Letter about quartering in February CTT

[lionelsoni]

See http://www.jghtech.com/html/dead-center-1.html through http://www.jghtech.com/html/dead-center-5.html for examples of usage of "front..." and "back dead center". The examples that I have found of "top dead center" as applied to reciprocating steam locomotives seem to be generally casual and non-technical.

Horizontal double-acting cylinders were virtually universal for a century before internal-combustion engines with their typically vertical single-acting cylinders became common. The term "top dead center" seems particularly ambiguous when adapted for steam locomotives, since neither end of the cylinder is closer to the top than the other and the maximum cylinder pressure occurs at each end.

[spankybird]

TDC normally refer to the point a max compression.

[palallin]

TDC (Top Dead Center) is applicable to all piston/cylinder applications, not just vertical ones, so the use of it in reference to steam locomotives cylinders was entirely correct and appropriate. The "top" of the cylinder refers to a location, not an orientation.

[lionelsoni]

Pete, Plummer acknowledged your last point in his original answer in October.

I agree that 10 degrees is quite a lot; but what I meant is that the locomotive would probably function just fine with as much as 15. I discovered when I changed the quartering on my 773 that Lionel's tolerance was more like 5 degrees. The wheels must have been quartered consistently by their production press, just not quite at 90 degrees. Once they were pressed onto the axles, the wheels were scored so that they could only be put back on at multiples of the axles' spline spacing. Unfortunately, I mixed up the wheels and had a hell of a time getting them sorted out and back on the correct ends of the correct axles again!

[SPFan]

A 10-15 degree tolerance is way off. I wonder if the author has ever done this. In my experience the tolerance is more like 1-1.5 degrees on a worm drive engine. On a spur drive engine the tolerance from side to side is not critical but the tolerance from axle to axle is. That is, instead of the correct 90 degree offset side to side, it really could be anything as long as its off by the same amount for all axles.

Pete

[lionelsoni]

The writer, Myles Marcovitch, purports to correct and expand on Ray Plummer's October column, but gets a few things wrong himself. He says, "In our electric trains, quartering does nothing!" Actually, Plummer got it nearly right when he pointed out that, "Good quartering is also essential on model locomotives with worm gearing on one axle." If you take "good" to mean within 10 or 15 degrees, Plummer is right. If the cranks are too close on, for example, a scale Hudson or an S2 steam turbine, the rods can bind, since they are the only things that transmit torque to the other drive wheels.

Marcovitch also incorrectly uses automobile terminology with "top dead center". On (most) steam locomotives, it's "front..." or "back dead center". And I think it unlikely that, as he says, without quartering a piston at front dead center would bend the piston rod, any more than that it would with quartering. In normal operation, that is precisely where the piston force is greatest anyway, with the full boiler pressure on the piston.

Incidentally, the Lionel General provides an interesting example of clever design of the model by deliberately not quartering the drivers. The crossheads are connected together under the boiler and, moving in unison, move a piston inside the boiler, which, functioning as a cylinder, pumps air through the smoke generator in the smokestack.