Advancing mechanism and system utilizing same for raising and lowering a work platform

A rack (16) extends along a support column (12) for a work platform (10) which is guided for up and down movement along the support column (12). A pair of independently movable supports (32, 34) are provided. A separate extendible/retractable hydraulic cylinder (64, 68) is interconnected between each support (32, 34) and a frame portion (30, 88, 90) of the work platform (10). Each support (32, 34) includes an extendible/retractable lock element (100) having an end portion which when the lock element (100) is extended is in locking engagement with either tooth portions of a pinion wheel (56) carried by the support (32, 34), or with the rack (16). In use, the lock element (100) is extended to lock the support in position relative to the support column (12). At the same time, the hydraulic cylinder (64, 68) which is connected to the support (32 or 34) is extended or retracted to raise or lower the work platform ( 10). At the same time, the lock element (100) that is associated with the second support is retracted into an unlock position. The second cylinder (64, 68) is moved to a starting point so that at the end of the stroke of the first cylinder (64, 68) the second lock element (100) can be advanced to lock its support in position on the rack and the second lock element (100) can be retracted to free its support for movement along the rack, and the second cylinder (64, 68) can be operated to advance the work platform (10) and the first cylinder (64, 68) can be returned to its start position.