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Cable
cars inspire many questions, such as: "What makes the cable cars
move?" "How do the cars take hold of the cable?" "How
do they go around corners?" "What happens when one cable line
has to cross another?" The following is a series of brief
descriptions of how the cable cars are able to run up and down San
Francisco's hills.
Photo: During rush hours cars were dispatched every 15 seconds up Market Street from the Ferry Building by the United Railoads of San Francisco, who took over the City's largest cable car system in 1902, the Market Street Railway, (1905) |
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The
Cable Beneath the Streets. Four loops of wire rope
are run at a constant 9 ½ miles an hour from the Washington-Mason
powerhouse in channels beneath the streets. A system
of pulleys supporting the cables from below allows the cables to
move in the channels. At the end of a line, the cable
is turned by a large pulley called a sheave (pronounced "shiv").
Cable channels can be seen from the Museum Sheave Room. Photo: In a very unusual sight, the cable is seen resting on the street after being removed from the channel because of a fray. |
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How the Cable is Powered. At the Washington-Mason powerhouse each cable has its own drive machinery -- a 510-horsepower DC electric motor, gears to reduce the speed of the motor to the proper cable speed, and a set of three sheaves. Museum visitors can observe the powering of the cable from the Museum Observation Gallery. |
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How
Cable Cars grip and let go of the Cable.
The cable car's grip – essentially a 300-pound-plus pair of
pliers – extends through a slot between the rails and grabs
hold of the cable to pull the car along. With the grip handle at 12
o'clock (straight up), the cable is in the grip but not being grabbed.
As the gripman pulls the grip handle to the rear the jaws tighten
down on the cable and the car starts moving. The harder the gripman
pulls the handle back, the more pressure the grip exerts on the cable
until the cable car is moving at the same speed as the cable –
9½ mph. When the gripman "sinks" the grip to reach full cable speed, the handle always comes back all the way. It takes far less grip (pressure) to reach cable speed on the level than on hills and there is an adjuster on the back of the grip handle to allow for easy operation on level ground. Before steep grades, the gripman adjusts the grip so its jaws can exert more pressure on the cable (as much as 30,000 psi). It takes far more strength and leverage for the gripman to sink the grip when it is adjusted properly for the steepest grades. The more passengers, the more power it takes to sink the grip. If the gripman fails to adjust the grip properly for the grade two things can happen. Too tight and he can't sink the grip fully. Too loose and the car does not grab the cable tightly enough. In either instance, the car starts slipping on the cable. The gripman must stop and, on the conductors bell signals, back the car down to the bottom of the hill, adjust the grip and try it again. There is considerable wear on the mild steel dies which are the part of the grip that actually close on the cable. They act a bit like a clutch in a motor vehicle but the dies wear out in about four days. If they wear out while the car is in service, the grip is replaced on the street. In March 1971 Muni started to replace its existing grips with a new design that was bigger, heavier, and safer than the prior model. The “T- Model” design, named for Don Troya, chief cable car grip builder uses interchangeable parts in contrast to the individually machined and custom-fitted parts on the former grips. Each of the new grips contains 306 pounds of steel and five pounds of brass. Almost 200 pounds of that steel is in high-strength castings. The new grips have more parts than the former grips – 62 different types of parts and 149 separate pieces. |
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How Cable Cars Stop. To stop a cable car the grip person must "let-go" of the cable. Each car has three types of brakes: the wheel brake, the track brake, and the emergency slot brake. Each wheel has its own wheel brake, a soft steel shoe that presses against the wheel to help stop the car. On Powell Street cars the lever on the rear platform operates the rear truck wheel brakes, while the gripman’s foot pedal in the front compartment engages the wheel brakes on the four front wheels. Track brakes are two-foot-long blocks of soft Monterey Fir (previously, Muni used soft pine), located between each of the four sets of wheels. Only high quality dry Monterey Fir without any moisture, knots, splinters or cracks is used. When the grip person pulls back on the track-brake lever, the blocks press against the tracks to help stop the car. When the grip person pulls back on the red emergency brake lever (very rarely done) the slot brake, a 18-inch steel wedge, is forced into the slot between the rails abruptly stopping the car. Museum visitors can a view soft pine wood track brake. |
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How the Powell and California lines cross. At this crossing, Powell Street cars have to “let go” of the cable because the Powell Street cable runs beneath the California Street cable. The Powell cars heading towards Market Street coast across California Street, picking up the cable by means of a dip in the track, or "take rope" as it is sometimes called. Northbound Powell Street cars coast downhill three blocks before picking up the cable at Jackson Street. The Cal Cable line, which had the prior franchise, had the right to run its cable above the Powell Street company's, avoiding the problems associated with letting go and picking up the cable. The blueprint below illustrates how the Powell Street and California Street lines cross. The cable car crossing at Powell & California and Mason and Jackson are the world’s only cable car crossings. The lower cable is the Powell Street, showing that Powell Street grips must be released from the cable before passing over the California Street line. The stationary depression pulleys force down and hold the Powell cable below both its normal distance below the street and that of the California cable. Powell cars at the "Let-Go" plate must have their grip disengaged from the cable. The bumper and bell provide a warning to the gripper, if they have failed to "Let-Go." If a Powell car should stop in the intersection, then it must be pushed by hand to a place where it can either coast or pick up the cable.
Photo: The cable car intersection of Powell and California is controlled by a signal tower. This is necessary since the grip person on an uphill car cannot see automobiles or cable cars on the other street and may not have time to make an emergency stop at the intersection. |
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Cable Cars Go Around Curves. Cable cars can go around corners
by two different methods. In the first, the "let-go" or
"drift" curve, the grip person simply releases the cable
to let the car coast around the corner. Once the car is around the
corner, the cable can be taken up again. However, if the car is
going both around a curve and up a hill a "pull" curve
is required. In a pull curve, a series of horizontal pulleys guide
the cable around the turn and car remains attached to the cable.
A chafing bar, located above and just to the outside of these pulleys,
prevents the grip from striking the pulleys and reduces the lateral
strain on the grip shank exerted by the pull of the cable. As a
car approaches a pull curve a slight reverse curve positioned the
grip, which pulls the cable up and away from its normal resting
place against the pulleys, outside the chafing bar. The car proceeds
around the turn at full grip, traveling at the speed of the cable. |
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Left: A close up view of a cable car bell that is used as a warning device of an approaching cable car. The cable car bell is an intergal part of cable car operations. An important part of cable car operations is the cable car bells. The most famous bell sounds are those made by the gripman to warn people, motor vehicles and other cable cars that the cable car is approaching. Bells are also used for the conductor and gripman to communicate with each other. For this purpose a smaller bell is used, called a conductors' bell. Below are the current bell codes: (Note: You can view a short video clip of a recent bell-ringing contest by going to the Museum's video section and clicking on clip #7.) |
| Conductor
to Gripman |
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| One
Bell |
Car
Running |
Stop
smoothly @ next regular stop |
| One
Bell |
Car
Starting |
Stop
immediately, but smoothly |
| Two
Bells |
Car
Standing |
Go
Ahead |
| Three
Bells |
Car
to stop immediately, but smoothly |
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| Four
Bells (two-two, two-two) |
Back
slowly, road behind is clear. While backing: One bell stop traffic
coming; two bells back path is clear. Four-bells is given continuously
by conductor, until gripman signals with roof gong (two bells)
that no further backing is necessary. |
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| Gripman
to Conductor |
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| One
Bell |
Apply
rear brake |
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| Two
Bells |
Release
rear brake |
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| Three
Bells |
Conductor
to apply slot brake (No. 61 line)
On Powell Cars this is an emergency, conductor give me everything you've got on rear brake, or gripman is to apply slot blade, hold on and warn passengers |
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| Four
Bells |
Desire
to back car |
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