DIY Vacuum Actuator?

Hi Guys! 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)
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On Tue, 26 Aug 2008 17:29:50 -0400, "Chet"

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 riders.
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 >>--
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Chet wrote:

Look at a cruise control throttle actuator.
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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.
Pete Stanaitis ---------------------
Chet wrote:

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diaphrams
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 closely. 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 the tube.
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 return spring.
Mine lasted eight years of daily interstate driving without a leak.
LLoyd
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Lloyd, Many thanks for the info on your DIY vacuum actuator. I'm sure if I were looking at your unit your description would match perfectly. 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 commercial substitute! I'll keep them on file in case my DIY version fails to perform...
Chet

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Sure... it'll probably be a day or so before I can do one, but I'll draw it up. It's dead-simple to make.
LLoyd
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Lloyd, you da man!!!!!
Chet
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I emailed one to you, and put it in the dropbox, as well.
"Sock Actuator.jpg"
LLoyd
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On Wed, 27 Aug 2008 21:14:45 -0500, Lloyd E. Sponenburgh wrote: ...

Ie,
http://metalworking.com/dropbox/Sock_actuator.jpg
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 matter?
-jiw
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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.
LLoyd (Chet? You listening?)
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Hi Lloyd! 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. ; - )
Chet
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have
works
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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.
LLoyd
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On Aug 28, 6:44 am, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:

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.
Dave
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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!
Chet
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of
With that size pipe, and depending upon the vaccum, it would be as much as (about) 26 pounds.
(probably less, since you aren't pulling down to 10^-8 Torr. <G>)
LLoyd
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BTW, Chet,
The sock-type actuator doesn't have any force advantage over a diaphram type. The advantage is its essentially unlimited length of stroke.
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 1" stroke.
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.
LLoyd
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Lloyd,
Thank you for the additional info! It's appreciated!
Would you mind if I posted your info on another website, http://www.chaski.org/homemachinist /
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.)
Chet
"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message

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suggested
of
type
a
force
the
Since it's embarrasing to miss that simple a problem -- YES, I'd like to re-do the drawing first... <G>
LLoyd
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I emailed you a corrected version, and put it back in the dropbox as "Sock-style_actuator_1.jpg"
LLoyd
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