Steam & Excursion > Unexpected realities of plain bearing driver axles

New plain bearings on locomotive driver axles take time and miles to 
become reliable, cool running,  axle bearings.

So, first thing to complete before bringinig axle and brass together 
is to scrape the sharp edges off of the new brass at the edge 
that parallels the axle surface.  If not tapered, the sharp edge 
of the brass will peel-away the oil film you're attempting to get 
between the brass and the steel axle.  The edge can act like your
windshield-wiper, and neatly clean-off the oil film you're trying 
create.

Ideally, the brass bearing and the steel axle should never contact each other,
when rolling down the track; it is intended that a very thin lubricant
layer is separating the weight bearing components.
However, newly machined surfaces are never as finely-polished
as is needed for cool operation.

There is a break-in period of hundreds of miles needed to get new surfaces 
to run 'cool', and give trouble-free service. You're polishing new surface to new 
surface --- that takes time and distance.  At time of installation, there remains 
tiny rings left by tye cutting tools used when cutting the final finish 
in the lathe.

So, experienced roundhouse machinists would scrape away at the sharp edge of the brass
to form a wedge-like interface at the axle --- the intent is to form and wedge 
a uniform layer of lubricant ( oil or grease & maybe a mixture) across the 
entire area between axle and the bearing brass.  Often times the scraping
tasks were performed by young machinist apprentices, as part of their hand-on 
training. 

Many railroads, like SP would apply a smooth layer of soft-metal 
'babbit' onto the brass to allow the axle to smoothly wear the babbit away,
eventually, having polished axle & brass happliy rolling smoothly.

So, both edges must be srcaped evenly and about 3/8" wide, the entire 
length of the brass bearing, and along both leading and trailing edges.

When rolling, there must be a uniform, very thin film of oil/grease separating 
the steel axle and the brass bearing. There must be NO metal-to-metal 
contact. The dense grease, that used to be common and available,
maintained a layer of lubrication, even when sitting idled for weeks, on end.

Once all the new bearings have been scraped, the whole assembly can 
be placed in the frame, and the binder-bar joining the frame bottom-rail
for 2, strong, straight bars carrying both engine weight and tractive forces.

There must also be lubrication provided at the vertical faces of the 
driver hubs.  Can be either grease or oil lubrication.

Grease lubrication meant that a block of grease is spring supported 
against the axle, and top is curved to fit the axle surface.  
To evenly, and slowly distribute the grease, a steel screen 'meters'
the grease onto the axle surface. The grease block covers the entire
bottom of the axle, ensuring that lubrication covers the entire exposed
axle surface.

Grease-lubricated bearings operate at a 'warm' running temperature,
that may seem 'hot', but still be a happy bearing!  Different engines,
have different 'happy-states' and running temperatures.

If there are break-in problems. you've got to catch them early.
As axle & journal brass warm-up, the brass will expand as it 
heats-up.  Being trapped inside the (cold) steel journal box, 
the expansion is confined to the curved, axle 'face' of the brass bearing 'block'---
where it's rubbing on the axle... The thermal expansion forces the curved
surface of the brass to more tightly grab the steel axle ---- thus, impairing
the continuous lubrication film, and getting progressively more hot, as it rolls along.

Similarly, the steel axle begins to get warm and it, too, begins to swell
as it gets hotter. It takes longer to heat the axle.  You get smoking
and dangerous heat, at temperatures approaching 400F.
You can predict what happens as time progresses !

The axle swells dramatically, and the brass is REALLY getting HOT!
And virtually no lubrication is getting where it's needed!

So, during the break-in runs on new axles and bearings, you must 
catch bearing problems EARLY! 

The most common cause is that the crown brass was machined too tight,
to account for the inevitable heat-up period, during break-in runs. 
Here is where very tight clearances are a subtle trap. RR Machinists 
were used to very liberal clearances where the brass meets the axle...

So, let's take a hypothetical example of a 12-inch diameter axle and 
a mating brass bearing, covering only the top half of the axle.
If, during a hot-axle event, the brass heats-up, it expands, grabbing the axle
more tightly.  The brass may expand 1/8" , or more, clamping onto the axle.

Now, with lubrication impaired, the 12" diameter axle begins to swell inside 
the tighter-fitting, hot, brass -- the clamping forces multiply. And pretty soo, 
what had seemed to be a ample allowance of 1/4" to 3/8" clearance
has disappeared!   And you're running metal-to-metal, with virtually no lubrication!

So, when machining axles and brasses, you must allow for initial over-heating during 
break-in runs. Verify ample lubrication, and make sure the two running-surfaces 
are not closing-off lubrication from getting to where it's needed.

Give plenty of room for axle & bearing expansion. and expect cooler 
running temps in the future.  

Or, if it persists in running HOT, you'll have to drop the axle, 
and take a 'healthy cut' to remove more brass and allow for normal heating;
and still get more lubrication there, as your only 'cure' ...

​W.


not proofed, yet



Edited 6 time(s). Last edit at 04/14/24 23:26 by wcamp1472.

Its not good to smell grease when you're running.  Nor is it good to see anything dripping under the boxes.
Doyle told me of an older engine owner who before making the first move of the season would drop the grease cakes and pour STP on them.  Early on, I had a "consultant" tell me to coat the screen/grease cake with graphite powder.  I tried it on one box and had to replace cake/screen.  When the graphite got hot it solidified (just like it does on the smokebox/firebox).  I should have thought of that before doing it.  Replaced cake and away we went.  Was also asked to pour a lubricant known as Militec-C on the boxes.  Didn't really get a good chance, plus it was frightfully expensive.  I've never hear of any others using it either.
I sure wished there had been more retired shop men back then to ask questions when in doubt.
We did have a fireman who fired for Dad in steam days give us good advice on heating the oil to a ballpark temperature.  We had to find our way through different oils as Bunker C wasn't always around.   We burned #6,#5 and some stuff that looked like sewage and was very thin but was nuclear hot burning.  Only burned diesel once in a pinch and it was awful.  No BTUs.  Never tried waste or vegetable oil.
I feel like we were pretty damned good at learning by doing.  I do wish the PBJ oil method had been perfected back when we were running.

Thanks for your advice and the real-life experiences.
I never fired a loco that burned oil..

Coal was all-about the firebed, and uniform burn rates 
over the coal grate.  With the use of the brick-arch,
the free-flow, open area at the upper edge of the brick-arch 
had a flow area = to about 30% of the grate areas.

The brick arch slopes upward, from the throat-sheet. ( Front, lower
portion of the firebox, at the circular boiler shell).  The brick arch 
causes crowding of the air-flow, and restricts the velocity through 
70% of the grate.

The rear 30% of the grate gives a 'straight-shot' fir the air flow,
over the arch and into the upper boiler flues and tubes.

The air flow at the rear of the grates is virtually free of obstruction ---
- and as such, the coal burns at a much faster rate... the burn-rate at
the rear grate area means that the fuel is cconsumed about 3-times faster
than the other grate area.

If the fuel burns-out quickly, how does that affect firebox temperatures and 
air flow patterns?   Any thoughts about solutions for the disproportionate 
rates of air flow, through the grates ?

(Well-experienced folks, hold your responses; give others the chance 
to get most out of the discussion.  It's where new learning has its greatest 
impact.)

​W.

 

So Tom is the new guy in the machine shop. The foreman tells him to scrape the edges. Tom gets a little overzealous and over does it. Now what?

Posted from Android

If Tom follows the instructions, but gets overzealous, 
he makes a 'wider' scraped area....thus, he's extending the region 
where more lubrication is entrained, and the liberal amount gets 
caught & wedged, and lubrication is improved.

The more common fault, is not ENOUGH of a taper is scraped.
Too small a tapered area is more likely to cause problems.

I would give Tom praise and congratulations ...as well as using 
Tom's work as an example of how taper-scraping should be done.

Most scrape enough, and say "That's good enough".
Most of the time they're right.  But, then too, often times it's NOT
'good-enough'. Scraping brass is a lot of work.

I like Tom's attitude.

W.

 



Edited 1 time(s). Last edit at 04/15/24 14:57 by wcamp1472.

Thanks Boss.  I;ll buy you breakfast next week.  

LET'S  DO IT !

W.

 

 Being trapped inside the (cold) steel journal box, 
the expansion is confined to the curved, axle 'face' of the brass bearing 'block'---
where it's rubbing on the axle... The thermal expansion forces the curved
surface of the brass to more tightly grab the steel axle ---- thus, impairing
the continuous lubrication film, and getting progressively more hot, as it rolls along.THIS IS ENTIRELY WRONG AND REVERSE OF WHAT HAPPENS WHEN THE BRASS GET HOT.