JP2010266297A - Friction coefficient measuring instrument - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a friction coefficient measuring instrument for measuring a true friction coefficient of a specimen wherein the Coulomb's law does not hold in an interface and a friction coefficient depends on a relative speed between an ideal surface and a rough surface. <P>SOLUTION: This friction coefficient measuring instrument includes a movable stand 11 comprising a contact part 21 contacting with a part of the specimen 91, a control means 12 for moving the movable stand 11 with the specimen 91 kept in contact with the contact part 21, a connection means 13 connected to the specimen 91 to convey a force caused by friction between the specimen 91 and the contact part 21, a transducer 14 converting the force conveyed by the connection means 13 into an electric signal, and a measuring means 15 for sampling the electric signal from the transducer 14 in a prescribed cycle. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a friction coefficient measuring machine for measuring a friction coefficient of a sample whose rough surface is elastically deformed and whose friction coefficient depends on speed and apparent contact area.

  About various materials, such as a metal, a ceramic, and a polymer | macromolecule, measuring the physical property has been performed by providing the force of a tension direction using a sample and a movable stand. In the conventional measurement, it has been common to perform measurement in such a manner that the sample is destroyed by applying a force to the sample so that the sample and the movable base are not in close contact with each other and fixed.

  On the other hand, when it is desired to know the property at the interface of the material, a force generated between the object and the material is measured by bringing a part of the material interface into contact with another object. One of the representative properties among the properties at the interface of materials is the friction property. Friction is a phenomenon that occurs at the interface between one object and another. There are various materials that make up the object, but as shown in FIG. 9, an object made of a material having a flat ideal surface without any deformation and a rough object made of a material having a general surface roughness. Think about the case of touching.

In this case, it is apparent that contact occurs in the areas A1, A2,..., Ak in the loads W1, W2,..., Wk (k is a natural number) in FIG. Here, p is the yield pressure of the rough surface. When the rough surface is plastically deformed by the yield pressure p due to contact with the ideal surface, i = 1, 2,..., K, and the total of the contact area between the rough surface and the ideal surface (true contact If the area is Ar = A1 + A2 + ... + Ak,
Total load W = ΣWi = pΣAi = pAr (1)
Because
True contact area Ar = W / p (2)
Thus, the true contact area Ar is determined only by the total load W and the yield pressure p.

  Here, it is considered that the frictional force acting between the ideal surface and the rough surface is caused by temporary adhesion at these interfaces (for example, see Non-Patent Document 1). In other words, a material having a small surface free energy (also referred to as interface free energy) generally has a small frictional force.

Next, consider a case where the rough surface is in the plastic deformation region of FIG. The shearing force F that cuts off the adhesion causing the frictional force is S, where the shearing strength per unit area is S.
F = ArS (3)
It becomes. Substituting equation (2) into this,
F = W / p · S = S / p · W (4)
It becomes. From Equation (4), the frictional force is proportional to the load (drag) and is generally known as Coulomb's law. The coefficient of static friction μ _s is
μ _s = F / W = S / p (5)
It is expressed as a material eigenvalue.

  As a measurement method for measuring the property of the interface of the material, it is known that the measurement is performed statically under a model condition in which the rough surface is plastically deformed (see, for example, Non-Patent Document 2).

Hiroyuki Saito, Kenichi Takai, Toshika Takasawa, Goro Yamauchi: “Materials”, 46, 5 pp. 551-554 (1997) Shimadzu Corporation website (searched on February 23, 2009) http: // www. shimadzu. co. jp / TEST / products / mtrl03 / index. html

  On the other hand, when a polymer material such as rubber forms a rough surface, the rough surface is elastically deformed to generate a stress relaxation component, so Coulomb's law often does not hold at the interface. That is, when the deformation of the rough surface due to the yield pressure p is elastic deformation, Equation (2) does not hold, and the true contact area Ar is not determined only by the total load W and the yield pressure p. Further, when Coulomb's law does not hold at the interface, the friction coefficient often depends on the relative speed between the ideal surface and the rough surface. Thus, it is difficult for conventional measuring machines to measure the coefficient of friction of a material whose Coulomb's law does not hold at the interface and whose coefficient of friction depends on the relative speed between the ideal and rough surfaces. was there.

  Therefore, the present invention provides a friction coefficient measuring machine capable of measuring the true friction coefficient of a sample whose friction coefficient depends on the relative speed between the ideal surface and the rough surface even when Coulomb's law is not established at the interface. For the purpose. Here, the sample refers to a material whose coefficient of friction is to be measured.

  In order to achieve the above object, the friction coefficient measuring machine according to the present invention moves a sample on a contact surface, samples a change in force due to friction between the samples, and calculates a relative speed and a friction coefficient between the sample and the contact surface. The relationship was calculated.

  Specifically, the friction coefficient measuring machine according to the present invention includes a movable base having a contact portion that contacts a part of a sample, and a control for moving the movable base in a state where the sample and the contact portion are in contact with each other. Means, a connection means connected to the sample, and transmitting a force based on friction between the sample and the contact portion; a transducer for converting the force transmitted by the connection means into an electrical signal; and Measuring means for sampling the electrical signal at a predetermined period.

  This friction coefficient measuring machine slides the sample on the contact portion and measures the frictional force generated between the two at a predetermined cycle. Therefore, the present invention provides a friction coefficient measuring machine capable of measuring the true friction coefficient of a sample whose friction coefficient depends on the relative speed between the ideal surface and the rough surface even when Coulomb's law is not established at the interface. be able to.

  The contact portion of the movable base of the friction coefficient measuring machine according to the present invention may have a shape that holds a part of the sample in the longitudinal direction. Since the apparent contact area between the sample and the contact portion affects the frictional force, it is desirable to make the apparent contact area constant during the measurement of the friction coefficient. Here, the “apparent contact area” means an area obtained by orthogonal projection of the entire contact interface between the sample and the contact portion in the normal direction. For example, in the case of FIG. 1, the bottom area of the sample 91 corresponds to the “apparent contact area”. This contact part is made into the shape which hold | maintains a part of sample stably. This contact portion can make the apparent contact area with the sample constant during the measurement of the friction coefficient. Therefore, the present invention can provide a friction coefficient measuring machine that can make the apparent contact area with the sample constant regardless of the shape of the sample and can measure the true friction coefficient of the sample.

  Here, it supplements about the point in which the apparent contact area of a sample and a contact part influences frictional force. As described in FIG. 9, the true contact area is determined by the total load and the yield pressure. The yield pressure is a material specific value, but the true contact area is determined by the yield pressure and the number of true contact points of the area Ak. This true contact point is considered to occur statistically and accidentally rather than a peculiar point with respect to the contact interface, and the number of contact points exists at a constant rate with respect to the apparent contact area. Become. That is, it can be said that the change of the apparent contact area is the number of true contact points, that is, the change of the true contact area.

  When Coulomb's law of Formula (4) is established, the frictional force depends only on the load, so it is not necessary to consider the change in the true contact area. However, when Coulomb's law does not hold, specifically, when the material side is in an elastically deformed state, the frictional force changes when the true contact area changes. That is, the apparent contact area between the sample and the contact portion affects the frictional force. Therefore, in order to make the true contact area constant when measuring the frictional force, it is possible to measure an accurate friction coefficient by keeping the “apparent contact area” constant.

  Furthermore, the contact part of the movable table of the friction coefficient measuring machine according to the present invention is a flat surface, and a force in a direction normal to the contact part of the movable table and a direction approaching the contact part of the movable table is applied to the sample. It is preferable to further include an applying means. This friction coefficient measuring machine can measure the friction coefficient when the load is changed.

  Furthermore, the control means of the friction coefficient measuring machine according to the present invention moves the movable base while keeping the speed constant, and the force transmitted by the connecting means is stationary that appears alternately between the sample and the contact portion. The measurement means calculates the static friction coefficient and the dynamic friction coefficient between the sample and the contact portion by setting the predetermined period as the fluctuation period of the force transmitted to the transducer. be able to. This friction coefficient measuring device can measure a static friction coefficient and a dynamic friction coefficient from stick-slip vibration of a sample.

  Further, the control means of the friction coefficient measuring machine according to the present invention moves the movable base while increasing the speed at a constant acceleration, and the force transmitted by the connecting means is a dynamic friction between the sample and the contact portion. The measurement means can calculate a dynamic friction coefficient with respect to a relative speed between the sample and the contact portion. This friction coefficient measuring machine can measure the dynamic friction coefficient according to the relative speed between the sample and the contact portion.

  The present invention provides a friction coefficient measuring machine capable of measuring the true friction coefficient of a sample whose friction coefficient depends on the relative speed between the ideal surface and the rough surface even when Coulomb's law is not established at the interface. it can. In addition, this friction coefficient measuring instrument can measure the friction coefficient not only for samples that are plastically deformed by contact with the contact portion but also for samples that are not plastically deformed. Furthermore, this friction coefficient measuring machine can simultaneously determine dynamic friction coefficients at various degrees of progress by a single measurement.

It is a block diagram explaining the friction coefficient measuring machine which concerns on this invention. It is a block diagram explaining the friction coefficient measuring machine which concerns on this invention. It is a figure explaining operation | movement of the friction coefficient measuring machine which concerns on this invention. It is a block diagram explaining the friction coefficient measuring machine which concerns on this invention. It is a figure explaining the holding means of the friction coefficient measuring machine which concerns on this invention. It is a figure explaining the holding means of the friction coefficient measuring machine which concerns on this invention. It is a figure explaining operation | movement of the friction coefficient measuring machine which concerns on this invention. (A) is a figure in case a restoring force is smaller than a static friction force, and a sample moves with a movable stand. (B) is a figure in case a restoring force and a static friction force correspond. (C) is a figure in case a restoring force is larger than a dynamic friction force, and a sample slides on the contact part of a movable stand. It is a figure explaining the relationship between the frictional force which arises between a sample and a contact part, and time. It is a figure explaining the property of the surface of a sample.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
FIG. 1 is a block diagram illustrating a friction coefficient measuring machine according to this embodiment. The friction coefficient measuring machine includes a movable base 11 having a contact portion 21 that contacts a part of a sample 91, a control means 12 that moves the movable base 11 in a state where the sample 91 and the contact portion 21 are in contact, and a sample. 91, connecting means 13 for transmitting a force based on friction between the sample 91 and the contact portion 21, a transducer 14 for converting the force transmitted by the connecting means 13 into an electric signal, and an electric signal from the transducer 14 Measuring means 15 for sampling at a predetermined cycle.

  The movable table 11 has a contact portion 21 that contacts the sample 91. The contact portion 21 of the movable base 11 in FIG. 1 is a plane. The shape of this plane is arbitrary, but an example is a rectangular shape. An example is the same size and shape as the sample. A part of the sample 91 is a flat surface. This is the bottom surface of the sample 91. The bottom surface of the sample 91 is placed on the contact portion 21 of the movable table 11. 1 is arranged so that the contact portion 21 of the movable base 11 is horizontal, but the contact portion of the movable base 11 is in a state where the bottom surface of the sample 91 and the contact portion 21 are in contact with each other. 21 may be arranged diagonally or vertically.

  The control means 12 sends a signal to the movable table 11 in order to move the movable table 11. For example, the control means 12 sends a signal to the movable table 11 to cause the movable table 11 to linearly move in the direction A. The linear motion of the movable table 11 can be set at a constant speed or at a constant acceleration. Further, the movement of the movable table 11 can be a specific curved movement, and in that case, it can be a constant speed or a constant acceleration.

  Any transducer 14 can be used, but a strain gauge or a load cell can be used as an example. The transducer 14 and the sample 91 are connected by the connecting means 13. The connecting means 13 is preferably made of a material that is not easily deformed by an applied force. For example, when the movable base 11 moves linearly in the direction A, the connecting means 13 can be a stainless steel wire. Further, when the movable base 11 moves in a direction B or a curved line, it can be a metal rod.

  The measuring means 15 samples the electrical signal from the transducer 14 at a predetermined period, but the sampling period can be designed arbitrarily. For example, sampling can be performed 50 times (50 Hz) per second. This period is a sampling period that can be easily acquired because it is synchronized with a commercial power supply in the vicinity of Tokyo.

  When the movable table 11 moves linearly in the direction A, the sample 91 is connected to the transducer 14 by the connecting means 13, so the absolute position does not move, and the sample 91 slides on the contact portion 21. For this reason, the friction force generated between the sample 91 and the contact portion 21 is transmitted to the transducer 14 via the connection means 13. The transducer 14 mechanically / electrically converts this frictional force and outputs it to the measuring means 15. The measuring means 15 can measure the frictional force while the sample 91 slides on the contact portion 21 by sampling this electric signal at every sampling period.

  Next, the operation of the friction coefficient measuring machine will be described. Here, it is assumed that the transducer 14 is a load cell. First, the case where the control means 12 moves the movable stand 11 in the direction A while keeping the speed constant will be described. FIGS. 7A to 7C are diagrams illustrating the operation of the friction coefficient measuring machine. F is the frictional force, G is the gravity acting on the sample 91, N is the drag of gravity G, and V is the moving speed of the movable table 11. FIGS. 7A and 7B are diagrams when the moving speed V of the movable table 11 is not 0 and the sample 91 is moving together with the movable table 11 (when the relative speed is 0). At this time, a static frictional force is generated between the sample 91 and the contact portion 12. When the restoring force by the load cell spring is smaller than the static frictional force, the movable table 11 and the sample 91 move together.

  FIGS. 7B to 7C are diagrams when the restoring force of the load cell spring is larger than the static friction force, and the sample 91 is moving by the restoring force while receiving the dynamic friction force from the contact surface 21. FIG. When the restoring force by the spring of the load cell becomes smaller than the static friction force, the sample 91 and the movable base 11 again move as a unit by the static friction force. This phenomenon is a special case of generally known stick-slip vibration. At this time, the force transmitted by the connecting means 13 varies due to static friction and dynamic friction that alternately appear between the sample 91 and the contact portion 21.

FIG. 8 is a diagram for explaining the relationship between the frictional force generated between the sample 91 and the contact portion 21 and time. The horizontal axis indicates the time for the control means 12 to move the movable base 11. The vertical axis represents the frictional force generated between the sample 91 and the contact portion 12. With reference to FIG. 8, the change in the frictional force generated between the sample 91 and the contact portion 21 when the sample 91 is causing stick-slip vibration will be described. A function (i) in FIG. 8 shows a change in static friction force in a section where the sample 91 and the contact portion 21 are united by static friction and linearly move at a constant speed. The following equation holds in this section.
F = −jx + μN = 0 (j is a spring constant, x is a displacement from the equilibrium position)
μ / μ _s <1 (μ _s is the coefficient of static friction, μ is the coefficient of dynamic friction)
−jx <μ _s N (6)
In the region where the sample 91 is elastically changed, the dynamic friction coefficient μ changes according to the relative speed between the sample and the contact portion 21.

On the other hand, the function (ii) in FIG. 8 shows an example of the change of the dynamic friction force when the sample 91 slides on the movable table 11 by the restoring force of the spring. The following equation holds in this section.
F = −jx− [sgn] μ _d N
([Sgn] is +1 or -1, mu _d dynamic friction coefficient) (7)
Since the function of FIG. 8 is a periodic function with respect to time, it can be expanded by Fourier series, but can be treated as a sinusoidal vibration with a very rough approximation only by the first term. The load cell electrically outputs this varying frictional force. The measuring means 15 can calculate the static friction coefficient and the dynamic friction coefficient between the sample 91 and the contact portion 21 by setting the sampling period to be equal to or less than the stick-slip vibration period.

  Next, a case where the control unit 12 moves the movable base 11 in the direction A while increasing the speed at a constant acceleration will be described. FIG. 3 is a diagram for explaining the operation of the friction coefficient measuring machine. The horizontal axis indicates the time for the control means 12 to move the movable base 11. The vertical axis represents the moving speed of the movable table 11. When a constant acceleration linear motion in which the acceleration is not 0 is performed, the moving speed V of the movable table 11 changes every moment. The force transmitted by the connecting means 13 is generated by dynamic friction between the sample 91 and the contact portion 13. The measuring means 15 calculates a dynamic friction coefficient with respect to the relative speed V between the sample 91 and the contact portion 21. When the bottom surface of the sample 91 (corresponding to a rough surface) is in the elastic deformation range, since the premise for making the friction coefficient constant cannot be applied, in general, the friction coefficient must be treated as a function of speed, that is, time. .

  Therefore, the measuring means 15 samples the load cell in a very short time. In FIG. 3, the friction coefficient in the section from time t (k−1) to t (k), which is a predetermined sampling period, is represented by μk (k: natural number). The friction coefficient within this sampling period can be regarded as constant if the sampling period is short. The measuring means 15 can acquire the dynamic friction grandchild numbers μ1, μ2,..., Μk at each moving time t and moving speed (relative speed) V. These μ1, μ2,..., Μk serve as indexes indicating the frictional force (friction coefficient) at each speed V.

  Conventionally, the friction coefficient can be acquired only when the moving speed is constant. However, since the measuring unit 15 acquires the friction coefficient in the sampling period, the friction coefficient measuring machine 1 acquires the electric signal output from the transducer 14. The friction coefficient of various moving speeds can be acquired by the test of 1 time. The same measurement can be performed when the bottom surface of the sample is in the plastic deformation region.

(Embodiment 2)
FIG. 2 is a block diagram for explaining the friction coefficient measuring machine. The difference between the friction coefficient measuring machine in FIG. 2 and the friction coefficient measuring machine in FIG. 1 is that the friction coefficient measuring machine in FIG. The applying unit 22 applies a force in the normal direction of the contact portion 21 of the movable table 11 and in a direction approaching the contact portion 21 of the movable table 11 to the sample 91. That is, the applying means 22 applies a force that presses the sample 91 against the contact portion 21. The friction coefficient measuring machine in FIG. 2 can acquire the friction coefficient of the sample 91 in the same manner as the friction coefficient measuring machine explained in FIG. Furthermore, the friction coefficient can be acquired by changing the pressure between the sample 91 and the contact portion 21. When the application unit 22 is provided, it is necessary to consider the frictional force generated between the sample 91 and the application unit 22. The same effect can be obtained even if a weight is further added from above the sample 91.

(Embodiment 3)
FIG. 4 is a block diagram illustrating the friction coefficient measuring machine. The difference between the friction coefficient measuring machine of FIG. 4 and the friction coefficient measuring machine of FIG. 1 is that the friction coefficient measuring machine of FIG. When the bottom surface of the sample is in the elastic deformation range, the apparent contact area affects the frictional force measured by the apparent contact area between the sample and the contact portion. Therefore, it is necessary to perform measurement with a constant apparent contact area. The contact portion 21 of the movable table 11 has a shape that holds a part of the sample 91 in the longitudinal direction. Specific examples of the holding means 23 are shown in FIGS.

  FIG. 5 is a view for explaining the holding means 23 when the sample 91 is in the form of a sheet. The holding means 23 holds the sheet-like sample 91 so as to sandwich the front surface and the back surface thereof. In the case of the holding means 23 in FIG. 5, a portion that contacts the surface of the sample 91 and a portion that contacts the back surface of the sample 91 become the contact portion 21. For this reason, the holding means 23 can reduce the change of the relative position with the sample 91 and can reduce the change of the apparent contact area.

  On the other hand, when the linear sample 91 is fixed by the holding means 23 as shown in FIG. 5, the area of the portion where the sample 91 and the holding means 23 come into contact decreases. For this reason, when the relative position of the linear sample 91 and the holding means 23 is shifted and becomes oblique, the rate of change in the contact area increases.

  Therefore, the holding means 23 is supported not only by the upper and lower sides but also by using a thin line on the side surface so that the relative position with the wire does not change, or by attaching a contact portion to the side surface of the holding means 23, It is possible to prevent the relative position from changing. In this case, the thin line and the contact portion on the side surface preferably have a known coefficient of friction. Further, it is desirable that the contact portion is made of a certain material and that the friction coefficient is smaller than the friction coefficient predicted for the sample to be measured.

  When measuring the friction coefficient of the linear sample 91, the holding means 23 as shown in FIG. 6 may be used. The holding means 23 in FIG. 6 has a shape such that a linear sample 91 passes through the center. Since the holding means 23 holds the outer periphery of the linear sample 91, the change in the apparent contact area can be reduced.

11: Movable stage 12: Control means 13: Connection means 14: Transducer 15: Measurement means 21: Connection section 22: Application means 23: Holding means 91: Sample A, B: Direction

Claims (5)

A movable table having a contact portion that contacts a part of the sample;
Control means for moving the movable base in a state where the sample and the contact portion are in contact with each other;
A connection means connected to the sample and transmitting a force based on friction between the sample and the contact portion;
A transducer that converts the force transmitted by the connecting means into an electrical signal;
Measuring means for sampling the electrical signal from the transducer at a predetermined period;
A friction coefficient measuring machine comprising:   The friction coefficient measuring machine according to claim 1, wherein the contact portion of the movable table has a shape that holds a part of the sample in the longitudinal direction. The contact part of the movable table is a plane;
The friction coefficient measuring machine according to claim 1, further comprising an applying unit that applies a force in a direction normal to the contact portion of the movable table and a direction approaching the contact portion of the movable table to the sample. . The control means moves the movable base while keeping the speed constant,
The force transmitted by the connecting means varies due to static friction and dynamic friction that alternately appear between the sample and the contact portion,
The measurement means calculates a static friction coefficient and a dynamic friction coefficient between the sample and the contact portion by setting the predetermined period as a fluctuation period of a force transmitted to the transducer. The friction coefficient measuring machine according to any one of 1 to 3. The control means moves the movable base while increasing the speed at a constant acceleration,
The force transmitted by the connecting means is generated by dynamic friction between the sample and the contact portion,
The friction coefficient measuring machine according to any one of claims 1 to 3, wherein the measuring means calculates a dynamic friction coefficient with respect to a relative speed between the sample and the contact portion.

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