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Post by smokebox on Sept 21, 2007 21:37:23 GMT -5
Hello all!!! While I am supposed to learning how to do my new job in the Army I have been wondering........How do you do the math to compute the size of the pistons for a compound mally? I would like to make a 2 or 2 1/2" scale malley for 7 1/2" gauge and do not know the method for making the low pressure side. Any ideas?? ???smokebox  |
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Post by patyoung on Sept 23, 2007 16:40:41 GMT -5
Hello Smokebox,
I believe a thermodynamics engineer could probably answer this but my gut feeling is
that the determination was partially determined by engineering and part by experimentation.
I've been planning to build a small articulated (possibly a mallet but I think the added burden
of a high pressure and low pressure engineering concern out-weigh any benefits you get
from re-using the steam). In the June 1978 issue of Live Steam, W. Marshall Black discussed
his 3/4" scale 2-4-4-2 Little River mallet and mentioned that there is a supposed classical
ratio between the high/low cylinder diameters of 1 vs 1.5 or 1 vs 1.6. So for a 1 1/8" rear
(high pressure) cylinder, the front (low pressure) cylinder would be 1 3/4".
Let me know if I can help you with any other question and if you proceed with your mallet,
please share your experience (good and bad) with us.
Pat Young
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Post by doug on Sept 26, 2007 10:01:13 GMT -5
Smokebox,
The rule of thumb about 100 years ago for compound cylinder sizes was 1:2.25. This is about where you will find most compound locomotives, but is not a hard and fast rule. The Columbia River Belt line 2-4-4-2 has a ratio of 1:2.35, iirc. As a side note, I have seen prints for successful compound marine engines with ratios of 1:4, but suspect the greater ratio is due to the use of a condenser.
I was visiting with a steamboater last week who had a compound engine for his boat. He had gauges on both the steam cheasts, and commented that he tweeked the HP valve gear to get a high enough steam pressure in the receiver pipe to balance out the work both cylinders were doing. Keep in mind the receiver pipe pressure is counter acting the steam pressure on the HP piston, and must be subtracted from the calculation of how much work the HP cylinder is doing. At one time iirc, I figured out that on the 2-4-4-2 mentioned, with a boiler pressure of 200 psi, the receiver pressure would need to be around 60 psi for the LP cylinders to be doing as much work as the HP.
Consider using super heat for a compound loco model. We run at a lower pressure (ie, less heat in the steam), and also experience greater heat losses. Most model compounds I have seen have a problem exhausting steam, as it is usually condensed into water in the LP cylinders. The exception to this is one that does have superheat, and exhausts with a dry stack.
One big advantage to a compound mallet over a simple articulate is that the piping is much simpler, fwiw.
I don't beleive we had any ng mallets in this country, but other countries did have them. Harold Sinclair of NZ toured the west coast tracks about 15 years ago with a model of a 30" gauge 2-6-6-2 that I seem to remember as being built for a country around the Slavic region. At a scale of 3" to the foot, this was a big loco.
Hope this gives food for thought.
Doug E.
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Post by Harlock on Sept 26, 2007 12:36:45 GMT -5
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