Patent Publication Number: US-2006011429-A1

Title: Damper for damping vibration and washing machine having the same

Description:
This application claims the benefit of Korean Patent Application Nos. 10-2004-0054423, 10-2004-0054424, 10-2004-0054425 and 10-2004-0054426, filed on Jul. 13, 2004, which are hereby incorporated by references as if fully set forth herein.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a damper for damping vibration, and more particularly, to a damper, the damping force of which is adjustable, and a washing machine having the same.  
      2. Discussion of the Related Art  
      Generally, a washing machine performs washing, rinsing, and dehydrating operations by filling the lower portion of the inside of a tub with water and detergent, placing laundry in a drum installed in the tub, and rotating the drum.  
      Such a washing machine is an apparatus for eliminating dirt from laundry using proper detergent and mechanical force.  
      Accordingly, the washing machine usually comprises a cabinet forming an external appearance, an outer tub provided in the cabinet for containing washing water, and an inner tub rotatably provided in the outer tub for containing laundry.  
      Particularly, a washing machine in which a rotary shaft of the inner tub is approximately horizontal with the ground is referred to as a drum washing machine.  
      With reference to  FIG. 1 , the drum washing machine is an apparatus for eliminating dirt from laundry by rotating a drum using the driving force of a motor under the condition that detergent, washing water and laundry are placed in the drum installed horizontally with the bottom of a cabinet. The drum washing machine minimizes entanglement of the laundry and damage to the laundry, consumes a small quantity of the washing water, and has laundry-beating and rubbing effects.  
      More specifically, the drum washing machine comprises the cabinet  10  forming an external appearance, a tub  20  provided in the cabinet  10 , and the drum  30  rotatably provided in the tub  20 .  
      Here, an opening for placing the laundry in the cabinet  10  is formed through the front surface of the cabinet  10 , and a door  11  for opening and closing the opening is installed at the opening.  
      A rotating device  40 , such as a motor, for rotating the drum  30  is installed in the rear of the tub  20 .  
      The tub  20  is horizontally installed in the cabinet  10  by suspension springs  50  for connecting the ceiling of the cabinet  10  and the upper part of the tub  20 , and a damper  60  for connecting the bottom of the cabinet  10  and the lower part of the tub  20  is installed on the lower part of the cabinet  10 .  
      The damper  60  is a unit for damping the vibration of the tub  20  generated due to the rotation of the drum  30 . The damper  60  usually comprises a damper cylinder having a cylindrical shape, a shaft inserted into the damper cylinder and reciprocating, and a friction member fixed to the shaft.  
      Here, the friction member reduces the vibration of the tub  20  by means of the friction with the damper cylinder.  
      The above drum washing machine sequentially or selectively performs washing, rinsing, and dehydrating operations, thereby washing the laundry.  
      When the dehydrating operation of the drum washing machine is started, the drum  30  is rotated at a high speed by the rotating device  40 , thus dehydrating the laundry.  
      In an initial stage of the dehydrating operation, i.e., in the initial driving of the rotating device  40  and the drum  30 , the tub  20  is excessively vibrated (hereinafter, referred to as “excessively vibrated state”).  
      As the spin speeds of the rotating device  40  and the drum  30  are gradually increased, the vibration of the tub  20  is gradually decreased.  
      Thereafter, when the rotating device  40  and the drum  30  reach normal spin speeds, the vibration of the tub  20  is greatly decreased compared to the initial stage of the dehydrating operation, and is stabilized to reach a normally vibrated state.  
      The shaft of the damper  60  reciprocates in the lengthwise direction of the damper cylinder in the damper cylinder by means of the vibration of the tub  20 , and the friction member reciprocates together with the reciprocation of the shaft and rubs against the inner wall of the damper cylinder.  
      The vibration of the tub  20  is damped by the frictional force between the friction member and the damper cylinder.  
      Hereinafter, the relation between the number of vibration of the tub and the spin speed of the drum and the relation between the vibration-transmitting force (hereinafter, referred to as TR force) of the damper and the spin speed of the drum according to variation of the damping force of the damper will be described with reference to  FIGS. 2 and 3 .  
      Here,  FIG. 2  is a graph illustrating the relation between the number of vibration of the tub and the spin speed of the drum according to variation of the damping force, and  FIG. 3  is a graph illustrating the relation between the TR force of the damper and the spin speed of the drum according to variation of the damping force.  
      With reference to  FIG. 2 , when the spin speed of the drum  30  is less than a designated value (ω 1 ) in the initial stage of the dehydrating operation, the tub  20  to which a damper having a high damping force is applied and the tub  20  to which a damper having a low damping force are vibrated as illustrated in the graph of  FIG. 2 . The numbers of vibration of the tubs  20  are approximately the same after the excessively vibrated state, i.e., in the normally vibrated state, regardless of the damping forces of the dampers.  
      Further, with reference to  FIG. 3 , when the dehydrating operation is continued so that the number of rotation of the motor, i.e., the number of rotation of the drum  30 , is increased more than a designated value (ω 1 ), the tub  20  reaches the normally vibrated state.  
      When the tub  20  is supported by the damper having a low damping force in the normally vibrated state of the tub  20 , the TR force transmitted to a main body of the washing machine by the damper having the low damping force is maintained less than a designated degree. On the other hand, when the tub  20  is supported by the damper having a high damping force, the TR force is increased as time goes by, thereby continuously increasing the vibration and noise.  
      Thus, in the excessively vibrated state of the tub, i.e., in the initial stage of the dehydrating operation, the damper having the high damping force is advantageous to damp the vibration and the noise. Further, in the normally vibrated state of the tub, the damper having the low damping force is advantageous.  
      Accordingly, the development of a damper for efficiently damping vibration using damping force varying according to a degree of the vibration of an object to be vibrated, such as a tub of a washing machine, has been required.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention is directed to a damper for damping vibration and a washing machine having the same substantially obviate one or more problems due to limitations and disadvantages of the related art.  
      An object of the present invention is to provide a damper having a structure for minimizing the transmission of vibration and a washing machine having the same.  
      Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
      To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a damper includes a damper cylinder; a shaft inserted into the damper cylinder through one end of the damper cylinder, and reciprocating in the lengthwise direction of the damper cylinder; and a friction member provided on the shaft for damping vibration by friction with the inner wall of the damper cylinder, and moving vertically to the lengthwise direction of the damper cylinder.  
      Preferably, the shaft may include a frictional force-adjusting device for moving the friction member to adjust the frictional force between the friction member and the inner wall of the damper cylinder.  
      More preferably, the frictional force-adjusting device may include a pressure unit for pressing the friction member to move the friction member towards the inner wall of the damper cylinder to a designated distance.  
      The pressure unit includes a first pressure member; a second pressure member provided at one side of the first pressure member, and positioned opposite to the first pressure member; and an interval adjuster for moving at least one of the first and second pressure members in the axial direction of the shaft in order to move the friction member.  
      When the first and second pressure members are close to each other, the frictional force between the friction member and the inner wall of the damper cylinder is increased.  
      For this reason, the first and second pressure members are disposed in the axial direction of the shaft, and at least one of the first and second pressure members includes at least one inclined portion inwardly inclined towards the other one of the first and second pressure members.  
      For example, the side surface of at least one of the first and second members is inclined such that the cross sectional area thereof is decreased close to the other one of the first and second members.  
      Preferably, the pressure unit may further include at least one slip member provided in the friction member to correspond to the inclined portion, and moving outwardly along the inclined portion to press the friction member when the first and second pressure members are close to each other.  
      The friction member moves towards the center of the shaft and is separated from the inner wall of the damper cylinder when the first and second pressure members are distant from each other.  
      Preferably, the slip member may include an elastic member for returning the friction member to the center of the shaft when the first and second pressure members are distant from each other.  
      The elastic member has a ring-shape, and is provided on the outer cylindrical surface of the slip member.  
      In the case that the pressure unit includes a plurality of slip members, the pressure unit may further include guide ribs provided on the outer cylindrical surface of at least one of the first and second pressure members for preventing the concentrated movement of the slip members in the circumferential direction.  
      The interval adjuster moves the second pressure member.  
      The interval adjuster includes at least one connection rod passing through the first pressure member fixed to a body of the shaft, and connected to the second pressure member; and a driving unit for driving the connection rod to move the second pressure member.  
      The driving unit rectilinearly may move the connection rod to move the second pressure member.  
      The driving unit includes an electromagnet provided at the other side of the first pressure member for rectilinearly moving the connection rod using magnetic force so that the second pressure member moves towards the first pressure member when current flows.  
      The connection rod includes a metal member provided between the first pressure member and the electromagnet.  
      The driving unit includes a wire connected to the connection rod; and an actuator connected to the wire.  
      The driving unit includes a motor for rotating the connection rod, and the second pressure member rectilinearly moves by the rotation of the connection rod.  
      The connection rod passes through the second pressure member, and is connected to the second pressure member by screws.  
      The pressure unit may further include a guide pin, one end of which is inserted into the first pressure member and the other end of which is inserted into the second pressure member, for guiding the movement of the second pressure member.  
      Alternately, the interval adjuster may include an electromagnet provided on the first pressure member; a metal member provided on the second pressure member; and a guide rod passing through the second pressure member for guiding the movement of the second pressure member.  
      Preferably, the guide rod may include a stopper, one end of which is connected to one side of the electromagnet and the other end of which prevents the second pressure member from being separated from the guide rod.  
      The friction member may have a ring shape surrounding the outer cylindrical surface of the shaft.  
      Further, preferably, the friction member may be made of an elastic material.  
      The friction member contacts the inner wall of the damper cylinder when the number of vibration of an object to be vibrated is more than a designated value, and is separated from the inner wall of the damper cylinder when the number of vibration of the object is less than the designated value.  
      The frictional force between the friction member and the inner wall of the damper cylinder when the spin speed of an object for generating vibration is more than a designated speed is lower than that when the spin speed of the object is less than the designated speed.  
      One of the damper cylinder and the shaft is connected to a tub of a washing machine, and the other one of the damper cylinder and the shaft is connected to a cabinet of the washing machine.  
      It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
       FIG. 1  is a schematic sectional view of a conventional drum washing machine;  
       FIG. 2  is a graph illustrating the relation between the number of vibration of a tub and the spin speed of a drum according to variation of damping force;  
       FIG. 3  is a graph illustrating the relation between the TR force of a damper and the spin speed of the drum according to variation of damping force;  
       FIG. 4  is a sectional view of a damper in accordance with a first embodiment of the present invention;  
       FIGS. 5A and 5B  are a partial and longitudinal sectional view and a transversal sectional view illustrating a friction member of the damper shown in  FIG. 4 , which is separated from the inner wall of a damper cylinder;  
       FIGS. 6A and 6B  are a partial and longitudinal sectional view and a transversal sectional view illustrating the friction member of the damper shown in  FIG. 4 , which contacts the inner wall of the damper cylinder;  
       FIG. 7  is a sectional view of one embodiment of a pressure unit of the damper of the present invention;  
       FIG. 8  is a perspective view of another embodiment of the pressure unit of the damper of the present invention;  
       FIGS. 9A and 9B  are sectional views of a damper in accordance with a second embodiment of the present invention;  
       FIGS. 10A and 10B  are sectional views of a damper in accordance with a third embodiment of the present invention; and  
       FIGS. 11A and 11B  are sectional views of a damper in accordance with a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
      Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
      First, with reference to FIGS.  4  to  8 , a damper in accordance with a first embodiment of the present invention will be described.  
      Here,  FIG. 4  is a sectional view of the damper in accordance with the first embodiment of the present invention.  FIGS. 5A and 5B  are a partial and longitudinal sectional view and a transversal sectional view illustrating a friction member of the damper shown in  FIG. 4 , which is separated from the inner wall of a damper cylinder.  FIGS. 6A and 6B  are a partial and longitudinal sectional view and a transversal sectional view illustrating the friction member of the damper shown in  FIG. 4 , which contacts the inner wall of the damper cylinder.  FIG. 7  is a sectional view of another embodiment of a pressure unit of the damper of the present invention.  FIG. 8  is a perspective view of yet another embodiment of the pressure unit of the damper of the present invention.  
      The damper of the present invention, which will be described hereinafter, may be applied to general washing machines or apparatuses generating vibration as well as the above drum washing machine. In the following description of the present invention, a detailed description of known functions and configurations of a washing machine, to which the damper of the present invention is applied, will be omitted, and the same or similar elements of the washing machine are denoted by the same reference numerals as those of the conventional washing machine using a damper.  
      First, with reference to  FIG. 4 , the damper  600  in accordance with the first embodiment of the present invention comprises a damper cylinder  610 , a shaft  620  inserted into the damper cylinder  610 , and a friction member  630  provided on the shaft  620 .  
      The damper cylinder  610  has a hollow cylindrical structure. One end of the shaft  620  is inserted into an opened end of the damper cylinder  610 , and the shaft  620  reciprocates in the lengthwise direction of the damper cylinder  610  when the tub  20  is vibrated.  
      Here, the other end of the damper cylinder  610  is connected to the lower part of the tub  20 , and the other end of the shaft  620  is connected to the bottom of the cabinet  10  to absorb the vibration generated from the tub  20 .  
      Alternately, the other end of the damper cylinder  610  may be connected to the bottom of the cabinet  10 , and the other end of the shaft  620  may be connected to the lower part of the tub  20 .  
      Hereinafter, for convenience of description, the damper  600  provided with an upper portion  610   a  connected to the lower part of the tub  20  and a lower portion  610   b  connected to the bottom of the cabinet  10  will be described.  
      The friction member  630  is movable vertically to the lengthwise direction of the damper cylinder  610 , and damps the vibration of the tub  20  by the friction with the inner wall of the damper cylinder  610 .  
      Accordingly, the shaft  620  includes a frictional force-adjusting device for adjusting the frictional force between the friction member  630  and the inner wall of the damper cylinder  610  by moving the friction member  630 .  
      With reference to  FIGS. 5A and 5B  and  FIGS. 6A and 6B , the frictional force-adjusting device will be described in more detail, as follows. The frictional force-adjusting device of the damper  600  of the first embodiment includes a pressure unit  640  for pressing the friction member  630  towards the inner wall of the damper cylinder  610  to move the friction member  630  by a designated distance towards the inner wall of the damper cylinder  610 .  
      In this embodiment, the pressure unit  640  includes a first pressure member  641 , a second pressure member  642  provided at one side of the first pressure member  641 , and an interval adjuster for adjusting the interval between the first pressure member  641  and the second pressure member  642 .  
      The first pressure member  641  is provided on one end of the shaft  620 , i.e., the upper end of the shaft  620 , and the second pressure member  642  is provided above the first pressure member  641  such that the second pressure member  642  is positioned opposite to the first pressure member  641 .  
      Accordingly, the first pressure member  641  and the second pressure member  642  are disposed in the axial direction of the shaft  620 , i.e., the longitudinal direction.  
      The interval adjuster moves at least one of the first pressure member  641  and the second pressure member  642  in the axial direction of the shaft  620  to adjust the interval between the first pressure member  641  and the second pressure member  642 , thereby causing the friction member  630  to contact the inner wall of the damper cylinder  610  with designated force.  
      In this embodiment, the first pressure member  641  is fixed to a body of the shaft  620 , and the second pressure member  642  is installed on the body of the shaft  620  such that the second pressure member  642  moves vertically. Thereby, when the first pressure member  641  and the second pressure member  642  are close to each other, the frictional force between the friction member  630  and the inner wall of the damper cylinder  610  is increased.  
      Alternately, the second pressure member  642  may be fixed to the body of the shaft  620 , and the first pressure member  641  may be installed on the body of the shaft  620  such that the first pressure member  641  moves vertically.  
      Although this embodiment illustrates the first pressure member  641  and the second member  642 , which are provided on the upper end of the body of the shaft  620 , the positions of the first pressure member  641  and the second member  642  are not limited thereto.  
      At least one of the first pressure member  641  and the second pressure member  642  includes at least one inclined portion inwardly inclined towards the other one of the first pressure member  641  and the second pressure member  642 .  
      For example, a plurality of inclined portions, which are inwardly inclined from the lower part of the first pressure member  641  to the upper part of the first pressure member  641 , may be formed on the side surface of the first pressure member  641  such that the inclined portions are spaced by the same interval in the circumferential direction. A plurality of inclined portions having symmetrical shapes to those of the inclined portions of the first pressure member  641  may be formed on the side surface of the second pressure member  642 .  
      In this embodiment, the first pressure member  641  has a shape such that the cross sectional area of the first pressure member  641  is decreased from the lower portion of thereof to the upper portion thereof on which the second pressure member  642  is provided, and has a side surface inwardly inclined from the lower portion thereof to the upper portion thereof. The second pressure member  642  is symmetrical with the first pressure member  641 . Accordingly, inclined portions  641   a  and  642   a  are respectively formed on the side surfaces of the first and second pressure members  641  and  642 .  
      Although not shown in the drawings, only one of the first pressure member  641  and the second pressure member  642  may have the above shape.  
      Here, the vertical cross sections of the inclined portions  6411  and  642   a  may have various shapes, such as rectilinear and concave shapes.  FIG. 7  illustrates one embodiment of the pressure unit including first and second pressure members  741  and  742  having inclined portions  741   a  and  742   a , the vertical cross sections of which have concave shapes.  
      Preferably, at least one slip member  643  for moving the friction member  630  outwardly so that the inclined portion  641   a  of the first pressure member  641  corresponds to the inclined portion  642   a  of the second pressure member  642  is provided in the friction member  630 .  
      Each of the slip members  643  includes upper inclined planes corresponding to the inclined portions  642   a  of the second pressure member  642  and lower inclined planes corresponding to the inclined portions  641   a  of the first pressure member  641 .  
      Accordingly, when the first pressure member  641  and the second pressure member  642  are close to each other, the lower and upper inclined planes of the slip members  643  are pressed by the inclined portions  641   a  of the first pressure member  641  and the inclined portions  642   a  of the second pressure member  642 .  
      More specifically, when the first pressure member  641  and the second pressure member  642  are close to each other, the lower and upper inclined planes of the slip members  643  slip along the inclined portions  641   a  of the first pressure member  641  and the inclined portions  642   a  of the second pressure member  642 .  
      Thereby, the slip members  643  outwardly move towards the inner wall of the damper cylinder  610 , and press the inner surface of the friction member  630 , thereby causing the friction member  630  to contact the inner wall of the damper cylinder  610  with designated force.  
      Preferably, the inclined portions  641   a  of the first pressure member  641 , the inclined portions  642   a  of the second pressure member  642 , and the upper and lower inclined planes of the slip members  643  are made of a material having a smoother surface than the friction member  630  and the inner wall of the damper cylinder  610 .  
      When the first pressure member  641  and the second pressure member  642  are distant from each other, i.e., when the second pressure member  642  moves upwardly, it is preferable that the friction member  630  moves towards the center of the shaft  620 . Thereby, the frictional force between the friction member  630  and the inner wall of the damper cylinder  610  is gradually decreased, and thus the damping force of the damper  600  is reduced.  
      Preferably, at least one slip member  643  includes an elastic member  643   a  for returning the friction member  630  towards the center of the shaft  620  when the first pressure member  641  and the second pressure member  642  are distant from each other.  
      For example, the elastic member  643   a  may be a metal spring having a ring shape or a rubber band provided on the outer cylindrical surface of the slip member  643 .  
      A groove  643   b  is formed in the outer cylindrical surface of the slip member  643  in the circumferential direction, and the ring-shaped elastic member  643   a  is inserted into the groove  643   b.    
      When a plurality of the slip members  643  are provided in the friction member  630 , as shown in  FIG. 8 , guide ribs  641   b  for preventing the concentrated movement of the slip members  643  in the circumferential direction are formed in the outer cylindrical surface of at least one of the first pressure member  641  and the second pressure member  642 .  
      Preferably, the guide ribs  641   b  are separated from each other in the circumferential direction by the same interval. Thereby, it is possible to maintain the proper interval between the slip members  643 .  
      The friction members  630  pressed and moved by the slip members  643  may be prepared in the same number as that of the slip members  643 , or may be one ring-shaped elastic member surrounding the outer cylindrical surface of at least one slip member  643  as described in this embodiment.  
      The interval adjuster of the damper of the first embodiment, as described above, moves the second pressure member  642  towards the first pressure member  641 .  
      For this reason, the interval adjuster includes connection rods  644  passing through the first pressure member  641  and vertically moving, and a driving unit for driving the connection rods  644 .  
      Upper ends of the connection rods  644  are connected to the second pressure member  642 , and vertically move along guide holes  641   c  formed through the first pressure member  641 . When the connection rods  644  rectilinearly move downwardly or upwardly, the second pressure member  642  moves downwardly or upwardly.  
      Accordingly, the friction member  630  is pressed by the slip members  643 , and moves towards the inner wall of the damper cylinder  610  or in the opposite direction. Thereby, the frictional force between the friction member  630  and inner wall of the damper cylinder  610  is adjusted.  
      In this embodiment, the driving unit of the interval adjuster moves the connection rods  644  by magnetic force.  
      More specifically, the driving unit includes an electromagnet  645  provided below the first pressure member  641 .  
      The electromagnet  645  rectilinearly moves the connection rods  644  downwardly using magnetic force so that the second pressure member  642  moves towards the first pressure member  641  when current flows.  
      For this reason, the electromagnet  645  includes a core  645   a  installed on the upper part of the shaft  620 , a bobbin  645   b  installed outside the core  645   a , and a coil (not shown) wound on the bobbin  645   b  several times. When current is applied to the electromagnet  645  to generate magnetic force, the connection rods  644  are attracted downwardly by the magnetic force of the electromagnet  645 .  
      Preferably, each of the connection rods  644  includes a metal member  644   a  formed at the lower end thereof between the first pressure member  641  and the electromagnet  645 .  
      The first pressure member  641  is fixed to the core  645   a . More specifically, the first pressure member  641  includes a fixing rod  641   d , which passes through the metal members  644   a  of the connection rods  644  and is provided with the lower end connected to the core  645   a.    
      The friction member  630  selectively contacts the inner wall of the damper cylinder  610  by the above-described frictional force-adjusting device.  
      That is to say, when the tub  20  is in the excessively vibrated state, the friction member  630  contacts the inner wall of the damper cylinder  610  so as to increase the damping force of the damper  600 . Then, when the tub  20  reaches the normally vibrated state, the friction member  630  is separated from the inner wall of the damper cylinder  610  so as to decrease the damping force of the damper  600 .  
      Hereinafter, a process for damping the vibration of a washing machine using the above damper of the present invention will be described.  
      First, when the washing machine is operated, the rotating device  40  rotates the drum  30 , and laundry is washed by the rotation of the drum  30 .  
      Then, the tub  20  is vibrated by the rotation of the rotating device  40  and the drum  30 .  
      Here, the vibration of the tub  20  is damped by the damper  600 , and the transmission of the vibration of the tub  20  to the cabinet  10  is minimized.  
      When the dehydrating operation of the laundry is started, the tub  20  is excessively vibrated in the initial stage of the dehydrating operation.  
      In the excessively vibrated state of the tub  20 , power is supplied to the electromagnet  645  to generate magnetic force, and the metal members  644   a  installed on the lower parts of the connection rods  644  move towards the electromagnet  645  by the magnetic force.  
      Simultaneously, the second pressure member  642  connected to the upper ends of the connection rods  644  move downwardly, and force is applied to the slip members  643  by the first pressure member  641  and the second pressure member  642  so that the slip members  643  move towards the inner wall of the damper cylinder  610 .  
      Accordingly, the friction member  630  is pressed by the slip members  643 , and contacts the inner wall of the damper cylinder  610 , and the vibration of the tub  20  is damped by the frictional force between the friction member  630  and the inner wall of the damper cylinder  610 .  
      When the dehydrating operation is continued for a designated time so that the spin speed of the drum  30  rotated by the motor  40  is more than a designated speed, the tub  20  reaches a normally vibrated state.  
      In the normally vibrated state of the tub  20 , when the friction member  630  is pressed towards the inner wall of the damper cylinder  610  by the same force as that of the excessively vibrated state, the vibration of the tub  20  transmitted to the cabinet  10  of the washing machine through the damper  600  is more increased.  
      Accordingly, when the tub  20  reaches the normally vibrated state, power is not supplied to the electromagnet  645 , thereby eliminating the magnetic force applied to the metal members  644   a.    
      Thereby, the force for pressing the friction member  630  provided outside the slip members  643  is eliminated, and the slip members  643  are returned to the original positions towards the center of the shaft  620  by the elastic members  643   a  provided on the outer cylindrical surfaces of the slip members  643 . Further, the friction member  630  elastically contracts, and moves to its original position, i.e., towards the center of the shaft  620 .  
      That is to say, the friction member  630  is separated from the inner wall of the damper cylinder  610 , a designated distance (t) is formed between the friction member  630  and the inner wall of the damper cylinder  610 , and the TR force of the tub  20  to a structure of the washing machine, particularly, the cabinet  10 , through the damper  600  is decreased. Thereby, the dehydrating operation is quietly and smoothly performed.  
      In order to operate the above damper  600  in the optimum state, preferably, a vibration sensing device (not shown) for sensing the vibration of the tub  20  and a control unit (not shown) for controlling the damper  600  according to the vibrated state of the tub  20  are installed at designated positions of the washing machine.  
      When the vibration sensing device senses the vibration of the washing machine and transmits a signal to the control unit, the control unit controls current supplied to the electromagnet  645 , thereby adjusting the damping force of the damper  600 .  
      Otherwise, an alternate method for adjusting the damping force of the damper  600  may be employed. In this method, a time taken to reach the normally vibrated state of the tub  20  is input in advance to the control unit. Before the tub  20  reaches the normally vibrated state, current is supplied to the electromagnet  645  so as to increase the damping force of the damper  600 , and after the tub  20  reaches the normally vibrated state, current is not supplied to the electromagnet  645  so as to decrease the damping force of the damper  600 .  
      Hereinafter, dampers in accordance with other embodiments of the present invention will be described with reference to  FIGS. 9A  to  11 B.  
      Some parts of dampers in these embodiments, which are substantially the same as those of the damper in the first embodiment, are denoted by the same reference numerals even through they are depicted in different drawings, and a detailed description thereof will thus be omitted because it is considered to be unnecessary.  
      First, a damper  600   a  in accordance with the second embodiment of the present invention will be described with reference to  FIGS. 9A and 9B .  
      With reference to  FIGS. 9A and 9B , the damper  600   a  comprises a connection rod  744  passing through the first pressure member  641  and the second pressure member  642  and vertically moving.  
      For this reason, vertical guide holes (not shown) are respectively formed through central portions of the first and second pressure members  641  and  642 .  
      Here, the diameter of the guide holes formed through the first and second pressure members  641  and  642  is larger than that of the connection rod  744 .  
      The lower end of the connection rod  744  is connected to an actuator (not shown), and is connected to a wire  745  for drawing the connection rod  744  downwardly.  
      A head  744   a  having a diameter larger than that of the guide hole of the second pressure member  642  is formed on the upper end of the connection rod  744 .  
      Accordingly, when the wire  745  draws the connection rod  744  by means of the driving of the actuator, the upper surface of the second pressure member  642  is pressed by the head  744   a  so that the second pressure member  642  moves downwardly.  
      Then, the slip members  643  are pressed by the first pressure member  641  and the second pressure member  642 , slid outwardly along the inclined portions  641   a  and  642   a  of the first and second pressure members  641  and  642 , and press the friction member  630  towards the inner wall of the damper cylinder  610 .  
      The connection rod  744  may be fixed to the second pressure member  642 . In this case, the connection rod  744  does not include the head  744   a.    
      Other configurations and operating method of the damper  600   a  are substantially the same as those of the damper of the first embodiment, and a detailed description thereof will thus be omitted because it is considered to be unnecessary.  
      Hereinafter, a damper  600   b  in accordance with the third embodiment of the present invention will be described with reference to  FIGS. 10A and 10B .  
      With reference to  FIGS. 10A and 10B , the damper  600   b  comprises a connection rod  844  passing through the first pressure member  641  and connected to the second pressure member  642  in order to vertically move the second pressure member  642 .  
      More specifically, the connection rod  844  vertically passes through the second pressure member  642 .  
      For this reason, vertical guide holes (not shown) are respectively formed through central portions of the first and second pressure members  641  and  642 .  
      Particularly, a female screw thread is formed in the guide hole of the second pressure member  642 , and a male screw thread corresponding to the female screw thread is formed on the outer cylindrical surface of the connection rod  844 . The female screw thread of the guide hole of the second pressure member  642  is engaged with the male screw thread of the outer cylindrical surface of the connection rod  844 .  
      The connection rod  844  is rotated by a rotating device, such as a motor  845 .  
      Although this embodiment illustrates the motor  845  provided below the first pressure member  641 , the position of the motor  845  is not limited thereto. For example, the motor  845  may be fixed to the inside of the first pressure member  641 .  
      Although the connection rod  844  is directly connected to a rotary shaft (not shown) of the motor  845 , the connection method of the connection rod  844  is not limited thereto. For example, the connection rod  844  may be connected to the rotary shaft of the motor  845  by a belt.  
      In order to prevent the first pressure member  641  from being influenced by the connection rod  844  when the connection rod  844  is rotated by the motor  845 , preferably, the diameter of the guide hole formed through the first pressure member  641  is larger than that of the connection rod  744 .  
      The first pressure member  641  and the second pressure member  642  are connected by a guide pin  846 , one end of which is inserted into the first pressure member  641  and the other end of which is inserted into the second pressure member  642 .  
      For this reason, a first groove (not shown) having a designated depth, into which the lower end of the guide pin  846  is inserted, is formed in the upper surface of the first pressure member  641 , and a second groove (not shown) having a designated depth, into which the upper end of the guide pin  846  is inserted, is formed in the lower surface of the second pressure member  642 .  
      Preferably, the diameter of at least one of the first and second grooves is larger than that of the guide pin  846 .  
      The guide pin  846  guides the vertical movement of the second pressure member  642 , and simultaneously prevents the second pressure member  642  from being rotated together with the rotation of the connection rod  844 .  
      Accordingly, when the connection rod  844  rotates in one direction, the second pressure member  642  is not rotated by the guide pin  846 , but moves upwardly or downwardly, i.e., rectilinearly moves in the longitudinal direction of the damper cylinder  610 .  
      As described in the damper of the first embodiment, the frictional force between the friction member  630  and the inner wall of the damper cylinder  610  is adjusted by the vertical movement of the second pressure member  642 .  
      Other configurations and operating method of the damper  600   b  are substantially the same as those of the damper of the first embodiment, and a detailed description thereof will thus be omitted because it is considered to be unnecessary.  
      Hereinafter, a damper  600   c  in accordance with the fourth embodiment of the present invention will be described with reference to  FIGS. 11A and 11B .  
      With reference to  FIGS. 11A and 11B , the second pressure member  642  of the damper  600   c  moves by the magnetic force of an electromagnet  945 , similarly to the damper  600  of the first embodiment.  
      More specifically, the damper  600   c  includes the electromagnet  945  provided on the first pressure member  641 , and a first metal member  946  provided on the second pressure member  642 .  
      Here, the electromagnet  945  passes through the first pressure member  641  so that the surface of the upper end of the electromagnet  945  coincides with the surface of the upper end of the first pressure member  641 .  
      The electromagnet  945  includes a core  945   a  installed on a body of the shaft  620 , a bobbin  945   b  installed outside the core  945   a , and a coil (not shown) wound on the bobbin  945   b  several times.  
      Although the core  945   a  of the damper  600   c  of this embodiment is fixed to the upper end of the body of the shaft  620 , the position of the core  945   a  is not limited thereto.  
      The electromagnet  945  further includes a second metal member  945   c  surrounding the outer cylindrical surface of the bobbin  945   b  provided with the coil wound thereon.  
      The second metal member  945   c  uniformly distributes the magnetic force generated when current flows along the coil, and improves the magnetic force of the electromagnet  945 .  
      The first metal member  946  serves to move the second pressure member  642  towards the first pressure member  641  when current flows along the electromagnet  945 .  
      For this reason, preferably, the first metal member  946  is fixed to the second pressure member  642 . More preferably, the first metal member  946  is fixed to the lower part of the second pressure member  642 .  
      In order to guide the vertical movement of the second pressure member  642  according to the supply of current to the electromagnet or cut-off of the supply of current, a guide rod  944 , which passes through the second pressure member  642  so that one end of the guide rod  944  is connected to one end of the electromagnet  945 , is provided.  
      More specifically, the guide rod  944  vertically passes through the central portion of the second pressure member  642  so that the lower end of the guide rod  944  is fixed to the core  945   a  of the electromagnet  945 .  
      In order to smoothly move the second pressure member  642 , preferably, a guide hole, through which the guide rod  944  passes, is formed through the second pressure member  642 . Here, the guide hole has a larger diameter than that of the guide rod  944 .  
      Preferably, a stopper  944   a  for preventing the separation from the second pressure member  642  is formed on the guide rod  944 .  
      The stopper  944   a  is provided on the other end of the guide rod  944 , i.e., the upper end of the guide rod  944 , and has a larger diameter than that of the guide hole.  
      Accordingly, when the supply of current to the electromagnet  945  is cut off, the second pressure member  642  is distant from the first pressure member  641  by force applied to the friction member  630 , and a designated distance between the second pressure member  642  and the first pressure member  641  is maintained by the stopper  944   a . That is, the second pressure member  642  moves upwardly to a designated distance, and is then stopped by the stopper  944   a.    
      Other configurations and operating method of the damper  600   c  are substantially the same as those of the damper of the first embodiment, and a detailed description thereof will thus be omitted because it is considered to be unnecessary.  
      Although the above embodiments illustrate the friction member separated from the inner wall of the damper cylinder in the normally vibrated state of the tub, the position of the friction member is not limited thereto.  
      That is, according to vibration characteristics of an object to be vibrated, the friction member may contact the inner wall of the damper cylinder by a small force in the normally vibrated state of the object rather than in the excessively vibrated state of the object.  
      Effects of the damper of the present invention will be described, as follows.  
      First, the damper of the present invention adjusts the damping force thereof according to the vibration degree of an object to be vibrated and/or the number of rotation of the object, thereby improving a vibration-damping effect and a noise-preventing effect.  
      Second, the damper of the present invention decreases the transmission of the vibration of the object, thereby preventing peripheral components from being damaged.  
      Third, the damper of the present invention has the simple configuration of a pressure unit for pressing a friction member, thereby being easily manufactured.  
      Fourth, the damper of the present invention does not require a power device for reducing the damping force of the damper, thereby reducing production costs.  
      Fifth, the damper of the present invention reduces or eliminates frictional force between a damper cylinder and the friction member in the normally vibrated state of a tub, thereby lengthening the life span of the friction member.  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions.  
      Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.