Repeat Problems 12.5 and 12.6 using a door weight of 250 N with the center of gravity 600 mm from the hinge. Spring torque for the door-open position as given in Problem 12.5 is to be 8 N m; net torque for the door-open position as given in Problem 12.6 is to be 16 N # m. The maximum allowable torsional stress is 350 MPa.
Problems 12.6
Suppose that the trap door in Problem 12.5 must be designed to open only 60° because there are clearance problems. Determine the length and diameter of torsion bar that would counterbalance 80 percent of the door weight when closed, and provide a net torque of 12 lb ft to hold the door against the 60° stop when open. Make a graph showing gravity torque, spring torque, and net torque all plotted against the door-open angle.
Problem 12.5
Figure P12.5 shows a fully opened trap door covering a stairwell. The door weighs 60 lb, with center of gravity 2 ft from the hinge. A torsion bar spring, extending along the hinge axis, serves as a counterbalance. Determine the length and diameter of a solid steel torsion bar that would counterbalance 80 percent of the door weight when closed, and provide a 6-lb ft torque holding the door against the stop shown. Use a maximum allowable torsional stress of 50 ksi. Make a graph showing gravity torque, spring torque, and net torque all plotted against the door open angle.
