Steam & Excursion > Counterbalancing Main Roads - Steam Engines question

Can anyone provide a tutorial on how main rods are counterbalanced ? (Wes?)

My understanding there is revolving weight and reciprocating weight to be considered. I think I might sort of understand that, but can someone dummy that down and explain?

Also this weight is important in determining the proportional and total weights on the crank pins.

I've heard that as steam designs were refined and speeds and masses increased that the standard pendulum formula was used. How was this used in practice?

T = 2π x √(L/g) where T= time, L = the length of the pendulum string from the pivot point to the center of mass of the bob and g = gravitational accerleration.  (that is 2 pi)

What is more critical, revolving weight or reciprocating weight.

Thanks ... 



Edited 5 time(s). Last edit at 11/15/24 13:26 by train1275.

Let's start with the easy part..
Revolving vs reciprocating ? 

Also, there is no mathematical formula for varying RPMs.
One formula solution fits only one, constant, driver RPM.

Emprical answer is the revolving mass, is more difficult to resolve.
Effective mass varies as the RPMs.  A kid with a rock and string is an
example of how RPMs make a difference.... at slow RPMs the string
is barely tight, and the rock is flying low to the ground.  
The same rock at higher spinning-speeds draws tighter on the string.
..and the rock is circling higher above the ground, and is straight-out.
( What would happen if the kid used an "elastic string"?) 

With revolving weights, it helps ( lower mass required) if you use larger
diameter drivers...  Again, because faster RPMs increase the apparent mass.  
A small weight, on an 80" driver, at 300 RPMs, does a respectable job of
counterbalancing.

The hidden problem is that the centrifugal force is the same for 360-degrees
of revolution.  Heavy weights lift the driver, axle and springs, clear off the rails -
-- as they rotate through the "12-o'clock quadrant".
And, the same pounding force is severely hammering the rails, across the 6-o'clock
quadrant.    The centrifugal force is equal through all 360-degrees.

Its the permanent rail damage that is most ruinous, in improperly-balanced drive wheels.
Small diameter drivers are a particular problem because they need greater 
( relative mass) .... account of lower RPMs.

Beacuse RPMs can vary over a great range, it's very difficult to pick a mass
that is the best 'compromise', in your mathematical calculations.
One number does not fit all RPMs.

A fixed mass is best counterbalanced at one, steady RPM.  
So, it's hard to pick one set of values and RPM, that satisfies
your equation; but, then spin the drivers at double the 'ideal', calculated, weights.

The driver-centers are cast with hollow voids inside what looks like 
a single 'counterweight' value.   The voids have additional weight of molten lead 
added, then steel cover-plates are welded across the back of the driver 
center, to enclose the lead weight.  This also allowed 'adjusting' the amount 
of lead added, following road tests, at varying RPMs.

Its much easier arriving at suitable compromises, with big diameter drivers
like on the later SantaFe 4-8-4s, or NYC Niagaras, & Hudson's.  
Trying to get a J-class to ride smooth, at 100-per, with 70" drivers is relatively
impossible.

You could take today's 611-loco to the fastest MPH that you dared --- someting 
like 110 MPH ---- but, then a big, high-drivered 4-8-4 could cruise on-by,
at a lower RPM, but higher track speed!  and pass a struggling 611...

The BEST arrangement is similar to the PRR T-1 class, because all of the 
weights and siderod and piston masses are smaller, less mass, and you've
got those large, 80-inch  drivers --- all riding much smoother, and lower RPMs

There are undocumented reports of PRR T-1s hitting 140 MPH! 

​W.

 



Edited 4 time(s). Last edit at 11/15/24 15:48 by wcamp1472.

No one can explain balancing. That is, no one
alive can say what's the best way to balance
a given engine.

As you probably know, the mass of the
cylinder and piston can't be balanced by
a weight on the wheel without unbalancing
the rotating weights. So how to compromise?
In, say, the 1920s designers would balance
the rotating weights, and then add enough
extra counterbalance to balance 40-50%
of the reciprocating weight. The extra
weight pounded the rail, naturally, so in
later years they reduced the percentage.
One reason they could get away with that
was that the total weight of a 4-8-4 was
more then a 2-8-2 so a given unbalance
has less effect.

Another thing no one can tell you: how
to calculate the "reciprocating weight"
of a main rod. We've seen pictures of them
penduluming main rods, but we don't know
what calculation they did after that.

(I suppose they hung the rod from the front end?
Was the rate of swing supposed to tell them
where the rod's center of mass was, or what?
How would that tell them how much of the
weight counted as reciprocating? Dunno.)

If it wasn't clear to everyone: in the pendulum
formula you gave, the time T is the time for a
a back-and-forth swing, not just from one side
to the other.



Edited 1 time(s). Last edit at 11/15/24 16:26 by timz.

By the 1940's, the Lima order files were roughly 30-40% filled with calculations and data on weights and such.

Posted from Android

Have you seen this yet?

https://babel.hathitrust.org/cgi/pt?id=mdp.39015004535301&seq=273

He says the center of percussion is what you need
to find in the main rod, not the center of mass.
What's that? Dunno.

timz Wrote:
-------------------------------------------------------
> Have you seen this yet?
>
> https://babel.hathitrust.org/cgi/pt?id=mdp.3901500
> 4535301&seq=273
>
> He says the center of percussion is what you need
> to find in the main rod, not the center of mass.
> What's that? Dunno.

Thanks for this !  I'll have to try and disget it, but it looks at first glance to offer some good basics.

> timz Wrote:
> --------------------------------------------------
> -----
> > Have you seen this yet?
> >
> >
> https://babel.hathitrust.org/cgi/pt?id=mdp.3901500
>
> > 4535301&seq=273
> >
> > He says the center of percussion is what you
> need
> > to find in the main rod, not the center of
> mass.
> > What's that? Dunno.

Well it offers more than good basics !! 

About everything you would want to know about counterbalancing. Good find, thanks !!

The Center of Percussion is discussed in several good videos, just google it. Basically it is the point on a stick or rod that when struck perpendicularly it has no hinge effect.
For baseball players, it is the sweet spot on the bat where there is no sting felt when hitting the ball.


 



Edited 1 time(s). Last edit at 11/18/24 05:08 by train1275.

Anyone have access to this publication ?

Counterbalance Tests of Locomotives for High Speed Service
Published 1944
Author: Association of American Railroads - Mechanical Division.



Edited 1 time(s). Last edit at 11/19/24 09:58 by train1275.

Yeah, I forgot what the center of percussion is.
If an object is stationary and you fire a bullet at it,
it will start to rotate if the bullet hits near its top end,
or it will start to translate if the bullet hits near its
center of mass. In one case the bottom of the
object moves one way, and in the other case
the bottom moves the other way. Center of
percussuin is where the bullet has to hit so
the object's bottom remains stationary for
an instant. E.g., in the batter's hands.