is approximately the thickness of the arms 39 and 41 of the t beam 24 since as may be seen in fig1 and 2 these arms are inserted between the upper and lower ball elements 21 and 20 in the preferred embodiment spacing 34 a 34 is generally about 10 mm thus the upper balls 20 roll on the upper faces 42 of the t beam arms 39 and 41 while the lower balls 21 roll on the lower faces 43 of these arms turning again to fig1 a second bracket or support 44 is secured to the upper surface of the upper frame 31 this second bracket 44 connects the pivotable frame 12 to the fixed cross bar 22 by means of an adjusting drive assembly 46 this adjusting drive 47 assembly 46 is comprised of a ballscrew drive and a direct current gear motor 48 the motor itself is secured to the cross bar 22 by any suitable securement means such as clamps flanges angle irons or the like thus actuation of the gear motor 48 will cause the ballscrew drive 47 to rotate so as to pivot frame 12 about bolt 28 this pivoting movement is made easy by rolling cooperation between upper and lower balls 21 and 20 on arms 39 and 41 of the spaced t beams 24 a pair of spaced upper support angles 51 are secured to a rear portion 49 of the.