third lower roller assembly differs in that they are not comprised of a shaft which extends through both the side plates 70 and 72 rather as seen in fig5 and 6 the third lower roller assembly comprises a mount 100 secured to leg 63 of angle iron 62 fixedly received in mount 100 is one end of a shaft 101 shaft 101 extending through side plate 70 and being secured thereto by nut 106 rotatably mounted on shaft 101 is a sleeve 108 sleeve 108 carrying a wheel 102 and being rotatable around shaft 101 in a similar fashion and as seen in fig5 and 6 a second such lower roller assembly is disposed between side plate 72 and one leg 65 of angle iron 64 as can be seen the upper and lower roller assemblies are positioned on carriage 14 such that the flanges 66 and 68 are sandwiched between the wheels flanges 66 and 68 effectively forming a rail or track for the wheels of the roller assemblies thus carriage 14 via the roller assemblies and flanges 66 and 68 can move reciprocally along the length of track member 12 while carriage 14 has been described in connection with the use of roller assemblies to enable carriage 14 to move along track member 12 it is to be understood that the use.