Side rod bushing clearance

[amadlinger]

Hi all,

As a rule of thumb: what is the 1. desirable, 2. minimum, and 3. maximum allowable clearance for the side rod bushings between the OD of the crankpins and the ID of the bushing?

Crankpins are 0.625" OD (material is hardened D2), bushings are Alloy 932 bronze, method of lubrication will be grease, and this is a 1-1/2" scale, 7-1/4" gauge B&M 2-8-0 that we are talking about.

Many thanks!

Sincerely,
Adam

[Fender]

In my limited experience, I would say about 0.002” minimum. If it gets up to maybe 0.020” the “ticking” gets annoying. (On full-size locos, it’s a “clonk”!) Probably grease helps fill the gap better than oil.

[FLtenwheeler]

The prototype I am modeling used used 1/32". That scales down to 0.004".

Tim

[Fender]

Also, on my mogul the bearings are a press fit into the main and side rods. The press fit reduces the bore of the bearing, so it needs to be reamed out after pressing.

[Miserlou57]

Should fits and clearances be directly scaled down?

This has actually been a big question of mine for awhile, so I’ll take the moment to slightly hijack the thread and ask in a more general sense. I have competing thoughts in my head but not a clear answer.

For example, if you had a press fit on a full size, you wouldn’t divide that amount by 8 would you? But when you have a clearance like the rod/pin fit, I would assume the driving factor is more based on geometry and the expected travel of the rod..?

Would love some advice here. Thanks in advance

[cbrew]

I bored my rod bears .006 over with a .002 pressed fit in the rods, which close up the bore a couple thou. remember too tight, she will bind up over un even trackage

[ChuckHackett-844]

As a rule of thumb: what is the 1. desirable, 2. minimum, and 3. maximum allowable clearance for the side rod bushings between the OD of the crankpins and the ID of the bushing?

Adam

Don't forget to account for side-play in the drivers. This varies by scale and wheel arrangement but, for example, my 7.5" gauge Northern hast 0.200" driver axle side play which could mean as much as 0.400" out-of-line from one rod end to the other - of course this 0.400" is not likely since the drivers will all go to the same side normally (except possibly a derailment - in which case too tight might make moving it forward/reverse more difficult due to binding).

The worst (normal) case on my engine would be the main rod running from the wrist pin to driver #2 with one end fixed and the other shifting 0.100" each way.

For plain bearings they will wear in but if you make them too tight at first she will be stiff in corners.

When I replaced my rod bearings they were well worn ('wollowed'?) in the horizontal plane but not in the vertical.

Sorry, I don't have exact measurements for you ...

Chuck Hackett

[ChuckHackett-844]

remember too tight, she will bind up over un-even trackage

Good point Chris ... I forgot to mention that one ...

Chuck Hackett

[ChuckHackett-844]

Should fits and clearances be directly scaled down?

.... I would assume the driving factor is more based on geometry and the expected travel of the rod..?

Remember, things like side-play requirements (curvature, track kinks, etc.) and track vertical curvature (AKA: Bumps ...) are WAY out of scale in our locomotives and must be taken into account.

I doubt if an engine built to exact scale would make it around any track I've seen.

Chuck Hackett

[Dick_Morris]

Would the clearance be different for Delrin?

[NP317]

Delrin is naturally more flexible than metals, so I would expect it to move more in stress situations.
I do not have scientific measurements to back up my opinion. Just experience machining and using it.
~RN

[Kimball McGinley]

I have to believe Delrin is too stiff to "give" enough to prevent binding on curves and bumps. It might eventually cold-flow and egg-out to the shape needed faster than bronze, however.

This same topic hit Trainorders today; the words of wisdom from the Strasburg RR were; "...too loose is better than too tight - better to hear them than smell them!