Best Of Spokane, Portland and Seattle (SP&S) 0-6-0 Project

[CNR5529]

On a real (two cylinder) steam locomotive the pins have to be offset at 90 degrees to eliminate dead spots and to balance the whole shebang. Four points on a circle evenly split between piston travel extremes on each side, two and two. On models, we tend to follow suit to be prototypical, although there's no mechanical reason to do so (and since we cannot see both sides of a steam locomotive at the same time, perhaps no aesthetic reason, either, other than it just seems wrong not to). So I wonder whether on a model, given the necessary slop in axle bearings, the 90 degree offset actually creates unnecessary lateral forces on the wheel/axle assembly and whether we'd be better off, mechanically speaking, with some other ratio; 120? 180? 0?

That logic is true for locomotives where all driving axles are driven by a gear train. The slightest amount of misalignment in quartering will bind the mechanism, and there is no mechanical reason, as you pointed out, to have the rods 90 degrees out of phase in this instance. This is an easy way to get rid of binding on models where the rod movement interferes with the wheel rotation which is already mechanically driven by other means.

In this case though, only one axle is driven by a gear, and the other two axles are only mechanically driven by the rods. Here, having the rods at 0 or 180 degrees would cause the mechanism to bind every time they were in the same plane as the axles (3 and 9 o'clock at each revolution). That said, the motion will be most fluid when the rods are furthest away from that plane (12 and 6 o'clock). This means that 90 degree quartering is optimal for smooth operation because it always ensures that one rod is moving towards the most fluid range of its motion, while the other is at risk of binding. 120 degrees would be even better, but only if there was a third rod to connect the axles (at 0, 60 and 120). The idea is to divide the half rotation power stroke evenly, if that makes sense.

[peteski]

Otto, if the drivers are all gear-driven (rods are strictly cosmetic) then not having the rods 90 degrees off on each side would not be a big problem, but if the rods are actually used to rotate drivers then if the quartering is not at 90 degrees the  rods will most likely lock up at certain point during the rotation due to the push/pull forces involved.  At least that is the way I see this.

[mmagliaro]

The others are all on the right track... ha ha.

With rod-driven drivers, in a model, it is optimal to set them at 90 degrees.  Think about it this way.

When one rod goes completely horizontal, through the center of all the drivers.  It now relies on the rod on the opposite side to reliably determine which way all the wheels go.  The opposite rod should be at top dead center in so that ALL of its force is directed in one direction - horizontally - to push the wheel the right way.  At any other angle, the horizontal motion of the second rod is partially wasted because it isn't tangential to the wheel.

If it's not too far off, it works fine.  But when you get too much off 90, it will never work.

The smaller the drivers are,  and the closer the crankpins are to the center, the more critical this is.   When the crank is far out from the middle, you get a big "lever" that makes it easy to move the wheel.  The closer in you get, the more important it is to keep things at 90 and not waste any force. 

So, for example, if you were crazy enough to hand-make little small-diameter brass drivers with crankpins very close to the center, things have to be really perfect to make it work. 

[Cajonpassfan]

Lol, nobody would be that crazy Nice, nice work Max!
Okay, I understand, I think. I was of course thinking of a fully (or mostly) gear-driven chassis, where the side rods would be purely cosmetic, going for a ride so to speak...
Otto K.

[mmagliaro]

Here's a short update, folks.

After a LOT of work, checking quartering, rod lengths, frame straightness, etc, I did some careful shimming of a bearing pocket or two, and I have reduced the occasional lifting in the motion.   It is not perfect, but it is the best I can possibly do.  I do not think anyone would notice the remaining imperfections while running on the track (unless of course, you are looking at brutal close-up macro level video  ).

So here's some brutal close-up macro level video 

In the course of doing this, I had to sever the plastic gear tube right down the middle, separating the two rear drivers, Dremel off the remnants of Loctite and plastic from each half-axle, then bore out a new plastic axle tube, put the drivers back together, requarter them....  let's just say it was a lot of work.

But it DID end up running better, because I could hone the quartering more precisely.  The rear gear on that axle tube was also a little beat up from all the fitting and testing in the early days of this engine, so it's just as well that it has a nice clean gear on it.

Meanwhile, some more progress... just a few items.

First, I realized that I could no longer get the boiler to slide off the front after I put on the yokes for the valve gear!  WHOOPS.  The little firebox lower sides attached to the boiler were preventing the boiler from sliding past the yokes.  So I carefully, surgically, cut those off, and made finished firebox sides, now attached to the FRAME instead of the boiler.
The lower firebox sides are clearly visible in the above video.

I also made the smokebox front, which is a project to itself.

[narrowminded]

Impressive work as usual!   The detail is outstanding!

[Chris333]

So the important question... How many drill bits have you broken so far? 

[mmagliaro]

So the important question... How many drill bits have you broken so far? 

Ha!  That would have been a great metric to keep track of. 
(I would guess I've broken around 10-12 bits, mostly #80, but not all... and used about 20 Xacto #11 blades... at least!)
Oh... and milling cutters (end mills)... probably 6-8 by now.   When I was first using the 1/32" ones, I wasn't very good at it... they last longer for me now.

[Chris333]

I would of never thought end mills. Since they are mounted nice and square with even pressure.

Good to know even you break bits

[narrowminded]

   When I was first using the 1/32" ones, I wasn't very good at it... they last longer for me now.

I haven't broken my .015" end mill yet.  Nor my $120 special miniature dovetail cutter with the spindly shank that plunges into a 1.2mm hole for cutting an internal 20 degree reverse angle pocket that works like a snap ring groove.  Mount up a high speed spindle as we talked about.  Pay for it with unbroken drills and end mills.   I'm serious.

[Cajonpassfan]

Max, I continue to be amazed at your doggedness (and skills). The little critter is an amazing product of both, especially given its diminutive size, and it runs great. Wow!
One comment, if I may: don't know if you're done with the number plate yet, but it seems too big relative to the headlight and smokebox...?
Otto K., following with admiration

[mmagliaro]

Max, I continue to be amazed at your doggedness (and skills). The little critter is an amazing product of both, especially given its diminutive size, and it runs great. Wow!
One comment, if I may: don't know if you're done with the number plate yet, but it seems too big relative to the headlight and smokebox...?
Otto K., following with admiration

Thanks, man!
On the number plate size, I am just guessing based on photos and some HO brass model.  It is pretty darn big, about 20% of the width of the boiler, as near as I can eyeball, which is how I sized mine.  I do hear what you are saying.  If I keep looking at pictures and decide it's too big, I might cut it down some.

[mmagliaro]

I haven't broken my .015" end mill yet.  Nor my $120 special miniature dovetail cutter with the spindly shank that plunges into a 1.2mm hole for cutting an internal 20 degree reverse angle pocket that works like a snap ring groove.  Mount up a high speed spindle as we talked about.  Pay for it with unbroken drills and end mills.   I'm serious.

There is a "high speed" kit for the X2/Micro-Mark mill that replaces the gear drive with a belt drive and get the top spindle speed up from 2500 to about 4600. That's still not "high speed" (where I'm thinking 10,000 or more to run those little end mills).  Do you think 4600 would help, or that much of an increase not really be worth it?  The kit is about 150 bucks, and it supposedly runs much quieter than the gear drive, which would be nice.

[Cajonpassfan]

Thanks, man!
On the number plate size, I am just guessing based on photos and some HO brass model.  It is pretty darn big, about 20% of the width of the boiler, as near as I can eyeball, which is how I sized mine.  I do hear what you are saying.  If I keep looking at pictures and decide it's too big, I might cut it down some.

Max, I felt awkward making my comment, so I dug into my pile of steam stuff, and guess what I found?! Had no idea I had it....NP drawing 5954-A, October 1907....
I hope this helps.... I read the diameter as 17 1/2"
Otto

[mmagliaro]

Otto... awkward???  Nonsense.

THANK YOU for looking up that drawing.  While you were doing that, I had already decided that you were right, so I made a new number plate.  The original was 0.125".  I made the new one 0.100".

17-1/2" just HAPPENS to be ... drumroll...  .109" in N Scale.    So I'm within a shade of your drawing measurement.
What a lucky guess...

The new look.  I agree with you.  This size is better.  The key was when you said "in relation to" the headlight.  That's the whole ballgame right there.  It doesn't matter so much that the circle is the perfect size, but it has to be about the same or a little smaller than the headlight in order to look right.  Proportion is more imporant than exact size in these circumstances.