considering all the problems i've read about when building model turntables, i wonder how precisely aligned real turntables were. What kind a gaps were there between the ends of the rails on a turntable bridge and the track on the ground it was aligned with? It must be comparable to the gaps the wheels must bridge near the frog on a turnout, several inches?
thanks for the bandwidth
greg - Philadelphia & Reading / Reading
Interesting question.
No way the gap was several inches. An inch or less, I'd say.
Now when you use the magic word "aligned", that's a bit of a different story. The aligning was done by eye by the turntable operator (most likely the hostler who was moving the loco). I'm guessing within a half an inch might be acceptable. There was most likely a great incentive to line the tracks properly--putting a loco on the ground because of imprecision was probably not a great excuse to bring to one's boss.
Ed
On prototypical turntables the gap could be four inches or more and the difference in elevation almost as much, especially at remote locations out on the line where there was no roundhouse or shop forces, and a Wye might not fit.
These tables might turn only an engine or two a week on a branch, depending.
The rail ends would get battered down significantly as the wheels spanned the gaps.
Thank You!
Kootenay Central On prototypical turntables the gap could be four inches or more and the difference in elevation almost as much, especially at remote locations out on the line where there was no roundhouse or shop forces, and a Wye might not fit. These tables might turn only an engine or two a week on a branch, depending. The rail ends would get battered down significantly as the wheels spanned the gaps.
There were (and still are) turntables in many locations. Some were at the ends of branchlines and used for "occasional" turning. They most likely weren't built and/or maintained to the standards of more heavily centralized and used turntables.
Note the second picture down at this link:
http://historical-travels.com/2009/08/11/rediscovering-steam-railroading/
You'll see a quite small gap. This would be preferred by railroads because there would be less battering of the rail ends than with a larger gap.
One thing that would militate against a small rail gap is rail expansion from temperature changes. While the short tracks going into a roundhouse aren't going to expand in length much due to heat, the leads could grow a lot in length--too far and they make the table unusable. So lead tracks might need a larger gap than roundhouse tracks. I think if I were faced with this kind of problem in the real world, though, I'd put an expansion joint in the lead near the table, and still try to have a small gap at the end of the table.
So, I guess it might matter what kind of a turntable one was modeling. Backwoods--go with a 4 inch gap. Large engine terminal--as small as possible.
gregc It must be comparable to the gaps the wheels must bridge near the frog on a turnout, several inches?
It must be comparable to the gaps the wheels must bridge near the frog on a turnout, several inches?
It isn't comparable. In theory (and pretty much in fact) there IS no gap near the frog of a turnout. The wing rails are designed to support the wheel as it passes over the gap at the frog.
There is a gap at the "point" end of a stub switch, however. See picture on right at link below:
http://www.housatonicrr.com/downshifting.htm
And here is a rather awesome picture of a Santa Fe loco being turned at Sandusky, Ohio on the PRR. Note the gap, which is wider than on my previous example--I'm guessing a couple of inches--maybe more. Note also the peened over rail ends. The wider the gap, the more the peening. And again, note that the radial tracks have splice bars in them. The short pieces were either replacements for overpeened ends or were made that way for later replacement when the damage became too much.
http://www.columbusrailroads.com/photogallery/v-don-5-PRR-5022-CLMB.jpg
PS: from another angle (It's a 2-10-4 from the ATSF class 5011):
http://www.columbusrailroads.com/photogallery/v-don-4-PRR-5022-CLMB.jpg
Note that that 8-wheel humongutruck under the Santa Fe tender is jacked up so the loco could be turned on a turntable too short for it. The UP had to do something similar when Challengers started running in short turntable country.
NYC got around that by building the 4-10 pedestal tenders (used on Niagaras, later retrofitted to Hudsons) with huge rear overhangs.
The turntables I'm most familiar with had rail gaps that ranged from 20mm in high summer to 60-70mm in the dead of winter - same turntables, both seasonal extremes.
Chuck (Modleing Central Japan in September, 1964)
Very little gap here (magnifiy your computer screen for a better look).
tomikawaTT The turntables I'm most familiar with had rail gaps that ranged from 20mm in high summer to 60-70mm in the dead of winter - same turntables, both seasonal extremes.
Interesting!
For a 100 degree F change in temperature, a 60' turntable rail should contribute to the gap difference about 1/32 inch (1 mm)(and assuming I ran the calcs right). So, there should be an awfully long bit of steel rail without an expansion joint (half a mile); or there is (more likely, I think) ground movement from such things as water retention in clay.
A segment of my house's foundation rises about an eighth of an inch when the clay underneath is wet.
7j43kFor a 100 degree F change in temperature, a 60' turntable rail should contribute to the gap difference about 1/32 inch (1 mm)(and assuming I ran the calcs right).
i calculate about 1/2" (7.3 10^-6 in / in * degF). and that's only the rail on the turntable.
gregc i calculate about 1/2" (7.3 10^-6 in / in * degF). and that's only the rail on the turntable.
Yeah, that number looks better than mine. Let me see:
7.3 x 10 to 6 inch per inch per degree =
.0000073 inch per inch per degree =
.0000876 inch per foot per degree =
.005256 inch per 60 feet per degree =
.5256 inch per 60 feet per 100 degrees
There's that half inch! Sorry about the earlier mistake.
Since there are two gaps, that's 1/4 inch per gap. Or 6 mm.
Chuck reported approximately a 50 mm difference between temperature swings. That leaves 44 mm (1 3/4 inch) of gap to the approach rail to account for. So then the approach rail must be a "single" (without expansion joints) piece 1.75/.5256 x 60 feet = 200 feet long.
The above assumes a 100 degree F temperature difference between the two original measurements.
That's doable. Again sorry about the math error.
On the Live Steam front, my rules of thumb about expansion for 100 feet of rail, 100 degrees F temp swing are 3/4" for steel and 1-1/2" for aluminum... that's just the estimate that I use when laying out expansion joints in track.
There used to be a great video on Youtube that showed military experiments on derailing trains in WWII, but it has unfortunately been removed. Really, you had to see it to believe it! They used explosives to remove sections of rail, and then pushed a steam loco (2-8-0 IIRC) with a string of gons over it with a diesel at the back. Mind you, this was on straight track, but the train stayed on the rails over some ridiculous gaps at a good rate of speed... it's hard to imagine a consolidation making it over an 18" gap! Anyway, having seen that film, I have no problem with 4" or more gaps at the end of the turntable, since the locomotive should be moving very slowly and the tracks are straight.
As far as alignment, that is more critical. However, the turntables I've seen use a "Dog" at the end to line up the turntable with the lead. It was an "H" shaped piece of steel that rested inside the webs of the rails just above the spikes. When the end of the turntable was lined up with the lead, it would be slid out so it was half on the lead, half on the table. This assured that the table was lined correctly and that it would stay in line during the movement of the locomotive.
I sure do wish that video was still on Youtube...
- James
EDIT - On the turntable in the picture a few posts earlier, I noticed it doesn't have the dogs for alignment, but it does have the latch mechanism seen just to the side of the brackets for the "Armstrong" poles. It accomplishes the same thing; alignment is fixed, and turntable won't rotate during the locomotive's movement.
Depends, but the gaps should be as small as practicable. This is the WSOR turntable at Janesville, WI.
Note the lock lever that helps keep things in place.
Mike WSOR engineer | HO scale since 1988 | Visit our club www.WCGandyDancers.com
7j43k Interesting question. No way the gap was several inches. An inch or less, I'd say. Now when you use the magic word "aligned", that's a bit of a different story. The aligning was done by eye by the turntable operator (most likely the hostler who was moving the loco). I'm guessing within a half an inch might be acceptable. There was most likely a great incentive to line the tracks properly--putting a loco on the ground because of imprecision was probably not a great excuse to bring to one's boss. Ed
Usually there was a sliding steel plate that would fit between the rails at one end or the other of the turntable. Once the table was lined up, this plate would be moved into position to stop any further rotation of the turntable. So while aligning the table depended on the judgement of the operator, using the plate ensured that it was (and stayed) properly aligned. Most times when a locomotive ended up in the pit it was carelessness, laziness or faulty assumptions to blame.
While the plate held the table in proper horizontal line, there could be noticeable vertical variation on a center-balanced turntable as it rocked under the weight. Larger tables would usually have three-point support. The horizontal gap could be several inches. You had to allow for possible movement of the rails on the radial tracks, as well as thermal expansion on both them and the turntable deck.
John
How is this for gaps?
http://www.proto87.com/model-railroad-track-holding.html
Pete
I pray every day I break even, Cause I can really use the money!
I started with nothing and still have most of it left!
That is exactly the film I was talking about before! Thanks for posting it.