The present invention relates to lift cranes, and more particularly to a luffing jib backstop arrangement for lift cranes.
Lift cranes include a boom attached to the crane bed upper works, and many times include a luffing jib pivotally attached to the top of the boom. In a tower crane configuration, the boom of the crane extends essentially vertically from the crane bed. The top end of the luffing jib may be raised and lowered, thus changing the angle of the luffing jib with respect to the boom. This is accomplished by the crane operator using winches and cables, controlled from the crane cab. Loads are suspended from a cable carried by a sheave at the top end or head of the luffing jib. The load may be positioned by raising and lowering the luffing jib and/or (except for tower cranes) the boom, retracting or letting out a length of cable, or a combination of the foregoing.
Cranes with luffing jibs generally incorporate backstop pendants to limit the operating angle of the luffing jib with respect to the boom. This is a safety precaution because the boom and luffing jib are constructed and rigged to operate only where the luffing jib extends from the boom at an angle less than 180.degree.. By limiting the operating angle of the luffing jib, the backstop pendants prevent the luffing jib from going over backwards upon the sudden release of the load, upon a wind gust, or when the jib/strut/pendant system center of gravity is to the rear of the hinge point and overtakes the jib dead weight (unloaded jib). The backstop pendants are generally attached at one end to the boom in the area of the boom head and at the other end of the pendant to the luffing jib in the area of the butt end of the luffing jib. Because the pendants are of a fixed length, the pendants prevent the luffing jib from rotating with respect to the boom beyond the safe operating angle, which is less than 180.degree. for all cranes.
The present invention is directed to the problem associated with attaching the backstop pendants to the crane prior to operational deployment of the crane. For instance, because the pendants are of a fixed length and are generally attached in the areas of the boom head and luffing jib butt, the luffing jib must be rotated with respect to the boom to an angle within the desired operational limits of the crane before the pendants can be attached. Since the boom and the luffing jib are usually laid out along the ground during assembly and prior to deployment, this rotation must be accomplished by elevating the boom head and luffing jib butt. The boom head and luffing jib butt, and thus the pendant attachment points, are then a significant distance above the ground. A worker then has to be positioned far above the ground in order to attach the pendants, usually by means of a very tall ladder or a man-lift, with the attendant logistical and safety concerns.
One former approach to solving this problem involved attaching the pendants to the luffing jib head or the boom butt. Ground level access to the attachment points was thus permitted even though the boom head and luffing jib butt were raised in the air. However, according to this particular solution, the pendants must span nearly the length of the luffing jib or boom. Because of their length, the pendants add significantly to the weight of the crane structure. This additional weight results in reduced lift capacity. Also, multiple pendants of different lengths are then required if the length of the boom or luffing jib on the crane is changed.