interconnected to sprockets 18 by chains 20 screws 17 extend past the base end of the section to mate with engagers 13 as shown in fig2 screws 17 extend to meet receivers 23 of engagers 13 refer now to fig3 which shows screw 17 removed from receiver 23 receivers 23 are hollow for receiving the end of screws 17 and have tapered edges 24 and slots 25 for receiving pins 22 protruding from screws 17 when screw 17 and receiver 23 are mated the rotation of engager 13 causes screw 17 to rotate through the engagement of receiver 23 and pins 22 refer now to fig9 a cutaway view of an engager 13 is shown receiver 23 is attached to plate 37 which may slide but not rotate with respect to engager 13 to compress spring 38 positioned inside engager 13 the spring affords the system a degree of flexibility to account for possible variations in screw 17 length or position additionally upon retraction of a section initially pins 22 may not line up with slots 25 and may contact receiver 23 at the upper edge thus depressing spring 38 subsequently upon rotation of engager 13 and receiver 23 relative to screw 17 pins 22 will slip into slots 25 to mate screw 17 with receiver 23 referring back to fig2 note that as shown all screws 17 are mated with engagers 13 and the column is fully retracted except for the innermost section 30 which is partially extended screw 17a is drivingly connected to screw 21a by drive sprocket 18a driven sprocket.