JP2007211874A - Electric-driven linear actuator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize an electric-driven linear actuator having an excellent low torqueability at low temperature and sufficient durability, by grease lubrication. <P>SOLUTION: The electric-driven linear actuator keeps a grease composition of a thickener of lithium soap, base oil having a kinetic viscosity of 15-50 mm<SP>2</SP>/s at 40°C and a chemical compound having layer structure enclosed to lubricate a screw spindle 3a and a nut member 14a. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electric linear actuator, for example, as a driving device for automatically switching a gear ratio of an automobile transmission or for automatically connecting / disconnecting a clutch, and more specifically, to improve operating characteristics at a low temperature. And a technology for ensuring a sufficient lubrication life.

2. Description of the Related Art Conventionally, as an electric linear actuator that expands and contracts using an electric motor 2 as a drive source and displaces various articles, those described in patent publications and the like have been known. Conventionally known electric linear actuators are equipped with an electric motor 2 that can be rotated forward and backward supported by a housing 1, and the rotation of a screw shaft 3 a that is rotatably supported in the housing 1. The nut member 14a engaged with the screw shaft 3a is displaced in the axial direction, and the base end portion is coupled to the nut member 14a, and the output shaft member 16 having at least a base end configured in a cylindrical shape is displaced in the axial direction. It is something to be made. In each of these prior arts, the periphery of the base of the output shaft member 16 is covered with a cylindrical cover to prevent foreign matter from entering the threaded portion between the screw shaft 3a and the nut member 14a. . Further, the outer peripheral surface of the output shaft member 16 and the inner peripheral surface of the tip end portion of the cover are in sliding contact with each other. The ball screw has a ball (generally a steel ball) interposed between the screw shaft 3a and the nut as a rolling element, and the friction loss during operation is almost negligible. Since the relative rotation between the shaft 3a and the nut member 14a is not accompanied by sliding, there is a feature that the wear of the component is very small. A ball screw of the total ball type is provided between a screw shaft having a male screw groove formed on its outer periphery, a nut having a female screw groove facing the male screw groove formed on its inner periphery, and a male screw groove and a female screw groove. And a circulation passage (tube, top, etc.) for circulating the ball from one end side to the other end side of the female screw groove formed in the nut. At the time of relative rotation between the screw shaft and the nut, the ball rolls with respect to both the male screw groove and the female screw groove and circulates from one end side to the other end side of the female screw groove through the circulation passage.
JP-A-8-322189 Japanese Patent Laid-Open No. 9-327149 JP 2004-138164 A U.S. Pat.No. 5,669,264

  When the electric linear actuator is used for an automobile, for example, since it is required to start smoothly even in a low temperature environment, it is necessary to have a low torque at a low temperature. Further, in the above-mentioned ball screw of the total ball shape, when balls that roll in the same direction between both screw grooves come into contact with each other, a relative slip occurs at a speed twice as high as the rolling speed. Sufficient durability is required because it may wear out more than the permissible limit in a short period of time, or may cause problems such as seizure of balls and screw grooves due to frictional heat. However, conventionally used grease compositions for lubrication (hereinafter also referred to as grease) have been difficult to simultaneously satisfy low torque characteristics at low temperatures and sufficient durability. Therefore, an object of the present invention is to achieve an electric linear actuator excellent in low temperature and low torque and having sufficient durability by grease lubrication.

In order to solve the above problems, in the present invention, a housing, an electric motor that can be rotated forward and backward supported by the housing, and a screw shaft that is rotatably supported by the housing and rotated by the electric motor, An electric linear actuator comprising a nut member that engages with the screw shaft in a state in which the rotation of the screw shaft is prevented and is displaced in the axial direction of the screw shaft as the screw shaft rotates. A grease composition containing a thickener and a base oil having a kinematic viscosity at 40 ° C. of 15 to 50 mm ² / s is enclosed to lubricate the screw shaft and the nut member. To do.

  Further, the grease composition may contain at least one solid lubricant selected from a compound having a layered structure, polytetrafluoroethylene, and ultrafine particles having a primary particle size of 50 nm or less.

Uses a base oil with a kinematic viscosity at 40 ° C of 15 to 50 mm ² / s as a constituent of the grease composition, providing excellent torque characteristics at low temperatures, and sufficient durability by using lithium soap as a thickener Therefore, an electric linear actuator having low temperature and low torque and sufficient durability can be achieved by grease lubrication. Further, the grease composition contains at least one solid lubricant selected from a compound having a layered structure, polytetrafluoroethylene, and ultrafine particles having a primary particle size of 50 nm or less, thereby further improving durability. Can be achieved.

  Mineral lubricants and synthetic lubricants can be used as the base oil of the grease composition of the present invention. Although it does not restrict | limit especially as a mineral type lubricating oil, Paraffin type mineral oil, naphthenic mineral oil, and those mixed mineral oil can be used. Synthetic lubricating oil is not particularly limited, but synthetic hydrocarbon oil, ether oil, ester oil, and the like can be used. Specifically, poly-α-olefin oil or the like is used as the synthetic hydrocarbon oil, dialkyl diphenyl ether oil, alkyl triphenyl ether oil, alkyl tetraphenyl ether oil or the like is used as the ether oil, and diester oil or neopentyl is used as the ester oil. Type polyol ester oils, or complex ester oils and aromatic ester oils thereof can be used. These base oils may be used alone or in appropriate combination.

The kinematic viscosity of the base oil is preferably 15 to 50 mm ² / s at 40 ° C., more preferably 20 to 30 mm ² / s. If it is less than 15 mm ² / s, the oil film becomes thin and it is difficult to obtain sufficient durability. If it exceeds 50 mm ² / s, the low temperature torque may increase. In addition, the base oil preferably has a viscosity index of 100 or more to ensure sufficient fluidity at low temperatures.

  In the present invention, lithium soap is used as a thickener. Lithium soap has superior adhesion to steel compared to other thickeners, so that it is easy to retain grease on the rolling surface even under reciprocating motions including low speed regions such as electric linear actuators. Excellent in properties. Examples of the lithium soap include lithium stearate, lithium 12-hydroxystearate, and lithium composite soap.

  The penetration of the grease composition (according to Japanese Industrial Standard K2220) is preferably in the range of 175 to 385, more preferably in the range of 200 to 295. If it is smaller than 175, it becomes too hard to expect a sufficient lubricating effect, and if it is larger than 385, it may leak from the linear motion device. In order to satisfy the above-mentioned miscibility range, the content of the thickener in the grease composition of the present invention is preferably 5 to 20% by weight.

  In the present invention, as a constituent of the grease composition, at least one selected from a compound having a layered structure, polytetrafluoroethylene (hereinafter also referred to as PTFE), and ultrafine particles having a primary particle size of 50 nm or less. A solid lubricant can be suitably used. These solid lubricants enter the friction contact surface and prevent direct contact between the screw shaft and the nut member, between the screw shaft and the ball, and between the nut and the ball, thereby further preventing damage to each member. It can contribute to the improvement of durability. Examples of the compound having a layered structure include molybdenum disulfide, hexagonal boron nitride, tungsten disulfide, graphite, mica, and the like. Examples of ultrafine particles having a primary particle size of 50 nm or less include metal oxides such as silicon dioxide, titanium dioxide, and zinc oxide. In particular, ultrafine particles with a primary particle size of 20 nm or less and a hydrophobic surface treatment are excellent in dispersion stability in grease and easily enter the frictional contact surface. preferable.

  The amount of the solid lubricant added to the grease is preferably 0.1 to 5% by weight with respect to the total amount of the grease composition from the viewpoint of improving durability. When it is less than 0.1% by weight, sufficient durability cannot be obtained. If it is more than 5% by weight, the grease tends to become hard, so that an appropriate oil film forming ability of grease cannot be obtained. Further, two or more solid lubricants may be mixed and used. For example, by using a mixture of a compound having a layered structure and ultrafine particles having a primary particle size of 50 nm or less, ultrafine silicon dioxide enters the gap in the molybdenum disulfide layer having a relatively large primary particle size of about several μm. In order to form a dense surface film, excellent durability can be obtained.

  Furthermore, in the grease composition of the present invention, various additives may be mixed as desired in order to further improve various performances. Examples of extreme pressure agents include organic zinc compounds such as zinc dialkyldithiophosphate, zinc diaryldithiophosphate and zinc dialkyldithiocarbamate, organic molybdenum compounds such as molybdenum dialkyldithiophosphate, molybdenum diaryldithiophosphate and molybdenum dialkyldithiocarbamate, Carbamine compounds, phosphates, phosphites and the like can be used.

  As the antioxidant, a commonly used antioxidant can be used. For example, an amine such as phenyl-1-naphthylamine, a phenol such as 2,6-di-tert-dibutylphenol, a sulfur, an antioxidant such as zinc dithiophosphate can be used.

  Antirust agents include organic sulfonates such as alkali metals and alkaline earth metals, alkyls such as alkyl and alkenyl succinic acid esters, alkenyl succinic acid derivatives and partial esters of polyhydric alcohols such as sorbitan monooleate. Rust can be used.

  Oiliness agents such as fatty acids and animal and vegetable oils can be used as the oiliness agent. As the metal deactivator, a metal deactivator such as benzotriazole can be used.

  These various additives can be used alone or in admixture of two or more. In addition, the addition amount of these various additives will not be specifically limited if it is a grade which does not impair the objective of this invention.

[Low temperature torque measurement]
A torque converter was installed between the electric motor of the electric linear actuator shown in FIG. 1 and the screw shaft, and the torque was measured under a constant axial load condition. The test conditions were: axial load 1500N, ambient temperature minus 20 (−20) ° C, stroke length 30mm, operating speed 60mm / s, and comparative evaluation was performed by calculating the specific low temperature torque with the low temperature torque of Comparative Example 1 as 1. . The test results are shown in Table 1.

[Durability test 1]
A peeling durability test was conducted on the electric linear actuator shown in FIG. The test conditions were an axial load of 1500 N, an ambient temperature of 80 ° C., a stroke length of 30 mm, and an operation speed of 60 mm / s → 1 s repeated stop. After the specified number of cycles (300,000 cycles), the specimens were disassembled and observed, and those with no peeling on the raceway surface were OK and those with peeling were judged as NG. The test results are shown in Table 1. It should be noted that the penetration of grease in each of the examples and comparative examples (according to Japanese Industrial Standard K2220) was adjusted by the amount of thickener added.

［耐久試験2］
上記耐久試験と同様の試験条件において、ねじ軸上のグリースが黒色に変色するまでのサイクル数を耐久寿命として、耐久寿命の比較評価を実施した。試験グリースとして、実施例1のグリースをベースグリースとして各種固体潤滑剤を添加したグリースを調製した。比較例2の耐久寿命を1として、比耐久寿命を算出し比較評価を行った。各種固体潤滑剤を用いた場合の評価結果を表2に示す。また、実施例1のグリースと共に固体潤滑剤として一次粒径12nmの超微粒子（二酸化珪素）を用い、固体潤滑剤をグリース組成物全量に対して、0.01、0.1、0.5、0.8、1.0、5.0重量％添加した場合の評価結果を図2に示す。なお、各種固体潤滑剤の一次粒径は、二硫化モリブデンが3μm、PTFEが0.1μm、二酸化珪素が12nmであり、いずれも疎水化処理が施されている。

[Durability test 2]
Under the same test conditions as the above-mentioned durability test, a comparative evaluation of the durability life was carried out with the number of cycles until the grease on the screw shaft was changed to black as the durability life. As a test grease, a grease was prepared by adding the various solid lubricants using the grease of Example 1 as a base grease. With the endurance life of Comparative Example 2 as 1, the specific endurance life was calculated and subjected to comparative evaluation. Table 2 shows the evaluation results when various solid lubricants are used. Further, ultrafine particles (silicon dioxide) having a primary particle size of 12 nm were used as the solid lubricant together with the grease of Example 1, and the solid lubricant was 0.01, 0.1, 0.5, 0.8, 1.0, 5.0 wt. FIG. 2 shows the evaluation results when% is added. The primary particle sizes of the various solid lubricants are 3 μm for molybdenum disulfide, 0.1 μm for PTFE, and 12 nm for silicon dioxide, all of which have been hydrophobized.

表1より、リチウム石けんを増ちょう剤とし、基油動粘度50 mm²/s以下の基油を用いることにより、低温低トルク且つ十分な耐久性が得られることが分かる。

From Table 1, it can be seen that low-temperature, low-torque and sufficient durability can be obtained by using lithium soap as a thickener and using a base oil having a base oil dynamic viscosity of 50 mm ² / s or less.

  Table 2 shows that the use of the solid lubricant of the present invention can improve the low temperature, low torque and durability.

  FIG. 2 shows that the durability can be further improved by adding the solid lubricant in an amount of 0.1 wt% to 5 wt%.

It is a fragmentary sectional view of an electric linear actuator. It is an evaluation result of specific durability life and specific low temperature torque.

Explanation of symbols

1 Housing
2 Electric motor
3a Screw shaft
14a Nut member
16 Output shaft member

Claims (3)

A housing;
Forward and reverse electric motor supported by the housing;
A screw shaft rotatably supported by the housing and driven to rotate by the electric motor;
A nut member that engages with the screw shaft in a state in which the rotation of the screw shaft is prevented and is displaced in the axial direction of the screw shaft as the screw shaft rotates;
A grease composition containing a thickener made of lithium soap and a base oil having a kinematic viscosity at 40 ° C. of 15 to 50 mm ² / s in the electric linear actuator provided with the screw shaft and the nut member An electric linear actuator, which is sealed to lubricate.   2. The electric linear actuator according to claim 1, wherein the lithium soap is at least one selected from lithium stearate, lithium 12-hydroxystearate, and lithium composite soap. The grease composition contains at least one solid lubricant selected from a compound having a layered structure, polytetrafluoroethylene, and ultrafine particles having a primary particle size of 50 nm or less, or 3. The electric linear actuator according to any one of 2 above.

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