Abstract: An automatic signaling system from a tractor-trailer being accidentally lifted with the cargo by the overhead traveling crane includes a height-change sensor for triggering a pulse generator which drives a strobe light that is seen by a discriminator mounted above on the crane. The discriminator is coupled into the hoisting circuitry only and does not affect the "lowering" circuitry; it will interrupt hoisting only if the discriminator sees strobe flashes of a particular predetermined frequency and pulse duration.

Abstract: In a cargo container lifting crane employing a lifting spreader for lockingly suspending a cargo container by means of four corner twist locks, an improved electrical interlock system for interlocking the twist lock manual control switch located in the cab of the lifting crane, said interlock system including normally closed, parallel connected proximity switches which are disposed adjacent the twist locks of the lifting spreader and which are connected to a relay whereby the relay is de-energized to close a relay circuit which supplies power to the twist lock manual control switch when all of the four corner proximity switches indicate full engagement of all twist locks into the corresponding locking parts of the cargo container. An adjustable time delay is provided between full lifting spreader engagement and application of control switch power, and a by-pass switch is provided for overriding the interlock system.