Tim, if you aren't far along enough in the curriculum yet., sorry to ask:

The thread about the 643 brings up a question:

The story I was told about dynamic augment is that it is a result of the attempt to balance a horizontally reciprocating mass (the rods) with a rotating mass (counterweights). Somehow that doesn't sound right because the rods move vertically through a circular motion as well.

The bottom line as it was reloated to me was balance could only be acheived at a single speed. True?


Can anybody put that in terms a guy who never had a class in statics, let alone dynamics could understand?

Also
Lets assume for a minute that somebody took an existing engine and cast/forged? new lightweight rods. Would this reduce the imbalance(s) at all speeds or merely change the speed at which balance was attained?


The bottom line: If somebody were to spend money to restore an engine , would making new lighter weight rods (if possible) do anything to increase operability by reducing dynamic augment (at some or all speeds)?

Weeeeeellllllllllll,

Before I say what I think is right and make myself look dumb (something I'm all too good at doing), let me check some sources. In the meantime, anyone a little more seasoned than me wanna take a shot? :-)

Re: "Can anybody put that in terms a guy who never had a class in statics, let alone dynamics could understand?"


The weight of the rods as they hang on the wheels is entirely offset by the counterweights, which are placed exactly opposite to the rod connections to the wheels.

The weight of the piston, piston rod and the part of the main rod weight that is carried by the crosshead is partly offset by the excess counterweight in the wheels, or "overbalance."

The centrifugal force of the overbalance in the horizontal direction is overly offset by the piston, piston rod, and the part of the main rod that is carried by the crosshead.

The centrifugal force of the overbalance in the vertical direction pounds the track, and the resulting force on the rails is called "dynamic augment." Higher speed brings on higher dynamic augment. It is important to understand that, because the pins on the wheelsets are 90 degrees apart, the dynamic augment pounds the track on one side at a time, in a pound-pound-lift-lift sequence.

The net combined effect of overbalance and the push-push-pull-pull sequence of the pistons is the side-to-side or lateral floating motion you see on an approaching working engine.

Cross-counterbalancing sought to offset lateral motion by placing an additional weight on the main driver on the opposite side of the engine, in the same angular position as the pin on the first main driver. The effect is to offset the dynamic pull of the first main driver's main counterweight by using the leverage of its separation from the wheel it is acting upon.

Of course, adding cross-counterbalances disturbed the original main counterweights, so the main counterweights had to be re-adjusted, etc.

Counterbalancing rotation and reciprocation was an imperfect art....

Yep, according to my textbooks and a couple steam engineering sources, that about explains it.

Thanks Bruce!

As far as the lightweight rods on an existing engine, assuming the existing wheels (and therefore existing counterbalances) were utilized, the counterbalance would most likely be calibrated to overcompensate for the weight of the new lighter rods. This would definately increase the dynamic augment. And obviously as speed increased, the engine would pound the rails even more.

And for your bottom line...

In my not-even-close-to-expert opinion, lighter weight rods would only be advantageous on a restored locomotive if the counterbalancing was re-calibrated (which more than likely would require new wheels). However if a locomotive is having MAJOR wheel work done anyway, this may be something to look into. There are many newer types of metals and alloys that can definately take the stresses placed on rods while being much lighter than the forgings of old...

EDIT: forgings, not castings...what was I thinking? (thanks Finders!)

I'm not going to go as far as to say I'm seasoned or an expert, but I believe Tim that rods are forgings, not castings as you mentioned. Rods were hammered out to their rough shape usually by a steam hammer, and then machined to proper dimensions. Castings would have been too brittle to stand the abuse that rods took. Forgings that had been previously beaten while in a red hot state were able to take these stresses. Do you think the guy in this picture is deaf?

Also, remember that up till the thirties, counterbalancing was done by rule of thumb rather than calculated, there was a society of engineers that had an approved method for this though I don't have their name at hand. This got Baldwin in trouble with both the ALC Northerns and the NH Hudsons which were over counterbalanced. So by putting light weight rods on an existing engine, you may be making the imbalance worse rather than better. If one is really bent on light weight rods, a better middle of the road approach would be to only make the main rods light weight and leave the existing side rods as is. Since the side rods are a rotating mass their weight can be and probably is fully counter balanced. So the only counter balances that need be adjusted would be the wheel set that the main rod attaches too.

Ed

I'll just post that link over here, too, and that includes the conceptual formulas for counterbalancing.

This article got one of my structural engineers through the whole concept. I think you want to track down Mr. Leach! See page 5 of this. I sure learned a lot - this was truly an educational effort.

http://www.rrhistorical-2.com/rlhs/rlhs ... nl24-3.pdf

I'm certainly no mechanical PE, but the other basic concept is that you can really only design counterbalance effectively for one speed. Big variables that kill you are high tractive effort, small drivers, and more of them; hence the specific problems with high-tractive effort, low-drivered 10-wheelers.

The whole problem is certainly logarithmic with rotational speed and mass of the rods. Less of an issue on the vast majority of FRA 1 & II tourist RR track out there limited to 15/25mph. Start getting it back up to 40 and it becomes and issue on a locomotive balanced for a lower or higher number.

I suggest you get the book "Mastering Momentum" edited by Louis Sillcox. This is a collection of papers written just after the war on railroad subjects. I believe most of them are from MIT. There is a very good paper in this collection on dynamic augment. The book is availabe used, if you search the web, for around $25.00.

I'd say yes, he probably is deaf....

I spent two college summers working at a small steel mill (where my dad worked full time) in SouthWestern Pennsylvania back in the 1980s. I had a two week assignment in the blacksmith shop where there was a small steam hammer. That was the scariest piece of equipment that I've ever seen operate in my lifetime. I'll never forget the sound that thing made!!! It shook the ground far away from the shop.

JimE
http://jrevans.fbody.com/trains.html