Should a breakage occur in suspending bands or tape 5 or in the spring-drums or other devices designed to furnish power whereby sash 1 may be automatically raised, spring will rock bell-crank lever 57 upon stud 58, forcing friction member 62 upward, and screws 64, extending through cam-slots 63, will throw the same laterally and press it firmly into engagement with fin 13, thus preverging thereby