Abstract:
A caster structure has a screw hole formed at the middle of a carrying plate and provided for passing, screwing, and pressing a screw, and then screwing and pressing tightly with a nut, so that the installed screw has a smaller thickness to lower the center of gravity of an installed wheel in addition to giving a dual nut fixing effect. The caster has bearings which are mainly thrust bearings for bearing a relatively larger axial gravity to prevent the bearing from being damaged by gravity and the balls from falling out. The bearings are limited and positioned securely to prevent installation gaps and dusts, to maintain a flexible rotation of the caster.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a caster structure and, more particularly, to the caster passing through the screws of different parts to prevent loosening the screws and providing a safe fixing effect by dual nuts. The screws have a small assembling thickness. Thus, the center of gravity of a bed surface of a cart is lowered to provide a more stable movement when the cart is pushed, and the bearing is limited and positioned to protect and prevent the components of the bearing from falling off and to provide a dust-resisting effect. 
     Description of the Related Art 
     A caster structure is disclosed in R.O.C. Pat. No. M349338 (hereinafter referred to as Prior Art 1) and R.O.C. Pat. No. M364616 (hereinafter referred to as Prior Art 2). In the Prior Arts 1 and 2, a screw is passed through components of a caster, and the rear end of the screw is fixed by a nut in order to connect the components securely. However, the screw is fixed by a single nut, and a heavy object is usually carried at the bottom of the cart. Thus, their bonding strength is relatively insufficient. The bearings adopted by the Prior Arts 1 and 2 are ball bearings. The application of the ball bearing has a larger lateral bearing stress, and a weaker axial bearing stress. When the ball bearing is applied to the caster with an axial load, inner and outer rings of the ball bearing are often damaged by the large vertical pressure, and the balls may even fall out. As a result, the cart may be stuck to cause an unsmooth movement of the cart. 
     In view of the aforementioned drawbacks of the conventional casters, a feasible design is provided to overcome the drawbacks of the prior art. 
     SUMMARY OF THE INVENTION 
     Therefore, it is a primary objective of the present invention to provide a caster passing through the screws of different components to provide a safe fixing effect and to prevent the screws from being loosened similar to a dual-nut effect. The installed screw has a small thickness to lower the center of gravity of a bed surface of a cart, to provide a stable movement when the cart is pushed. The bearing is one capable of bearing a relatively large axial force, and the bearing is limited and positioned during the installation to protect the components, to prevent the bearing from falling out, and to provide an anti-dust effect. 
     To achieve the aforementioned object, the present invention provides a caster having a carrying plate installed at the top of the caster, a screw hole formed at the middle of the carrying plate for passing a screw from bottom to top through different components and then screwing and connecting with a nut, and a stop structure for limiting and stopping the nut and the rear end, so that the nut cannot be rotated anymore. Therefore, the invention stops the rear end of the screw by the effect similar to a dual-nut effect to prevent the components from being loosened and to provide a safe fixing effect. 
     The present invention is also characterized in that the bearing is a thrust bearing which has a large axial force to bear a larger axial load of the cart and to prevent the bearing from being damaged by the axial load or falling out. The bearing is embedded into a concave cavity of the caster and adjacent stacked components to reduce the installation gap of the bearing and to provide an anti-dust effect of the bearing. 
     The technical characteristics, contents, advantages and effects of the present invention will be apparent with the detailed description of preferred embodiments accompanied with related drawings as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a caster structure in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is an exploded view of a caster structure in accordance with a preferred embodiment of the present invention; 
         FIG. 3  is a front sectional view of a caster structure in accordance with a preferred embodiment of the present invention; and 
         FIG. 4  is a side sectional view of a caster structure installed with a wheel in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIGS. 1 to 3  for a caster in accordance with a preferred embodiment of the present invention, the caster  1  comprises the following elements: 
     A bipod  10  includes a main body  11  and two support legs  12 . A through hole  110  is formed at the middle of the main body  11 , an upper cavity  111  is downwardly and concavely formed on a top side of the main body  11  and concentrically configured with the through hole  110 , and a lower cavity  112  upwardly and concavely formed on a bottom side of the main body  11  and concentrically configured with the through hole  110 . A protrusion  1111  is formed between a middle position  1110  on an internal bottom side of the upper cavity  111  and the edge of the through hole  110 , so that a limiting interval  1112  is formed on the internal bottom of the upper cavity  111  and from the middle position  1110  to the inner wall on a side of the upper cavity  111 . The support leg  12  is fixed to the bottom side of the main body  11  and is disposed on both left and right sides separately. The top end of the support leg  12  is fixed (soldered) with the main body  11 , the bottom end of the support leg  12  is extended downwardly and obliquely, and the bottom end of the support leg  12  has a pivot hole  120  for installing a wheel  6  (as shown in  FIG. 4 ) between the two support legs  12 . Both left and right ends of a shaft  60  of the wheel  6  are passed into the pivot holes  120  respectively to achieve the locking and fixing effects. 
     An upper bearing  2   a  and a lower bearing  2   b  are thrust bearings. The upper bearing  2   a  is installed in the upper cavity  111 . The upper bearing  2   a  has an external diameter matched with the internal diameter of the upper cavity  111 , and the inner and outer sides of the bottom of the upper bearing  2   a  are limited and positioned by the limiting interval  1112  (as shown in  FIG. 3 ). The lower bearing  2   b  is embedded into the lower cavity  112 , and the lower bearing  2   b  has an external diameter matched with the internal diameter of the lower cavity  112 , so that the outer wall of the lower bearing  2   b  abuts the internal sidewall of the lower cavity  112 . The upper bearing  2   a  and the lower bearing  2   b  have the same structure comprising an upper ring  20   a ( 20   b ) in a hollow circular form and arranged in upper and lower rows, a lower ring  21   a ( 21   b ) disposed under the upper ring  20   a ( 20   b ), and a plurality of balls  22   a ( 22   b ) installed between the upper ring  20   a ( 20   b ) and the lower ring  21   a ( 21   b ). The installation of the upper bearing  2   a  and the lower bearing  2   b  maintains a flexible rotation of the bipod  10  and the wheel  6  to the left and right sides. 
     A carrying plate  3  is installed at the top of the main body  11  and provided for covering the upper cavity  111  and locking and fixing a bed surface of the bottom end of the cart. A screw hole  30  is formed at the middle position of the carrying plate  3  and configured to be corresponsive to the through hole  110 . A groove  31  is downwardly and concavely formed on a top side of the carrying plate  3  and disposed at a peripheral surface of the screw hole  30 . A conical abutting surface  32  is protruded downwardly from the bottom side of the carrying plate  3  and configured to be corresponsive to the groove  31  (or disposed on at the periphery of the screw hole  30 ). The bottom side of the carrying plate  3  covers the top side of the upper bearing  2   a , and the middle portion of the abutting surface  32  may be embedded into the upper bearing  2   a . A portion of the abutting surface  32  is pressed by a top end A of the inner wall of the upper ring  20  of the upper bearing  2   a  as shown in  FIG. 3 . 
     A screw  4  is passed from bottom to top and sequentially through the lower bearing  2   b , the through hole  110  and the upper bearing  2   a . Then the rear end of the screw  4  and the screw hole  30  of the carrying plate  3  are screwed and pressed tightly with each other. Since the carrying plate  3  has the screw hole  30 , a function similar to a nut is provided, so that after the screw  4  is screwed all the way into the screw hole  30  of the carrying plate  3 , the rear end of the screw  4  is passed upwardly out from the top of the screw hole  30 . A nut  5  is screwed with the rear end of the screw  4 , so that the nut  5  is disposed adjacent to press the carrying plate  3  tightly. After the nut  5  is pressed tightly and embedded into the groove  31 , the top end of the nut  5  and the rear end of the screw  4  are aligned precisely with the top side of the carrying plate  3  as shown in  FIG. 3 , and the screw  4  connects different components of the caster  1 . Finally, a stop structure is provided for limiting and stopping the nut  5  and the rear end of the screw  4 , so that the nut  5  will not be rotated anymore. The stop structure is formed by having a concave hole  40  formed at the rear end of the screw  4 , and extending a punching rod  62  of a punching machine  61  into the concave hole  40  to expand the concave hole  40 , so that the outer wall of the concave hole  40  expands to latch the nut  5 , so that the nut  5  is stuck and cannot be rotated anymore to prevent the screw  4  and the nut  5  from being loosened. In addition, the screw  4  has a head  41  with an external diameter greater than the internal diameter of the lower bearing  2   b , so that the head  41  has an effect of supporting and sealing the bottom end of the lower bearing  2   b . The outer wall of a limit ring  43  between the head  41  and a starting position of a threaded section  42  is a smooth surface, and the limit ring  43  has an external diameter matched with the internal diameter of the lower ring  21   b  of the lower bearing  2   b , so that the limit ring  43  and the internal side wall of the lower cavity  112  can laterally abut the inner and outer sidewalls of the lower ring  21   b  to position the lower bearing  2   b  tightly. 
     After the aforementioned elements are combined sequentially as shown in  FIGS. 1 and 3 , the caster  1  and wheel  6  are coupled as shown in  FIG. 4 . The application of the present invention has the following advantages and effects. 
     1. The carrying plate  3  has a screw hole  30 , so that when the screw  4  is passed from bottom to top to lock and connect each component, a dual-nut effect is achieved for the screw hole  30  of the carrying plate  3  formed at the rear end of the screw  4  and the nut  5 . In addition, the stop structure is provided for limiting the rotation of the nut  5  to prevent the screw  4  from being loosened and to improve the safety of use. 
     2. The outer walls of the upper bearing  2   a  and the lower bearing  2   b  are abutted by the inner walls of the upper cavity  111  and the lower cavity  112  respectively, and the lower end, the upper end and the hollow inner wall of the upper bearing  2   a  are abutted by the protrusion  1111  and the abutting surface  32  respectively (as shown in  FIG. 3 ), so that both inner and outer peripheral surfaces of the upper bearing  2   a  are abutted and so that no gaps will be formed. The bottom side of the lower bearing  2   b  is supported by the head  41  of the screw  4 , the inner wall of the lower ring  21   b  is abutted by the outer wall of the limit ring  43 , and the upper ring  20   b  is attached closely with the internal bottom of the lower cavity  112 , so that the lower bearing  2   b  is positioned securely and so that no gaps will be formed. Such arrangement provides an excellent seal after the upper bearing  2   a  and the lower bearing  2   b  are positioned, to prevent dust from sticking onto the balls or hindering the flexible rotation of the caster  1 . 
     3. The upper and lower bearing  2   a ,  2   b  are thrust bearings with a large axial bearing capacity, so that when the caster  1  is applied for a large axial bearing force, the structure of the upper and lower bearings  2   a ,  2   b  of the caster  1  will not be damaged by gravity easily. 
     4. The screw  4  is locked from bottom to top, so that the head  41  with a smaller thickness faces the installation end of the wheel  6 , and so that the external diameter of the wheel  6  will not be limited by the length of the screw  4  that much. After the caster  1  and the shaft  60  of the wheel  6  are installed, they are close to each other to lower the center of gravity, to improve the stability of pushing the cart. 
     While the invention has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the invention set forth in the claims.