location notably the power cable 67 from the electric motor 66 is preferably threaded through the associated rear cross members 56 to communicate with the central bracket 58 and a controller c preferably on the uppermost riser assembly 24 the inner pulley 68 and the outer pulley 70 include a toothed surface to engage the toothed belt with a minimum of slippage the example toothed surface includes a plurality of vertically extending teeth 73 the inner pulley 68 and the outer pulley 70 rotate about respective axes generally parallel to the vertical leg 52 the electric motor 66 includes a shaft 75 directly connected to the inner pulley 68 the shaft 75 rotates about an axis a that is perpendicular to the direction of movement i of the toothed timing belt 72 the direction of movement i establishes a belt plane associated with the toothed timing belt 72 the toothed timing belt 72 preferably faces away from but is engaged with each adjacent horizontal leg 50 of the next inner riser assembly 14 24 fig3 d that is the toothed timing belt 72 of the belt drive system 64 on the horizontal leg 50 of the outermost riser assembly 24 faces inward toward its own horizontal leg in direction ii the belt 72 however is engaged with the horizontal leg 50 of.