Lift mask carriage mounting arrangement

Claim 1

In a lift mast carriage mounting arrangement of the type including a support assembly having an upright pair of laterally spaced and inwardly facing channular beams, each beam having a pair of rail surfaces and an interconnecting web surface, the rail surfaces being substantially normal to the respective web surface; a carriage assembly having an upright pair of elongated legs, an upper pair of guide rollers, a lower pair of guide rollers, an upper pair of side thrust rollers, and a lower pair of side thrust rollers, the legs being disposed for movement between and longitudinally along the channular beams on the rollers, the upper and lower pairs of guide rollers individually having rolling planes longitudinally oriented and substantially normal to the rail surfaces; wherein the improvement comprises: means for coupling with said lower pair of side thrust rollers for transmitting side thrust forces from the carriage assembly to said channular beams only when said upper guide rollers and said upper side thrust rolllers are elevationally disposed beyond said channular beams, said means including a pair of canted guide rollers mounted on said legs elevationally intermediate said upper and lower pairs of guide rollers and individually having a rolling plane angularly oriented relative to said rail surfaces and said web surfaces, said canted guide rollers being substantially free of force transmitting contact with said channular beams when said upper guide rollers are elevationally disposed within said channular beams said rolling planes of said canted guide rollers being angularly inclined at least about three degrees relative to said rolling planes of said upper and lower pairs of guide rollers.