Driver Thrust Bearing

[cbrew]

Most 7.5 scale locos and full size locomotives don't have thrust bearings, and just rely on the hub liners for lateral wear. Like Bill says, you could easily end up with a locomotive that is too tight laterally if you add a thrust bearing without making other adjustments etc.

I can't think of a single locomotive from RRSC, Little engines etc that have thrust bearings on the axles. happy to be corrected if wrong.

the drawings do call out one between the snap ring and the inner edge of the journal box for the Allen drivers, I did not like this idea so i went a different route.

[Bill Shields]

Now Harold...I have specifically avoided using the T word throughout..

I have seen some locos where the frames and bearings are dimensioned to handle 7.25" gauge...with spacers added to handle 7.5" track.

Especially on locos that have been regauged.

[cbrew]

ok i fixed the issue, when you write code for 8 to 10 hours, the brain is jelly.
but here is the major issue that comes to light when one has the privilege to build a locomotive.
I just finished looking over the drawings i have for my crap.
the drawing for the journals call out 2", the frame also calls out 2".. absolution nothing for clearance. anyone that has built these models will know as as soon as the first drive dips into a low spot in the rail, the rolling gear will bind up and often lock up depending where the angle of the connecting rods just happen to be at.

[Bill Shields]

There you go . Zero clearance with nominal tolerance

After all...14 / 2 = 6 depending on your tolerance..
.

[cbrew]

[Harold_V]

ok i fixed the issue, when you write code for 8 to 10 hours, the brain is jelly.

Understood. Besides, because the word "tolerance" is tossed about endlessly by unknowing individuals, it's hard to not get caught up in its (improper) use. Fact is, it is misinformation, and I try to keep that off the board.

None of my comments are intended to be an insult, and I thank you for your polite reply.

That said, it would be a kindness for the balance of the comment to be corrected. The insinuation that "locomotives built by engineers that held to extreme tolerances roll about 25 feet off the steaming bay and slide to a stop." is misleading. Again, that's not true. The engineer may well have provided insufficient clearance, but tolerance isn't the problem, not unless he had wide tolerance and managed to get overlapping dimensions such that the designed clearances were eliminated by too loose of a tolerance.

See what I mean?

Look at it this way. Your 2" journals you mentioned are a good place to set an example. Let's assume that you want ten thou clearance. You prefer it not be greater, but you know it must be no less than eight. In such a case, you'd specify the two diameters to be set nine thou different, and to ensure that you didn't get the wrong fit, you'd specify a TIGHT TOLERANCE, such as ± .001". If one chose to specify a loose tolerance, say ±.010, it's clear that you could find your fit so tight it couldn't be assembled, or so loose it was troublesome. That's what loose tolerance does for you.

Tight tolerance isn't your enemy---it's your friend. Use it appropriately and many of your problems will evaporate.

Bill---I took note (when I posted) that you hadn't mentioned tolerance. You, being an engineer, should know what I'm talking about better than most others, so I'm not surprised.

H

[cbrew]

the example i gave does not list out a tolerance and the prints do not call out any clearance, so it is easy to assume to hold to that number if one never built a locomotive. if the journal and frame is machined to 2.000 as called out on the prints. yes it will run on the stand. but as soon as the locomotive rolls out on rough track it will bind up. that is a true statement, no question about that.
that is the detail i am driving home.

[Greg_Lewis]

I have this foggy recollection* that some set of locomotive drawings I saw had a page of notes on which it was stated that all dimensions were nominal and that clearance and tolerance were up to the builder. Whether this is the best way to handle it would be a subject for another debate, but woe to the beginner with no experience who came upon those plans.

_______
*At my age, all my recollections are foggy.

[NP317]

My experience rebuilding, maintaining and operating full sized steam locomotives quickly taught me that a well-worn logging locomotive was unlikely to derail on the crappy track that was their home, and ran with cooler bearings than rebuilt ones with close clearances. Clank more: Of course, and that was comforting to hear from the Engineer's seat.
(Insert "tolerance" joke here...)
RussN

[daves1459]

Since alignment and squareness have become a point of discussion I did some surveying today.

First I checked the squareness of the front face of the lead axle pedestals. I clamped a precision square to the right outside face and placed a .005" shim between the square blade and right pedestal front face. Next I passed various shims between the blade and left pedestal face. I could pass a .006" shim, but a .007" shim had drag. Therefore, I suppose, the pedestals are square with the frame within .002". I would think .002" is acceptable.

Next I checked the parallelism of the rear axle pedestals front face to that of the front pedestals. I held a 12" parallel to both faces with machinists shims and measured the distance between parallels. The left side parallel was .007" further rearward than the right. Is .007" too much. The fore to aft journal box to clearance inside the pedestals is .003" to .004".

The reason I have thrust washers is the original design of the loco and build was for 7 1/4" gauge. Sometime over the years since 1953 it was converted to 7 1/2" gauge using 1/8" washers between the driver hub and axle shoulder, The washer was big enough in diameter to bear on the journal box face.

I reworked the journal thrust face and made new bronze thrust washers. The side clearance on the front axle is now .051" and .113" on the rear. Would those clearances be adequate? Too large or too small? I can make the thrust washers to any thickness.

The last photo shows a pair of new thrust washers and reworked journal boxes. It was suggested earlier to machine grooves on the thrust faces. I marked the location of four radial grooves on one of the washers stopping 1/16" from the outside diameter. I was thinking using a 3/16" ball end mill and making four grooves per side .010" deep indexing the pattern 45 degrees between sides.

The last question is what to use for lubricant? Oil or grease? What kind and any particular additives?

Thanks, Dave

[daves1459]

I forgot to attach the photos

[Bill Shields]

Ah HA...a 7.25 conversion! Thought so...

Most needle bearings like oil but considering the speed and loading...I just use generic axle grease on mine....like on a universal joint of a car...and yes I have several locos with needle bearing journals but will never build another that way.

Any lube is better than dry and seriously doubt that a needle in this service would ever fail because of the wrong lube.

Alignment is as close as it needs to be for the frames.

What I always check is axle positioning with the axles in place and all the journals pushed FORWARD in the running position. This is what is important in the long run...not the frames.

I have fixed locos that had horribly out of square frames that had the axle boxes bored to accommodate.. and they ran just fine.

When you add up all the Clearances...you might be surprised where things sit. Tolerances have a nasty way of adding up against you.