a hospital bed has two mechanical drive mechanisms connected between each end of a bed support frame and a base , each of which has a respective actuator . selective operation of each of the drive mechanisms with the actuators permits either end of the bed support frame to be inclined to the trendelenburg and reverse trendelenburg positions at any height of the hospital bed . the drive mechanisms transfer forces for raising and lowering the bed support frame but do not transfer force when the bed is lowered onto an obstacle . when an obstacle is encountered , a switch interrupts operation of its respective actuator .