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Tools and Machinery of the Granite Industry, Part III
Chronicle of the Early American Industries Association, Inc., The, Dec 2006 by Wood, Paul
The Lane Manufacturing Company's electric, overhead bridge crane (ca. 1895) had two non-reversing electric motors, essentially replacing sheaves in the old flying rope design-one motor on the trolley to power the fall rope hoist and to move the trolley and one motor on the bridge to move the bridge (Figure 16). Lane also began to replace wooden parts with steel. Electricity was conveyed to the bridge by three bare conducting cables that ran the length of the shed rails and were continuously contacted by a set of three wheels on the bridge as it moved back and forth along the length of the shed. Electricity was conveyed from the bridge to the trolley by three bare conducting cables that ran the length of the bridge and were continuously contacted by a set of three wheels on the trolley as it moved back and forth along the bridge. Like the flying rope crane, the operator sat in the trolley. The operator had two wheels to control the hook and the trolley. The wheel on the left controlled the hoist and hook-turned one way the hook was raised, turned the other the hook was lowered, and in the center position the hook was stationary. There was a lever to the operator's right that selected one of two hoist-operating speeds-high and low. The wheel on the right controlled the trolley-turned one way the trolley moved in one direction, turned the other way the trolley moved in the opposite direction, and in the center position the trolley was stationary. There were two brake pedals-the left one braked the hoist and the right one braked the trolley. The trolley and the hook could be moved simultaneously. Bridge movement was controlled by a rod that ran along on top of the bridge beam on the operator's left side. If the rod was pulled one way the bridge moved in one direction, and if pulled the other way the bridge moved in the opposite direction, and if centered the bridge remained stationary.
Pawling & Harnischfeger of Milwaukee, Wisconsin, made the next major advance in traveling bridge cranes by the use of three motors-one for the bridge, one for the trolley, and one for the fall rope and hook. All the motors were direct connected and had variable-speed controllers allowing the clutches and "frictions" to be eliminated. This greatly simplified the design and led to greater reliability and less down time. In addition, P&H provided its cranes with operator cabs suspended under and at one end of the bridge, which allowed much better visibility for the operator. F. R. Patch Manufacturing Co. of Rutland, Vermont, manufactured a four-motor bridge crane. The fourth motor was for the fast lifting of light loads. This crane had a bridge speed of 200 to 300 feet per minute, a trolley speed of 100 to 150 feet per minute, and a hoisting speed of 12 to 30 feet per minute. Modern overhead bridge cranes have dispensed with an operator riding on the crane by providing hanging wire controls or hand-held radio controls so that an operator on the floor can both load and unload the hook as well as operate the crane.
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