Abstract:
A wheel device and a chair equipped with the same which do not require replacement of a conventional retractable wheel, but instead enable the wheel to be placed in the operating state at all times, as required. The wheel device comprises a wheel and a holder member for holding the wheel. An urging member is arranged between the wheel and the holder member to constantly urge the wheel to be projected in an operable position. A retraction permitting member is adapted to permit the wheel to be retracted while overcoming the urging force of the urging member when a load is exerted in the direction of gravity on a member on which the wheel is mounted. The retraction permitting member includes a setting mechanism for setting the wheel to be in an un-retractable state. The chair is equipped with the wheel devices mounted to a leg portion thereof.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a wheel device adapted to chairs and a chair equipped with the same. More particularly, the invention relates to a wheel device of the type in which a wheel operates under the no-load condition and retracts under the loaded condition, wherein the wheel is capable of being kept from retracting as required even under the loaded condition, and a chair to which such a wheel device is attached. 
     There has heretofore been placed in the market a chair for school children, in which wheel devices are attached to the lower ends of legs of the chair so that the chair can be easily moved and that the wheels retract relative to the legs when the chair has received the load due to sitting so as to prevent the wheels from working, to thereby enable school children of a relatively lower age to use the chair with safety. 
     The chair of this type features its convenience and has been favorably accepted in the market. As the children who are the users grow up, however, the following demands must be satisfied. For children grown up to the upper grades in school, the chair that moves on wheels while carrying a child sitting on it no longer causes danger, or the chair can be safely used by a child himself or herself. Rather, it is requested that the user be permitted to move within distances of a short range while sitting thereon. To comply with this request, the purchasers buy for their children who have grown up a new chair equipped with universal casters which are operable at all times. 
     In this case, however, it does not mean that the chairs used when the children were in the lower grades in school are no longer usable; i.e., adjustment of the height of the seat level permits the chairs to be used by the children who have grown up. Therefore, simply purchasing the new chairs for convenience is a wasteful expenditure. Attempts have been made to use the chair purchased when the child was of a lower age even after the child has grown up instead of buying a new one. This attempt is commercially accomplished by a chair in which fixed ground pieces and casters are replaceably attached to the lower ends of the legs of the chair so that the fixed ground pieces are attached to the lower ends of the legs for use when a child is in the lower grades in school, and that the fixed ground pieces are removed and, instead, the casters are attached when the child has grown up to the upper grades in school, whereby he or she is allowed to move over narrow distances while sitting thereon. 
     According to this construction, however, the chair is used without the wheel device when the child is in the lower grades in school. Except when the child is sitting, therefore, this chair is inconvenient such as when it is to be moved. In addition, the replacement operation is cumbersome. More than that, the fixed ground pieces are replaced by the casters in many cases after the chair has been used for two or three years. Therefore, the user must store the casters for years and it often happens that the casters are missing by the time when they are to be used. If the casters capable of solving such problems were developed, they can be used not only for the chairs for school children but also for chairs for persons who need care taking, chairs for use in offices and the like. In this respect, therefore it has been urged to positively solve the above-mentioned problems. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of such circumstances. Accordingly, it is an object of the present invention to provide a novel wheel device which enables a wheel to operate at all times as required without replacement of a conventional retractable wheel device, and a novel chair equipped with such a wheel device. 
     In accordance with one aspect of the present invention, a wheel device is provided. The wheel device includes a wheel having an axle, a holder member for holding the wheel, an urging member, arranged between the wheel and the holder member, for constantly urging the wheel to be projected in an operable position, and a retraction permitting member adapted to permit the wheel to be retracted while overcoming the urging force of the urging member when a load is exerted in the direction of gravity on a member on which the wheel is mounted. The retraction permitting member includes a setting mechanism for setting the wheel to be in a retraction blocked state. 
     According to this invention, even a retractable wheel can be used so as to select the state of blocking the retraction in which the wheel is forcibly maintained to work. 
     In a preferred embodiment of the present invention, the holder member includes a bearing that directly supports the axle of the wheel and a casing that supports the bearing. 
     According to this embodiment of the invention, the holder member is constituted of the bearing and the casing for supporting the bearing. Therefore, a conventional most typical design structure can be utilized, and the wheel can be smoothly moved in a vertical direction. 
     In a preferred embodiment of the present invention, the holder member is mounted in the form of a universal caster on the member on which the wheel is mounted. 
     According to this embodiment of the invention, since the holder member is a so-called universal caster, the direction of movement is not limited. 
     In a preferred embodiment of the present invention, the urging member is directly disposed between the bearing and casing or substantially between the bearing and casing while another member is interposed between the bearing and casing. 
     According to this embodiment of the invention, a conventional most typical design structure can also be utilized. 
     In a preferred embodiment of the present invention, the setting mechanism includes a setting piece that acts on the bearing for the wheel so as to set between a state for permitting the retraction of the bearing and a state for blocking the retraction of the bearing, depending on the attitude of the setting piece which is set. 
     According to this embodiment of the invention, even a retractable wheel can be used so as to select the state for blocking the retraction in which the wheel is forcibly maintained to work. 
     In a preferred embodiment of the present invention, the setting piece in the setting mechanism is provided on the casing above the bearing in a manner to be rotatable about a rotational axis perpendicular to the axle, and the setting piece is provided with a retraction permission recess and a retraction blocking protrusion in portions on a side thereof adjacent to the bearing, whereby the setting piece is turned to change over the setting attitude thereof so that either the retraction permission recess or the retraction blocking protrusion is made face the upper side of the bearing. 
     According to this embodiment of invention, such a construction which is a so-called bayonet mechanism permits the retraction blocked state or retraction permitted state of the wheel to be selected by turning the setting piece. 
     In a preferred embodiment of the present invention, the setting mechanism includes a turn range-setting mechanism provided on slide surfaces of the casing and setting piece to more reliably set the retraction permitted state and the retraction blocked state. 
     According to this embodiment of the invention, the retraction permitted state and the retraction blocked state of the bearing can be reliably and easily changed over. 
     In a preferred embodiment of the present invention, the angle for turning the setting piece is 90 degrees. 
     According to this embodiment of the invention, the setting piece is turned 90 degrees in the bayonet-like structure to select the retraction blocked state or the retraction permitted state of the wheel. 
     In a preferred embodiment of the present invention, at least one of the retraction blocking protrusion of the setting piece and the bearing that comes into contact with the retraction blocking protrusion is provided with a change-over guide portion so that the setting piece is allowed to smoothly turn even in a state where the bearing is slightly entering into the retraction permission recess of the setting piece. 
     According to this embodiment of the invention, the setting piece can be smoothly turned even in a state where the wheel is sinking to some extent due to the own weight of the chair, i.e., even in a state-where the axle is entering to some extent into the retraction permission recess. 
     In a preferred embodiment of the present invention, a bearing surface and a protuberance are formed on the change-over guide portion of the bearing for stably supporting the urging member. 
     According to this embodiment of the invention, the urging member can be stably supported even when the urging member has moved to some extent with the turn of the setting piece. 
     In a preferred embodiment of the present invention, the setting piece includes a shift piece mounted on an upper end thereof at a position above the casing. 
     According to this embodiment of the invention, the retraction permitted state and the retraction blocked state of the wheel can be selectively and easily changed over by operating the shift piece. 
     In accordance with another aspect of the present invention, a chair is provided. The chair includes a seat portion, a leg portion and wheel devices constructed as described above and mounted to a lower end of the leg portion. The wheel devices are set to be in an operating state under the no-load condition, and are selectively set to be in either a non-operating state or an operating state under the loaded condition. 
     When the chair having such construction is used by a school child in the lower grades in school for whom safety must be maintained while he or she is sitting on it, the wheels are retracted so as to prevent the chair from being inadvertently moved. When the child grows up to the upper grades in school and the chair that moves to some extent may no longer cause danger, the chair may be so used that the wheels are maintained to work at all times. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a perspective view showing an embodiment of a chair equipped with wheel devices according to the present invention; 
     FIG. 1B is a perspective view showing an embodiment of the wheel device of the present invention; 
     FIG. 2 is an exploded perspective view illustrating the wheel device shown in FIG. 1B; 
     FIG. 3A is a perspective view illustrating, in cross section, some parts of the wheel device in a disassembled manner, when the wheel device is in the retraction permitted state; 
     FIG. 3B is a perspective view illustrating the wheel device in the retraction blocked state; 
     FIG. 4 is a vertical sectional view showing the wheel device when it is viewed from the front central direction; 
     FIG. 5 is a vertical sectional view showing the wheel device; 
     FIG. 6 is a perspective view illustrating another embodiment of the wheel device according to the present invention, which wheel device is of the router hole-fitted type; 
     FIG. 7 is a perspective view illustrating a further embodiment of the wheel device according to the present invention, which wheel device is of the side-attached type; 
     FIG. 8 is a vertical sectional view illustrating a still further embodiment of the wheel device according to the present invention, in which wheel device the wheel swings; 
     FIG. 9 is a perspective view illustrating the appearance wheel device shown in FIG. 8; 
     FIG. 10 is a vertical sectional view illustrating a yet further embodiment of the wheel device according to the present invention, which wheel device includes the structure which vertically moves along a caster pin; 
     FIG. 11 is a perspective view illustrating the appearance of the wheel device shown in FIG. 10; 
     FIG. 12A is a perspective view illustrating a further embodiment of the wheel device according to the present invention; 
     FIG. 12B is a partly enlarged elevational view in section, showing the wheel device of FIG. 12A; 
     FIG. 12C is a perspective view illustrating the appearance of the chair in which the wheel devices of FIG. 12A are incorporated; 
     FIG. 13 is a perspective view illustrating a setting mechanism in another embodiment of the wheel device according to the present invention; 
     FIG. 14 is a vertical sectional view illustrating a setting mechanism in a further embodiment of the wheel device according to the present invention; 
     FIGS. 15A to  15 C are vertical sectional views illustrating a setting mechanism in a still further embodiment of the wheel device according to the present invention; 
     FIGS. 16A to  16 C are diagrams illustrating the state of a change-over guide portion in the setting mechanism in the wheel device of the present invention; 
     FIG. 17 is a perspective view illustrating the wheel device equipped with a wheel cover; 
     FIG. 18A is an exploded perspective view illustrating an embodiment of a slide setting mechanism for changing over the retraction permitted state and the retraction blocked state by sliding a setting piece; 
     FIG. 18B is a sectional view showing an essential part of the slide setting mechanism of FIG. 18A; and 
     FIGS. 19A and 19B each are a diagram illustrating another embodiment of the wheel device which directly prevents the turn of the wheel in the retracted state. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described by way of diagramed embodiments with reference to FIG. 1A to FIG. 19B, wherein like reference numerals designate like or corresponding parts throughout. 
     FIG. 1A illustrates an embodiment of a chair  2  equipped with wheel devices  1  according to the present invention. It will be noted that the member on which the wheels are mounted referred to in the claims stands for the chair  2  as a whole or a leg portion  21  of the chair  2  to which the wheels are directly attached. More particularly, the chair  2  is mainly constituted of a seat portion  20  and the leg portion  21 , the seat portion  20  being mounted on the leg portion so as to freely rotate with respect to the leg portion  21 . The leg portion  21  includes a post  22  that rises vertically to directly support the seat portion  20 , and, for example, five extended legs  23  on the lower part thereof. The wheel devices  1  of one embodiment of the present invention are attached, for example, in the form of universal casters to the lower surfaces at the ends of the extended legs  23 . 
     The most preferred embodiment of the wheel device of the present invention will now be described with reference to FIGS. 1B to  5 . As described above, the wheel device  1  is of the type of a universal caster, and the wheel  11  is of the type of double wheels attached on both sides with respect to a casing  10 . First, the casing  10  is formed by injection-molding a synthetic resin or the like and is a narrow and flat hollow member. The holder member for the wheels  11  referred to in the claims substantially stands for the casing  10  or the one that includes the casing  10  and a bearing  13  that will be described later or a holder member shown in FIGS. 12A to  12 C merged with some of the leg portion  21  as will be described later. 
     The casing  10  has a caster pin  101  vertically raised at a position near one end thereof in the back-and-forth direction. To meet the requirement of strength, it is desirable that the caster pin  101  have a metal member press fitted therein. More specifically, the caster pin  101  has a flange portion  101 A at the central portion thereof, a casing insertion portion  101 B formed in the lower portion thereof, and a chair-side insertion portion in the upper portion thereof. 
     The casing  10  is substantially hollow on the inside thereof to form a bearing-receiving portion  102 , and has slit-like axle guide holes  103  formed in the sides thereof in the vertical direction. The wheels  11  are supported by the casing  10 . More particularly, the wheels  11  are supported by the casing  10  through an axle  12  penetrating through both of the right and left wheels  11  and a bearing  13  supporting the axle  12  at the central portion thereof. The bearing  13  has nearly a rectangular shape in cross section as viewed from the side, and has sectors each having a center angle of 90 degrees and being laterally expanded from opposite sides of the central circular portion as viewed on a plane. That is, the bearing  13  has clearance portions  131  for retraction along the side peripheries thereof, and has extended portions  132  for blocking the retraction. Corresponding to the constitution of the slit-like axle guide holes  103 , the bearing  13  together with the wheels  11  is attached to the casing  10  so as to move up and down. In the rotation permitted state of the wheels  11 , the wheels  11  and the axle  12  are integral together, whereas the rotation is permitted between the axle  12  and the bearing  13 . 
     A skid piece  14  is fitted in the casing  10  under the bearing  13 . The skid piece  14  prevents the end of the casing  10  and the like from coming into direct contact with the floor surface or the like when the wheels  11  are retracted, and enables smooth slide. The skid piece  14  has a smooth lower surface formed to have a partial spherical shape serving as a skid portion  141 , and has an engaging portion  142  at an upper portion thereof so as to be fitted in the casing  10  while keeping away from the bearing  13 . 
     A setting spring  15  is provided above the axle  12  and constitutes the urging member that is referred to in the claims to constantly urge the bearing  13  downward. The setting spring  15  in this embodiment is a coil spring which comes at its lower end in contact with the upper surface of the bearing  13  and is supported at its upper end by the casing  10  serving as a fixing member. In this embodiment, however, the setting spring  15  is not in direct contact with the casing  10 , but instead is in contact with a setting piece  170  in a setting mechanism  17  which is one of the major structures of the present invention. As will be described later, the setting piece  170  is turned. If the end of the setting spring  15  is left as it is cut, a sharp edge thereof may bite into the setting piece  170  when it is turned, leading to impairing the smooth turn. It is therefore desirable that at least the end (upper end) of the setting spring  15  on the side adjacent to the setting piece  170  be bent inward and downward so as not to create the above-mentioned problem. It is further considered that with the turn of the setting piece  170 , the setting spring  15  may laterally move to some extent. It is therefore desirable that a protuberance  13   a  be also formed on the upper surface of the bearing  13  to restrict the position of the setting spring  15  and to stably hold it. 
     The setting mechanism  17  will now be described in detail. The setting mechanism  17  includes a setting piece  170  mounted in the casing  10  above the bearing  13  in a manner to be rotatable about a rotational axis extending in a vertical direction. The setting piece  170  is provided with a spring-receiving portion  171  for receiving the setting spring  15  formed in the central portion thereof, and the lower portion thereof is formed in the shape of a fork so that the setting piece  170  further has retraction permission recesses  172  which are cut-away portions and retraction blocking protrusions  173  having a relatively protruded shape. The retraction permission recesses  172  and the retraction blocking protrusions  173  are alternately formed at angular pitches of 90 degrees in the direction of turn of the setting piece  170 . 
     The rotational angle of 90 degrees was selected to be the optimum angle by taking into consideration easy use and shapes of parts. However, what is important is that in setting the retraction blocking state and the retraction permission state of the wheels  11 , the above two states can be set by turning the setting piece  170  in a certain angle in relation to the bearing  13 . Therefore, the design can be suitably changed to select the two states by a 45 degrees turn or to select the two states by a 180 degrees turn, etc. The retraction permission recesses  172  have the size large enough for the bearing  13  to be retracted. 
     It is desirable that the following constitution be employed between the retraction blocking protrusions  173  in the setting piece  170  and the retraction blocking extended portions  132  in the bearing  13 . Though the operation state between the two will be described later, the setting spring  15  in the wheel device  1  is slightly compressed in a state of receiving the own weight of the chair  2  and, hence, the bearing  13  slightly enters into the retraction permission recesses  172  of the setting piece  170 . Even when it is attempted to turn the setting piece  170  in this case, the turn of the setting piece  170  is blocked by the bearing  13 . As shown in FIGS. 16A to  16 C, therefore, the corners of each retraction blocking protrusion  173  of the setting piece  170  and the corners of each extended portion  132  of the bearing  13  which comes into contact with the retraction blocking protrusion  173  are rounded and are used as a change-over guide portion  136  in the bearing  13  and as a change-over guide portion  176  in the setting piece  170 . The change-over guide portions  136  and  176  may be formed on either the bearing  13  or the setting piece  170 , or may be formed on both of them as in the illustrated embodiment. When the smooth rounded change-over guide portion  136  is formed on the bearing  13 , the contacting surface for the setting spring  15  is also rounded. It is therefore desirable that a nearly horizontal surface  13   b  be positively provided around the periphery of the protuberance  13   a  as shown on an enlarged scale in FIG. 16C in order to more stably support the setting spring  15 . 
     The upper end of the setting piece  170  is arranged so as to be exposed through an opening at the upper end of the casing  10 , and a shift piece  174  is attached to the upper end thereof. As will be described later in detail, the shift piece  174  functions to select the retraction permission state and the retraction blocking state upon changing the attitude of the setting piece  170  by 90 degrees. Change-over indications  104  for indicating such positions are formed on the casing  10 . In this embodiment, a change-over indication  104 A or “UNLOCK” indicates the retraction permission state and a change-over indication  104 B or “LOCK” indicates the retraction blocking state. It is possible to employ other suitable indications, for example, the change-over indication  104 A as “OFF” to represent the retraction permission state and the change-over indication  104 B as “ON” to represent the retraction blocking state. Instead of providing the above-mentioned shift piece  174 , a shift groove  174   b  such as an engagement groove for a plus or minus screw driver D, a groove for a coin (see FIGS. 6 and 15A to  15 C), or the like may be formed in the setting piece  170  at a position where it is directly exposed to outside, so that the setting piece  170  can be turned. 
     In operating the shift piece  174 , further, it is desirable that a turn range-setting mechanism  18  be formed on the setting piece  170  and on the casing  10 , so that the position of the change-over indication  104 A or  104 B is correctly selected, and that the shift piece  174  is not set at an intermediate position. As an example, the turn range-setting mechanism  18  is formed on both slide surfaces of the setting piece  170  and the casing  10  as shown in FIG.  2 . More specifically, the range-setting mechanism  18  includes a turn range-setting protuberance  18 A formed on the slide portion of the setting piece  170  and a turn range-setting recess  18 B formed by cutting in an arcuate form about one-fourth turn so that the turn range-setting protuberance  18 A is allowed to turn in the casing  10  through an angular range θ, i.e., only 90 degrees, to accomplish the reliable change-over operation. Alternatively, a position guide mechanism such as click-stop mechanism or the like may be provided between the shift piece  174  and the casing  10 . 
     The above-mentioned embodiment of FIGS. 1A to  5  has dealt with the wheel device of the type of double wheels having wheels  11  attached to both sides of the bearing  13 . The wheel device, however, may be of the single wheel type having a single wheel  11  attached to the central portion of the bearing  13 . The wheel  11  is a narrow cylindrical member that comes in line contact with the floor surface. Not being limited thereto only, however, the wheel  11  may be a ball member that comes in a point contact. 
     The wheel device  1  according to the preferred embodiment of the present invention has construction as concretely described above. To attach the wheel devices  1  to the chair  2  which is a member on which the wheel devices  1  are to be mounted, the caster pin  101  is inserted in the lower end of the extended leg  23  of the chair  2  so as to be secured therein. When the chair  2  is used in this state, the wheel device  1  works in a manner as described below. 
     (1) Use in the Retraction Permission State 
     In this case, first, an end of the shift piece  174  attached to the setting piece  170  constituting the setting mechanism  17  is set to the unlocking side of the change-over indication  104 , i.e., set to the change-over indication  104 A which is “UNLOCK”. Then, as shown in FIG. 3A, the retraction permission recesses  172  in the setting piece  170  are set in a direction along the axle  12 , whereas the retraction blocking protrusions  173  are located at positions deviated from the bearing  13  in the back-and-forth direction with the bearing  13  sandwiched therebetween. In such a state, the bearing  13  supporting the axle  12  is allowed to retract upward against the setting spring  15  since a space sufficient for retraction has been maintained above the bearing  13 . When nobody is sitting on the chair  2 , therefore, the wheels  11  are urged downward by the setting spring  15 , the whole chair  2  is permitted to be moved while the wheels  11  are in contact with the floor. When a person sits on the chair, the wheels  11  are deviated upward against the setting spring  15  due to the load from the upper side. Accordingly, the skid piece  14  comes in contact with the floor together with the wheels  11 . 
     (2) Setting to the Non-retracting State 
     Next, to set to the non-retracting state, the shift piece  174  is turned 90 degrees to the “LOCK” ( 104 B) in the change-over indication  104 . Due to this operation, the setting piece  170  integral with the shift piece  174  changes its attitude by 90 degrees, and the retraction blocking protrusions  173  of the setting piece  170  come in contact with just the top of the upper portion of the bearing  13 . Therefore, the bearing  13  is blocked by the retraction blocking protrusions  173  from moving upward, and the wheels  11  are set to be maintained in a protruded state at all times. Then, irrespective of whether a person is sitting on the chair  2  or not, the wheels  11  are maintained protruded so as to work (see FIG.  3 B). 
     (3) Action of the Change-over Guide Portion 
     In turning the setting piece  170 , provision of the change-over guide portions  136  and  176  makes it possible to smoothly turn the setting piece  170  even in a state where the wheels  11  are slightly retracted due to, for example, the own weight of the chair  2  or even in a state where the bearing  13  is entering to some extent in the retraction permission recesses  172  of the setting piece  170 . That is, despite the bearing  13  is entering to some extent into the retraction permission recesses  172  in the setting piece  170 , the smooth rounded surfaces of the change-over guide portions  136  and  176  permit the change-over guide portions  136  and  176  to be abut against each other, to thereby enable them to smoothly turn as shown in FIGS. 16A to  16 C. In addition, upon forming the substantially horizontal bearing surface  13   b  and the protuberance  13   a  on the change-over guide portion  136  of the bearing  13 , the setting spring  15  is supported more stably even when the setting spring  15  is moved to some extent with the turn of the setting piece  170 . 
     In the foregoing was described the construction of the wheel device  1  and the chair  2  equipped with the same according to a preferred embodiment of the invention. The invention, however, can be realized in manners of the following embodiments. 
     (1) Wheel Device 
     FIG. 6 shows a router hole-fitted type fixed caster to which the present invention is adapted. For instance, a fitting groove is formed by a router in a leg portion  21  or the like of a wooden chair  2 , and a wheel device is fitted therein. In this case, as schematically shown, a setting mechanism (not shown) which is similar to that of the above described embodiment is provided between a casing  10  and a bearing (not shown) so that a wheel  11  is set not to retract at all times. In this instance, for example, when a setting piece (not shown) constituting the setting mechanism is to be turned from the upper side, an operation hole  4  is pierced in the leg portion  21  of the chair, and a screw driver D or the like is fitted into the shift groove  174   b  in the shift piece  174  connected to the setting piece to turn it. 
     According to an embodiment shown in FIG. 7, the present invention is adapted to a caster of the so-called side-attached type. Though not described in detail, a setting mechanism (not shown) can be provided between a casing  10  and a bearing (not shown) supporting an axle (not shown) of a wheel  11  like in the above-mentioned embodiment. In the wheel device of the side-attached type and in the wheel device  1  of the above-mentioned first embodiment (shown in FIGS. 1A to  5 ), the setting piece  170  of the setting mechanism  17  may be turned by using the shift piece  174  extending sideward as shown in FIG.  7 . Though the setting mechanism is not shown in FIGS. 6 and 7, the one concretely illustrated with reference to FIGS. 1A to  5  or the one shown in FIG. 8 that will be described later, can be used for the embodiments of FIGS. 6 and 7. 
     Moreover, in a wheel device  1  of the universal caster type shown in, for example, FIGS. 8 and 9, when a wheel  11  is supported by a swing arm-like bearing  13 , a setting mechanism  17  may be provided above the bearing  13  at its free end. FIGS. 10 and 11 illustrate a wheel device  1  of the universal caster type in which a setting spring  15  for vertically moving a wheel  11  is provided around a caster pin  101 . In this case, a setting piece  170  of a setting mechanism  17  is turned like a bayonet to select the retraction permitted state and the retraction blocked state. 
     In the above-mentioned embodiments, the setting spring  15  acting as the urging member is in the form of a coil spring and is set right above the wheel or wheels  11 , and is kept in contact with the bearing  13 . However, the setting spring  15  serving as the urging member may be a Leaf spring in order to directly support the axle  12  as shown in FIGS. 12A to  12 C. In this embodiment, the holder member for holding the wheel  11  stands for the leg portion  21  of the chair  2 , since the leg portion  21  exhibits the action of holding. In this embodiment, the setting mechanism  17  of the above-mentioned type can be used though this embodiment which utilizes engagement of the form of an inverted pawl which is another type. 
     In an embodiment shown in FIG. 17, the wheels  11  are covered by neither the casing  10  nor the leg portion  21  of the chair  2  but are exposed to outside. In this case, a wheel cover  10   a  is provided to extend substantially halfway around the wheels  11  in the casing  10 . When a person who sits is a young child, this construction effectively prevents the cloths or toes from being inadvertently caught by the wheels  11 . 
     (2) Setting Mechanism 
     The setting mechanism can be realized in other forms. In the above-mentioned embodiment, the setting piece  170  acts like the so-called bayonet to select the retraction permitted state or the retraction blocked state. In effect, the wheel(s)  11  may be blocked from being retracted upward. Therefore, various other embodiments can be employed in addition to the embodiments described above. That is, the setting mechanism is shown in FIG. 13 is relatively simple, wherein a setting piece  170  which is a pin for blocking the swinging of a bearing  13  is inserted like a bar through a hole  105  in the side of a casing  10  to prevent the retraction. Such a mechanism can be adapted to the fixed caster of the so-called side-attached type shown in FIG. 7 as well as to the casters of any other type. An embodiment shown in FIG. 14 employs a cam which is used as a setting piece  170 . The protruded portion or apex of the cam serves as the retraction blocking protrusion  173  and the land portion other than the apex of the cam serves as the retraction permitting recess  172 . 
     Further, described below is the setting mechanism  17  of the embodiment in which the axle  12  is directly supported by the setting spring  15  as shown in FIG.  12  and described above. In this embodiment, a setting piece  170  is inserted in the leg portion  21  of the chair  2  from the upper side, which leg portion  21  serves as a holder member. Then, as shown on an enlarged scale in FIG. 12B, a retraction permission recess  172   a  of the shape of an inverted pawl and a retraction block protrusion  173   a  are formed in two steps in the vertical direction on the inner side wall of the leg portion  21  that serves as the holder member. A pair of engaging pawls  175  of the shape of an inverted pawl are formed on the opposite side portions of the setting piece  170 , so that the depressed position of the setting piece  170  can be set in two upper and lower stages. 
     In a state where the setting piece  170  is pushed down, the lower surface of the setting piece  170  urges, from the upper side, the setting spring  15  positioned right on the axle  12 , so that the setting piece  170  stays at the position where the engaging pawls  175  and the retraction blocking protrusions  173   a  are brought into engagement together, whereby the wheel  11  is forcibly set in a state of being protruded. To unlock this state, shift buttons  174   a  are depressed to release the engagement between the engaging pawls  175  and the retraction blocking protrusions  173   a  so as to pull the setting piece  170  up. 
     In the embodiment of FIGS. 1A to  5  described above, the setting piece  170  is turned a predetermined angle to select the retraction permission state or the retraction blocking state. According to the setting mechanism  17  of an embodiment shown in FIGS. 18A and 18B, a setting piece  170  is slid back and forth a predetermined distance. That is, a bearing  13  for directly supporting an axle  12  is formed in a crossing manner as viewed on a plane. Correspondingly, a reception groove  170   a  is formed in the setting piece  170  so that the bearing  13  can be fitted therein, whereby the setting piece  170  is slid to select the retraction permitted state or the retraction blocked state. In this case, the position at which the receiving groove  170   a  of the setting piece  170  is matched with the bearing  13 , creates the retraction permitted state and the position at which they are deviated relative to each other creates the retraction blocked state. 
     To slide the setting piece  170 , a so-called knock-type slide-setting mechanism  30  is used. As shown, for example, in FIGS. 18A and 18B, this mechanism  30  includes a guide portion  31  that is fixed to a casing (not shown) on the operation side of the setting piece  170 , a position-setting piece  32  fitted into the guide portion  31 , a knock piece  33  for pushing the position-setting piece  32 , a push button  34  for directly pushing the knock piece  33 , and a return spring  35  for urging the setting piece  170  at all times on the non-operation side of the setting piece  170 . The guide portion  31  is hollow, has splines  36  formed in the inner peripheral surface thereof, and has a receiving portion  37  formed on the surface on the non-operation side, i.e., on the fore side in the depressing direction, wherein every other splines  36  having dissimilar lengths. The position-setting piece  32  is fitted in the guide portion  31  so as to turn a predetermined angle only when it is disengaged from the splines  36 . The position-setting piece  32  is rotatably supported by a rod  38  or the like so as to maintain a predetermined distance from the setting piece  170 . The position-setting piece  32  has, for example, four engaging protrusions  39  formed on the rear side in the depressing direction to come into engagement with the receiving portion  37 . Each of the engaging protrusions  39  has an inclined surface  39   a  inclined in one direction at an end thereof on the operation side. The knock piece  33  is mounted so as to cover the position-setting piece  32 , and has pushing portions  40  which have sharp ends and which are formed on the peripheral surface on the fore side in the depressing direction so as to push the engaging protrusions  39 . 
     Such a knock-type slide-setting mechanism has been widely employed for a variety of switches and writing utensils, and the manner of operation thereof will be briefly described below. First, when the push button  34  is depressed, the pushing portions  40  of the knock piece  33  come into contact with the respective inclined surfaces  39   a  of the engaging protrusions  39  of the position-setting piece  32 . As the push button  34  is further depressed, the engaging protrusions  39  are guided by the splines  36  formed in the inner peripheral surface of the guide portion  31 , and the position setting piece  32  moves in the depressing direction. When the ends of the splines  36  are reached, the engagement is no longer maintained. As the push button  34  is further depressed, the position-setting piece  32  is turned the predetermined angle along the inclined surfaces  39   a  of the engaging protrusions  39  and is slightly moved in the depressing direction. The rotational angle in this case is set to a pitch of the splines  36 . Due to the force of the return spring  35 , the engaging protrusions  39  come into contact with the receiving portion  37  on the next splines  36 . After every depression of the push button  34 , the position-setting piece  32  is turned by one pitch of splines  36 . The setting piece  170  slides back and forth depending on the length of the receiving portions  37  of the splines  36 , and comes into a halt at a respective one of positions in the back-and-forth direction. 
     The setting mechanism  17  may be further constituted in a manner as shown in FIGS. 15A to  15 C. The feature in this case is that the setting mechanism  17  can be operated from the lower side of the casing  10 . That is, the bearing  13  of the wheels  11  is provided with a pair of extended portions  132  formed to oppositely protrude therefrom and a pair of clearance portions  131  formed at positions where the extended portions  132  do not exist. On the other hand, the setting piece  170  is rotatably attached to, for example, the skid piece  14  under the bearing  13  by using a clip C or the like. On the upper part of the setting piece  170  are formed the retraction blocking protrusions  173  extending like pawls facing inward to each other so as to engage with the extended portions  132  of the bearing  13 . Further, the portions where the retraction block protrusions  173  do not exist are used as retraction permission recesses  172 . The state where the wheels  11  accompanying the axle  12  can be retracted upward or not, is set depending upon whether the extended portions  132  of the bearing  13  are engaged with the retraction blocking protrusions  173  of the setting piece  170 . In this embodiment, the setting piece  170  is turned by using a shift groove  174   b  in which a coin can be inserted. 
     (3) Retracted State 
     Described below is a further embodiment which permits the wheel device  1  to be retracted. In the above-mentioned embodiment of FIGS. 1A to  5 , the retracted state of the wheel device is set so that the wheel device  1  is retracted against the urging member such as the setting spring  15  or the like and the wheels  11  are allowed to freely rotate. It is, however, also allowable to employ a known structure disclosed in, for example, Japanese Utility Model Application Laid-Open Publication No. 171703/1983 entitled “Wheel Device for Legs of Chair”. That is, as shown in FIG. 19A, a gear G is formed on the inside of the wheel  11 , whereas a rack R is formed near the upper end of an axle guide hole  103  of a casing  10  to mesh with the gear G. In the retracted state, the gear G is meshed with the rack R to directly prevent the turn of the wheel  11 . Alternatively, a stopper mechanism may be such that the gear G is formed on the whole inner circumference of the wheel  11  and the rack R is formed near the lower end of the axle guide hole  103  of the casing  10  to mesh with the gear G as shown in FIG.  19 B. 
     (4) Chair 
     The chair  2  of the present invention is equipped with the wheel devices  1  described above with reference to FIGS. 1A to  5 , or with the wheel devices  1  according to other embodiments described above with reference to FIGS. 6 to  19 B. 
     According to the present invention which has the technical features as described above, even the retractable wheel device can be set to the retracted state or to the retraction blocking state. Further, the chair equipped with the wheel devices according to the present invention meets a wide range of uses or demands as well as ensures safety. 
     While preferred embodiments of the present invention have been described with a certain degree of particularity with reference to the drawings, obvious modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.