I run a 1/8 scale live steam locomotive.
While testing the vacuum brakes on my tender, I blew out the diaphrams that
activate its brakes.
The original unit was used as a vacuum advance for old-time carburetors and
is now an endangered species.
Anybody have success making their own from scratch?
If so, would you be willing to share the process?
Thanks in advance,
Chet (engineer of ANCR Mogul #9401)
Haven't built one, but a distributor vacuum advance was expected to
move a few grams of linkage, not transmit enough force to (in concert
with two dozen others) stop several hundred pounds of train and
You might have to come up with your own design - make two shell
halves that bolt together with 6/32 screws to roughly duplicate the
shell of your vacuum advance, get some fabric reinforced neoprene
gasket material - think fuel pump diaphragm, and two fender washers
sandwiched in the middle on a rod nut for connecting an actuating arm.
Go take apart an old mechanical fuel pump for ideas on how to build
the working section - they transmitted the levels of force you need.
Or look at a truck air brake actuator for ideas. Scale down.
Or see if you can find a miniature air cylinder that is the right
size and shape for the job, and you can just buy them and drop them in
instead of reinventing the wheel. Clippard or Cincinnati or Bimba...
On that note, you can get a lot more power from air than vacuum,
even under "Direct Air" - or duplicate the full WABCO Relay system in
scale, and if the train brake pipe breaks everything stops.
--<< Bruce >>--
How about using a vacuum operated windshield wiper motor instead? They
have a lot more power than a vacuum advance would have. This "old
stuff" isn't gone, you just have to know where to look. In Minneapolis
Mn there's a place called "Little Dearborn" where you can get almost
anything for old Fords at least. I didn't say "cheap", though.
I'd also try an outfit like Bimba who makes air cylinders. They make
em pretty small and the difference between air pressure and vacuum isn't
all that great.
Also, there are companies that make vacuum sensing switches. We used
to use several open bellows types. There are your bellows.
YES! I built my own cruise-control in the 1980s. I built a "sock"
type actuator. It's really simple.
Get an inner tube of the un-inflated diameter about what you want.
Find a piece of metal or plastic tubing into which it will fit
Find or fabricate a disk that will fit inside the inner tube when it
is doubled over inside itself.
Cement/fasten/clamp the inner tube to the disk. Then turn the whole
assembly inside-out, so the disk pulls the "inner" portion down inside
the "outer" portion.
Now slide the whole tube/disk affair inside your metal tubing, and
fold the free end of the inner tube over and around the butt-end of
Seal it with a little adhesive between the outer surface of the metal
tube and the inner tube fold-over. Add over that a cap that fits
tight. Apply vacuum to the cap end.
You may also install a return spring inside, or rely upon an external
Mine lasted eight years of daily interstate driving without a leak.
Many thanks for the info on your DIY vacuum actuator.
I'm sure if I were looking at your unit your description would match
BUT not having one in my hand, I must confess the description leaves me a
bit confused : - (
Any chance you could e-mail a drawing or equivalent???
This is clearly a case of needing to see the 'picture' before I can see the
'light' : - )
And thanks to all the others who offered great suggestions for obtaining a
I'll keep them on file in case my DIY version fails to perform...
On Wed, 27 Aug 2008 21:14:45 -0500, Lloyd E. Sponenburgh wrote:
As you've drawn it, it looks like pulling a vacuum would tend
to collapse the membrane. Conversely, it looks like a proper
setup for pneumatic operation.
If the push rod and vacuum inlet had their positions swapped,
pulling a vacuum would pull the membrane out against the cylinder,
improving the seal. Conversely, pneumatic operation would tend
to collapse the membrane.
In short, from your earlier description I had envisioned the
membrane being on the other side of the vacuum, ie, with the
vacuum inlet on the left and push rod entering on the right.
So, have I got things backwards, or maybe the direction doesn't
Y'know, I might have mis-remembered the arrangement -- it was, like,
1978-to-1982 when I built it, and I was copying another device I
couldn't afford to buy.
I think you're right about which end the vacuum goes on. I didn't
catch that when I drew it, but it would make more sense.
(Chet? You listening?)
You bet I'm listening!!!!!
And I must confess to being a bit confused. Both ways seem to have the
potential to work correctly.
Guess I'll have to plan on making a couple and see which one works best!
I'll let everyone know which one won the competition. ; - )
No.. the way I drew it, it would only work under positive pressure.
Reversing the end the rod works through will allow it to work under
vacuum. Otherwise, when vacuum is applied (to the end I drew), the
sock would simply collapse inward.
On Aug 28, 6:44 am, "Lloyd E. Sponenburgh"
Looks like it would work fine to pull the actuator bar in to the
device, it just needs an external spring or such to pull it back out.
But, it also looks like the sock would collapse on itself, so pulling
vacuum such that the sock is expanded against the walls (from the
other end) would probably be better for smoothness and longevity.
I knocked together a quickie version of the modifications suggested for
Lloyd's sock-type actuator.
I was blown away by it's power.
I used some 1.5" pvc with caps and a bicycle inner tube.
Hooked up a small vacuum pump, and couldn't not believe the amount of 'pull'
on the rod!
The sock-type actuator doesn't have any force advantage over a
diaphram type. The advantage is its essentially unlimited length of
Unlike a diaphram type, the throw of which usually cannot be more than
an inch or so in practical versions, you could make a sock-type
actuator with a two or three foot stroke just as easily as one with a
That can help your avoiding cludging up stroke-amplifying (and force
dividing) bell-cranks and other such devices to get the stroke of the
cylinder to match the throw of your throttle linkage.
Thank you for the additional info! It's appreciated!
Would you mind if I posted your info on another website,
I also asked for help on their Live Steam forum, but without getting a
successful design like yours.
Needless to say, you'll get full credit for the design and info... (Think
you'd want to modify your original drawing to show the reversed rod and
vacuum input? If not, I'll post it as it is with the suggested changes.)
"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message
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