apparatus for braking relative vertical movement between two vertical members includes linkage connected either directly or indirectly to a cable pulley - mount that is spring loaded and moves cams that wedge in between two vertically moveable members when cable tension is not present at the pulley . a broken cable , for example , will create a situation where there is no cable tension between a plurality of cable pulleys , the lack of cable tension moves the mechanical cam style linkage to engage the braking members .