on the lower plate 16 the carriage 22 is supported on the lower plate 16 by means of a roller bearing assembly 75 which rides on the upper surface of the outer flange 16b and a pair of roller bearing assemblies 67 mounted on each side of the beam 30 in a position to engage the bottom of the lower plate 16 the assembly 75 is mounted on a vertical plate 66 which is secured to the plate 37 the roller bearing assembly 75 includes a pair of roller bearings 68 which are mounted on an axle 70 which is mounted in a hole 71 in plate 66 the hole 71 is slightly larger than axle 70 so that the roller bearings 68 are free to ride on the upper surface of flange 16b means are provided on the carriage 22 to positively locate the tower assembly 10 in a fixed position on the rack such means is in the form of a plurality of holes 73 provided in an equally spaced relation around the lower flange 16b a locating pin 72 is supported on the end of a lever arm 74 in alignment with a hole 76 in plate 37 the lever arm 74 is pivotally mounted on a pin 78 mounted on the top of a cover 80 the pin 72 is dropped through a hole 76 in plate 37.