Steam & Excursion > “ witness marks”..

Old time practice:
On reconditioning or re-machining worn or damaged parts, 
the machinists used lathes and boring machines, as well as 
shapers, and other machine tools..

When re-machining something like an egg-shaped, worn driver
crank pin, the machinist cuts to a new surface on over 99.9%  of the pin.
He leaves a tiny scratch or groove as a witness of the original surface.
You always leave a hair-line trace of the original surface & diameter.

That remaining 'groove' is a witness-mark of the original pin diameter.
And, only just enough material was removed to leave a tiny mark of
the origiinal pin surface...

That tells all concerned that only just enough material was 
removed, and not "over-machined", with a 100% 'new surface.

Its crucial to just remove enough metal to make a new surface;
but, the witness mark tells us it was not cut deeper than the original
surface, with a gouge, or other injury,  that was the reason why a
new surface was cut.

Worn, gouged, or dented things like a damaged cylinder liners,
always were re-machined just deep enough, yet a clear scratch
or small damaged area that was on the original bore, remains.
They will always leave a small groove in the original bore.

Again, that will tell other location mechanics that the part 
had been cut & re-machined sometime earlier in it's career,
as evidenced by the 'witness-mark'.

Rusted areas on crankpins, etc., are better just polished, smooth 
surface, but 1,000s of small pitted areas act like small "lubricant reservoirs".
So, it's not necessary to reduce the pin's original diameter to eliminate all
small pits.  It doesn't have to be entirely re-machined, 

As long as the surface area has only pits or gouges that are below 
the surrounding area. All areas  that have raised defects and damage
that is higher than the original surface you always want those defects 
smoothed-over;  but, again you can leave scrapes and rust-pits to 
increase areas with hundreds oil-retention 'pockets' over the surface.

NOTE: With pitted or  damaged races or roller bearings, such bearings must 
be replaced, with new.

But, lightly pitted piston rods, or similar surfaces can be continued in service,
with the pits holding tiny reservoirs of oil or lubricant.

Some, crankpins are fitted with 'floating" brasses, which have mutlitple 
drill-holes that retain rod grease.  When applying new brasses, 
those that have voids through the brass & have lube holes over the surface
require special attention.   ALL those brasses' grease holes need to
manually packed with new, stiff rod grease, before applying to the locomotive.

So,  rusty pins need not be further damaged ( by unnecessary re-machining);
but, can be continued in service, keeping their full diameters,  with hundreds of
surface "lube-holes" --- a beneficial outcome, and saves time and money 
in the restoration.  Rusty surfaces pins don't look nice, but because they are 
not over-machined, they'll keep their owners happy for longer periods.

Yes, you can be overzealous and make all surfaces clean 
and shiny with machining --- but, the better surface has those
thousands of tiny oil reservoirs .... and will have a longer life,
compared to a new set of surfaces, wearing against each other.

W.



Edited 4 time(s). Last edit at 03/26/24 07:09 by wcamp1472.

Part 2...

Another challenge peculiar to crankpins is that 
cutting a new surface on pins using a single machine, individually
can result in other than 90-degree relationship to all the other crankpins.

Separately machined crankpins may have a nice new surface,
but, it might get re-machined out of line with true 90-degree 
separation, from it's opposite driver.

The BEST way to re-surface crankpins is by using a crankpin-truing .
machine --- "quartering machine'.
it has two cutting heads set at exactly 90-degrees apart.
So that worn pins can be re-machined & resurfaced and brought
back to a true 90-degree relationship to the opposite side.
Preserved, operable, quartering machines are a very few,
they are very rare.

Typically, when a new set of driver tires are applied 
to a locomotive, the crankpins will get properly machined 
in a driver quartering machine.

Crankpins wear unevenly, in an egg-shaped pattern, because the 
greatest piston thrusts are at beginning of power strokes.
So that, over time, the front and back surfaces wear more quickly,
than the 90-degree areas.

To bring everything back to 'perfect', 'quartering machines' are 
used to restore the bearing surfaces and maintain that 'perfect'
90-degree relationship.... between all the drivers in one loco-frame.

W.



Edited 2 time(s). Last edit at 03/26/24 09:05 by wcamp1472.

Wheels have a witness mark so as to show what diamater not to go past when truing. 

engineerinvirginia Wrote:
-------------------------------------------------------
> Wheels have a witness mark so as to show what
> diamater not to go past when truing. 

Actually that "witness groove" on diesel locomotive wheels is for accurately measuring the diameter and/or tread thickness. No way does wheel truing ever reach that "witness groove".

Yes,  ( diesel locomotives) it is typically a circular groove in the face of the tread.

It can be tricky turning all the drivers for one steam locomotive,
to the exact same circumferences.  Different circumferences put
strains on the  siderods, as they revolve around the crankpins

The tire with the greatest wear will dictate the new dimension 
that ALL the drivers for that specific loco must be cut-to.

All the "running-surface", driver-diameters and circumferences must be the same.
Generic 80-inch driver diameters can have widely varying circumferences,
if there's a lot of miles on the engine.

In 1975, when 4449 had its tires re-machined at the shops in North Little Rock, Ark,
( former MoPac?)
All 8 drivers had to be turned to the same circumferences. #1 wheelset was the thinnest.*

Their shop had an underfloor Hegenscheidt wheel-truing machine --- it could turn 
all wheels to whithin 1/4 of a "tape" of circumference difference between the 8 drivers.
( 80" driver diameters = 251.32 inches in circumference...)

The drivers could be machined --- all to the same circumference,  without having
to drop each wheelset separeately.

W.

( In Portland, after AFT, Doyle applied all 8, new driver tires...also, over time,
he converted all the axles to roller bearings, except for the 4449's driver 
axles...).



Edited 5 time(s). Last edit at 03/27/24 15:22 by wcamp1472.

HotWater Wrote:
-------------------------------------------------------
> engineerinvirginia Wrote:
> --------------------------------------------------
> -----
> > Wheels have a witness mark so as to show what
> > diamater not to go past when truing. 
>
> Actually that "witness groove" on diesel
> locomotive wheels is for accurately measuring the
> diameter and/or tread thickness. No way does wheel
> truing ever reach that "witness groove".

Indeed, Hotwater !
You would have nicely turned wheels in order to condemn them. I always reviewed potential wheel true to make sure we had "enough meat". I'd rather do a changeout  than true too thin. Sometimes the wheel true guys were too eager to get in some overtime by doing a true "it's just gotta be done, can we go ahead?"  Sometimes there was no good alternative, or the monthly budget could not be adjusted and we did it to get by til whenever the next 92 day fell but if there was any doubt to getting to that marker the unit was set aside a few days. I also saw more than a few dumbsh*ts run wheels to condemning and then think there were going to interchage it for contract wheel work. That never ended well. Also you can eat a lot of meat if the flanges have gotten too bad and have to be careful what you do. I'd also sometimes change out wheels a little early, especially when I had backshop time to do so. Maybe at 1-1/2" for example. That way I had some used seed stock for quick repair change out for a handbrake slid flat or whatever might be needed for that tread match. I guess when I looked at the thread title that was where my mind went ! Proper and managed use of wheel true can extend wheel life but poor judgement can turn an otherwise useful set of wheels to the scrap bin.



Edited 1 time(s). Last edit at 03/26/24 09:34 by train1275.

Train 1275...

I liked your stories from "the wheel shop"...
Some put a smile on my face.

😀

wcamp1472 Wrote:
-------------------------------------------------------

> When re-machining something like an egg-shaped, worn driver
> crank pin, the machinist cuts to a new surface on over 99.9%  of the pin.
> He leaves a tiny scratch or groove as a witness of the original surface.
> You always leave a hair-line trace of the original surface & diameter.

I just don't see how that is done with a lathe.

Axles are done in a lathe...crankpins are sometimes refinished 
with a portable, motor driven cutting tool.

Typically, an individual, portable machine is supported and clamped 
to the driver, and the cutting tool revolves around the crankpin.

It makes a nice smooth crankpin, but it might be off-center 
from its true 90-degrees relationship to it's other wheel's crankpin.

A similar portable tool is used the re-bore the inside of valve and cylinder 
bushings while mounted in the locomotive.

However, for proper 90-degree cutting process it takes a large, floor mounted
"Quartering Machine" with two separate cutting tools ---- 90-degrees apart.  
And the cutting-heads are adjustable according to the diameters of the crank pin circles,
and the driver diameters, which determines the piston stroke of the cylinders..


​W.

 



Edited 2 time(s). Last edit at 03/26/24 19:48 by wcamp1472.

wcamp1472 Wrote:
-------------------------------------------------------
> Typically, an individual, portable machine is
> supported and clamped 
> to the driver, and the cutting tool revolves
> around the crankpin.
>
> It makes a nice smooth crankpin, but it might be
> off-center 
> from its true 90-degrees relationship to it's
> other wheel's crankpin.
>
> A similar portable tool is used the re-bore the
> inside of valve and cylinder 
> bushings while mounted in the locomotive.

If you try to round off an egg shaped profile, I do not see how you are able to save any of the original diameter.

You are correct, so with plain-bearings, the renewable brass sleeves are 
the major "wearing-bearing"; so, they apply thicker brass bushings
to make up for the lost diameter, from crankpin turnings.

When constructed, the crankpins are well over-sized, in anticipation
that they will be 'turned' in the field , multiple times in the life of the 
locomotive.

Some railroads, like NKP, cut the brass rod bearing sleeves into 3 or 4 segments ...
so that adds a longer life to the brass bearing elements.  The outer ends 
of the brass elements have a threaded bolt hole so that a plain bolt
can be inserted and used as a 'handle' to change-out & remove the worn brasses, 
when renewal time comes.

Replacing the siderod brasses was a quick and easy process, and very routine.
Different thickness-dimension ( to account for various diameters) new brasses
are prepped, and in the stockroom --- when needed.

Remember, that plain bearing locos allow the axles to shift slightly...
That allows the drivers' inner hubs and hub-liners ( vertical, brass faces of the 
journal brasses) to be free from constant-contact... That's important to keep
the too-tight hubs & bearings from overheating.

However, axles that shift from their neutral positions, make the distances
between the neighboring crankpins get slightly longer.  

So, to accomodate the varying lengths of siderods, liberal allowances in
the brass rod bushings allows for the changing rod-lengths, when navigating
curves in the track, as the axles shift with respect to the other axles...
That loose-ness can be heard in the ringing rods when on straight track,
and a light throttle, and low cylinder pressures.

When too-tight tolerances, get stressed further, as when the axles shift, 
things get overheated very quickly.   Years ago, UP's 844 had problems 
with new rod brasses, that were too tight, and could not adjust for the
necessary axle-shifting while navigating curves.

The old adage about rod bearings is still very true:
" If you hear them, then you won't smell them!"

​W.



Edited 1 time(s). Last edit at 03/26/24 22:08 by wcamp1472.

Curious as to when a witness groove or scratch of any type on a journal becomes a critical opportunity for a stress riser to raise its' ugly head and lead to a pin failure?  Or am I missing something here?  As previously stated, a witness groove on a diesel wheel is an entirely different animal used for aiding measurement of wheel diameter.



Edited 1 time(s). Last edit at 03/28/24 10:17 by RoadForeman.