It has been known for some thirty years that permanently mounted truck cranes would be advantageous and economically feasible for the rapid and safe unloading of flatbed trucks. The need for such permanently mounted truck cranes increased as truck load bearing capabilities went upward and as the standard flatbed truck length approached forty feet.
Some of the early truck cranes were essentially a self-powered cart on wheels. The wheels of such carts were placed in specially designed slots in the truck bed, the slots being at or near one or more I-beams which run the length of a standard flatbed truck.
Prior U.S. patents which illustrate the state of the art in this technology are U.S. Pat. Nos. 3,501,031; 3,082,889; and 3,093,248.
The prior art truck cranes were of course a vast improvement over preceeding unloading methods which required much time and mechanical labor to put machinery in place for unloading a large flatbed truck.
However, prior art truck cranes which roll along the bed of the truck have historically suffered at least one serious drawback, i.e. they could not move along the bed of the truck while simultaneously carrying a load. Thus, in practice, the crane would be rolled into position by means of its own motor unit, the load would be lifted, and then the crane boom would be extended as far as safely possible. The load would then be lowered and released, the boom retracted, and the crane would be rolled into position to again pick up the load and move it still further down the length of the flatbed truck. This "inch-worm" process continued until the loads were in the desired position to be lifted one last time by the truck crane and swung off the truck.
It is again emphasized that this complex process was required since the rolling truck crane bases of the prior art, utilizing by necessity a relatively low-power engine, were not and are not capable of moving under load conditions. Once the load is picked up, they are simply not able to overcome the roller friction forces resulting from a heavy load. Thus, the "inch-worm" process referred to above has become a standard unloading practice in the industry over the past 18 years.
Such process is not only time consuming but poses increased danger to the operator and the load itself since, each time a load is picked up and put down again, a risk of damage or injury from load tipping or operator error is possible.
From the foregoing, it is apparent that a rolling base truck crane which is capable of moving under full load conditions would be of highly significant advantage in the art.