the technical features and major effects of the slider assembly of the present invention will be clearly shown via the following embodiments . the slider assembly of the present invention can move on a slide rail e along an axial direction s in a linear and reciprocating manner thanks to the circulating rolling movement of rows of rolling elements r , as explained in more detail below with reference to the drawings , particularly fig8 , in which the direction perpendicular to the page is defined as the axial direction s . referring to fig1 and fig2 for the first embodiment of the present invention , a middle retaining member 1 extends along the axial direction s and is provided with a groove 11 along the axial direction s . the groove 11 has an opening 111 . the inner surface of the groove 11 includes two opposite lateral surfaces 112 and 113 and a bottom surface 114 connecting the two lateral surfaces 112 and 113 . a plurality of positioning blocks 12 are evenly arranged on each of the two lateral surfaces 112 and 113 , wherein the positioning blocks 12 on the lateral surface 112 correspond in position to the positioning blocks 12 on the lateral surface 113 respectively . the middle retaining member 1 is made of an elastic material such as plastic or metal . the positioning blocks 12 on the two lateral surfaces 112 and 113 increase in size toward the bottom surface 114 . the middle retaining member 1 is further provided with two retaining surfaces 13 for retaining the rolling elements r . each of the two retaining surfaces 13 has a guide groove 131 for guiding a retaining chain z ( see fig8 ). referring to fig3 , fig4 , and fig5 , the middle retaining member 1 is installed on a slider 2 . the slider 2 has an inverted square u - shaped cross - section with two feet 21 . a mounting portion 22 is formed between the two feet 21 so that the slider 2 can sit astride the slide rail e . the two feet 21 each has an inner side that is provided with two rail surfaces 211 and 212 . between each pair of the rail surfaces 211 and 212 is a protrusion 23 which projects in a transverse direction y , extends along the axial direction s , and is formed with a plurality of positioning holes 231 evenly arranged along the axial direction s and corresponding to the positioning blocks 12 of the middle retaining member 1 . the positioning holes 231 penetrate the protrusions 23 in a direction defined as a connecting direction c , which is perpendicular to both the axial direction s and the transverse direction y . the positioning holes 231 can be made from below the slider 2 without interference by any other parts and therefore feature ease of production . reference is now made to fig6 and fig7 . to install the middle retaining member 1 to the slider 2 , an external force f is applied to open the opening 111 of the groove 11 of the middle retaining member 1 , and then a pushing force p is applied to fit the groove 11 of the middle retaining member 1 onto the protrusion 23 of the slider 2 , thus bringing the positioning blocks 12 to the positioning holes 231 . after that , the middle retaining member 1 is released , allowing the positioning blocks 12 to engage in the positioning holes 231 completely , and thereby fixing the middle retaining member 1 to the protrusion 23 . the connection between the positioning blocks 12 and the positioning holes 231 in the connecting direction c is advantageous in that not only is the manufacturing process made easy , as mentioned above , but also production cost can be lowered due to the reduced number of components and ease of assembly . it should be pointed out that , referring again to fig1 , the correspondingly connected positioning holes 231 and positioning blocks 12 in this embodiment are evenly arranged in the axial direction s along the protrusion 23 of the slider 2 and the middle retaining member 1 respectively . if the middle retaining member 1 in this embodiment is equally divided along the axial direction s into two end portions 14 and a middle portion 15 between the two end portions 14 , the positioning blocks 12 are evenly distributed in each of the two end portions 14 and the middle portion 15 . in fig1 , the middle retaining member 1 has a length l , and the positioning blocks 12 are provided on the middle retaining member 1 at a fixed interval d and are therefore evenly distributed in the end portions 14 and the middle portion 15 . as such , connection between the middle retaining member 1 and the protrusion 23 takes place along their entire lengths , and all the connected positions are equally restrictive . the strength of connection between the middle retaining member 1 and the protrusion 23 is therefore increased , allowing the two components to withstand greater stress in the connected state . referring to fig8 , the assembly process of the slider assembly of the present invention further includes connecting an upper retaining member 3 and a lower retaining member 4 to the slider 2 and placing rows of rolling elements r in place , wherein each row of the rolling elements r is retained in the retaining chain z . the two ends of the slider 2 are then covered with end caps respectively . ( the end caps , which are not shown in the drawing , are well known in the art and therefore will not be described in detail herein .) once the slider assembly is fitted astride the slide rail e , a linear guide rail is completed and ready to be used . in order for the rolling elements r to drive the slider assembly along the slide rail e , referring to the right - hand side of fig8 , an upper load channel and a lower load channel are formed jointly by the retaining surfaces 13 of the middle retaining member 1 , the rail surfaces 211 and 212 of the slider 2 , retaining surfaces 31 and 41 respectively provided by the upper retaining member 3 and the lower retaining member 4 , and rail surfaces e 1 and e 2 provided by the slide rail e . the retaining chains z are guided by the guide grooves 131 of the middle retaining member 1 and guide grooves 311 and 411 respectively provided by the retaining surfaces 31 and 41 of the upper retaining member 3 and the lower retaining member 4 . fig9 and fig1 show the second embodiment of the present invention . in this embodiment , the middle retaining member 5 has generally the same configuration as the middle retaining member 1 in the first embodiment . the differences between the two embodiments are shown in the drawings and detailed as follows . the middle retaining member 5 has a groove 51 whose inner surface is provided with a plurality of evenly arranged and corresponding positioning blocks 52 . each of the positioning blocks 52 of the middle retaining member 5 is provided with a recess 53 . when it is desired to install the middle retaining member 5 to the protrusion 23 of the slider 2 , there is no need to apply the force f ( see fig6 ) for opening the groove 51 ; only the pushing force p is required to press the positioning blocks 52 against the protrusion 23 . this is because the positioning blocks 52 can readily engage in the positioning holes 231 of the protrusion 23 through elastic deformation and restoration caused by the pushing force p and enabled by the recesses 53 , which provide the positioning blocks 52 with room for deformation . the third embodiment of the present invention is shown in fig1 , in which the middle retaining member 6 has generally the same configuration as the middle retaining member 1 in the first embodiment except that the positioning blocks 62 in the groove 61 of the middle retaining member 6 are alternately arranged . this embodiment has the same effects as the first embodiment . fig1 shows the fourth embodiment of the present invention and more particularly a middle retaining member 7 and a slider 8 . the middle retaining member 7 extends along the aforesaid axial direction s . ( the extending of the middle retaining member 7 can be readily known from the previous embodiments and therefore is not shown in fig1 .) the middle retaining member 7 has a groove 71 whose inner surface includes two opposite lateral surfaces 711 and 712 . in addition , the middle retaining member 7 has a plurality of through holes 72 extending through the two lateral surfaces 711 and 712 . the through holes 72 are distributed in the same manner as the positioning blocks 12 in the first embodiment . like the slider 2 in the first embodiment , the slider 8 has two opposite protrusions 81 . each protrusion 81 has a plurality of through holes 811 corresponding respectively to the through holes 72 . to assemble , the groove 71 of the middle retaining member 7 is mounted on the corresponding protrusion 81 of the slider 8 , before a pin 9 is inserted through each pair of the corresponding through hole 72 and through hole 811 to connect the middle retaining member 7 and the protrusion 81 . this embodiment can achieve the same effects as the first embodiment . while the operation , use , and effects of the present invention have been fully disclosed through the foregoing embodiments , it is understood that the embodiments are not intended to restrict the scope of the present invention , which encompasses all simple , equivalent changes and modifications based on the claims and specification of the present invention .