Steam & Excursion > 3 Legged Milking Stools

All conventional steam locomotives sat on the track like three legged milking stools.  Each leg of the stool is a complex arrangement of levers, springs, and hangers to tie a group of wheels, their axles, and bearings into system forming a single unit to support the frame (the seat of the stool) with even distribution of the weight of the locomotive regardless of curves and irregularities in the track.

This drawing explains how it works on a 2-8-2 locomotive, but the principle is the same for every locomotive from an 0-4-0 to a 4-8-8-4!

-LD



Edited 3 time(s). Last edit at 10/11/20 18:48 by LarryDoyle.

---

Very NICE...

A complete rendition of the principles of suspensions.

Thanks

​W.
 

Trying to figure this out...
That transverse link at the front and rear driver.  Does that behave like a sway bar in an automobile. That is, help stabilize the frame during cornering. Or, maintain stability from a "bump" on only one side.  Any insight is appreciated.
...bill

Stupid question, but so be it:

What's the advantage to equalizing, versus having each axle sprung indepedently?  Is it just to avoid having the rods bind up?

It has no effect on the rods. It does maintain weight distribution uniformly regardless of track irregularities though.

-LD

Probably the simplest example of equalized suspension is used for tandem axles of highway trucks, as shown in the picture below.  The springs are linked by a rocker arm that assures equal loading of the two connected wheels.  Otherwise, one axle or the other would be supporting most of the load while passing over a non-flat surface, like the entrance to a driveway.  Or if the height of the 5th wheel of a truck tractor doesn't match the height of the tandem axle suspension at the rear of the trailer it is towing, one of the tandem axles will be carrying more load than the other.  Axle loadings are subject to legal limits, and individual axle weights are checked at weight stations along highways.  So weight equalization is essential for truckers.

The same concerns apply to rail vehicles, including steam locomotives.  Even if the track didn't deviate significantly from flat, adjusting the springs to achieve equal load distribution of multiple independently sprung axles would be a never-ending nightmare.



Edited 2 time(s). Last edit at 10/11/20 00:29 by Elesco.

---

RE: K4s 3750..

NOT STUPID!!  Very logical question...

Its a matter of costs of inverstment in  the roadbed.   In Britain, the early days of
railroads vast amounts money and labor was expended on building the roadbeds
virtually flat, without short steep grades.  The used extensive 'fills' made of immense 
numbers of cubic yards of earth and they built, long expensive bridges, tunnels, etc...
all to make the railroad as flat, & level as possible.

Their intent was to use locos of modest size, cars of modest capacity, and trains of modest size,
moving over long, easy grades of modest inclinations.  Their light locos were built on plate-steel
frames, with coil springs above each axle .... as you hypothesize.   They were not equalized.
Very few modern locos built in Britain were constructed using American-style 
loco axle equalization.  ( Also, use of vacuum train-braking technology limited train lengths to 
comparatively short trains..).

In the U.S., rails were laid quickly, curvy, and  hilly.  Engines were built bigger and bigger.
so equalizing was found to be absolutely necessary... on uneven track, axle loadings could  vary
immensely; so, if simple coil springs were used...meaning that Tractive effort ( towing capacity)
 changed with how many powered axles were actually load-bearing --- as the uneven tracks
varied in amount and steepness of the roughly laid rails.

As locos grew from 2-2-2 to 4-4-0, to 2-6-0, & 2-8-0s. and increasingly more axles..equalizing 
became more and more a necessity.  Very heavy train weights meant that non-equalized 
drivers became very slippery when suddenly only half are in solid contact with the rail ...
if not equalized.

With equalization, all drivers remain carrying the same weight -----  power and traction was shared
equally by each axle, regardless  of lumps in the track profile, and variations in rail cross-level profiles.  
Also, on uneven American tracks, with simple coil springs, British type locomotives become
very rough-riding.

​W.



Edited 8 time(s). Last edit at 10/11/20 09:19 by wcamp1472.

LD, that is by the way, a very nice drawing.  It clearly shows the the equalization scheme and the three independent supports.  You used a 3D CAD program, I presume.

Elesco Wrote:
-------------------------------------------------------
> LD, that is by the way, a very nice drawing.  It
> clearly shows the the equalization scheme and the
> three independent supports.  You used a 3D CAD
> program, I presume.

Thank you.
TurboCad 19 is the program I used for the graphic, saved to a jpg file which I imported to MS Paint to apply the callouts and text. Probably stone age technology, but it's what I've got and learned to use it.

-John

So, what's the subtle change I sneaked into the above graphic viz-a-viz the two posted last week, which substantially improved the engine????

He, he, he, (under my breath) ;^)

-LD

Centering rockers/devices front and rear

What isn't clear from the drawings is the twisting of the trailer truck leaf springs 
as the trailer truck slews left or right.

Again, the "large casting technology" for the manufacturing processes had not yet 
been developed, when the need for guiding trailer trucks was needed.
So, the frame designers of the times were left with 'built-up' alternatives using smaller
pieces, bolted together.

With the development of large-castings steel technologies ( in the 1920s), came such things 
as cast, one-piece trailer trucks, pilot truck's, and eventually, complete 'engine beds'
incorporating the cylinders, and boiler saddle, frame extensions,  etc.

The three-point support concept was easily visible with the cast, two and four-wheel  'Delta' trailer 
trucks of the General Steel Castings Company.... in the late 1920s and early 1930s.

Such manufacturing steps had to be perfected ( and thoroughly tested) before they could be
successfully applied to locos under construction in the erecting halls and loco factories.

W.

Wes hit on it, but didn't quite nail the change.

I changed the trailing truck from the Baldwin design Hodges truck to a Cole design by Alco.  The detail drawings below show the difference.

The Hodges design, as Wes pointed out, put a lot of twisting force on the leaf springs, which was damaging to the springs but put little to none of that force to stabilize the back of the engine in either forward or reverse at anything above yard speed.

Note that the Cole truck has a hinged floating plate between the top of the journal box and the leaf spring, so the journal can move sideways beneath the plate on curves and track irregularities without distortion of the spring.  A centering device, similar to the device in my photo in an earlier post of D&NM No, 14, transfers guiding force from the truck to the tailpiece of the locomotive frame is always used with this truck.  I have not shown detail of this device, but have indicated its position as the transverse horizontal cylinder beneath the frame rails.

That floating plate must be kept well lubricated, top and bottom.

This truck is very stable and rides very well in forward and reverse.  I know, firsthand, that locomotives so equipped can be run safely and confidently in reverse at 30 to 50 mph or more.

-LD

---

---

An early variant to the cast-one piece trailer truck was the PRR 'KW' truck, first used on the E-6, 4-4-2,
and all later PRR 2-wheel trailer trucks.  It was invented (later improved) about 1913.

However, its my belief that the PRR truck did not have any specific  'centering scheme' to aid in reverse moves.
PRR's design had two, spring loaded, (hollow) round, flat-based tubes, containing load-bearing coil-springs -'elephant's feet' *--
mounted ahead of the trailer truck axle, at the frame behind the rear driver.
It used the "elephant's feet" as a fulcrum in the spring-rigging scheme of equaization.

It, likewise, required constant grease lubrication.... I'm guessing that for back-up moves, the friction of the elephant's feet
sliders offerred some degree of loco guidance, but it was not an 'increasing resistance' leading effect, as with the later heart rockers, etc.

The KW truck was an important innovation, being a one-piece casting, used in a critical application.
The  GSC "Delta" design, of a decade later, is much better at frame-guiding forces, in reverseing movements...

W.

*See also the PRR GG1 weight bearing 'pads' used on each frame, in conjunction with the offset, pivot pins & bearing points --- more 3-point suspensions...



Edited 2 time(s). Last edit at 10/12/20 12:15 by wcamp1472.

Illbay Wrote:
-------------------------------------------------------
> Trying to figure this out...
> That transverse link at the front and rear
> driver.  Does that behave like a sway bar in an
> automobile. That is, help stabilize the frame
> during cornering. Or, maintain stability from a
> "bump" on only one side.  Any insight is
> appreciated.
> ...bill

The bar at the front driver serves to tie the pilot truck, the first two drivers on the left and the first two drivers on the right into an equalization unit - one leg of the stool.

The tie behind the last driver pair does little to nothing to cross-equalize, but rather it offsets the hangers to allow them to pass the frame rails to reach down to the lower truck equalizer lever.  The alternative would be to drop the rear spring hanger thru a slot milled vertically thru the frame rail.

-LD



Edited 1 time(s). Last edit at 10/12/20 11:08 by LarryDoyle.

This discussion is very interesting!

Regarding the Cole truck, a drawing in the 1922 Locomotive Cyclopedia shows there are multiple parts between the hinged plate and the top of the journal box, rather than flat sliding surfaces.  For one thing, there is a need to accommodate angular misalignment.  But I can't figure out what's there when they show all of the parts in one drawing.  (I need to get out and look at some locomotives when the virus is under control.)

Regarding the KW truck shown in the first picture below (again from the 1922 Locomotive Cyclopedia), it is apparent that the entire truck frame acts as an equalizing arm.  At the rear of the truck, there is a device mounted below the truck frame which I can't identify.  As Wes pointed out, all of the load on the trailing axle and half of the load on the rear drive axle are concentrated on two feet that contact the bottom of the engine frame.  They would be subject to sliding during lateral motion of the truck.

In the second picture, above the truck frame rear cross member, there is what might be a centering device.

---

---

Locomotives with a 4 wheel leading truck-----

So far our entire discussion has concerned 2 wheel lead trucks.  Four wheel (and the rare six wheel) leading trucks are not linked by equalization levers to the driving wheels   Instead, the 4 (or 6) wheel truck is attached to the frame by a single center pin in exactly the same manner as any freight or passenger car truck to form one leg of the "stool" all by itself.  All driving wheels on the left side of the engine the trailing truck are linked together to form the second leg, and likewise on the right for the third leg,  Even all the way up to 4-12-2's!

Articulated locomotives....

The equalization of the front engine (the portion that swivels) functions as if it were a separate locomotive, with the weight of the front portion of the boiler bearing down onto the swiveling frame on a sliding plate (again, well lubricated),  Little, if any, weight is transferred to the front frames via the hinge, though some did.

-LD

I knew little about equalization and this thread inspired me to figure it out. I know a lot of you intimately understand how all of this works, but if you don't, I found this article on line that helped me understand spring equalization. Very cool engineering, and lots smart ideas.
Take a peek. http://www.dougkerr.net/Pumpkin/articles/Spring_Equalization.pdf 

...bill