BACKGROUND ART Generally, a conventional power transmission device has employed a one-way clutch for transmitting the rotational force of an input shaft to an output shaft. An example of the one-way clutch is disclosed in detail in U. S, Pat. No. 4,162,000. However, such a conventional power transmission device can transmit the rotational force of the input shaft only in one direction to the output shaft but not the rotational force in the other direction. In addition, an undesired power transmission from the output shaft to the input shaft, which may cause damages to a power source, frequently occurs.

DISCLOSURE OF THE INVENTION It is, therefore, a primary object of the invention to provide a power transmission device which is capable of transmitting the rotational force of an input shaft in both directions to an output shaft and preventing the reverse transmission.

It is another object of the invention is to keep the rotation speed of the output shaft same as that of the input shaft.

In accordance with an aspect of the present

invention, there is provided a power transmission device comprising: a stationary housing rotatably supporting an input and an output shafts; a cylindrical member having a circumferential protrusion, the cylindrical member being coupled to the output shaft and connected to the input shaft in such a manner that the cylindrical member rotating relative to the output shaft axially moves; a first and a second one-way clutches fixed to an inner surface of the stationary housing in such a manner that their inner races are rotatable in opposite directions and the circumferential protrusion of the cylindrical member is located therebetween, whereby when the input shaft rotates in the direction same as the rotation direction of the first inner ring the cylindrical member is axially moved to come in a friction contact with the first inner ring so that the rotational force of the input shaft is transmitted to the output shaft through the cylindrical member, and when the input shaft rotates in the direction same as the rotation direction of the second inner ring the cylindrical member is axially moved to come in a friction contact with the second inner ring so that the rotational force of the input shaft is transmitted to the output shaft.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which: Fig. 1 shows a cross sectional view of a power transmission device in accordance with a preferred embodiment of the present invention; Fig. 2 illustrates a cross sectional view of a power transmission device in accordance with another preferred

embodiment of the present invention; Fig. 3 represents a cross sectional view of a power transmission device in accordance with further another preferred embodiment of the present invention; and Fig. 4 presents a cross sectional view of the outer race and the first inner race in Fig. 3.

MODES OF CARRING OUT THE INVENTION There is shown in Fig. 1 a sectional view of a power transmission device in accordance with a preferred embodiment of the present invention.

The power transmission device of the present invention comprises a stationary housing 10 rotatably supporting an input and an output shafts 20,30 through a plurality of bearings B, a first and a second one-way clutches 40,50 and a cylindrical member 70 having a circumferential protrusion 71.

The input and the output shafts 20,30 are disposed coaxially. The output shaft 30 and the cylindrical member 70 are provided with a male and a female threaded portions 32,72, respectively, and they are thread-coupled together through the threaded portions 32,72.

Tightly fixed on the input shaft 20 is a coupling member 80 having a flange 83 with an opening 84. The cylindrical member 70 has an axially extending portion 73 passing through the opening 84 of the flange 83 of the coupling member 80. Therefore, the cylindrical member 70 rotates with the input shaft 20 and axially moves relative to the output shaft 30.

The first and the second one-way clutches 40,50 are fixed apart from each other on an inner surface of the stationary housing 10 in such a manner that their inner races are rotatable in opposite directions and the circumferential protrusion 71 of the cylindrical member 70

is located therebetween. That is, the inner race of the first one-way clutch 40 is rotatable only in one direction (referred to"normal direction"hereinafter), and the inner race of the second one-way clutch 50 is rotatable only in the other direction (referred to"reverse direction"hereinafter).

When the input shaft 20 rotates in the normal direction, the cylindrical member 70 axially moves to come in a friction contact with the inner race of the first one-way clutch 40 so that the rotational force of the input shaft 20 is transmitted to the output shaft 30 through the cylindrical member 70, and when the input shaft 20 rotates in the reverse direction, the cylindrical member 70 axially moves to come in a friction contact with the inner race of the second one-way clutch 50 so that the rotational force of the input shaft 20 is transmitted to the output shaft 30.

On the other hand, if the output shaft 30 unintendedly rotates in the normal direction while the input shaft is stationary, the cylindrical member 70 moves toward and comes in a friction contact with the inner race of the second one-way clutch 50, thereby preventing the rotational force of the output shaft 30 from being transmitted to the input shaft 20. Furthermore, if the output shaft 30 unintendedly rotates in the reverse direction, the cylindrical member 70 moves toward and comes in a friction contact with the inner race of the first one-way clutch 40, thereby preventing the rotational force of the output shaft 30 from being transmitted to the input shaft 20.

In addition, if the output shaft 30 rotates faster than the input shaft in the normal or reverse direction due to an external factor, the cylindrical member 70 moves toward and comes in a friction contact with the inner race of the second or first one-way clutch 50,40, thereby

reducing the rotation speed of the output shaft 30 and keeping it same as that of the input shaft 20.

In order to prevent the slip between the cylindrical member 70 and the inner race of the first and the second one-way clutches 40,50, a material such as a felt, which has a great friction coefficient, may be attached to the side surfaces of the circumferential protrusion 71 of the cylindrical member 70 and/or the inner races of the first and the second one-way clutches 40,50. Alternatively, the circumferential protrusion 71 and the inner races of the first and the second one-way clutches 40,50 may have complementary serrations on their facing surfaces.

There is shown in Fig. 2 a power transmission device in accordance with another preferred embodiment of the present invention. The second preferred embodiment is similar to the first except for a coupling member 180 having an axially extending portion 181 and a cylindrical member 170 having a flange 173 with an opening 174 through which the axially extending portion 181 passes.

Furthermore, there is shown in Fig. 3 a power transmission device in accordance with the third preferred embodiment of the present invention. The third preferred embodiment comprises a stationary housing 310 rotatably supporting an input and an output shafts 320,330 through a plurality of bearings B, an outer race 390, a first and a second inner races 395,396 and a cylindrical member 370 having a circumferential protrusion 371.

The input and the output shafts 320,330 are disposed coaxially. The output shaft 330 and the cylindrical member 370 are provided with a male and a female threaded portions 332,372, respectively, and they are thread- coupled together through the threaded portions 332,372.

The output shaft 330 has a flange portion 331.

The input shaft 320 has an inwardly protruding portion 321 with an opening. The cylindrical member 370

has an axially extending portion 373 passing through the opening of the inwardly protruding portion 321 of the input shaft 320. Therefore, the cylindrical member 370 rotates with the input shaft 320 and axially moves relative to the output shaft 330.

The first and the second inner races 395,396 are snugly fixed an outer surface of the cylindrical member 370 in such a manner that their inner races are rotatable in opposite directions and the circumferential protrusion 371 of the cylindrical member 370 is located therebetween.

That is, the first inner race 395 is rotatable only in the normal direction and the second inner race 396 is rotatable only in the reverse direction with respect to the outer race 390, as shown in Fig. 4.

When the input shaft 320 rotates in the normal direction, the cylindrical member 370 axially moves and presses the first inner race 395 to come in a friction contact with the input shaft 320 so that the rotational force of the input shaft 320 is transmitted to the output shaft 330 through the cylindrical member 370, and when the input shaft 320 rotates in the reverse direction, the cylindrical member 370 axially moves and presses the second inner race 396 to come in a friction contact with the flange portion 331 of the output shaft 330 so that the rotational force of the input shaft 320 is transmitted to the output shaft 330.

On the other hand, if the output shaft 330 unintendedly rotates in the normal direction while the input shaft is stationary, the cylindrical member 370 moves and presses the second inner race 396 to come in a friction contact with the flange portion 331 of the output shaft 330, thereby braking the output shaft 330 and preventing the rotational force thereof from being transmitted to the input shaft 320. Furthermore, if the output shaft 330 unintendedly rotates in the reverse

direction, the cylindrical member 370 moves and presses the first inner race 395 to come in a friction contact with the input shaft 320, thereby preventing the rotational force of the output shaft 330 from being transmitted to the input shaft 320.

In addition, if the output shaft 330 rotates faster than the input shaft in the normal or reverse direction due to an external factor, the cylindrical member 370 axially moves to come in a friction contact with the second or first inner race 396 or 395, thereby reducing the rotation speed of the output shaft 330 and keeping it same as that of the input shaft 320.

In order to prevent the slip between the cylindrical member 370, the first and the second inner races 395,396 and the input and the output shafts 320,330, a material such as a felt, which has a great friction coefficient, may be attached to the side surfaces of the circumferential protrusion 371 of the cylindrical member 370 and/or both side surfaces of the first and the second inner races 395,396.

According to the power transmission device of the present invention, the rotational force of the input shaft in both directions can be transmitted to the output shaft, and the undesired transmission from the output shaft to the input shaft is prevented. In addition, the rotation speed of the output shaft can be kept same as that of the input shaft.

Although the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.