Movable open stairway for theatre - mounts string rails on T=shaped column heads including pin joined parallelograms setting treads horizontal

摘要

The movable open staircase is used for theatrical purposes, having a collapsible metal framed construction, and includes a pin-jointed parallelogram mechanism which sets the treads (21, 31) horizontal whatever the string rail (12) inclination. Telescopic posts (16) on trolleys each have a high levellcross bar (24) termination in stub axles (23) and fixed horizontal cantilever arms (37). Each axle locates the foot of a bracket (34) which is welded to the relevant rail, the head of the bracket being pinned (P) to a side gusset (32b) of a primary tread plank (21). A pin-jointed (39, 30) link (38) connects the free end of each arm to the higher end of each gusset to complete the mechanism.