please refer to fig5 . fig5 is an exploded diagram of a sliding structure 5 that is an embodiment of the present invention . the sliding structure 5 comprises a rack 52 , a gear unit 54 and a hinge 56 . the hinge comprises a first gear 561 , a first cam 562 , a second cam 563 , a spring 564 , a bolt 565 and a housing 566 . the gear unit 54 comprises a second gear 541 and a third gear 542 where the second gear 541 and the third gear 542 share a same axis . please refer to fig6 . fig6 is a diagram combining components of the sliding structure 5 in fig5 . the first cam 562 , the second cam 563 , the spring 564 and the bolt 565 are placed inside the housing 566 . the bolt 565 is placed inside the spring 564 ; the second cam 563 slides along the bolt 565 through a link with the first cam 562 . with the sliding of the second cam 563 the spring 564 is compressed . the rack 52 meshes with the second gear 541 of the gear unit 54 , and the first gear 561 of the hinge 56 meshes with the third gear 542 of the gear unit 54 . fig7 and fig8 are diagrams of motions of the sliding structure 5 . when the rack 52 moves , which drives the gear unit 54 to rotate , and the first gear 561 of the hinge 56 rotates . since the first cam 562 shares the same axis as the first gear 561 , the first cam 562 and the second cam 563 starts rotating . while rotating , the second cam slides downwards and compresses the spring 564 ( as in fig7 ). when in a situation as shown in fig7 , removing the force on the rack 52 allows using the hinge 56 to drive the gear unit 54 to rotate and the rack 52 to move . with a rebound of the spring 564 , the second cam 563 slides upwards and continues to rotate in correspondence to the first cam 562 . the first cam 562 continues to rotate and drives the gear unit 54 to rotate , hence moving the rack 52 . when the spring 564 recovers to its original length , all the rotating and moving stops , as shown in fig8 . please refer to fig9 and fig1 . fig9 and fig1 are diagrams of the sliding structure 5 realized as an electronic device having a sliding cover 9 . the electronic device having a sliding cover 9 comprises a first housing 92 and a second housing 94 . the first housing 92 comprises a display monitor 91 , and inside the first housing 92 is a first fixing component 95 . inside the second housing 94 is a second fixing component 96 and a third fixing component 98 . the rack 62 of the sliding structure 5 is placed on the first housing 92 , while the gear unit 54 and the hinge 56 are placed on the second housing 94 . the first fixing component 95 of the first housing 92 in fig9 detachably connects with the second fixing component 96 of the second housing 94 to fix relative locations of the first housing 92 and the second housing 94 . after completing the motion of the sliding structure 5 , as in fig1 , the first fixing component 95 of the first housing 92 detachably connects and fixes with the third fixing component 98 of the second housing 94 to fix final relative locations of the first housing 92 and the second housing 94 after the first housing has been fully extended . the sliding structure of the embodiment of the present invention does not occupy a large area and an overall length can be adjusted with different matching springs , so that a thickness of an electronic device occupied by the sliding structure can be altered according to design requirements . therefore design of the electronic device can be decreased , agreeing with a current trend . those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention . accordingly , the above disclosure should be construed as limited only by the metes and bounds of the appended claims .