I have the NMRA gauge and always thought my wheels etc. were within the gauge. But, I wonder, what is the tolerance for wheel spacing? It must be +or- some value. I have been using my (way less than good) eyeballs on the gauge. Is there a more accurate way? I have a good vernier caliper.
http://www.nmra.org/standards/sandrp/S-4_2ScaleWheels.html
There’s a max for K, and a min for B, a little more thinking than I want to do should pop the tolerance out of there!
The size of the slots for the wheels on the gauge should define the tolerance, as well, if one was inclined to figure it out.
I don’t ever recall see ing a +/- tolerance for the wheel gauge. The purpose of the metal NMRA gauge was so you could adjust the wheel spacing so the flanges of both wheels dropped into their slots. If one was out of the slot, it wasn’t right and should be adjusted accordingly. Using that along with the other various gauges on the NMRA gauge would insure that everthing was compatable. It is sort of a go, no-go gauge that is easy to use.
Now that’s not saying that wheels that don’t fall in these specs won’t run right. It just means that for the best performance with the fewest derailments, the wheel flanges should fall in the slots.
Bruce, the tolerances are quite small if you want to minimize errors. The problem is that while the gauge may offer you some assurance that your wheels are actually true to the standard, the rail paths may not be. As Joe Fugate tells us, and Tim Warris at Fast Tracks, few commercial turnouts are very good, let alone exceptional in terms of conforming to the NMRA standards, mostly at the frog and guards. So, let’s say that your wheels are smack dab in the bottoms of the grooves on your gauge. Great. What about the flange path on your A, B, and C turnouts? If you were able to watch the flanges on your in-gauge wheel sets run through the frogs, how far from the middle of the available pathway would they ride? So, a statistician like Chuck could explain that it’s all about probabilities. Your wheels gauged to the NMRA specs have a reasonably high probability of success on any one HO turnout, but the highest that figure may every actually be would be in the order of about 93%, and probably less. So, statistically, you can expect about 6% of your cars and engines to need adjustments, or your turnouts to need corrections.
Note that if any one turnout deviates from the standard by more than about 10%, the likelihood of several cars derailing, assuming they are only close to being in gauge, is very high. It would depend on which way the error lies. Too sloppy at the guard rails? Wheels set too wide by about half a millimeter? You can expect a derailment probably 30-50% of the time, and it would be by picking the frog point…probably.
I’m not sure this helps at all, but by sticking with the NMRA gauge, you get the highest probability that most of your wheelsets will be okay at any one turnout. Individual turnouts will have individually better and worse results if you were to compile a data set over time.
-Crandell
For wheel (flange) dimensions, the steel NMRA gauge is a simple go/no go. If the flanges fit in the slots when the gaauge is parallel to the axle, it fits the spec. You need to eyeball centering on the axle - and adjust BOTH wheels if they are in gauge but off center.
The critical tolerance on the gauge is the little step on the track gauge feeler. The corner of the railhead has to meet that step. Too tight is obvious. If the step falls past the railhead, the gauge is too loose.
Of course, checking dimensions is only part of the process. Changing out-of-tolerance dimensions is a whole different ball of wax.
Chuck (modeling Central Japan in September, 1964)