How they work - what's happening out of sight on the San Francisco Cable cars?

[Music] foreign [Music] Francisco's cable cars are 150 years old I've told their story in another film here we're going to take a look at what makes them tick or rather Clank Rattle and Roll the Subterranean cable moves at a steady nine and a half miles per hour the cars aren't permanently attached to it they grip on to start moving when the system opened the initial grips used a screw down mechanism but a few years later engineer William applezheimer developed the grip which is still in use today most of the grip is below the cable car's floor only the operating lever sticks up above it the lever has a catch handle attached to a ratchet so you need to pull the handle before you can move the lever the grip protrudes below rail level through the slot in the road there two grip Jaws enclose two dies which is the part of the grip which squeezes the cable in this example we're starting with the cable dropped which means the lever is all the way forward this is how you'll find it at a Terminus when it's time to depart the cable car conductor will pull a Trackside lever this lifts the cable into the jaws of the grip now the grip man can move the lever back so the Jaws tighten around the cable at first the cable is allowed to slip through the Jaws this makes a Midway stop at a station simple to get moving the lever is pulled back slightly and the dies tighten on the cable now we're off to reach top speed the lever is pulled back again so the dies are exerting maximum pressure on the cable because they suffer so much wear the dies have to be changed every few days dropping the cable and picking it up is more common than you might think look out for signs like this near a Terminus here the grip man is reminded to let go of the cable because they're approaching a turntable moving off the turntable the track goes into a little dip that's not just to help the car move but to bring the grip closer to the cable here's the conductor pulling the Trackside lever moving the cable into the jaws of the grip the cars on the California Street line can be worked from either end so they don't need a turntable to reverse they Coast into a headshunt and then the conductor still needs to pull up a lever to lift the cable into the grip Jaws as they depart they pass over sprung points moving them onto the correct running line under the slot the cable sits in a channel every few feet there's a pulley wheel to support it as the grip of the cable car moves forward it actually lifts the cable up slightly so there's no danger of the grip and the supporting pulleys colliding but cable cars are not made to run on the flat and in San Francisco they have to cope with frequent changes in gradient which does make things a little more complicated below the slot here's an example the cable still sits on pulleys but where the climb starts a pulley is actually placed on top of the cable it's known as a depression pulley and holds the cable down otherwise under tension it will rise up and rub on the underside of the slot as we've seen as the grip moves forward it lifts the cable off the pulley as it gets close to the depression pulley the effect is reversed and the cable is pushed off the bottom of the pulley however without some sort of mechanism the depression pulley is still in the way of the grip going past the early Engineers came up with a solution the depression pulley is mounted on a beam which has a pivot at one end as we've seen when the grip approaches the cable is pushed downwards when the grip reaches the beam it forces it to Pivot to the side moving the depression pulley out of the way the grip runs past without interference and when it's gone by the beam is sprung to Pivot back into place and the cable Rises up again to engage with it all of this happens out of sight automatically when the first San Francisco cable car line opened in 1873 it traveled in a straight line later Innovations mean the cable cars can now navigate sharp corners and there are two ways of doing this the first is called adrift or let go curve here's a tight corner with the running rails slot and cable the easiest way for the cable to change direction is to make a quarter turn around a large pulley before being guided back under the slot now headed in a New Direction but the cable car still needs to follow the rails of course as it approaches the curve the gripman drops the cable the car travels around the corner under its own momentum hence the term drift curve and then when it's back above the cable the grip man can reattach but if you're turning a corner and going uphill a drift curve won't help New Zealand developed the pull curve and it was imported back to San Francisco here the cable is guided on small individual pulleys as the cable car grip approaches the cable moves off the pulleys and a chafing bar located above and in front of them ensures the grip doesn't hit them now the cable car is under power for the whole turn again look out for the signs to remind the grip man here the car is approaching a pull curve and the instruction is to take rope you can spot a pull curve by the number of metal inspection hatches covering the small pulleys all the way around the corner climbing Powell Street look out on the right hand side for the small octagonal cabin this is the closest the system comes to having a signal box it's needed because the California Street and pal Street lines cross here the Powell Street cable goes underneath the California Street cable so it's vital the grip man on the pal car drops the cable so the two don't interfere with each other with just three cable car routes surviving one powerhouse can handle moving all the cables a motor Powers a large driving wheel called a sheave because it has grooves around its circumference next to it is an idler shiv which is unpowered the cable comes from the street and winds in a figure of eight around the sheaves to maximize the contact it has with them reducing slipping it then heads along the building to attention sheave before heading back to the street the tension sheave is mounted on a carriage with a counterweight as the tension changes with cars attaching and detaching it keeps the cable tight the cable can also stretch by a hundred feet over its working life so maintaining the correct tension is vital add the lengths of all four cables together and it comes to more than ten and a half miles under the Streets of San Francisco [Music] foreign [Music]

Channel: Tom Ingall

Views: 12,850

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Id: lazLKSLszuY

Length: 7min 47sec (467 seconds)

Published: Sat May 20 2023