Author Topic: Steam engine eccentric crank angle  (Read 1078 times)

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Bill H

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Steam engine eccentric crank angle
« on: September 11, 2016, 03:42:32 PM »
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Group:
Rebuilding some steam engine valve trains after replacing damaged driver sets (came from the factory that way). Trying to determine the correct angle of the eccentric crank. As an example, if the crank pin is at exactly six o'clock, and thus the counterweight is at twelve o'clock, it seems on some engines the eccentric crank is pointing to about two o'clock, in other engines about ten o'clock. Is there a correct angle dependent on the type of valve gear or?

Kind regards,
Bill

Big Train

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Re: Steam engine eccentric crank angle
« Reply #1 on: September 11, 2016, 04:31:06 PM »
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Who makes the engines?

Some manufacturers use keyed cranks, some don't. I'd check Spookshow's awesome website:

http://www.spookshow.net/locos.html

Maybe some of the photos can provide some guidance. If that doesn't help, sometimes you can find the exploded parts diagrams from various manufacturers online.

Or, I'm thinking if you can find the prototype locomotive photograph also online that may give you an idea. Just had a quick look at a Kato 2-8-2 Japanese prototype loco (It's the only one on my workbench right now that I can grab immediately). If the counterweight is at "top dead centre" at 12 o'clock, the crank angle is 10 o'clock and 11 o'clock, regardless of which side you look at.

I'm not sure if the Japanese engines are similar in design to North American engineering practice, so check your references.

Hope this helps.


Bill H

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Re: Steam engine eccentric crank angle
« Reply #2 on: September 11, 2016, 05:24:04 PM »
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Hi Big Train:
I did not mention the manufacturer, as it seems at least in my collection of steamers, that it is all over the place, regardless of the manufacturer. Further, I am not terribly convinced that some factory assemblers really pay that much attention to it.

I found an interesting discussion here http://www.trainorders.com/discussion/read.php?10,1812445 but I am not knowledgeable enough about steam engine linkages to determine what is correct for a specific engine - be it inside or outside emission. Perhaps one of the more knowledgeable steam gurus could jump in.

Kind regards,
Bill

peteski

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Re: Steam engine eccentric crank angle
« Reply #3 on: September 11, 2016, 05:40:03 PM »
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Most of the model crank pins with eccentric cranks are keyed (they are usually square-shaped) to install at the proper angle.  If they are not then you would need to refer to the prototype photos for the proper angle.

To make things worse, on real locos the crank leans in the same direction on both sides of the loco. But that means the model manufacturer would have to make separate mold for left and right wheel.  More often than not, manufacturer cheats and only makes one mold, so on the model one crank is always leaning the wrong way.
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randgust

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Re: Steam engine eccentric crank angle
« Reply #4 on: September 11, 2016, 08:02:57 PM »
+1
May help:

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There's inside and outside admission there, which actually doesn't seem to have much to do with it.

mmagliaro

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Re: Steam engine eccentric crank angle
« Reply #5 on: September 13, 2016, 06:33:48 PM »
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I almost hate to admit this, but I don't worry about it that much when reassembling a steam loco model,
other than to make sure that when the counterweight it at 12 o'clock, the eccentric crank is pointing "mostly straight up".
Sometimes it's at 10 o'clock, sometimes it's at 2, and sometimes, the crank points straight up to 12.
If the crankpin is keyed, as Peteski and others have pointed out, that usually makes the decision easy because
if you don't get it right, the wheels can't make it around without locking the rods.


Cajonpassfan

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Re: Steam engine eccentric crank angle
« Reply #6 on: September 13, 2016, 08:33:11 PM »
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OMG, is that thing made of wood?
Nice!

Otto K.