Conventional lift cranes include a rotatable body or upper works mounting the lift boom and machinery that rotate about a vertical axis on a lower works or body. If the crane is mobile, the lower works or body is typically crawler mounted. The lifting capacity of a mobile crane is largely determined by the geometry of the base since all of the compression and tilting loads must act through and around the mobile base to the ground. The constant demands for increasing crane capacity have been partly met by larger-sized cranes having bigger lower bodies, both for more strength and to further space the fulcrum or tipping point of the crane from the counterweight effective line of action. These larger cranes have also been provided with increasing amounts of counterweight carried on the rotatable upper works which resist the overturning moment of the larger loads.
A significant increase in crane capacity was achieved by providing a self-propelled crane with the support ring and extended boom carrier disclosed and claimed in U.S. Pat. Nos. 3,485,383; 3,878,944; and 4,196,816. In these designs, the weight of the crane and its load is transferred to the ground through a large diameter, track-like ring. As shown in these patents, and as practiced commercially for some years, the support ring is either blocked into place by timbers fitted or wedged beneath and completely around the ring or is supported by a plurality of jacks spaced around the periphery of the ring. While such cranes have increased counterweight and lift capacities, they are no longer mobile under heavy loads.
Further refinements in ring supported cranes are disclosed in U.S. Pat. Nos. 4,042,115; 4,103,783; 4,387,813 and 4,387,814. These patents disclose inter alia that a separate transporter mechanism may be run in and out of an otherwise stationary ring supported crane in order to move that crane between different locations or transporter mechanisms and/or idle crawlers or dollies may be installed beneath the ring under the boom foot and counterweight. Additional ring segments of even greater radius, which may also be mounted on mobile transporter mechanisms, are disclosed in U.S. Pat. Nos. 4,316,548; 4,358,021; 4,449,635 and co-pending application Ser. No. 259,932, filed May 4, 1981, now U.S. Pat. No. 4,601,402. Other arrangements for forwardly extending the boom foot or load fulcrum point are disclosed in "TransiLift" type cranes such as in U.S. Pat. Nos. 3,836,010; 4,170,309; 4,243,148; 4,537,317 and 4,555,032.
Conversely, attempts have been made to increase crane capacity by adding free-swinging counterweights such as disclosed in U.S. Pat. Nos. 3,202,299 and 3,209,920 or through the use of "Sky-Horse" type counterweight trailers such as disclosed in U.S. Pat. Nos. 3,842,984, 3,921,815 and 4,258,852 or suspended counterweight control systems as disclosed in U.S. Pat. No. 4,557,390. Reference may also be made to much earlier movable counterweight control systems for pedestal mounted cranes such as U.S. Pat. No. 524,619 wherein the counterweight position is directly dependent on the load line tension and U.S. Pat. No. 970,773 wherein the counterweight is swung out rearwardly in opposition to the forward reach of a jib.
None of these prior art arrangements, however, provides a fully satisfactory arrangement for increasing the lift capacity of a mobile crane while maintaining full mobility and maneuverability of the crane on its own crawler base and also permitting full swinging movement of the crane upper under both load and no load conditions without undue swinging and counterbalancing forces being created by the suspended counterweight mechanism.