Friction brake structure

To reduce abnormal noise production in a friction brake structure, the friction brake structure includes: a brake plate (20) fixed to a rotating shaft (15) of a rotary electric machine (1); a ring-shaped brake shoe (30) disposed facing the brake plate; and a brake shoe support plate (40) which engages with a fixing portion of the rotary electric machine so as to be movable in an axial direction, and which supports the brake shoe and is biased by biasing action so as to bring the brake shoe into sliding contact with the brake plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application filed under 35 U.S.C. 371 of International Application No. PCT/JP2015/085699, filed Dec. 21, 2015, which claims priority from Japanese Patent Application No. 2014-265847, filed Dec. 26, 2014, each of which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to a friction brake structure for a rotary electric machine such as a motor.

Background Art

FIG. 6shows a conventional friction brake structure for a motor. The motor includes a motor case100, a stator101, and a rotor102, and the stator101and the rotor102are provided in the motor case100. The rotor102is supported by a rotating shaft103. The rotating shaft103is supported on an output-side bearing106aand an opposite-to-output-side bearing106b. A brake plate104is fixed to the rotating shaft103. A plurality of brake shoes105are disposed so as to face the brake plate104. The brake shoes105are inserted into holes108drilled in a bearing housing portion107in the axial direction. The opposite-to-output-side bearing106bis mounted in the bearing housing portion107. Each brake shoe105is biased toward the brake plate104by a coil spring109so as to be in sliding contact with the brake plate104. The coil spring109is supported by a spring retainer plate110.

In the friction brake structure as described above, the sliding contact of the brake shoes105on the brake plate104provides braking action as well as holding torque when the motor stops.

Patent Literature 1 discloses fixing, to a ring-shaped support, a plurality of brake shoes as described above.

CITATION LIST

Patent Literature

Patent Literature 1: JP H11-089173 A

SUMMARY OF INVENTION

Technical Problem

In such a conventional friction brake structure, the brake shoes rotate along with the rotation of the brake plate, and such rotation might produce abnormal noises called brake squeal.

In view of the problem described above, the present invention has an object to reduce abnormal noise production in a friction brake structure.

Solution to Problem

To achieve the above object, an embodiment of the present invention provides a friction brake structure including: a brake plate fixed to a rotating shaft of a rotary electric machine; a ring-shaped brake shoe disposed facing the brake plate; and a brake shoe support plate which engages with a fixing portion of the rotary electric machine so as to be movable in an axial direction, and which supports the brake shoe while being biased by biasing action so as to bring the brake shoe into sliding contact with the brake plate.

A configuration may be employed in which a protrusion is provided in either one of a surface of the brake shoe opposite to a sliding contact surface with the brake plate, and a support surface of the brake shoe support plate, and an engaging portion for engaging with the protrusion is provided in the other one.

The brake plate may be fixed to the rotating shaft at a location outside the opposite-to-output-side bearing for supporting the rotating shaft.

An outside diameter of the brake plate may be smaller than an outside diameter of the opposite-to-output-side bearing, and the brake plate may be supported on an inner ring portion of the opposite-to-output-side bearing.

An outside diameter of the brake shoe may be smaller than an outside diameter of the opposite-to-output-side bearing

The friction brake structure may further include a conical coil spring as a mechanism for applying the biasing action to the brake shoe support plate.

The friction brake structure may further include a contact stop portion for restricting movement of the brake shoe support plate in the axial direction toward the brake plate, the movement occurring when an axial thickness of the brake shoe reduces below a predetermined value.

Advantageous Effects of Invention

As described above, the friction brake structure according to an embodiment of the present invention includes a brake plate fixed to a rotating shaft of a rotary electric machine; a ring-shaped brake shoe disposed facing the brake plate; and a brake shoe support plate which engages with a fixing portion of the rotary electric machine so as to be movable in an axial direction, and which supports the brake shoe while being biased by biasing action so as to bring the brake shoe into sliding contact with the brake plate.

Bringing the ring-shaped brake shoe into sliding contact with the brake plate while preventing the brake shoe from rotating together with the brake plate reduces a rotational moment caused by a friction force of the brake shoe, and thereby makes the spring constant of the coupled portion between the brake shoe and the brake plate less liable to change. As a result, self-excited oscillation of the brake shoe is suppressed, and thus production of abnormal noises called brake squeal can be reduced.

DESCRIPTION OF EMBODIMENTS

[Mechanisms of Abnormal Noise Production]

First of all, the inventor made a study on mechanisms of abnormal noise production, which will be described in detail below.

FIG. 1is a view illustrating the brake plate104and the plurality of brake shoes105in the motor shown inFIG. 6as viewed from the opposite-to-output side of the motor. InFIG. 1, the four brake shoes105, which are provided in the circumferential direction at even intervals, are denoted by the reference numerals1051to1054so as to be distinguishable from one another. The rotation direction of the brake plate104is indicated by arrow A1.

Abnormal noise production in a friction brake structure is a kind of self-excited oscillation in a coupled system.FIGS. 2A and 2Beach show the coupled system including a portion of the brake plate104, the brake shoe1052, and the coil spring109as viewed from the direction indicated by arrow Y ofFIG. 1. The elasticity, mass, and position of the brake plate104inFIGS. 2A and 2Bare represented by kB, mB, and xB, respectively. Also, the elasticity, mass, and position of the brake shoe1052inFIGS. 2A and 2Bare represented by k, mS, and xS, respectively. The elasticity of the coil spring109is represented by ks. InFIG. 2B, φ indicates an inclination angle of the brake shoe1052observed when the portion of the brake plate104has moved in the direction of arrow A2as a result of the rotation of the brake plate104. The moment of inertia of the brake shoe1052is represented by JS. The friction coefficient between the brake plate104and the brake shoe1052is represented by μ. The distance from the center of gravity G of the brake shoe1052to the sliding contact surface105ais represented by l0, and time is represented by t. The contact area between the brake plate104and the brake shoe1052is represented by A, and the spring constant related to the rotation around the center of gravity G of the brake shoe1052is represented by kφ. In the vicinity of the equilibrium point, these quantities have the following relationships:

Equation (1) is the equation of motion for the position of the brake plate. Equation (2) is the equation of motion for the position of the brake shoe. Equation (3) is the equation of motion for the rotation of the brake shoe.

Here, the integrand f(l) is a function expressing the pressure between the brake plate104and the brake shoe1052, in which l represents the coordinates on the sliding contact surface105ain the case in which the point of intersection Q between the axis of the brake shoe1052and the sliding contact surface105ais used as a reference point. The integrand f(l) can be approximated as follows:
f(l)=k(xB−xS−lφ)

Combining the above equations gives the following simultaneous equations:

Taking the Laplace transform of these simultaneous equations gives the following characteristic equation:

In general, it is known that if the matrix in the above characteristic equation is symmetric, it indicates the system is not a self-excited oscillation system, but if any pair of elements given by switching the row and column indices have opposite signs, it indicates the system is a self-excited oscillation system. In the characteristic equation shown above, the elements related to the rotation of the brake shoe are not symmetric, and each may possibly have an opposite sign to an element given by switching the row and column indices. This indicates a possibility that self-excited oscillation of the brake shoes may be generated in the rotation direction, which causes abnormal noises.

As described above, Patent Literature 1 discloses that the brake shoes are fixed to the ring-shaped support. In such case, as the brake plate rotates, the brake shoes might rotate slightly with respect to the ring-shaped support, which causes self-excited oscillation, and thus abnormal noises.

Embodiment

In light of the mechanisms of abnormal noise production described above, an embodiment of the present invention will be described below.

As shown inFIGS. 3 to 5, a motor1has a motor case10, which serves as a housing. In the motor case10, a stator11and a rotor12are provided. The rotor12is supported by a rotating shaft15supported on an output-side bearing13and an opposite-to-output-side bearing14.

The opposite-to-output-side bearing14is mounted in a bearing housing portion10awhich is integrally provided to the motor case10. In the bearing housing portion10a, a hole10bis drilled in the axial direction so as to communicate with the outside of the motor case10. The hole10b, which is approximately oval-shaped, has two flat surface portions10b1and10b2formed in the outer periphery so as to face each other. Thus, the hole10bhas a so-called D-cut shape.

A ring-shaped brake plate20is fixed to the rotating shaft15at a location outside the opposite-to-output-side bearing14. The rotating shaft15passes through a hole20aof the brake plate20. In addition, the brake plate20is supported on an inner ring portion14aof the opposite-to-output-side bearing14. The outside diameter of the brake plate20is smaller than the outside diameter of the opposite-to-output-side bearing14. The brake plate20rotates together with the rotating shaft15.

A ring-shaped brake shoe30is provided so as to face the brake plate20. The rotating shaft15passes through a hole30aof the brake shoe30. The outside diameter of the brake shoe30is smaller than the outside diameter of the opposite-to-output-side bearing14. The brake shoe30has four protrusions30cin an axial end surface30bopposite to the other axial end surface being in sliding contact with the brake plate20. The protrusions30care provided in the circumferential direction at even intervals. The brake shoe30may be made of a material, such as a PPS (polyphenylene sulfide) resin or a PTFE (polytetrafluoroethylene) resin.

The brake shoe30is supported by a ring-shaped brake shoe support plate40which is movable in the axial direction. The rotating shaft15passes through a first hole40aprovided at the center of the brake shoe support plate40.

The brake shoe support plate40has two flat surface portions40b1and40b2formed in the outer periphery40bso as to face each other across the first hole40a. These two flat surface portions40b1and40b2are provided so as to engage with the two flat surface portions10b1and10b2, respectively. Thus, the brake shoe support plate40has an approximately oval, so-called D-cut shape. The engagement of the two flat surface portions40b1and40b2of the brake shoe support plate40respectively with the flat surface portions10b1and10b2prevents the brake shoe support plate40from rotating around the rotating shaft15while allowing the brake shoe support plate40to move in the axial direction.

In addition, the brake shoe support plate40has four second holes40cprovided in the circumferential direction at even intervals so as to engage respectively with the four protrusions30c. The engagement of the four second holes40crespectively with the four protrusions30cfixes the brake shoe30onto the brake shoe support plate40.

A conical coil spring50is disposed on the outside axial end surface, which is opposite to the surface supporting the brake shoe30, of the brake shoe support plate40. The conical coil spring50is supported on a conical coil spring support plate60which is attached to the motor case10with screws61. The brake shoe support plate40is biased by biasing action of the conical coil spring50so as to bring the brake shoe30into sliding contact with the brake plate20.

Further, between the opposite-to-output-side bearing14and an outer peripheral portion40dof the brake shoe support plate40, a contact stop portion10cis provided. The outer peripheral portion40dis located radially outside the second holes40c. The contact stop portion10cprotrudes radially inward from the inner wall of the hole10bof the bearing housing portion10a. The contact stop portion10cis provided in order to restrict the movement of the brake shoe support plate40in the axial direction toward the brake plate20. Such movement is to occur when the brake shoe30is worn by sliding contact with the brake plate20enough to reduce the axial thickness of the brake shoe30below a predetermined value.

As described above, the brake shoe30is fixed to the brake shoe support plate40, which is prevented from rotating around the rotating shaft15while being allowed to move in the axial direction. Thus, while the motor1is driven, the brake shoe30is prevented from rotating along with the rotation of the rotating shaft15and the brake plate20, which allows for braking the rotation of the rotating shaft15. When the drive of the motor1stops, the braking force of the brake shoe30can quickly stop the rotation of the rotating shaft15. In addition, while the motor stops, a certain holding force for the rotating shaft15is exerted. As described above, this embodiment provides braking action while the motor is driven as well as holding torque while the motor stops.

By bringing the brake shoe30, which has a ring shape allowing securing of a sufficient sliding contact area, into sliding contact with the brake plate20, the brake shoe30is prevented from rotating together with the brake plate20. This reduces a rotational moment caused by a friction force of the brake shoe30, and thus makes the spring constant of the coupled portion between the brake shoe30and the brake plate20less liable to change.

Here, in each of Equations (4) to (6) expressing coupled oscillation, the first term on the right side expresses a restoring force that causes simple harmonic motion, and the coefficient of this term is the so-called spring constant. The inclination angle φ is included in the second or subsequent item, which brings the same effect as changes in the spring constant in the conventional technique shown inFIG. 6.

In contrast, according to the above embodiment, by making the spring constant of the coupled portion between the brake shoe30and the brake plate20less liable to change, self-excited oscillation of the brake shoe30can be reduced, and thus production of abnormal noises called brake squeal can be reduced.

Such effect of reducing abnormal noise production will be described in relation to Equation (7). Note however that since it seems to be difficult to simply use Equation (7), which includes three variables, without any modifications, the determination on whether stable or not will be considered below by using a modified equation including two variables obtained by reducing the elements in Equation (7). Suppose such modified equation can be expressed as follows:

It is known that when the product of off-diagonal elements is negative in this equation, it indicates that the system is a self-excited oscillation system.

According to the above embodiment, the ring shape of the brake shoe30allows the brake shoe30to secure a sufficient sliding contact area, and thereby to prevent or reduce the brake shoe30from rotating together with the brake plate20. In other words, unlike the conventional technique shown inFIG. 6, the inclination angle φ is zero or infinitely close to zero in this embodiment. Excluding the equation of motion for the rotation of the brake shoe from Equation (7) gives the following equation:

In Equation (9), the product of the two off-diagonal elements is positive, which indicates the system is not a self-excited oscillation system. This means that the above embodiment can reduce abnormal noise production in the brake.

In contrast, regarding the conventional technique as shown inFIG. 6, excluding XBfrom Equation (7) gives the following equation:

According to the conventional technique, because of the presence of the inclination angle φ, the following expression:
k∫ldA
which is included in the off-diagonal elements can take both positive and negative values. This presents a possibility that the product of the two non-diagonal elements may be negative, indicating that the conventional technique might permit self-excited oscillation, and thus permit abnormal noise production in the brake.

Friction against the brake plate20gradually wears the brake shoe30, and reduces the axial thickness of the brake shoe30. This thickness reduction moves the brake shoe support plate40, on which a biasing force is imposed by the conical coil spring50, toward the brake plate20. The brake shoe support plate40eventually comes into contact with the contact stop portion10c, which restricts the further movement of the brake shoe support plate40toward the brake plate20. When the brake shoe30is further worn, the brake shoe30is no longer in sliding contact with the brake plate20. This prevents or reduces spark generation, which is expected to occur if the brake shoe30is in such sliding contact even after being worn to some substantial extent.

In contrast, inFIG. 6, which is not provided with the contact stop portion10c, spark generation can be prevented or reduced by limiting the free height of the coil spring109such that the coil spring109is not compressed after the brake shoes105have been substantially worn. However, in such case, as the brake shoes105are worn, the compression height of the coil spring109increases, and accordingly the biasing force of the coil spring109decreases. This might lead to braking force reduction.

In the above embodiment, the presence of the contact stop portion10celiminates the need of limiting such spring free height. This makes it possible to select, as the conical coil spring50, a spring having a free height large enough to exert a sufficient biasing force before the brake shoe support plate40comes into contact with the contact stop portion10c. Thus, the above embodiment can provide a reliable braking force before the brake shoe support plate40comes into contact with the contact stop portion10c.

InFIG. 6, the brake plate104is provided axially inside the opposite-to-output-side bearing106b. The brake shoes105are disposed radially outside the opposite-to-output-side bearing106bso as not to interfere with the opposite-to-output-side bearing106b. As a result, the brake plate104has a relatively large outside diameter. In other words, the brake shoes105are brought into sliding contact with the brake plate104at relatively distant points from the axis of the rotating shaft103. This inevitably increases the circumferential speed of the sliding contact portions to a relatively high value.

The wear volume of a brake shoe is proportional to the product PV of the contact pressure P [MPa] caused by the biasing force F of a spring and the circumferential speed V [m/s] at the sliding contact surface of the brake shoe. However, after the circumferential speed V increases to a certain value or more, the wear volume increases to a large value without being dependent on P any longer. This makes it difficult to prolong brake life inFIG. 6.

In the above embodiment, the outside diameters of the brake plate20and the brake shoe30are both smaller than the outside diameter of the opposite-to-output-side bearing14. Thus, the sliding contact portion is relatively close to the axis of the rotating shaft15, which suppresses an increase in the circumferential speed V at the sliding contact portion. The closer to the axis the sliding contact portion is, the larger spring biasing force F is needed to provide a desirable braking force. However, in the above embodiment, the ring shape of the brake shoe30, which allows securing of a sufficient sliding contact area, limits the value taken by P. Therefore, the value of the product PV is reduced, and thus brake life can be prolonged.

Also, a single integrated brake shoe is provided instead of a plurality of brake shoes, and only one conical coil spring is provided as a biasing mechanism corresponding to the brake shoe. This allows for more efficient assembly.

In addition, though providing the brake structure axially outside the opposite-to-output-side bearing is considered to increase the size of the motor in the axial direction, such size increase can be limited by employing the conical coil spring.

Other Embodiments

In place of the conical coil spring50, any biasing mechanism such as an ordinary coil spring or any resilient mechanism may be provided.

The number of the protrusions30cis four in the above embodiment, but may be set to any number. It is only necessary to provide the second holes40cas many as the protrusions30c. The second holes40chave only to be engaging portions for engaging with the protrusions30c, and may either penetrate through the brake shoe support plate40from the support surface to its opposite surface, or do not penetrate to this opposite surface but instead form recesses. A still alternative configuration is also possible in which the protrusions30care provided to the brake shoe support plate40and the second holes40care provided to the brake shoe30.

The brake shoe support plate40does not necessarily engage with the bearing housing portion10a, which is a fixing portion provided in the motor1and is not expected to be moved by driving the motor1, but may engage with another fixing portion.

The friction brake structure may also be provided to a rotary electric machine other than the motor1.

Certain embodiments of a friction brake structure have been specifically described above. However, the present invention is not limited to such embodiments, and any modifications and alterations obvious to those skilled in the art will be all included within the technical scope of the present invention.

REFERENCE SYMBOLS LIST