# Articulated Locomotive Physics



## SlateCreek (Jan 2, 2008)

There's been some discussion lately of articulated locomotives.... and one interesting point that's come to light is that the front "engine" assembly generally doesn't bear the weight of the boiler from above, but via the flexible coupling between the front and rear engines. 

There are some drawings of how this works at: http://www.catskillarchive.com/rrextra/blwmal00.Html

What's interesting to me is the leverage that must be involved, particularly when you consider that you need to get as much adhesion out of the front "engine" as you do the rear one, or it's going to be breaking away and slipping all the time.

So how, in the model world, do you overcome the adhesion problem if bearing any of the boiler weight will cause tracking problems in the front truck? 

Matthew (OV)


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## ralphbrades (Jan 3, 2008)

Well the answer is very simple -but dire to calculate!!! 

*The flat saddle from the boiler rests on a longitudinal conical surface on the front power bogie*

You have to calculate the weight distribution on all wheels. Provided you have designed the articulation properly the line contact formed by the saddle and the cone will always remain within the gauge of the rails -thus stopping the front bogie from "rearing up".


regards

ralph


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## SlateCreek (Jan 2, 2008)

Except that the article mentions that the sliding surface is there just "in case of accident" and normally has an open air gap .... meaning there is no downward pressure on the front truck from the boiler above it.... which is what's confusing me. At least on, say, a two truck Shay, it's easy to see how each end is supported on a truck.... here, it's like one of those cantilevered patio chairs..... and even with them, there doesn't seem to be even weight distribution across the "foot" According to what I've been able to find, the axle loading for the front and rear engine assemblies is exactly the same, axle to axle.... and I'm not sure how that's possible. 

Matthew (OV)


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## Dwight Ennis (Jan 2, 2008)

Except that the article mentions that the sliding surface is there just "in case of accident" and normally has an open air gap 
I donno Matt - I searched the entire article linked in your first post, and I didn't see anything like that. Maybe I'm missing it, but I highlighted all occurrences of the word "accident" - and there weren't any occurrences.  

From what I read, the front of the boiler is either supported on sliding bearings, or if the boiler is articulating, the front is rigidly secured to the front frame.


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## SlateCreek (Jan 2, 2008)

Sorry, Dwight, and others: Right site, wrong page: 

http://www.catskillarchive.com/rrextra/mallet.Html 

Scroll down past all the valve and pipe stuff to: ADJUSTMENT OF THE ALIGNMENT OF THE FRONT ENGINE FRAMES for the whole thing and the diagram ... here's the part I was referring to:


_In designs of articulated compound locomotives having no front truck, where two sliding boiler bearings are employed, the front sliding bearing does not normally carry any load, but is merely an emergency stop in case of derailment or any unusual change in the alignment between the two frames. This bearing is so designed that when the front and rear frames are in proper alignment there is a clearance (ordinarily ¼ inch) between the upper and lower bearing and an equal amount between the upper bearing and the safety straps or clips, as indicated at Z and Z', Fig. 12._

Full Stop!

I think I've probably read that article and the associated drawing twenty times and missed that it's talking about a front truck with TWO sliding bearings.... and there not being much weight borne on the FRONT one. In posting that here, I finally saw it.... but will post it anyway, just so the rest of the conversation makes sense.

This makes a LOT more sense now.... but I'd still welcome commentary (exclusive of a commentary on my reading comprehension abilities!)

Matthew (OV)


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## Pete Thornton (Jan 2, 2008)

The Allegheny (2-6-6-6) in the B&O Railroad Museum has the weight of the front of the boiler resting on the front 'engine'. I seem to recall the chassis-mounted part is flat, and both parts are well greased.


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## Les (Feb 11, 2008)

Posted By SlateCreek on 02/02/2009 4:30 PM
Sorry, Dwight, and others: Right site, wrong page: 

[edited] 
.... but I'd still welcome commentary (exclusive of a commentary on my reading comprehension abilities!)

Matthew (OV) 









Aw, Matt, you take all the fun out of it!









Mallets make my head hurt to study them in all their glorious complication. 

Seriously, nothing with clarify thought like open discussion.

Now I have a question that leaves _me_ open to jeers--I've been contemplating a pair of 4-wheel power bricks from a couple of x-4-x engines I acquired. The notion of making an articulated X-4-4-X for my tight-radius layout struck me as rather clever. While I haven't scoured the web, I don't recall ever seeing such an arrangement--or recognizing it if I did. I envision a rod-type rather than a Shay or the like. I admire well-executed 'freelance' engines, but I want to remain somewhat prototypical.

Was there ever any such an animal?

Les


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## Semper Vaporo (Jan 2, 2008)

Les: 

Go here:

http://www.steamlocomotive.info/index.cfm

then click the "Search" link at the top on the left.

Then select the 0-4-4-0 or 0-4-4-0C "Wheel Arrangement" in the next screen that appears.

Lots of them!


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## SlateCreek (Jan 2, 2008)

http://narrowmind.railfan.net/ 

Scroll down to the articulated section. If you're into Narrow gauge, you're in luck. 

Matthew (OV)


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## Tim Hytrek (Jan 2, 2008)

Thanks Matthew for the info. 

Tim


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## Dwight Ennis (Jan 2, 2008)

Makes sense to me now too Matt.  Ain't gettin' old terrific? hehehe


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## Ray Dunakin (Jan 6, 2008)

I've been wanting to do a little 0-4-4-0 ever since I first saw a photo of that Mexican one a few years ago.


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## Les (Feb 11, 2008)

To all of you who posted re x-4-4-x:

Thank you for taking a moment. I sorta thought I those were a fig newton of my imagination.

Dwight, just outta the blue, you do a first-rate job as moderator. Here's an 'Attaboy' to stick on your monitor, instead of $$.









Attaboy --->







Monitor of the month!

Chuckle. (Been wanting to use that thingy).


Les


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## SteveC (Jan 2, 2008)

Yes, but do note that C.T.'s & Matthew's sites are not one and the same.


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## Les (Feb 11, 2008)

Which is why I thoughtfully used the word 'all'.....









Les


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## Dwight Ennis (Jan 2, 2008)

Posted By Les on 02/03/2009 8:31 AM
Dwight, just outta the blue, you do a first-rate job as moderator. Here's an 'Attaboy' to stick on your monitor, instead of $$.









Attaboy --->







Monitor of the month!

Chuckle. (Been wanting to use that thingy).


Les



Thanks Les. I appreciate it.


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