Swing Link Talk

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ccvstmr
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Swing Link Talk

Post by ccvstmr »

Been having discussions with a few local hobbyists about proper swing link orientation. Shouldn't matter if the application is for a steam loco front end or swing bolster equipped rolling stock. Seems as though there are more opinions than possibilities. So, thought I'd turn to the board for additional comments, insight and explanation.

The following drwg was put together to help describe the different configurations:
swing link config.JPG
Three different configurations shown: 1) links parallel, 2) links angled in at the top, and 3) links angled in at the bottom. Dimensions are shown for reference only and to help show the pivot points and swing link movement. Based on the link motion shown in the diagram, the wheels set would be entering a left hand curve which results in the links and swing bolster moving to the right.

If the purpose of the swing links is to transfer load as the loco or car enters a curve, it seems the "links angled in at the top" is the proper orientation as the load is transferred (more) to the inside wheel(s) as the link straightens while preventing those wheels from lifting. Another benefit is the swing bolster elevates the the piece of equipment to pitch inward on that curve. The "links angled in at the bottom" does just the opposite...would reduce the load on the inside wheels and have a tendency to pitch the loco or car outward (not good). Parallel links...does not offer proper load transfer.

So, I put it to the board contributors to offer their opinions and experience. Thanking all in advance. Carl B.
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Steggy
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Re: Swing Link Talk

Post by Steggy »

ccvstmr wrote:If the purpose of the swing links is to transfer load as the loco or car enters a curve, it seems the "links angled in at the top" is the proper orientation as the load is transferred (more) to the inside wheel(s) as the link straightens while preventing those wheels from lifting.
Your supposition is correct, and what you describe follows typical automobile suspension geometry. In all cases, the intention is to make the centrifugal force acting outward on the locomotive or car to work against itself. A similar principle was used in the days of fully independent automobile front suspension to reduce the tendency for the front end of the car to nose down during braking. This so-called anti-dive feature is a byproduct of the unequal length A-arm suspension system that dominated in the days before the introduction of the (technically inferior but cheaper) MacPherson strut. The same principle is present in the rear suspension of rear wheel drive cars, except there it is called anti-squat.

Aside from maintaining stability in a curve, the lateral equalization effect of a negative trapezoidal linkage has a tendency to produce more even wheel wear on equipment running mostly in one direction on a circular track.
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FLSTEAM
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Re: Swing Link Talk

Post by FLSTEAM »

swing link.pdf
(10.18 KiB) Downloaded 353 times
Here is the pilot swing link I used on my Mogul. Copied from a full size engine.

Jb
jscarmozza
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Re: Swing Link Talk

Post by jscarmozza »

I'm building a 1" scale GG1, according to what I found on line regarding their trucks, they had a rocker system instead of links; so that's what I made. It seemed to work fine on a couple on trial runs made with the incomplete chassis, I since added lateral coil springs to dampen the motion, I haven't tried to run it with the springs so I'm not sure if that was a good idea. Originally I was going to copy the cardioid swing links used on my LE Atlantic, but they're difficult to machine and don't seem to swing freely. John
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Bill Shields
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Re: Swing Link Talk

Post by Bill Shields »

rockers do one thing very well...they fall out....

had rockers on my 3/4" Hudson for about 2 months...

1 month to realize they were a PITA
1 month to make captive roller replacements that have been in for 30 years.
Too many things going on to bother listing them.
jscarmozza
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Re: Swing Link Talk

Post by jscarmozza »

I never had them fall out, but when I moved the chassis around or took the load off the trucks the rockers would flop around. I corrected the problem by adding a coil spring between the pivot plate and truck frame on each side. That kept the pivot plate centered and the rockers aligned in their seats, now I can handle the trucks without any problem. The rocker bottoms are hinged to the frame, the tops have seats in the bottom of the pivot plate and are connected to the plate and each other with a bar. The bar keeps the rockers parallel and has a slightly oversized slotted hole at the plate connection to facilitate freedom of movement. It works well.
jscarmozza
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Re: Swing Link Talk

Post by jscarmozza »

I was looking at my rocker system today, it's a parallel system. I understand the benefits of the trapezoidal configuration and I'm looking at how I can modify the pivot plate to toe in the rockers at the tops. Thanks Carl and Dino for describing how a swing link is supposed to work. John
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Re: Swing Link Talk

Post by Smokey N Steamer »

Looking at photos of EMD's Blomberg truck, with its swing hangers angled at the bottom, is that a flawed design based on your study?

Image

I'm confused. By "angled at the bottom" are you referring to the middle or bottom row in your original illustration? Also, you describe the illustration as the truck is entering a left-hand curve, but the illustration says "on right curve." Can you clarify this?
ccvstmr
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Re: Swing Link Talk

Post by ccvstmr »

Well guys, 1st...thanks for the responses. 2nd...tried to find mention of swing links in other publications lurking around the library. Didn't find anything in my Kalmbach Car Builder's Cyclopedias. However, I found something in Mayers Locomotive Construction book. Several examples were illustrated. All but one showed the swing links angled in at the top. The only case described where the links were angled out at the top (similar to John B's example)...was for a single axle loco pilot truck that was equalized. Still, no explanation of the benefits of one configuration over another.

Smokey...the EMD Blomberg truck example is interesting. That too would behave similar to the bottom example in my initial drawing. Using the diagram of the Blomberg truck...and lets' assume we're facing front...as the loco nosed into a right hand curve...the bolster swings to the left. This (in my mind) shortens the vertical distance on the left side of the truck (as shown), but increases the vertical distance on the right side of the truck as that swing link approaches a true vertical orientation. To me...that's backwards as that would tend to pitch the loco body to the outside of the curve. Mind you...I realize that loco bolster is not moving much laterally. Still trying to understand why swing links would be configured differently (inclined inward vs inclined outward), and the benefits of each. Anybody else have ideas, examples, comments...other? Carl B.
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Steamin
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Re: Swing Link Talk

Post by Steamin »

One conclusion from previous discussions was to make the links parallel. The parallel design imparts a vertical force as the truck moves under the frame, which is desired, whereas If you have a parallelogram design, it also imparts a tilting force onto the frame (undesired).

See link for discussion: http://www.chaski.org/homemachinist/vie ... 88#p295188
jscarmozza
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Re: Swing Link Talk

Post by jscarmozza »

Well now that I have taken my trucks apart, should I leave them as a parallel system or adjust the rockers to tilt in at the tops? Is it possible that locomotive trucks function differently than car trucks, one staying flat and the other leaning into the curve? Just asking because I don't know. John
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Fender
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Re: Swing Link Talk

Post by Fender »

jscarmozza wrote:Well now that I have taken my trucks apart, should I leave them as a parallel system or adjust the rockers to tilt in at the tops? Is it possible that locomotive trucks function differently than car trucks, one staying flat and the other leaning into the curve? Just asking because I don't know. John
I would be cautious about making changes to an existing setup, if it works well now. A friend, who is building a pacific, took his loco to the track for the first time, and discovered that his angled links were causing the wheels on one side of the lead truck to lift off the rails, derailing the engine. He determined that the angles were causing the problem.
Another thing, does your truck have swing links, heart links, or rockers? The heart links have two pivot points at the top, and are generally parallel to each other. (heart links have a heart-shaped cutout at the top that hang from double pins, while swing links just have a single pin at the top and bottom).
Dan Watson
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