Steam & Excursion > Counterbalancing, Part 2

Later in loco engineering improvements, like after 1938(?)....
And a sophisticated engineering concept --- with benefits only apparent
when running at high track-speeds. 

The engineering & addition of cross-counter balancing was applied to
pairs of driver-axle assemblies.

Cross-counter balancing was used as a way to attempt to reduce the 
effect of lop-sided driver counterweights --- the two counterweights are
90-degrees apart, so when spinning at track speeds, you get massive
imbalances, compared to a situation where you could have crankpins
at 180-degrees.
( 'subject',  for a later discussion...)

Because of the 90-degree offset between the two crankpins on an axle-set, 
if left to roll freely on the roundhouse floor, the driver-set will come to a rest
with both counterweights closest to the floor --- in an angular compromise of
the position of the two weights.  They are massively heavy.

It was found that the imbalance of counterweights, while spinning down the main line
could be mitigated by adding a smaller weights opposite to the "main-mass" of 
a driver's counterweight --- on the other end of the same axle.

So, it was also revealed that a reduced weight has an equal off-setting weight,
if mounted --- at a distance-- on an axle-end opposite to the primary  counterweight.   

You will find two added, small, block-like counterweights --- if looking at the opposite
driver --- at the end-points of a driver's primary, massive counterweight.
So, you can see the cross-counterweights on the opposite driver-set at the 
end-points of the counterweight on the mate-driver,  mounted on the same axle-set.

If you inspect carefully, you'll see that the smaller counterweight is 180-degrees opposite
the main counterweight on the other driver,   The 90-degree offset of the two crankpins,
means that there will also be a small counterweights added to the mated-driver.

When rotating at high driver RPMs, the added cross-counterbalances tend to off-set 
the effects & impact of the whirling main counterweights. --- the engine rides smoother,
and pounds the rails to a lesser extent.

As cross-counter balancing became more popular, manufacturers concealed 
the cross-counterweights by slightly repositioning the main counterweight so
that the shape is moved slightly away from its 180-degree position.  

The combined masses will have the benefits, without the smaller cross-weights 
being so obvious.   The only clue is that when looking at a driver-set, the curved 
counterweight is not exactly opposite it's crankpin.

​W.
 



Edited 1 time(s). Last edit at 11/16/24 02:53 by wcamp1472.

In case it wasn't clear: cross-balance improves the balance of the rotating masses. The recip masses are still as much of a problem as ever.

Thanks Wes good information.

But just how was it done is what I've been wondering. Like if someone gave you a locomotive and said cross balance it. Maybe it's a lost art,

Posted from Android

I'm no design engineer, I wouldn't know where to begin.
We have instrumentation today that they never had.
Also, the realistic upper operating speeds allowed today,
would be very modest!

But, also the best weight is a "ball-park" compromise between RPMs,
driver diameter, weight of reciprocating weights, etc.  The "effective" masses 
increase with increased RPMs.

Later, in the development, the most beneficial weight Is reductions in 
the reciprocating pieces was the adoption of hollow piston rods.
Together with lighter pistons, and lighter crossheads, designed for
improved weight-reduction, were the biggest contributors to reciprocating
weights-reduction.

Crossheads carry the outer end of the reciprocating piston rod, and the crosshead's 
wrist-pin carries the front bearings of the main rod.

​W.



Edited 2 time(s). Last edit at 11/16/24 08:09 by wcamp1472.

Radial acceleration of a point on a rotating object is equal to the radius from the rotation axis multiplied by the square of the rotation speed.  That apples to everything in the locomotive machinery — including the side rods and the counterweights - so if you balance at one speed, you’re balanced at all speeds.  To make the numbers work out, rotational speed is usually expressed as radians/sec, which is equal to revolutions/sec multiplied by 2*pi.

The radius effect benefits counterweights on large diameter drive wheels, which can achieve the needed balance forces with less mass because they have higher radial acceleration.

Reciprocating acceleration is the same as radial acceleration at the crank pin, except of course acting in only one direction.

The main rod is a combination of the two.  I don’t know how the pendulum concept comes in, but it might be some clever way of breaking down the main rod mass into rotating and reciprocating components.

Imbalance forces are equal to accelerations multiplied by mass.  As mentioned earlier in this thread, balance weights on the drivers are a compromise between balancing the vertical rotating imbalance and the horizontal imbalance, which is the sum of rotating and reciprocating.  So you end up with a net over-balance vertical and net under-balance fore-and-aft.

The vertical over-balance forces are reacted by a combination of the suspension springs and the rail.  The share of loads is a function of how stiff the springs and the track are, including the condition of the ties and roadbed. I don’t have any idea what the real numbers are, but the common assumption is that the vertical over-balance forces are absorbed by the rail.  This is called dynamic augment.

The fore-aft under-balance forces are reacted by the journal boxes and the locomotive frame.

As Wes mentions above, later developments included substantial reduction in reciprocating mass.  Timken designed new pistons, piston rods, and cross-heads in conjunction with their roller bearing rods, clainimg a reciprocating mass reduction of over 50%.  This in turn allowed the use of lighter counterweights which reduced dynamic augment, allowing higher running speeds.  Following figures are from the 1938 ocomotive Cyclopedia, page 701.

---

---

Thanks for all the good info, I'll need a few days to chew through this stuff but it is starting to give some clarity to my questions.

I appreciate the added comments and technical details, and explanations.
I am continuing the learning, every day.
Thanks for contributing.

W.

 



Edited 1 time(s). Last edit at 11/16/24 17:20 by wcamp1472.

Elesco Wrote:
-------------------------------------------------------
> As Wes mentions above, later developments included
> substantial reduction in reciprocating mass. 
> Timken designed new pistons, piston rods, and
> cross-heads in conjunction with their roller
> bearing rods, clainimg a reciprocating mass
> reduction of over 50%.  This in turn allowed the
> use of lighter counterweights which reduced
> dynamic augment, allowing higher running speeds. 
> Following figures are from the 1938 ocomotive
> Cyclopedia, page 701.
>Great research and development!
>and I thought the computations for pressure and flow thru hydraulic circuits was confusing! 



Edited 1 time(s). Last edit at 11/16/24 22:04 by Mike6640-2.

timz posted this on the other thread out there related to my original question on counterbalancing and it is a very thorough explanation for those interested in going down the technical rabbit hole.

https://babel.hathitrust.org/cgi/pt?id=mdp.3901500

Railway Locomotive and Historical Society did a wonderful article a few years ago on the dynamic augment issues on 10-coupled power related to driver diameter, also rod mass.

Bottom line was that the Burlington early 2-10-4's (M-4) Baldwin '27 were pretty terrible; small driver diameter (64") and heavy rods, anything above 35mph damaged track and the dynamic augment calculations in that article were pretty terrible.   Then B&LE ordered similar 2-10-4's of which 643 was in the group, WWII with and without lightweight rods, same kind of issues which (along with the weight) was how B&LE simply had to upgrade the entire railroad and keep the speeds down.   Meanwhile, ATSF, with bigger drivers (74") could keep them in balance and ran them as higher-speed freight.

I've never seen any other article that honed in on the recipricating mass vs. driver size as it related to lighter-weight alloys for the rods during WWII.  Very educational.

Hey, I found it, and its still downloadable.   
https://rlhs.org/WP/wp-content/uploads/nl24-3.pdf

 



Edited 1 time(s). Last edit at 11/18/24 07:01 by randgust.

randgust Wrote:
-------------------------------------------------------

>
> I've never seen any other article that honed in on
> the recipricating mass vs. driver size as it
> related to lighter-weight alloys for the rods
> during WWII.  Very educational.
>
> Hey, I found it, and its still downloadable.   
> https://rlhs.org/WP/wp-content/uploads/nl24-3.pdf
>
>  
That is an interesting article.  The author is (or was, 20 years ago) very knowledgeable about the history of locomotive development.  Thanks for posting.