Abstract:
A clamping apparatus includes upper and lower platens; one or more tiebars connecting the platens; and an intermediate platen between the upper and lower platens for movement relative to and along the tiebars. Upper and lower mold halves are provided on the upper and intermediate platens, respectively. A linkage connects the lower and intermediate platens. The linkage includes upper and lower links connected with each other for rotation on an intermediate shaft. The upper and lower links are pivotably supported on first and second shafts fixed to the intermediate and lower platens, repectively. The intermediate shaft is operatively connected with a drive mechanism so that the drive mechanism transmits a driving force to the linkage, and the lower platen is moved relative to the intermediate platen. A set of a radical needle bearing and a thrust bearing is used for at least one of the intermediate, first, and second shafts.

Description:
TECHNICAL FIELD 
     The present invention relates generally to a clamping apparatus, and more particularly to a mold clamping apparatus used for an injection molding machine. 
     BACKGROUND OF THE INVENTION 
     Typically, a mold clamping device has upper and lower mold halves for movement relative to each other to define a mold cavity by closure of the mold halves. The device is controlled so that a predetermined force clamping the mold halves is obtained and sustained for a predetermined time. 
     A toggle-type clamping arrangement is typically used for moving one of the mold halves relative to the other along a longitudinal axis of the device. In an exemplary arrangement, one end of each of a pair of links in a toggle linkage is supported for rotation on pivot pins. One of the pivot pins is secured to a movable unit having an upper mold half while the other is secured to a stationary unit having a lower mold half. The other end of each of the pair of links is supported for rotation on an intermediate pivot pin, which is connected operatively with a drive capable of moving the intermediate pin in a direction transverse to the longitudinal axis. For, example, a ball screw is used in the drive mechanism. With this construction, the transverse movement of the intermediate pivot pin allows the movable unit to move up and down (in the longitudinal direction) relative to the stationary unit. 
     In this clamping device using such a linkage, in order to support a relatively large clamping force, a radial needle bearing having an excellent load resistance is preferably used as a bearing mounted, for example, between a link and pivot pin. However, there are cases where thrust is generated because the rollers skew between the outer and inner rings of the bearing, which may damage components such as the bearing, the link or the ball screw. 
     SUMMARY OF THE INVENTION 
     Hence, it is an object of the present invention to provide a clamping apparatus with high reliability capable of preventing such damage of the components. 
     It is another object of the present invention to provide a radial needle bearing that serves to prevent such damage effectively. 
     To achieve the above object, a clamping apparatus of the present invention includes upper and lower platens; one or more tiebars for connecting the upper and lower platens; an intermediate platen provided between the upper and lower platens for movement relative to and along the tiebars. Upper and lower mold halves are provided on the upper and intermediate platens, respectively. A linkage is provided for connecting the lower and intermediate platens. A drive mechanism drives the linkage so that the lower platen is moved relative to the intermediate platen, so that the upper mold half on the upper platen is moved relative to the lower mold half on the intermediate plate. The linkage includes upper and lower links connected with each other for rotation on an intermediate shaft. The upper link is pivotably supported on a first shaft fixed on the intermediate platen. The lower link is pivotably supported on a second shaft fixed on the lower platen. The intermediate shaft is operatively connected with the drive mechanism so that the mechanism transmits a driving force to the linkage. A set of radial needle bearing and thrust bearing is used for at least one of the intermediate, first and second shafts. 
     According to the apparatus, where the thrust may be generated when the rollers of the radial needle bearing in the linkage skew, the trust can be applied to the trust bearing, so that the components of the linkage are prevented from being damaged. 
     A radial needle bearing of the present invention includes a set of needle rollers each having spherical axial ends; and holding members facing said axial ends of the rollers for restraining against axial movement of the rollers. The holding members are separated from outer and inner rings of the bearing. Even if the rollers skew, the rollers can be brought into contact with the holding members through the spherical ends. With this construction, a stress concentration does not occur as in the case where needle rollers each having cylindrical or flat ends are used and, as a result, the rollers that skew may be brought into contact with the holding members through the edges. This has the advantage to improve the fatigue life of the holding members. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side elevation view of a first embodiment of the clamping device according to the present invention; 
     FIG. 2 is a schematic cross sectional view of a toggle arrangement, taken along the line II—II in FIG. 1; 
     FIG. 3 is a schematic cross sectional view of a toggle arrangement, similar to FIG. 2, of a second embodiment of the clamping device according to the present invention; 
     FIG. 4 is an enlarged partial schematic sectional view of a toggle arrangement of a third embodiment of the clamping device according to the present invention; 
     FIG. 5A is a view showing needle rollers having spherical opposite ends in FIG. 4; and 
     FIG. 5B is a view showing needle rollers having cylindrical or flat opposite ends. 
    
    
     Corresponding reference numbers indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring now to the drawings, and particularly to FIGS. 1 and 2 thereof, there is shown a clamping device  2  that includes an intermediate platen  4  secured to a base not shown. The intermediate platen  4  has a lower mold half  6  on the upper surface. Four parallel tiebars  8  (only two tiebars are shown.) are slidably inserted through the holes formed in the intermediate platen  4 . An upper platen  12  is secured via nuts  10  to the upper portions of the tiebars  8 . The upper platen  12  has an upper mold half  14  on the lower surface. A lower platen  18  is secured via nuts  16  to the lower portions of the tiebars  8 . 
     The lower platen  18  is connected through a clamping system  20  with the intermediate platen  4  so that the drive of the system  20 , which will be described below, allows the lower platen  18  and the upper platen  12  connected thereto via the tiebars  8  to move up and down (in the longitudinal direction of the device  2 ) relative to the intermediate platen  4 . 
     In this embodiment, the clamping system  20  is in the form of two double-toggle arrangements (one of them is shown.) each including a left toggle linkage and a right toggle linkage, with each toggle linkage being the mirror image of the other. 
     The linkage  20  is shown in greater detail in FIG.  2 . The linkage has upper and lower links  22  and  24 . The upper end of the upper link  22  is supported rotatably through a radial needle bearing  26   a  on a shaft  28  extending transversely from the support member  4   a  provided on the lower surface of the intermediate platen  4 . The lower end of the lower link  24  is supported rotatably through a radial needle bearing  26   b  on a shaft  30  extending transversely from the support member  18   a  provided on the upper surface of the lower platen  18 . The lower end of the upper link  22  and the upper end of the lower link  24  are supported rotatably on a transverse shaft  32  through the radial needle bearings  26   c  and  26   d , respectively. It is to be noted that, although each of the radial needle bearings  32   a - 32   d  in the drawing has an outer ring formed with two annular flanges, the structure of the radial needle bearing is not limiting of the invention. In the drawing, reference number  33  represents a holding member for fixing a radial needle bearing to the corresponding link. As known in the art, the rollers of a retainerless needle bearing are easier to skew than those of a needle bearing using a retainer. The radial needle bearing used is a retainerless bearing, although it is not limiting of the invention. 
     The one ends of the two shafts  32  of the two left linkages  20  (which are located on the left side in FIG. 1) are connected with a traveling nut  34 . Likewise, the one ends of the two shafts  32  of the two right linkages  20  (which are located on the right side in FIG. 1) are connected with another traveling nut  34 . The two traveling nuts  34  are threadedly connected with a ball screw  36  extending in a transverse direction. The grooves of the ball screw  36  for receiving balls (not shown) are formed to allow the simultaneous inward or outward movement of the two traveling nuts  34  as the ball screw  36  is rotated. The rotation of the ball screw  36  allows the linkage  20  to be driven so that the lower platen  18  is moved upwardly or downwardly. 
     The other ends of the shafts  28 ,  30  and  32  are threadedly connected with locknuts  38   a ,  38   b  and  38   c , respectively. Thrust bearings  40   a - 40   g  are provided between the support member  4   a  of the intermediate platen  4  and upper link  22 , between the upper link  22  and locknut  38   a , between the traveling nut  34  and upper link  22 , between the upper link  22  and lower link  24 , between the lower link  24  and locknut  38   b , between the support member  18   a  of the lower platen  18  and lower link  24 , and between the lower link  24  and locknut  38   c , respectively. Although a needle bearing is used as thrust bearing in the embodiment, a ball bearing or slide bearing may be used instead. Note that the thrust needle bearing  40  is configured so that the needle rollers thereof are held by a retainer and put directly between the side components without bearing rings. 
     The upper links  22  of the two linkages  20  that are located on the left or right side in FIG. 1 are connected with each other via a connecting spacer (connecting member)  41   a , as shown in FIG.  2 . Likewise, the lower links  24  of the two linkages  20  that are located on the left or right side in FIG. 1 are connected with each other via a connecting spacer (connecting member)  41   b , as shown in FIG.  2 . 
     The ball screw  36  carries a driven pulley  42  on its one end. A motor  44  carries and drives a driving pulley  46 . A belt  48  is wound around the pulleys  42  and  46  to transmit the rotation of the motor  44  to the ball screw  36 . 
     With the clamping device  2  so constructed, the motor  44  is rotated in a given direction to transmit a driving force via the driving pulley  46 , the belt  48 , the driven pulley  42  and then the ball screw  36  to rotate the ball screw  36 . Accordingly, the traveling nuts  34  and shafts  32  of the linkages  20  fixed thereto are moved inwardly to allow the lower platen  18  to move downwardly. The tiebars  8  are slid downwardly relative to the intermediate platen  4  so that the upper platen  12  fixed to the tiebars  8  is lowered until the upper mold half  14  on the upper platen  12  is brought into contact with the lower mold half  6  on the intermediate platen  4 . The upper mold half  14  is additionally moved downwardly until a predetermined clamping force is obtained. Note that the clamping force can be measured for example by measuring the amount of movement of the tiebars  8 . 
     After the mold halves  6  and  14  are clamped, a molding process is performed. Then, the motor  44  is rotated in the reverse direction so that the traveling nuts  34  and the shafts  32  of the linkages  20  fixed thereto are moved outwardly to allow the lower platen  18  to move upwardly. Accordingly, the upper platen  12  fixed via the tiebars  8  to the lower platen  18  and the upper mold half  14  are lifted. 
     If the rollers of the radial needle bearings  26  skew during the operation of the clamping device  2 , since the thrust can be applied to the thrust bearings  40 , a force tending to bend the upper and lower links  22  and  24  is much smaller than in the case where the thrust bearings  40  are not incorporated in the linkages  20 . If the rollers of the bearings  26  skew, the links  22 ,  24  and the shafts  28 ,  30 ,  32  are applied with the same load of thrusts in opposite directions. The thrust applied to the links  22  and  24  is applied via the thrust bearings  40  to the components  4 ,  18 ,  38  and  36 . Since these components are fixed to the shafts  28 ,  30  and  32 , the thrust applied to the shafts  28 ,  30  and  32  and the thrust to the links  22  and  24  are balanced in the shafts  28 ,  30  and  32 . Therefore, components other than the shafts  28 ,  30  and  32  such as the ball screw  36  (which is extended in a direction perpendicular to the thrust) are applied with substantial no thrust, resulting in a very low possibility of damages caused by the thrust. In addition, the spacers  41   a  and  41   b  that connect the links  22  and the links  24 , respectively, of the two linkages  20  arranged on the left or right side in FIG. 1 allow the rigidity of the linkages  20  to be improved. The thrust which may generate in the radial needle bearings  26  of one of the linkages  20  can be applied not only to the thrust bearings  40  of the one linkage  20  but also to the thrust bearings  40  of the other linkage  20 , which results in providing the clamping apparatus  2  with higher reliability. Moreover, the locking of the locknuts  38   a ,  38   b  and  38   c  to pre-pressure the links  22 ,  24 , the shafts  28 ,  30 ,  32  and the thrust bearings  40  can restrain variations in loads applied to each components. This has the advantage to reduce the displacement and stress amplitude of each component and therefore improve the accuracy of clamping and prevent damages of the components. 
     Second Embodiment 
     Referring now to FIG. 3, the clamping device of the second embodiment according to the present invention will be described hereinafter. The toggle arrangement of the clamping device is similar to that in FIG. 2 except for the details described below. 
     Specifically, sets of radial needle rollers  50   a - 50   d  are provided between the upper link  22  and shaft  28 , between the upper link  22  and shaft  32 , between the lower link  24  and shaft  32 , and between the lower link  24  and shaft  30 , respectively. 
     In order to restrain the radial needle rollers  50   a - 50   d  against axial movement, annular spacers (annular holding members)  52   a - 52   g  are mounted between the support member  4   a  of the intermediate platen  4  and set of rollers  50   a , between the set of rollers  50   a  and locknut  38   a , between the traveling nut  34  and set of rollers  50   b , between the set of rollers  50   b  and set of rollers  50   c , between the set of rollers  50   c  and locknut  38   b , between the support member  18   a  of the lower platen  18  and set of rollers  50   d , and between the set of rollers  50   d  and locknut  38   c , respectively. For rotational movement of the radial needle rollers  50  relative to the spacers  52 , there is a play between the ends of the rollers  50  and the ends of the spacers  52  facing the rollers  50 . 
     Bearing steel may be used as a material of the spacers  52 . Each of the annular spacers  52  has an inner diameter slightly larger than the outer diameter of the opposed shaft  28 ,  30  or  32 , so that it is not fixed on the opposed shaft. Therefore, if the radial needle rollers  50  skew so that the trust may be applied to the corresponding spacers  52 , a stress is not concentrated on the contact region between the spacers  52  and the opposed shaft  28 ,  30  or  32 . 
     In the embodiment, each of the annular spacers  52  has an outer diameter smaller than the bearing bore diameter of the opposed link  22  or  24  defining an outer ring of the bearing. However, the annular spacers  52  may be in contact with the link  22  or  24  as long as they are not fixed to each other. 
     Third Embodiment 
     Referring now to FIG. 4, the clamping device of the third embodiment according to the present invention will now be described hereinafter. The toggle arrangement in FIG. 4, illustrating in enlarged dimension the shaft  32  and the components facing the shaft  32 , is a modified arrangement of the linkage shown in FIG.  3 . Each of the needle rollers  150  of the linkage has spherical ends  150   a  so that, even if the rollers  150  skew as shown in FIG. 5A, the rollers  150  can be brought into contact with the opposed spacers  52  through the spherical ends  150   a . With this construction, a stress concentration does not occur as in the case where needle rollers  250  each having cylindrical or flat ends as in FIG. 5B are used and, as a result, the skewing rollers  250  may be brought into contact with the opposed spacers  52  through the edges  250   a . This construction has the advantage to improve the fatigue life of the spacers  52 . 
     According to the present invention, where the thrust may be generated when the rollers of the radial needle bearing in the linkage skew, the components of the linkage are prevented from being damaged, which results in providing a clamping apparatus with high reliability.