when the sealing strips 52 are in engagement with the floor 14 the elevating and lowering mechanism 34 includes an elongated threaded bolt 64 having its lower end swivelly connected to the bight portion 48 of the u shaped member 46 as indicated by reference numeral 66 the swivel connection may be of any suitable detachable type of connection which rotatably connects the bolt 64 to the bight portion 48 of the u shaped member 46 the bolt 64 extends up through an aperture 68 in the web 58 and the bolt 64 is threaded through a thread block 70 fixedly secured in the peripheral frame 26 as illustrated in fig2 and 6 the upper end of the bolt 64 is provided with a polygonal head 72 disposed in a recess 74 formed in the panel 12 in which the recess is defined by a box like housing 76 extending inwardly from the outer surface of the panel member 24 so that the recess 74 is open to the exterior surface of the panel member 24 thereby providing access to the head 72 of the bolt 64 so that the bolt 64 can be rotated by a suitable powered wrench manual ratchet wrench or the like thus by rotating the bolt 64 the u shaped member 46 may be elevated and lowered when the u shaped member 46 is lowered as illustrated in fig2 the bight portion 48 and the seals 52 thereon are positioned below the ball caster 36 and the ball caster 36 is elevated out of contact with the floor surface 14 when the u shaped member 46 is elevated as illustrated.