engineers often use less time consuming approximations that are "good enough".
greg - Philadelphia & Reading / Reading
I've noticed.
Ed
gregc selector A #6 frog diverges one unit toward the diverging route for every six units of length let's make that unit 1/10" see http://www.catskillarchive.com/rrextra/tkwk10.Html previously posted do all all turnouts have to be the same
selector A #6 frog diverges one unit toward the diverging route for every six units of length
let's make that unit 1/10"
see http://www.catskillarchive.com/rrextra/tkwk10.Html previously posted
do all all turnouts have to be the same
this diagram illustrates the NMRA turnout dimensions described in RP 12.3 as of a few years ago (i don't believe the radius is correct, but the lead length (middle number) is accurate).
i hope you agree that there appear to be some inconsistencies
However, it looks like the NMRA has updated their spec, NMRA RP-12.2 Turnout Dimensions. And while at least the lead lengths for the smaller turnouts appear more correct, there still seems to be some inconsistencies (easier to see graphically)
#4 6 13/16#5 7 5/8#6 8 3/8#7 11 3/8#8 12 3/16#9 12 15/16#10 13 5/8
when using equations from the Catskill site, the turnout dimensions change more consistently.
selector gregc do all all turnouts have to be the same From the frog point, yes. The frog number is what matters, and the ratio of divergence AT the frog is the basis for the frog 'number'.
gregc do all all turnouts have to be the same
From the frog point, yes. The frog number is what matters, and the ratio of divergence AT the frog is the basis for the frog 'number'.
deviations in the closure rail, either an inconsistent radius or straight sections leading to the frog, can result in turnouts of different (lead) lengths.
while the frog # should be all that matters, it looks like model turnouts, including curved turnouts as discussed, have some deviations. At this point, how can they meet NMRA specs when those specs seem to be changing?
Are you looking at the correct specs? If you go to the STandards and RPs sectioon ont he NMRA site, there's a ZIP file full of PDFs under RP13 which has individual PDFs for each scale in straight and curved turnouts. The HO one ate 12.32 for straight and 13.33 for curved turnouts, updated 2015.
Manufacturer dimensions can and will vary - this is only an RP, not a Standard, so strict adherence to the numbers in the tables is not a requirement.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
thanks.
I wasn't aware that the RP had been updated in 2015. My notes are from 2014.
while the rp 12.31 values are better(?) than the rp 12.3 values from 1961, i still see inconsistencies from frog # to #.
the lead length doesn't increase proportionally (nor exponentially) (see jumps between #6 and 7 and #10 and 11).
I don't understand the value of what they call the "curved rail radius (11)" which I assumed means the closure rail and is shown in red, drawn to the frog. It doesn't seems to align with the frog and alignment varies from # to #.
I haven't figured out what the ordinate points are referring to. They didn't line up with the curve rail radius. And the RP specifies a section of straight track. Any section of straight track will reduce the closure rail radius.
The Catskill site describes prototypical methods which i believe describe a constant radius closure rail from points to frog.
Not so sure it should? The angle to frog number formula is angle = 2 arctan (1/2x) where x is the frog number, and that is anything BUT a linear function.
gregc engineers often use less time consuming approximations that are "good enough".
Well, I don't know if this will help or hinder the discussion but I'll toss it out for your perusal:
ICS_frog_33 by Edmund, on Flickr
ICS_frog_34 by Edmund, on Flickr
ICS_frog_35 by Edmund, on Flickr
ICS_frog_36 by Edmund, on Flickr
ICS_frog_37 by Edmund, on Flickr
ICS_frog_38 by Edmund, on Flickr
If you're not put to sleep yet, I can post the chapter on crossovers, too.
Regards, Ed
rrinkerThe angle to frog number formula is angle = 2 arctan (1/2x) where x is the frog number, and that is anything BUT a linear function.
yes. the relationship between frog number and angle is non-linear.
rrinkerNot so sure it should?
presumably this is referring to my comment to
gregcThe Catskill site describes prototypical methods which i believe describe a constant radius closure rail from points to frog.
the following figure (posted yet again) illustrates how a #6 (or any) turnout can have different lead lengths and closure rail radii. The blue line indicates a constant radius curve and the red line a straight section of rail. number at left is closure rail radius and middle number the lead length.
The Catskill site has a single set of equations for determining the lead length and closure rail radius where the closure rail radius is constant up to the frog for a specified frog #. This means the tangent line of the closure rail is aligned and parallel with the frog where it meets the frog.
I don't believe a prototype would do it any other way except in a special case.
while a shorter turnout with a #6 frog angle may be necessary in a special case (e.g. industry spur), the drawback is a tighter closure rail radius which may limit the locomotives that can handle that specific turnout.
RP 12.31 specifies a #6 turnout lead length of 6.5" for which the shortened closure rail radius is 30". I'll guess that this was a pragmatic compromise; a shorter turnout and a 30" radius that is more than adequate for most modelers.
But, as the following figure illustrates, a properly laid out #5 turnout has a closure rail radius of 33" and is just a bit longer, 6.6", than an RP 12.31 #6 turnout.
gregci'm working on laying out a curved turnout and I come up with larger frog numbers than the commercial turnouts of the same sizes. Do commercial turnouts use stock frogs?
i believe the diagram illustrates how a curved turnout can be made relatively short and with more typical frog numbers by laterally offsetting the centers of the curves. (thanks DaveB)
RR_Mel I agree with Carl, I worked up a Peco curved turnout on my CAD a few years ago and couldn’t come up with their specs as printed. I’m glad I bought the turnouts before I did the drawing otherwise they wouldn’t have fit my CAD driven track work. Mel
I'll have to have a look back thru these discussions at a later date. Appears to be a really educated discussion of the subject, but a little above my pay grade at this time (above my understanding).
Brian
My Layout Plan
Interesting new Plan Consideration
Best they can do is match whatever the manufacturer says the specifications are. If the manufacturers misstate their dimensions, well... but they generally aren't off by a huge amount, which means if you don't try to squeeze in ever last fraction of inch, the plan will still fit as drawn in the CAD program.
rrinker Best they can do is match whatever the manufacturer says the specifications are. If the manufacturers misstate their dimensions, well... but they generally aren't off by a huge amount, which means if you don't try to squeeze in ever last fraction of inch, the plan will still fit as drawn in the CAD program.
I've found (as well as others) that these 'mis-stated curve radii' by the manufacturers are sometimes CONSIDERABLE different than real life.
And as GregC has mentioned somewhere on one of these discussions of curved turnouts, the placement of these turnouts is MORE CRICTIAL than the placement of ordinary turnouts due to their double curves whose connections with other tracks can be thrown off considerable from drawing phase/computer design verses actual model layout real life (I think I said that somewhat correctly?,...I could not find his quote).
Yes, I am trying to cram a lot of trackage into my freight yard/ladder design, but don't real railroads attempt to do the same?
The thing the CAD can do is make sure the tracks extending off any of the turnout legs matches the radius, instead of possibly being kinked or not exactly matchng because the sectional track piece is 18" radius and the turnout is 18.4" radius.
Frankly, if I couldn;t get accurate dimensions of certain brands of track peices, I wouldn't use them, and go for an alternative method, such as a pinwheel ladder made with Atlas #4, which are really a 4.5 frog, ok for switchers and smaller cars. If I had to make my yard that compact, I'd forget all ideas of long cars.