"Heart Link" Operation Principle

[McReef]

Hey All,

I am newly joined, though I have done a bit of reading here up until now.

I am in the process of designing and putting together a 7.25 gauge steam "outline" locomotive. This site in particular has been a goldmine of information, thank you for that! It was here that I finally, I think, figured out how lead truck centering devices are intended to function, there is precious little information floating around out there describing this particular aspect. I do still have a few questions however. The first is whether I have put the pieces together properly in my head and have the right idea concerning the general workings involved. I have put together a little animation to illustrate my interpretation and will attach it below, please let me know if I am way off the mark.

Assuming I have the principals involved correctly worked out, that leaves me with the finer details to work out...

First, is the heart link generally considered to be the best way to go, or would a double ramp arrangement perhaps be a better course to pursue? Six of one, half dozen of the other?

If I were to stick with the heart link arrangement, it occurs to me that the performance of this device would be drastically affected by the spacing of the two pins at the top of the link. A greater spacing of the pins would seem logically to impart greater resistance to displacement. Is this considered to be a critical design criteria that needs to be carefully calculated, or is a "best guess" configuration likely to result in acceptable performance?

I realize it is entirely possible that I am presenting the group with overly general questions, or may be leaving out some crucial bits of data that would be required to properly answer my questions. With that in mind, I ask for your patience with my ignorance, and that you feel free to set me straight if I am out of order in any way.

Thank you in advance for any help you can give,
-Dave

[Fender]

On the prototype drawings for my mikado, the two pins are actually side-by-side, so the separation is at a minimum. Putting the pins too far apart would be a mistake, in my opinion. Their main purpose is keep the loco from hunting left and right on straight track, and to guide it into broad curves at some speed. On sharper curves (and with lower speeds) when the sideways displacement is greater, the role of the lead truck is much less.

[johnpenn74]

The correct thing to ask here is how is the motion of a heart link different from a swing link (assume verticle for this discussion). In both cases heart or swing, the links use gravity pulling down to create a horizontal vector of force the will push the lead truck back to the center and minimize the hunting of the engine. The difference lies in the resulting path. Heat links have a "V" type motion so once centered it tends to resist the the horizontal forces a little more and stay centered. The swing motion links, do the same be to a lesser degree because the swing link motion is more of a semicircular path, so there is more hunting.

Scroll down the threads a little ways and there is a good discussion of Swing link behaviors about a month or so old.

JP

[McReef]

Thanks guys.

On the prototype drawings for my mikado, the two pins are actually side-by-side, so the separation is at a minimum. Putting the pins too far apart would be a mistake, in my opinion. Their main purpose is keep the loco from hunting left and right on straight track, and to guide it into broad curves at some speed. On sharper curves (and with lower speeds) when the sideways displacement is greater, the role of the lead truck is much less.

The spacing of the pins seems to be a place of some significant variance from what I can find on these systems. It seems as though some have quite closely spaced pins, while others appear to have a wider arrangement, or the equivalent of such. It is hard to work out how much centering force is appropriate, perhaps this will all be left to a bit of trial and error to see what works for my particular application...

The correct thing to ask here is how is the motion of a heart link different from a swing link (assume verticle for this discussion). In both cases heart or swing, the links use gravity pulling down to create a horizontal vector of force the will push the lead truck back to the center and minimize the hunting of the engine. The difference lies in the resulting path. Heat links have a "V" type motion so once centered it tends to resist the the horizontal forces a little more and stay centered. The swing motion links, do the same be to a lesser degree because the swing link motion is more of a semicircular path, so there is more hunting.

Scroll down the threads a little ways and there is a good discussion of Swing link behaviors about a month or so old.

JP

Yes, I did give that thread a read, and quite a few others here. That one in particular wa helpful in putting together the overall function in my head.

From what I gather, in general, it seems that the heart links are the most commonly applied, though rocker arrangements and double inclines are not unheard of by any means. I guess I'll just have to put something together and see how it works.

[daves1459]

Many years ago I built a Little Engines 0-6-0. It was a good puller. However. under heavy load, particularly going up grade, the front end would pitch side to side in sync. with the thrust on the crank pin generated by the pistons . No derailment occurred because of it as far as I cam recall, but, the pitching back and forth was a bit disconcerting. So I decided to extend the front of the frame and add a two wheel lead truck. The first design I built used swing links that slanted inwards at the top of the link. That design reduced the side to side movement somewhat but not much. It became apparent that initial side to side resistance was minimal. It wasn't until the links were 15 to 20 degrees out of there centered position that much force was generated to return the loco to the center position. So I revised the design to the use of heart links. With the heart links the side to side motion was completely eliminated and the overall ride of the loco was very much improved. Apparently the three point contact of the heart link has more resistance to movement out of the central position than a swing link. As to what is the optimal spacing of the two top pins of the heart link and their ratio to the length of the link I can not say. I copied the design I used directly out of a Locomotive encyclopedia. If you think it will help contact me direct and I'll return a photo copy of the design I selected.

Dave S.