Patent Application
20140339029
This application is based on and incorporates herein by reference Japanese patent applications No. 2013-105357 filed on May 17, 2013.
The present invention relates to a magnetic functional fluid and to a damper or a clutch using the magnetic functional fluid.
A magnetic functional fluid is a functional fluid that responds to a magnetic field. As this type of magnetic functional fluid, there is, for example, a magnetorheological (MR) fluid in which micron-sized ferromagnetic particles are dispersed in a dispersion medium, such as oil, as disclosed in NPL 1. This is formed by ferromagnetic particles that have a large particle diameter, namely, by dispersed particles that are all micron-sized. The ferromagnetic particles have a multi-magnetic-domain structure. Then, it is known that the MR fluid shows Bingham fluid-like viscous properties, which show yield stress.
Further, in addition to the above, there is a magnetic functional fluid as disclosed in PTL 1. In PTL 1, ferromagnetic particles having a large particle diameter of 0.5 μm to 50 μm and ferromagnetic particles having a small particle diameter equal to or less than 25 nm are both dispersed, as dispersed particles, in a dispersion medium that has electrical insulation properties, such as kerosene or silicone oil. Both the large particle diameter ferromagnetic particles and the small particle diameter ferromagnetic particles are sphere shaped, and the small particle diameter ferromagnetic particles have a single magnetic domain structure, since the particle diameter is equal to or smaller than 25 nm.
PTL1: Japanese Patent Application Publication No. JP-A-2002-170791
NPL 1: “Magnetic Fluids”, Hiroshi Yamaguchi, MORIKITA PUBLISHING Co. Ltd., 2011.
At present, as a vibration control device that uses a magnetic functional fluid, a damper that uses the above-described MR fluid is commercially available. This damper is a damper that uses the viscosity of the fluid, and can increase the damping force of the damper by increasing the viscosity of the fluid.
There are methods to increase the viscosity of the above-described MR fluid, such as causing the volumetric proportion of the dispersed particles to increase or increasing the diameter of the dispersed particles. However, there is a limit to the volumetric proportion of the ferromagnetic particles that can be mixed into a dispersion medium, and it is known that if this limit is exceeded, the fluid ceases to function as a fluid. Further, if the diameter of the particles is increased, dispersion stability of the particles deteriorates and the particles settle.
This problem can be said to similarly apply to the magnetic functional fluid of the above-described PTL 1. In particular, in the magnetic functional fluid of the above-described PTL 1, instead of the fluid in which the dispersed particles are all the ferromagnetic particles having the large particle diameter, the fluid is formed by replacing some of the large particle diameter ferromagnetic particles with ferromagnetic particles that have a small particle diameter. When a comparison is made between the fluid of PTL 1 and the fluid in which the dispersed particles are all the ferromagnetic particles having the large particle diameter, when a volume concentration of the dispersed particles is the same, the viscosity becomes smaller the greater the amount of replacement particles.
In light of the foregoing, it is an object of the present invention to provide a magnetic functional fluid in which a greater degree of viscosity is possible than the above-described known magnetic functional fluid when a comparison is made with a same volumetric concentration of dispersed particles. Further, it is another object of the present invention to provide a damper and a clutch that use this type of the magnetic functional fluid.
In order to achieve the above-described object, according to a first aspect of the present invention, magnetic functional fluid includes dispersion medium; and dispersed particles which are dispersed in the dispersion medium, wherein the dispersed particles includes: first ferromagnetic particles having an average particle diameter of 0.5 μm to 50 μm; and second ferromagnetic particles each having a needle-like shape, each having a smaller particle size than the first ferromagnetic particles, and each having a length ratio of a long axis to a short axis of 2 or more.
Similarly to the magnetic functional fluid of the above-described PTL 1, the magnetic functional fluid according to the first aspect is a fluid formed, in contrast to a fluid in which all of the dispersed particles are ferromagnetic particles having a large particle diameter, by replacing some of the ferromagnetic particles having the large particle diameter with ferromagnetic particles having a smaller particle diameter. However, the magnetic functional fluid according to the first aspect is different to the magnetic functional fluid of PTL 1 in that, when a comparison is made with the fluid in which the dispersed particles are all the ferromagnetic particles having the large particle diameter and when the volumetric concentration of the dispersed particles is the same, the viscosity increases the greater the amount of replaced particles.
Therefore, according to the first aspect of the present invention, it is possible to provide a magnetic functional fluid in which a greater degree of viscosity is possible than in the above-described known MR fluid and the magnetic functional fluid of PTL 1.
In the magnetic functional fluid according to a second aspect, the second ferromagnetic particles have an average particle diameter of 50 nm to 300 nm.. Further, the magnetic functional fluid according to a third aspect has properties that are different to those of the known magnetic functional fluid. As described in a third aspect, the magnetic functional fluid has Bingham fluid-like viscous properties when a magnetic field is not applied and has pseudoplastic fluid-like viscous properties when a magnetic field is applied.
According to a fourth aspect of the present invention, a damper using viscous resistance of working fluid includes: the magnetic functional fluid according to one of claim 1 which serves as the working fluid; and magnetic field application device which applies magnetic field to the working fluid.
According to a fifth aspect of the present invention, a clutch for transmitting rotation of an input shaft to an output shaft via working fluid includes: the magnetic functional fluid according to one of claim 1 which serves as the working fluid; and magnetic field application device for applying magnetic field to the working fluid.
According to the fourth and fifth aspect of the present invention, a damper and a clutch are provided that use the viscous properties of the magnetic functional fluid according to the first to third aspects.
Hereinafter, embodiments of the present invention will be explained based on the drawings. Note that in each of the following embodiments, portions that are the same or equivalent to each other are explained using the same reference numeral.
The magnetic functional fluid of the present invention is a fluid in which, as dispersed particles (i.e. dispersoid), first ferromagnetic particles and needle-shaped second ferromagnetic particles are dispersed in a dispersion medium. Each of the second ferromagnetic particles has a needle-like shape. The particle diameter (particle size) of the second ferromagnetic particles is smaller than that of the first ferromagnetic particles.
An organic base oil, such as polyalphaolefin, is used as the dispersion medium. The dispersion medium is not limited to the organic base oil, and water or another dispersion medium may be used, as long as the effects of the present invention are obtained.
The first ferromagnetic particles are micron-sized particles, that is, they are particles having an average particle diameter (average particle size) of 0.5 μm to 50 μm. This is because particles having a particle diameter that is large to a certain extent are used as the first ferromagnetic particles and particles having a particle diameter (particle size) larger than 50 μm are likely to settle. The shape of the first ferromagnetic particles is, for example, spherical. The material of the first ferromagnetic particles is a material that shows ferromagnetic properties and is, for example, Fe, Co, Ni or an alloy that is formed from two or more of these elements.
The second ferromagnetic particles have a smaller particle diameter than the first ferromagnetic particles, and have a multi-magnetic domain structure or a single magnetic domain structure. For example, particles having an average particle diameter (an average particle diameter in a long axis direction) that is equal to or more than 50 nm and equal to or less than 300 nm are used. Note that as long as the particle diameter of the second ferromagnetic particles is smaller than that of the first ferromagnetic particles, the second ferromagnetic particles may be micron-sized. It is preferable that the size of the second ferromagnetic particles is a size that enters between adjacent first ferromagnetic particles in a chain cluster of the first ferromagnetic particles which is shown in
A volumetric proportion of the dispersed particles (the first and the second ferromagnetic particles) to the fluid as a whole is set within a range in which fluid-like properties are obtained, and can be set as 30 vol %, for example. Further, a volumetric proportion of the second ferromagnetic particles to the fluid as a whole is set in a range at which the second ferromagnetic particles can be added, and can be set as 2 to 10 vol %, for example.
One of a thickener, which suppresses particle settling, and a surfactant, such as oleic acid, which improves dispersibility, may be added to the magnetic functional fluid as necessary, or both the thickener and the surfactant may be added.
Here, as disclosed in PTL 1, it is known that the ferromagnetic particles in the dispersion medium have a magnetic moment when a magnetic field is applied, and the magnetic interaction force that acts between the particles causes the formation of chain clusters.
In contrast to this, when the needle shaped ferromagnetic particles are used as the dispersed particles, as in the present invention, the needle shaped ferromagnetic particles have the same shape as that of two or more spherical ferromagnetic particles aligned in a cluster. Thus, even in a state in which there is no magnetic field, there is a similar effect as with short chain clusters, and the viscosity of the fluid increases.
Further, as shown in
As a result, it is thought that the magnetic functional fluid of the present invention has the following properties. Namely, in contrast to the fluid in which the dispersed particles are all the ferromagnetic particles having the large particle diameter, the magnetic functional fluid of the present invention is formed by replacing some of the ferromagnetic particles having the large particle diameter with the ferromagnetic particles having the small particle diameter, in a similar manner to the magnetic functional fluid of the above-mentioned PTL 1. When the volumetric proportion of the dispersed particles is taken as a constant and some of the micron-sized large ferromagnetic particles are replaced with the small-sized smaller ferromagnetic particles, in the magnetic functional fluid of the above-mentioned PTL 1, the viscosity decreases the greater the amount of replacement ferromagnetic particles whose diameter is equal to or less than 25 nm. In contrast to this, the magnetic functional fluid of the present invention has the property that the viscosity increases the greater the amount of the replacement ferromagnetic particles 2. In this manner, according to the magnetic functional fluid of the present invention, when a comparison is made when there is a same volumetric concentration of the dispersed particles, it is possible to increase the viscosity to a greater extent than the above-described known MR fluid and the magnetic functional fluid of the PTL 1.
Further, although a detailed reason is not clear at the present time, the magnetic functional fluid of the present invention has viscous properties similar to those of a pseudoplastic fluid when a magnetic field is applied, as well as viscous properties similar to those of a Bingham fluid when the magnetic field is not applied, due to the first and second ferromagnetic particles. This is explained in more detail later in a working example.
(Damper using Magnetic Functional Fluid)
The interior of the cylinder 11 is filled with the working fluid 15. An orifice portion 16 is formed between the outer peripheral portion of the piston 12 and the inner surface of the cylinder 11. A damping force is generated by the viscous resistance of the working fluid 15 that arises when the piston 12 that is fixed to the shaft 13 is moved. The coil 14 is magnetic field application means that applies a magnetic field to the working fluid 15. In the present embodiment, the coil 14 is wound around a bobbin 17 such that it covers the whole outer peripheral portion of the cylinder 11, and it is possible to apply a magnetic field to the working fluid 15 over the whole interior of the cylinder 11 by passing an electric current through the coil 14.
The damper 10 of the present embodiment uses the magnetic functional fluid having the above-described composition as the working fluid 15, and can thus change the damping force as a result of the magnetic field application, as explained in the working example below. Further, the damper 10 has damping force characteristics in which the damping force becomes small in the case of a low frequency domain and a small amplitude.
The magnetic functional fluid of the present invention has viscous properties similar to those of a pseudoplastic fluid when a magnetic field is applied, namely, it has viscous properties in which viscosity in a low-speed area becomes rapidly lower when the magnetic field is applied, as in the explanation of the working example below. Thus, the clutch 30 of the present embodiment has a property by which the torque transmitted to the output shaft 32 becomes rapidly lower in line with a reduction in rotation speed of the input shaft 31. In other words, the clutch 30 has a property by which a transmission rate rapidly decreases in the low-speed area.
Each of working fluids having respective compositions shown in a Table 1 were prepared and used as the working fluid 15 in the damper 10 shown in
In Table 1, a fluid 1 is a comparative example that corresponds to the known MR fluid. Fluids 2 to 5 are working examples of the present invention. The μm sized ferromagnetic particles listed in Table 1 are the first ferromagnetic particles and the needle shaped ferromagnetic particles are the second ferromagnetic particles. For each of the fluids, the volumetric proportion of the dispersed particles is constant at 30 vol % and each of the fluids uses the first ferromagnetic particles having an average particle diameter of 1.2 μm and the second ferromagnetic particles having an average particle diameter of 100 nm and an average of 4 as the length ratio of the long axis to the short axis. However, a mixture ratio of the first and second ferromagnetic particles is changed for each of the fluids. The first and second ferromagnetic particles are both formed of iron powder, and a magnetic powder that is generally used as the magnetic powder of a magnetic tape was used as the second ferromagnetic particles. Further, polyalphaolefin was used as a dispersion agent, smectite was used as a thickener and oleic acid was used as a surfactant.
Then, the damping force was measured when the piston 12 was forcibly oscillated at each of constant frequencies (1 to 10 Hz) with an amplitude of ±4 mm. At that time, both a case in which there was no applied magnetic field and a case in which there was an applied magnetic field were measured. (a) and (b) in
From the above, as can be seen with reference to
Further,
From
In order to compare the above results with the magnetic functional fluid disclosed in PTL 1, three types of fluid having compositions shown in a Table 2 were prepared (fluids 6, 7 and 8 that are comparative examples). The fluids 6, 7 and 8 have the same volumetric proportion of dispersed particles as in the case of the fluids shown in Table 1, namely 30 vol %, but the volumetric proportion of the μm sized ferromagnetic particles and the 10 nm sized ferromagnetic particles has been changed. Note that the dispersion medium used is the same as the fluids shown in Table 1.
Then, the fluids 6 to 8 were used as the working fluid 15 in the damper 10 shown in
From the above results of the experiments, it was confirmed that the magnetic functional fluid of the present invention has completely reverse properties to those of the magnetic functional fluid disclosed in PTL 1.
Fluids 9 and 10 that are shown in a Table 3 were prepared and an experiment was performed in the same manner as Experiment 1. The μm sized particles listed in Table 3 are the first ferromagnetic particles and the needle shaped ferromagnetic particles are the second ferromagnetic particles. The second ferromagnetic particles used are particles having an average particle diameter of 150 nm, and the length ratio of the long axis to the short axis is in a range between 5 and 10. The other materials used were the same as those in Experiment 1. The fluids 9 and 10 are working examples of the present invention, and the mixture ratio of the first and second ferromagnetic particles is the same as that of the fluids 3 and 5 of Table 1.
The results obtained were the same as the results of Experiment 1, as shown in
As shown in
It should be noted that the present invention is not limited to the above-described embodiments and working examples and various modifications are possible without departing from the scope of the claims. Further, in each of the above-described embodiments, it goes without saying that the elements that form the embodiments are not necessarily essential, apart from a case in which it is particularly stated that they are essential and in which it is thought that they are clearly essential in principle. In addition, in each of the above-described embodiments, it goes without saying that the number, value, amount and range etc. of the structural elements of the embodiments are not limited to the number specified herein, apart from a case in which it is particularly stated that they are essential and in which they are clearly limited to a particular number in principle. Furthermore, in each of the above-described embodiments, when a shape or a positional relationship etc. of a structural element is mentioned, the shape and the positional relationship etc. is not limited, apart from a case in which the shape and the positional relationship etc. is clearly stated and in which the shape and the positional relationship etc. are limited to a particular shape and positional relationship in principle.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-105357 | May 2013 | JP | national |
