Bridge Cranes, how do they work?

[doctorwayne]

.......................Maybe our good friend Dr Wayne will illuminate us....

If I'd been paying attention, I at least would've tried.

At the steel plant where I worked, the bridge cranes, like those shown in the photo above, were removed many years ago, as most Great Lakes ore and coal boats are self-unloaders.

A neighbouring plant down the street from this still has similar large bridge cranes, although they're of a different design and may not be for coal or ore.
Since I never worked in the raw materials area, I can't tell you much about how they were used: basically, they used a large clamshell bucket to dig the coal or ore from the boat's holds, then deposited it in a large storage yard, where it was later loaded into trucks or railcars for delivery to the coke ovens. As Val mentions, the idea was to get the boat unloaded so it could go for another load - the shipping season is restricted in the winter, so plants need to stockpile enough raw materials to last until shipping resumes in the spring.

While these cranes are quite large, they operate like most bridge cranes. The actual bridge is the long, truss bridge-like structure which spans the storage yard - most bridge cranes inside the mill buildings, because of their smaller spans, have bridges constructed of built-up (welded or riveted) box girders, as deep as is required for their span and intended loading.
The bridge travels laterally on tracks, so it's possible for multiple cranes to work on the same boat (lake boats can be 700'-800' in length), speeding-up the turn-around time. In the photo in the earlier post, the near end of the bridge supports are on a pair of rails, with their mounting structure spanning what appears to be a railway track - this may be for loading hoppers, either directly from the boat or from the stock piles. However, there doesn't appear to be any guide chute to direct the material into a railcar, so this feature may no longer be in use. Most plants now use conveyor belts for this purpose.
The crane's rails are generally of a larger profile than most railroad tracks, and are generally bolted, with clips, to heavy I-beams set in concrete. all of the crane's wheels are flanged on both sides, to keep it on the track and to minimise the amount that the bridge can skew (one end gets ahead of the other). The wheels are driven by electric motors, acting through reduction gear boxes.
On the mill cranes with which I'm familiar, only the wheels on one side of the bridge were powered - that is, powered wheels on only one side, with one at each end. In the photo below, the crane bridge has a powered wheel on the near corner of the bridge (visible under the boarding platform), and another on the same side of the bridge, but at the opposite end, hidden by the grey electrical cabinet. The wheels on the unseen side of the bridge are merely idlers.



Here's a look from the opposite direction, showing the same side of the bridge with the other powered wheel closest to the camera:



The larger span of the prototype raw materials crane, however, may mean that there are powered wheels at all four corners of the supporting towers.

Electrical power for the bridge motors (and all other equipment on the crane) is provided by collector rails (heavy angle irons, mounted on insulators - like an upside-down "L"). In the prototype photo, they can be seen on the concrete retaining wall, on the near side of the storage yard. They're protected from dropped coal and ore by an overhanging projection of the wall. Collector shoes (pivotting steel blocks) slide atop these energised rails, gathering power, which is routed through various electrical panels (the grey boxes on my model) depending on the end use.
In the photo of my crane, above, the four bridge collector rails are mounted on the runway support columns, visible beneath the runway members in the foreground of the first photo, and on the column supports of the far side in the second photo.

Within the bridge of that prototype material handling crane is a trolley, which also runs on rails. As you can see from the ironwork between the side trusses of the bridge, much of the trolley is suspended below the bridge, including the operator's cab. It gathers its power from more collector rails running the length of the bridge. With the bridge able to travel laterally and the trolley transversely, it's easy to see that the clamshell's load can be dumped (or picked up) anywhere within the yard. Access to the operator's cab is via the stairways at the far (inland) end of the bridge, with a drop-down extension (to allow access to the full width of the boat) at the outboard end of the bridge. The bucket is also powered by electric motors, as is the trolley.
While they're not visible in the photo, there are also lights beneath the length of the bridge and on the trolley, as these machines are often in use 24 hours a day. Generally, the operator sits just inland of the bucket and its associated cables, allowing a good view into the hold of the boat being unloaded - often, the lower front wall of the cab is glass, angled under the operator's feet, to provide a clear view.

I hope this, however belatedly, has been of some help.

Wayne