Weight balancing a steam loco ?

[SkipGear]

I have an older Bachmann 0-6-0 that I did some experimenting with. While I added weight to the front end, the center of gravity was still closer to the third driver than I would have wished.

I then removed weight from behind the third driver which moved the CG close to the middle. I found that it actually increased pulling power.

Martin Z

It depends on the version of the 0-6-0. The traction tire has been all over the place on that loco. The current versions have the TT driver in the middle. The previous version had the TT driver in the #1 position. Before that, they were in the #3 (rear axle) position. I have a version with the TT driver in the front that will pull 15 + cars with a small lead "tool box" on the pilot. The first thing I do to the current version is move the TT driver from the #2 to the #3 position. Moving the driver on the current version will get it up to 15+ cars with no added weight because of the tailheavyness (is that a word?) of the loco.

[nscaleSPF2]

If I may be allowed to add to the discussion.  Ever since I read up1950s comment:

As a trains drag pulls on the drawbar , and where the drawbar is attached vertically and what vertical slop it has , the static weight on drivers may differ when under load .

And saw Peteski's sketch:



I started to think.  This is not always a good thing for an engineer, but here goes.  Peteski's analysis is 100% correct, providing that the drawbar is located at the same height as the center of the drivers, and is horizontal, like so:



Now if the drawbar is located higher than the center of the drivers, and is horizontal, we have this:



The drawbar force will tend to lift the forward driver and put more load on the rear driver while the load on the pilot and trailing trucks will not be affected very much.  Max pointed out that this can happen to the prototype in an earlier post.

Conversely (how's that for an engineering term?), if the drawbar is located below the center of the drivers, and is horizontal, we have:



This will actually tend to put a little more load on the forward driver, while removing some from the rear driver.

The analysis gets a little more complicated if the draw bar is not horizontal.  The fore/aft location of the drawbar attachment on the engine comes into play.

So what does all this mean to Max?  It means that, after he has added as much weight to the engine as he can, he can experiment with the vertical location of the drawbar to optimize the tractive effort of the engine.

Hope this helps.
Jim Hale

[mmagliaro]

I will let you know how it goes when I get to the point of pull-testing this model.
I am going to try to attach the drawbar on the centerline of the drivers (at axle height), because that
is the most stable position.

[SkipGear]

Always try to get the drawbar as far forward as possible also. It will have less leverage effect in both verticle and horizontal planes. Don't forget, locos need to go around curves and the farther the drawbar connection point is away from the drivers, the more leverage it has. The farther away from the drviers the more likely the loco will want to nose off the track in a curve, especially when pulling a heavy load.

[mmagliaro]

Always try to get the drawbar as far forward as possible also. It will have less leverage effect in both verticle and horizontal planes. Don't forget, locos need to go around curves and the farther the drawbar connection point is away from the drivers, the more leverage it has. The farther away from the drviers the more likely the loco will want to nose off the track in a curve, especially when pulling a heavy load.

Good point, and yes, I agree Tony.  (That's another brilliant plus in the Kato Mikado's design, of course.  They ran that drawbar
OVER/THROUGH the trailing truck and attached it right behind the rear driver).

[randgust]

The entire discussion about the drawbar position is possibly even more significant than weight balancing.    I had no end of trouble with my LL 2-8-4 with the split drawbar post under the cab.   It lifted and pried the front driver set off of curves under load.   I redesigned the drawbar position to imitate the Kato 2-8-2; i.e. pull from behind the rear driver, as low as possible - by making a new drawbar that fit in the screw hole of the trailing truck and went up and over top of the trailing truck.   Worked/works great and solved the entire tracking problem.

My rebuilt Kato 2-8-2 (PRR L-1) is massively balanced tail-heavy.  But with that drawbar where it belongs behind the rear axle and low on the frame it tracks and pulls well.   Pull torque pulls the locomotive DOWN in the front, not up.  And with the traction tire on the rear driver and that honkin' cast boiler, it's the standing tractive effort champ that I've seen for a single steam locomotive.

Meanwhile, a new Atlas 4-4-0 really likes to torque over the entire locomotive under load to the outside rail of the curve.    Still studying that one.  The lead truck really doesn't do anything except hopefully not derail.  It's a much worse problem under load than just running on its own.    I'm thinking this is exactly why the prototype needed a lead truck; not just to lead the drivers through a curve, but to try to hold the phenomenon to a minimum, the attempt of the lead axle to twist out and try to climb the outside rail.