Cable Car: How they work

THE GRIP

Cable Cars in San Francisco have gone through a series of evolutionary changes since the first line opened in 1873. These have involved the mechanical equipment and design of the cars themselves, as well as those along the track and in the powerhouses. One of the first changes was made to the grip itself. Hallidie, in conjunction with his draftsman William Eppelsheimer, designed a mechanism that employed a large hollow screw fastened to the floor of the dummy car. A large hand wheel enabled the gripman to raise and lower the grip, while a smaller, upper hand wheel ran through the middle of the device to operate the jaws of the grip itself, to grab and release the cable in the slot below.

This early prototype was unreliable and problematic, and was not widely used on other cable car lines; only the Union Street car employed it.
Some historians give most of the credit for this grip to Eppelsheimer who, in any case, went on to develop other important innovations in cable rail design. Henry Casebolt and his chief engineers Asa Hovey and T. Day made several design changes to the Sutter Street Railroad, in an attempt to evade patent fees and grip royalties. Day and Hovey developed a side grip with a lever and quadrant, similar to that on present day cars, the main difference being that the grip took the cable from the side, instead of the bottom. The lever and quadrant arrangement, which replaced the screw within a screw design of the Clay Street Hill Railroad, marked the most significant change to Hallidie's original technology, as it is used on all present day cable cars in San Francisco. The side grip eliminated the need for turntables as the gripman simply shifted position on the lever, depending which way the car was headed. The side grip continued to be improved for new lines, remaining popular for its gripping strength in spite of difficulties with let-gos and curves.

Henry Root, assistant engineer on Stanford's Central Pacific, was recruited to build the California Street Cable Railroad, and became a seminal figure in cable transportation. Root made many changes to the structure of tracks, conduit and powerhouse. He also designed a new type of side grip for the California Street line, but twenty-two "let-gos" along the later O'Farrell-Hyde-Jones route necessitated using the Eppelsheimer bottom grip, which was suited to picking up the rope using only a slight dip in the roadway to contact the cable. Eppelsheimer developed this grip, which is used on all present cable car lines in San Francisco, for the Geary Street Park & Ocean Railroad in 1879.

The Eppelsheimer bottom grip used the lever and quadrant developed for the Sutter Street Railroad as an improvement over Hallidie's screw grip. The grip is what makes the cable car move, as it is the link between the car itself and the moving cable under the street. The grip is attached to the floor of the car by a solid carry bar. The outer portions of the grip are composed of a center plate, crotch, and shank plates. When the center plate is lowered by the gripman pulling on the lever, hinges fastened to it are forced by rollers to smoothly tighten two semi-cylindrical dies against the cable, in a vise-like grip. This starts the car moving smoothly and pressure can be adjusted by the gripman pulling back or releasing the lever, which can also be adjusted to accommodate steep grades where more pressure is required.
The gripman can also adjust the lever to not only grip and release the cable, but also keep the cable in the grip but running free, by moving the lever to a halfway point on the quadrant. This is used when the car stops to let passengers on and off. A sand plate at the bottom of the grip protects the mechanism and also plays a role in guiding the grip around a pull curve.

The dies themselves take a lot of wear and tear and so are replaced at regular intervals of 3 to 4 days.


	The Grip
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