oil, the square dots indicate the data of the lubricating oil to which the amine additive was added to the conventional damper oil, and the triangular dots indicate the data of the lubricating oil to which the organomolybdenum additive was added to the conventional damper oil.

As can be seen from a comparison of the "data measured under load 24N" and the "data measured under load 70N" of fig. 3, the greater the load, the greater the friction force. From the results of fig. 3, it is understood that the friction force reduction effect is small at the load of 24N and the friction force is remarkably reduced at the load of 70N by adding molybdenum dithiocarbamate to the oil.

The case where the friction reduction needs to be achieved is a high load in which the friction increases. Thus, lubricating oil 23 used an oil containing an organomolybdenum additive that had significantly reduced friction under a load of 70N.

The mechanism of the increase in the friction force reducing effect based on molybdenum dithiocarbamate when the load is increased is considered as follows.

Molybdenum dithiocarbamate forms a surface coating containing molybdenum disulfide (MoS2) with an increase in pressure/load. Molybdenum disulfide has a layered crystal structure of a stack of molybdenum sandwiched by sulfur. Molybdenum has a strong bond with sulfur, but weak bonds between sulfur connected between the layers, and the layers easily slide under shear force, so that the coefficient of friction of molybdenum disulfide is low.

From this fact, it is considered that molybdenum dithiocarbamate generates a surface coating film containing molybdenum disulfide having a low friction coefficient under a high load, and the friction force is reduced.

In addition to molybdenum dithiocarbamates, organic molybdenum additives having the same effect as molybdenum dithiocarbamates can be used as additives for lubricating oils. Among various organic molybdenum additives, molybdenum dithiocarbamate is preferred for reasons such as not containing phosphorus (P) in the molecule and avoiding deterioration in durability.

Then, the inventors investigated the optimum addition amount of the organic molybdenum additive capable of reducing the frictional force thereof by adding to the oil.

Fig. 5 is a graph showing the effect of changes in the amount of organic molybdenum additive on the friction of lubricating oil. The vertical axis of fig. 5 represents the friction force, and the horizontal axis represents the slip duration. In addition, the triangular dots represent data without the addition of the organomolybdenum additive, the square dots represent data with the addition of 700ppm of the organomolybdenum additive, the open circles represent data with the addition of 1000ppm of the organomolybdenum additive, and the solid circles represent data with the addition of 2000ppm of the organomolybdenum additive.

The specific measurement method is as follows.

Molybdenum dithiocarbamate (MoDTC) was used as the organic molybdenum additive, and MRF (Magneto-Rheological Fluid) oil (model 126CD manufactured by LORD) was used as the oil. The following four were made as lubricating oils: 126CD only oil; oil with 700ppm MoDTC added to 126 CD; oil with 1000ppm MoDTC added to 126 CD; and oil with 2000ppm MoDTC added to 126 CD.

Using these four kinds of lubricants, four kinds of experimental devices similar to those of fig. 4 were prepared, and measurement was performed by applying a load of 70N in the same manner as that of fig. 4. From the results, it was found that when 700 to 2000ppm of the organomolybdenum additive was added to the oil, the frictional force was reduced as compared with the oil to which the organomolybdenum additive was not added, and the frictional force was minimized when 1000ppm of the organomolybdenum additive was added.

As described above, the amount of the organomolybdenum additive added to the lubricating oil 23 is preferably 700 to 2000ppm, and more preferably 900 to 1300 ppm.

< Sleeve >

Next, the friction reduction of the sleeve 22 will be described.

The inventors of the present invention have studied the influence of the material of the sleeve on the frictional force in the vibration damping device of the inverted structure.

Two types of conventional sleeves and modified sleeves were manufactured, wherein the conventional sleeves contained Polytetrafluoroethylene (PTFE) in an amount of 70 mass% and calcium fluoride (CaF2) and ferric oxide (Fe2O3) as other main components, and the modified sleeves contained polytetrafluoroethylene in an amount of 85 mass% and Perfluoroalkoxyalkane (PFA) in an amount of 15 mass%, respectively, to manufacture vibration damping devices having inverted structures using the two types of sleeves. The dimensions of both sleeves are, for example, 40mm inner diameter, 44mm outer diameter and 20mm height.

The distance between the sleeves of the two vibration damping devices is set to, for example, 115mm from the design viewpoint such as the rod diameter. The two vibration damping devices have the same configuration except for the sleeve.

A damper unit test was performed in order to compare the frictional forces generated by the two vibration damping devices. The shock absorber unit test referred to herein is a test in which clamps are provided on both upper and lower sides of a shock absorber, and a piston rod is moved up and down while applying a lateral force in a direction perpendicular to the axial direction of the shock absorber, thereby measuring a frictional force generated by the shock absorber. The following damper unit tests are all referred to as such test methods.

For example, a damper cell test was performed on each of the inverted-structure damping devices under the conditions of a frequency of 0.005Hz, an amplitude of ± 5mm, and a lateral force of 0N. As a result of the measurement, the friction force generated by the vibration damping device using the conventional sleeve was 208.9N, and the friction force generated by the vibration damping device using the improved sleeve was 199.9N. The friction of the vibration damping device is reduced by about 9N by modifying the sleeve in the absence of lateral forces. Thus, the sleeve 22 constituting the bearing portion 20 of the vibration damping device 10 of the inverted structure contains polytetrafluoroethylene and perfluoroalkoxyalkane.

As described above, if the frictional force can be reduced by improving the lubricating oil 23 and/or the sleeve 22, it is considered that not only smooth sliding between the cylinder 11 and the outer cylinder 12 can be achieved, but also wear of the sleeve 22 can be reduced.

In order to verify the effect of reducing the wear of the sleeve 22 due to the reduction of the frictional force, the modified sleeve and the sleeve having the same level of wear resistance as the conventional sleeve were immersed in oil grooves and brought into contact with metal, and a high load was applied thereto to perform a sliding test for 4 hours. The roughness of each sleeve surface was measured using a contact surface roughness meter after the sliding test. The contact surface roughness meter is an instrument that tracks the surface of a test piece with the tip of a stylus of a detector and electronically detects the vertical movement of the stylus. The average wear depth was calculated by comparing the sliding surface with the surface which had not slid after the sliding test, and as a result, the average wear depth of the conventional sleeve was 11 μm and the average wear depth of the modified sleeve was 4 μm. It was confirmed that the wear resistance of the sleeve containing polytetrafluoroethylene and perfluoroalkoxyalkane was improved.

As described above, in the vibration damping device in which the lubricant 23 is an oil to which an organic molybdenum additive is added and the sleeve 22 is a sleeve containing polytetrafluoroethylene and perfluoroalkoxyalkane so as to reduce the frictional force, the sleeve is less likely to be worn. Therefore, in such a vibration damping device, since the surface load of the sleeve can be increased, the frictional force can be further reduced by reducing the frictional sliding surface of the sleeve.

Here, a case where the plurality of sleeves 22 are used for the bearing portion 20 is considered. As shown in fig. 3 described above, the frictional force varies depending on the magnitude of the load (lateral force). It is considered that if the load applied to the bearing portion 20 by the lateral force received by the vibration damping device 10 differs depending on the position in the vertical direction in the bearing portion 20, the degree of wear of the sleeve 22 differs depending on where the sleeve is located in the bearing portion 20. For the sleeve 22 located at a position less prone to wear, the frictional sliding surface can be reduced to further reduce the frictional force.

Therefore, the inventors of the present invention have studied the adjustment of the size of the frictional sliding surface of each sleeve 22 in the case where a plurality of sleeves 22 are provided.

The inventors of the present invention have made a model of the vibration damping device 10 provided with the sleeve 22a on the upper side in the height direction and the sleeve 22b on the lower side in the height direction. Changes in the surface loads received by the sleeves 22a and 22b, which are associated with changes in the load received by the vibration damping device 10, are analyzed and compared by CAE (Computer Aided Engineering). The sleeves 22a and 22b to be measured are each formed in a substantially cylindrical shape, and have, for example, an inner diameter of 40mm, an outer diameter of 44mm, and a height of 20mm, and the distance between the sleeves of the vibration damping device 10 is, for example, 115 mm. The measurement results are as follows.

Fig. 6 is a graph comparing the load received by the upper sleeve 22a in the height direction with the load received by the lower sleeve 22b in the height direction. The horizontal axis represents the lateral force acting on the vibration damping device 10, and the vertical axis represents the force acting on the sleeve 22. The square dots indicate data of the upper sleeve 22a, and the triangular dots indicate data of the lower sleeve 22 b.

As can be seen from fig. 6, the upper sleeve 22a receives a positive load, the lower sleeve 22b receives a negative load, and the upper and lower sleeves receive opposite loads. In addition, when the load distribution in the sleeve 22 is measured, the load distribution of the sleeve 22a is biased toward the edge portion on the side on which the lateral force acts, and the load distribution of the sleeve 22b is the same for the entire sleeve 22 b. It can be said that the lower sleeve 22b is less likely to receive a load and the load is dispersed, and therefore, the risk of wear due to local excessive surface load is low.

As is apparent from the above description, when the vibration damping device 10 is provided with two or more sleeves 22, the area of the frictional sliding surface can be reduced in the lower sleeve 22 that is less likely to receive a load even if a lateral force acts on the vibration damping device 10. The frictional force generated by the sleeve 22 can be reduced by reducing the lower sleeve 22 as compared with the upper sleeve 22 to reduce the frictional force generated by the bearing portion 20.

As described above, when the plurality of sleeves 22 are provided in the vibration damping device 10, it is preferable to reduce the area of the frictional sliding surface of the sleeve 22 on the lower side in the height direction compared to the sleeve 22 on the upper side in the height direction. With this configuration, the frictional force generated in the bearing portion 20 can be reduced while stably holding the cylinder 11 by the plurality of sleeves 22.

Further, from the viewpoint of further reducing the frictional force, it is also preferable to omit the lower sleeve 22 that is not easily subjected to the load, and to dispose only one sleeve 22 immediately below the oil seal 21a and above the bearing portion 20 that is subjected to a large load.

In addition, if the direction in which the lateral force is applied is defined when the vibration damping device 10 is used, the side on which the surface load applied to the sleeve 22 by the lateral force increases is also defined. Thus, the sleeve 22 is preferably formed in a chamfered cylindrical shape so that the area of the frictional sliding surface on the side where the surface load of the sleeve 22 increases is large and the area of the frictional sliding surface on the opposite side is small.

Fig. 8 shows a schematic view of a chamfered cylindrical sleeve 22c having a height on the side to which a lateral force is applied higher than the height on the opposite side when the vibration damping device 10 is used. The arrows of fig. 8 indicate the direction of the lateral force applied to the vibration damping device 10. That is, in the sleeve 22c, as indicated by an arrow, the height of the surface on the side to which a large load is applied is higher than the height of the surface on the opposite side.

By setting the area of the frictional sliding surface on the side where the surface load increases as in the sleeve 22c, the necessary rigidity of the sleeve 22 can be ensured. On the other hand, by setting the area of the frictional sliding surface on the opposite side to be small, the surface load of the sleeve 22 can be reduced to reduce the frictional force. The mechanism of reduction of this friction force is considered as follows. That is, microscopic irregularities exist on the surface of the object. When the surfaces of two objects are in contact, what is actually in contact is the microscopic convex portion (actual contact portion), and the frictional force in a minute region can be expressed as the product of "shear strength of the material constituting the object" and "area of the actual contact portion". Thus, if the area of the frictional sliding surface is reduced as shown by the sleeve 22c to reduce the area of the actual contact portion, the frictional force generated in the minute region can be reduced. It is presumed that the sleeve 22c reduces the frictional force in this manner.

If the sleeve 22 is formed in the shape of a chamfered cylinder whose height on the side to which the lateral force is applied is higher than that on the opposite side, it is possible to achieve both the securing of necessary rigidity and the reduction of frictional force. The term "chamfered cylindrical shape" as used herein includes not only a strict chamfered cylinder but also a substantially chamfered cylinder.