1.  Field of the Invention
The present invention relates to a hinge apparatus for a mobile terminal, including a cellular phone, a smart phone.  More particularly, the present invention relates to a free-stop hinge apparatus for a folding-type mobile terminal for providing a stopping force when the folder is closed and opened. 
2.  Description of the Related Art
Generally, a mobile terminal is a broad term that includes devices such as a notebook computer, a PDA, a radiotelegraph, an MP3 player, a PMP, a cellular phone, a smart phone, and the like.  From among such mobile terminals, a cellular phone or a smart phone, in order to be commercially successful, has to have a light-weight and be small-sized for the convenience of carrying.  Especially, as a multimedia environment has been grown, items such as image communication, games, customized videos, TV-watching, and the like as well as simple voice communication or short message transmission, are now fairly common for most new mobile terminals.  Thus, mobile terminals have been developed into devices in which multimedia contents can be conveniently watched in more modes. 
Commonly used mobile terminals mainly include bar-type, folding-type, and sliding-type mobile terminals.  In particular, the folding-type mobile terminal is mounted with a hinge apparatus for rotatably connecting a main body to a folder.  A hinge apparatus employed in a folding-type mobile terminal, such as a cellular phone, is a semi-automatic hinge apparatus, and includes a hinge shaft, a hinge cam, a hinge spring, and a housing for receiving these components. 
In the above mentioned semi-automatic hinge apparatus, a main body is rotatably connected to a folder by the hinge apparatus.  Thus, the hinge apparatus provides a driving force for semi-automatically opening/closing the folder.  In other words, the semi-automatic hinge apparatus provides to the main body: (1) a force for closing the folder toward the main body within a range of about 0˜10°; (2) a force for opening the folder within a range of about 10˜170°; and (3) a force for stopping the folder within about 170°. 
In FIG.  1, a folding-type mobile terminal is shown.  As shown in FIG.  1, the folding-type mobile terminal includes a main body 10, a folder 20, and a hinge apparatus 30.  The hinge apparatus 30 semi-automatically opens and closes the folder 20 by rotatably connecting the folder 20 to the main body 10.  Typically, in an inner surface of the main body 10 includes a keypad 100 including multiple keys, and in an inner surface of the folder 20 includes a display unit (not shown) and a speaker device. 
The hinge apparatus 30 includes a hinge dummy (not shown), a hinge module (denoted by 35, in FIG.  2), and a plurality of hinge arms.  The hinge arms include a couple of side hinge arms 110 and 112, and a center hinge arm 210, which is disposed between the side hinge arms 110 and 112, and is coupled to the side hinge arms 110 and 112 by the hinge apparatus.  The hinge apparatus 30 includes one hinge dummy, and one hinge module 35, and the hinge dummy (not shown) and the hinge module 35 are embedded in the center hinge arm 210.  In FIG.  1, a hole 113 formed in one side hinge arm 112 is shown in an exposed state.  Through the hole, a part of a shaft portion 342 of a driving cam of the hinge module can be coupled with a shaft portion of the hinge dummy (not shown). 
In FIG.  2, the hinge module 35 for the folding-type mobile terminal is shown.  As shown in FIGS.  2 to 3c, the conventional hinge module 35 receives, within a hinge housing 310, a driving cam 340, a driven cam 350, a hinge spring 320, and a hinge shaft 330, which will be described later.  The hinge spring 320 provides a force for tightly fitting the driving cam 340 in the driven cam 350, and the hinge shaft 330 restrains the driving cam 340, the driven cam 350, and the hinge spring 320 in the hinge housing 310. 
FIGS.  4a and 4b show driving parts for such a conventional hinge module discussed herein above, specifically, the driving cam 340 and the driven cam 350, respectively.  The driving cam 340 and the driven cam 350, shown in the drawings, provide a stopping force to the folder at about 180°. 
As shown in FIG.  4a, the driving cam 340 comprises a member which is disposed to face the driven cam 350 and performs a cam motion to engage with the driven cam 350, and includes a couple of mountain-shaped protrusions 343 and 344.  Each of the mountain-shaped protrusions 343 and 344 protrudes from a main body 341 of the driving cam in the direction of the hinge axis.  Between the couple of protrusions 343 and 344 are a naturally formed couple of groove-shaped valley portions 345 and 346. 
As shown in FIG.  4b, in the driven cam 350, a couple of mountain portions 352, 353 and valley portions 354 and 355, (i.e. groove-shaped recesses), are formed.  The valley portions 354 and 355 are engaged with the protrusions 343 and 344.  When the protrusions 343 and 344 engage with the valley portions 354 and 355, a stopping force is provided to the folder.  The diagram of the open/close operation of the hinge apparatus is shown in FIG.  5. 
However, the above-described driving part of the hinge apparatus for the mobile terminal includes a couple of protrusions and a couple of valley portions, and provides a stopping motion of the folder at about 180°.  Thus, a stopping force cannot be provided except when the folder is closed and opened.  In other words, in the folding-type mobile terminal, the folder cannot be stopped except when being in a closed state and in an opened state, that is, at two positions (about 0° and 180°).  This causes inconvenience in the use in various modes of a mobile terminal because the folder can only be stopped at the aforementioned positions. 
Especially, the viewing-convenience of a multimedia environment currently favors the convenience to operate keys and to watch the data displayed on the display unit.  From the point of view of a user, various angles are required to watch the data displayed on the display unit. 
A hinge apparatus that provides a force for stopping the folder at various angles is called a free-stop hinge apparatus. 
A driving cam and a driven cam of such a conventional free-stop hinge apparatus are shown in FIGS.  6a and 6b, respectively. 
Referring now to FIGS.  6a and 6b, a driving cam 360 for the conventional free-stop hinge apparatus includes a single protrusion 362 and a flat portion 361, and a driven cam 370 includes a flat surface 371 a single valley portion 372.  The protrusion 362 of the driving cam 360 performs a cam motion by engaging with the valley portion 372 of the driven cam 370.  Such a hinge apparatus has a structure to provide a force for stopping the folder at about 0° or 360°.  The diagram of the open/close operation of the conventional hinge apparatus employing the driving cam 360 and the driven cam 370 is shown in FIG.  7. 
However, in the above described free-stop hinge apparatus, the cam motion is unstable, thereby making the open/close operation of the folder unstable.  The single protrusion of the driving part, and the single valley portion performing a cam motion by engaging with the single protrusion cause an unstable open/close operation of the hinge apparatus through an out-of-balance cam motion according to the open/close of the folder.  This state gradually imposes an excessive stress on the arms and a hinge module of the hinge apparatus, thereby causing a crack.