Re: Wooden rims

Mark Schecter said:

A Muzi said:

Wood is a singularly poor choice of material for a braking surface.>

Just as a point of interest, the cable cars in San Francisco use wooden brake
shoes bearing downward against the steel tracks as the friction material of
choice. It is cheap and easy to replace, which is needed every few days. But
the wood is the consumable, not the keeper part.

Dear Mark,

Here's a page about the innards and intricacies of the
triple braking system of San Francisco cable-cars:

http://www.cablecarmuseum.org/archive/Anat/Anat.html

The swine have "improved" the original site that Jobst
Brandt pointed out last year, so the good stuff is now
harder to find in the archives.

Carl Fogel

Mark Schecter said:

Quoted message said:

Wood is a singularly poor choice of material for a braking surface.

Quoted message said:

Just as a point of interest, the cable cars in San Francisco use
wooden brake shoes bearing downward against the steel tracks as the
friction material of choice. It is cheap and easy to replace, which
is needed every few days. But the wood is the consumable, not the
keeper part.

SF Cable Cars are not a good example of braking technology. SF Cable
Cars are practically devoid of working brakes, relying on the cable to
climb and descend at constant 9.5mph speed. The wooden blocks are
holding brakes used for boarding passengers while the cable is allowed
to slide through the grip. Cable Cars also have wheel brakes that are
hardly used because wheels slide easily and cause flat spots. The
front and rear wheel brakes are operated separately by the gripman and
conductor respectively. Besides, there is no need for a brake other
than the wood block parking brake, the wheel brakes being ineffective
in stopping the car even on milder grades.

http://www.sfcablecar.com/gripmans.html

On this site you can operate the various brakes and grip, as well as
the bell, which has the correct tone for a real cable car. The noise
you hear is the cable running over slot pulleys, something residents
along the route appreciate like the sound of surf at the beach. It's
background.

What the website model does not show, is the third lever that operates
the steel wedge which is the best example of failure in brake design;
the so called emergency brake. This "brake" is a steel wedge that can
be driven into the cable slot. It has a contact area of about a
square inch with no heat dissipating surface or cooling means. It has
never prevented a runaway from a collision at Bay Street (bottom of
the Hyde Street hill) and always shown to do what one should expect,
melt the wedge and weld to the cover plate upon impact with traffic.
That it requires a cutting torch to be removed, proves to the press
and onlookers that it has great stopping power. What is missed is
that molten steel is a good lubricant and that a brake must dissipate
energy.

The cause of these runaways is also obvious, since they nearly all
occur on the Hyde Street hill where there is a passenger pickup just
before the crest of the hill. Unless the gripman makes a special
effort, the car will not be up to synchronous speed with the cable
before the crest of the hill where grip must lift the cable from the
crown wheel under the street. At this point, a sliding cable can
readily peel out of the grip and occurs more so early in the life of a
grip that has not yet worn a pocket from cable slip.

Jobst Brandt
[email hidden]

Quoted message said:

Mark Schecter said:

A Muzi said:

Wood is a singularly poor choice of material for a braking surface.>

Just as a point of interest, the cable cars in San Francisco use wooden brake
shoes bearing downward against the steel tracks as the friction material of
choice. It is cheap and easy to replace, which is needed every few days. But
the wood is the consumable, not the keeper part.

Dear Mark,

Here's a page about the innards and intricacies of the
triple braking system of San Francisco cable-cars:

http://www.cablecarmuseum.org/archive/Anat/Anat.html

The swine have "improved" the original site that Jobst
Brandt pointed out last year, so the good stuff is now
harder to find in the archives.

Carl Fogel

Thanks for the link. There are a couple good pictures and more description at:

http://www.steamengine.com.au/railways/trams/sfcable/

His guess about the motors being 30-50 hp is way wrong, though, as the site
you gave says 510 hp DC.

Wonderful device, the cable car.

-Mark

Quoted message said:

Mark Schecter said:

Quoted message said:

Wood is a singularly poor choice of material for a braking surface.

Quoted message said:

Just as a point of interest, the cable cars in San Francisco use
wooden brake shoes bearing downward against the steel tracks as the
friction material of choice. It is cheap and easy to replace, which
is needed every few days. But the wood is the consumable, not the
keeper part.

SF Cable Cars are not a good example of braking technology. SF Cable
Cars are practically devoid of working brakes, relying on the cable to
climb and descend at constant 9.5mph speed. The wooden blocks are
holding brakes used for boarding passengers while the cable is allowed
to slide through the grip. Cable Cars also have wheel brakes that are
hardly used because wheels slide easily and cause flat spots. The
front and rear wheel brakes are operated separately by the gripman and
conductor respectively. Besides, there is no need for a brake other
than the wood block parking brake, the wheel brakes being ineffective
in stopping the car even on milder grades.

http://www.sfcablecar.com/gripmans.html

On this site you can operate the various brakes and grip, as well as
the bell, which has the correct tone for a real cable car. The noise
you hear is the cable running over slot pulleys, something residents
along the route appreciate like the sound of surf at the beach. It's
background.

What the website model does not show, is the third lever that operates
the steel wedge which is the best example of failure in brake design;
the so called emergency brake. This "brake" is a steel wedge that can
be driven into the cable slot. It has a contact area of about a
square inch with no heat dissipating surface or cooling means. It has
never prevented a runaway from a collision at Bay Street (bottom of
the Hyde Street hill) and always shown to do what one should expect,
melt the wedge and weld to the cover plate upon impact with traffic.
That it requires a cutting torch to be removed, proves to the press
and onlookers that it has great stopping power. What is missed is
that molten steel is a good lubricant and that a brake must dissipate
energy.

The cause of these runaways is also obvious, since they nearly all
occur on the Hyde Street hill where there is a passenger pickup just
before the crest of the hill. Unless the gripman makes a special
effort, the car will not be up to synchronous speed with the cable
before the crest of the hill where grip must lift the cable from the
crown wheel under the street. At this point, a sliding cable can
readily peel out of the grip and occurs more so early in the life of a
grip that has not yet worn a pocket from cable slip.

Jobst Brandt
[email hidden]

Jobst, thanks for the illumination. That site is really fun and illustrates
the mechanism for the layperson.

I didn't mean to suggest the cable car as an example of good brakes, merely as
an instance of the use of wood as the friction pad material. Yet, as the wheel
brakes are also not very effectual, if there were a better material for the
track brakes, woodn't they use it? For instance, sintered metallic or ceramic
as in auto brake shoes? In a certain sense, it seems that the design is
elegant in its sufficiency to the purpose, not perfect or ultimate, but pretty
darn good for the job at hand.

-Mark

Mark Schecter said:

Quoted message said:

http://www.sfcablecar.com/gripmans.html

Quoted message said:

Quoted message said:

On this site you can operate the various brakes and grip, as well
as the bell, which has the correct tone for a real cable car. The
noise you hear is the cable running over slot pulleys, something
residents along the route appreciate like the sound of surf at the
beach. It's background.

Quoted message said:

Jobst, thanks for the illumination. That site is really fun and
illustrates the mechanism for the layperson.

Quoted message said:

I didn't mean to suggest the cable car as an example of good brakes,
merely as an instance of the use of wood as the friction pad
material. Yet, as the wheel brakes are also not very effectual, if
there were a better material for the track brakes, woodn't they use
it? For instance, sintered metallic or ceramic as in auto brake
shoes? In a certain sense, it seems that the design is elegant in
its sufficiency to the purpose, not perfect or ultimate, but pretty
darn good for the job at hand.

As I said there is no real running brake on these cars and wood does
excellently for the parking brake. Unfortunately it gets a little
dynamic work as the car rolls to a stop on the level places where
passengers board. The wheel brakes are supposed to do that job and do
so fairly well but gripmen also know that the wooden pads do a fine
job of stopping the car from 9.5mph on the level. The lever that
operates the wooden blocks has enough force to lift the wheels off the
track when few passengers are on board. Having the grip in the slot
prevents the car from getting off track when this occurs.

Jobst Brandt
[email hidden]