The two counterweight sheaves were installed last week on the Portsmouth towers. These will accommodate the cables the attached the 4-million-pound lift span to the four counterweights. Below of photos of the second of these being placed on the Portsmouth West tower just after noon on Thursday.
| Before we go any further, let's take a look at how the bridge works. The sketch to the right shows the moving parts. The four counterweights are attached to the lift span with heavy cables that pivot around the counterweight sheaves shown above. The counterweights have additional cables that move them up and down, in turn lowering or raising the lift span. A second, much smaller sheave is located at the top of each tower for the control cable that pulls the weight upward (called the "downhaul rope" since it forces the lift span downward). Another control cable, the "uphaul rope" is used to pull the weight back down, lifting the span. These control cables both attach to a large drum at the base of the tower, as we'll see below. This keeps them in sync with each other. In the previous blog entry, you can see a photo of one of the small control sheaves being installed. These are not visible from the outside of the finished towers. |  |
The counterweight cages have been loaded with weights. Below is the view from above the Kittery East tower weight. The ten slots visible in the top will guide the cables connecting the lift cables to their attachment points. The cables visible in the photo are the control cables that actually pull the counterweights up and down. The two in the foreground of the photo are temporarily rigged for lifting the weight into place, but will be rerouted over the control sheaves on each side of the tower.
Below, the weights are visible within the counterweight cage. The small weights on top allow for fine tuning, while the larger weights are intended to be permanent.
Below is the view of the bottom of the counterweight in the Kittery East tower. The two cables at the top of the photo will be for pulling the weight downward (lifting the lift span), and the cables at the bottom of the photo go up, over the control sheaves and back down to the top of the weight to pull it upward.
In the Kittery West tower, the cables have been tensioned and wrapped around a drum that connects to an electric motor that drives the lift operation. The multiple wraps around the drum increase the friction between the drum surface and the cable.
Finally, a shot of the progress on the Kittery side. The rail deck, curving off to the right, is complete. The highway abutment and first span are also complete (terminating at the yellow platforms). In the middle foreground is Pier 14 (with the rusty temporary supports in front of it). Since this photo was taken, work is well underway on this span. This will leave just one more, at Pier 13 (out of frame at the bottom) to connect the highway to the towers.
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