The City by the Bay's cable cars are more than just a San Francisco treat, they're the last such manually operated trolleys in existence. And these massive engines are what keep them chugging up some of the City's steepest terrain.
The heart of the cable car system is the Powerhouse located at the corner of Washington and Mason Streets. Each of the trolley lines -- California Street, the Powel-Hyde, Powel-Mason and their common section -- all emanate from this terminal hub. Each line is independently powered by a 510hp (380kW) DC electric motor and include speed-reducing gears and three large, self-adjusting pulleys called sheaves. The gears work to normalise the cable's speed to 15km/h across all lines while the sheaves ensure the cables run through smoothly.
The cable itself is a 1.25-inch diameter steel rope comprised of six twisted strands, each of which are made of 19 individual wires encircling a sisal rope core. The cables vary in length from 9300 feet on the Powell line to 21,700 feet for the California. Each cable is slathered with a tar-like substance that works much like axle grease, wearing and breaking down in the cable's place. Even with this protective coating, the cables only last six to eight months on average. When one fails, the system is shut down that evening and a new cable is spliced into the feed.
According to the San Francisco Cable Car Museum:
Each of the different cable car lines once had its own powerhouse -or sometimes several-to drive the cable used on the line. The first power sources were steam engines powered by enormous amounts of coal each day, thus each powerhouse was equipped with boilers to heat the water needed to produce steam. During the decade after the 1906 quake, steam power was replaced by electricity. Before the 1982 restoration, a single 750 horsepower electric motor drove all the three remaining cable lines, with another identical motor for backup.
After restoration, four 510 horsepower engines were installed, one for each cable with its own separate gearbox. A set of six 14-foot diameter sheaves is driven by the motors, around which the cable runs in a figure-eight pattern to reduce slippage. Tension sheaves keep a constant tension on the cable and takes up slack produced by wear, car load and stoppage of cars on the line.
From the tension sheave the cable moves back under the power sheaves into an enclosed channel through which it passes out of the powerhouse and under the street to the tracks.
The individual cars grasp the running cable in what are essentially a giant pair of pliers that poke through the bottom of the car and extend under the street to the cable track. The gripman -- aka the driver -- controls the car's forward momentum by varying how tightly the grip grabs the cable. [SF Cable Car Wiki - Cable Car Museum 1, 2]
Image: Caroline Culler