Rex, you are correct. Anchor the pivot pin for the TT. Half the TT bridge will expand in each direction. If you leave a gap equal to the thickness of a dime at installation, your locos and rolling stock will roll right across it with not problems mechanically. I would recommend anchoring the approach track at the edge of the TT Pit and allow the expansion to go toward the engine hose. That should take care of your expansion issues.
Thanks for the info. This turntable is 4 ft plus the length of the round-house (4 ft) and has a 1 ft connector between the relatively stationary structures. It's straight. There is nowhere for the expansion to go but is that a problem? First of all I don't run trains in the winter because we're not there. The summer temps in the mountains rarely gets to 90 or down below 40. The turntable and roundhouse are both in the shade so no direct sunlight to worry about. If I install the two fixed structures at say 70 degrees and figure worst case of 100 degrees then the math says I could have up to .019 inches of gap to build into the connection between the turntable and the connector rail. I seriously doubt if I can build to that tight of a tolerance. My loco's and battery cars can surely jump that much gap. I just need to make sure I design in some gap or I'll have the turntable being pushed around. I believe the pivot point is the critical location. That will get locked down and everything else is relative to it. Am I missing something?
Rex
Andrew, et al,
Expansion is a dogma in model railroading. Everyone seems to wish to ignore it until it tears something up and then it is time to try to 'compensate' for it. Let me see if I can clarify thermal expansion.
The magic number, called the coefficient of expansion, for brass is .0000104 inches PER inch of length PER degree of temperature differential. So, if in the winter your coldest temperature is 0 degrees and you reach 100 degrees in the summer that will be a 100 degree temperature differential, NOMINAL ONLY. There is another factor that no one accounts for, that is SOLAR heating. From personal experience in the panhandle of Florida, I have measured 145 degrees ACTUAL rail temperature after peak solar heat. My guess is possibly as high as 160-165 degrees during the peak summer heat. SOO, lets do the math. if we are looking at 48 inches of length, and 165 degrees of actual thermal differential, that would be 45 times 165 times .0000104 will equal .082368 inches, or just short of 3/32 inches. Now this is a WORST CASE SCENERIO, but the math is accurate.
@Tom - depending on where you are, like possibly desert southwest where ambient temps can reach 120, I am sure the solar heating can be appreciative. Try putting your hand on the rail in the middle of the day and see if you want to keep it there. Calculating thermal expansion can be a real pain in the posterior if you want to get it right. Way too many hobbyists have problems because they try to 'anchor the track fast'. I have said this on other fora as well "Thermal expansion will not be denied. It WILL go some where." I do agree with you that the real number will be closer to your numbers being as we usually install our railroads in weather somewhere in the middle of the overall temperature range, meaning that the real numbers need to be calculated for both the expansion of temperatures above the installation temperature as well as the contraction on the temperatures below installation temps.
Good luck with your turntable.
Bob C.
Rex, no so sure you will be experiencing the 122 degrees F that Andrew talks about. Unless I’m totally off base I think you generally run in the neighborhood of +20(+)F to 80F. I would not expect much more than about 1/32 inch across that range. If you were to do the final installation on a very warm to hot day all should be well. I don’t see any need for expansion joints for the turntable. You will be needing a small gap from turntable to yard track so you should be good.
Tom Trigg
According to Stan Silvermans Handy Converter for Model Railroaders, 4 feet of brass track will expand 1/16 inch at 50 degrees Celcius. At 10 feet it would expand 1/8th inch. Brass, Stainless, nickel silver & aluminium will expand at that temperature & length the same.
For a few $ Stans converter deals with everything, electrics ,leds, wiring sizes, scale timber, converting scales in trains, time-speed-distance to work out how fast your loco travels in scale speed, How much track is needed for shape & hight of helixes, etc.
I dont have any connection to Stan, or his software converter, but am impressed with what is on it!
www.StansTrains.com
Andrew
Sandbar & Mudcrab Railway
The AZ Big Trains club had their spring open house this weekend. We visited 15 layouts out of more than 25 that were open. Three of the ones we visited had turntables. Two were gallows type and the other was a girder. Operation was a problem here in the desert. It seems the wide temperature changes make match-up almost impossible. The track inside the engine houses doesn't expand as much as the sun beaten turntable so the gap is variable. One owner is planning to cast his in concrete but I don't think that will work.
I seem to recall expansion joints being advertised although I can't find them now. Has anyone out there used them and would they work for this particular operation? Also how much will 4 feet of brass track expand with a 50 degree temperature change?
Thanks,
Very generally speaking, early turntables have the following basic components.
1. Some form of and heavy sized pivot pin centered in the turntable pit.
2. There were two small diameter ring rails (one up on the foundation of the turntable, and one down on the underside of the turntable). These rings were usually in the 5' to 8' diameter range depending on the size of the turntable.
3. Spider rollers between the above mentioned rings, which bore the majority of the weight of the locomotive, or whatever was being turned on the table.
4. There was an outer single ring rail, whose purpose was to support the end of the table when loading / unloading. There were usually two wheels, not flanged, on each end of the turntable that rode on the rails.
For maximum interest, I prefer a gallows type of turntable because I feel it has the most eyecatching interest.
FWIW
Bob C,
As far as turntables the only one i have seen was a small one and it had two single rail wheels at each end and was gear driven in the center
When I read about the evils of drinking I quit reading.
Another way of bending wood is to use pvs pipe with a sealed cap on on end. Fill pipe with household ammonia.
Soak the wood (up to 3/4" thick) & it will bend into a circle, spiral, corkscrew or what ever shape you need. I read about this from a model shipbuilding paper.
Also to timber is less liable to split as the natural oils in the wood are still there. You also have a longer time to shape the timber compared to steaming.
Saves on laminating timbers.
It's no secret. You just have to make the laminates thin enough going with the grain of the wood. In my case I cut 4 each 1/4" x 5/8" x 5' and then glued them back together with tite bond III and 1" staples in a jig held with clamps until the glue dried.
What do you know about turntable pits and rails?
great job looks great
how the heck do you guys bend your wood i have made several attempts at it unsuccessfully
To be a little more accurate I think I'm building a "pivot bridge" to operate as a turntable. Thus far I've built the bridge. I plan to use the lazy susan idea from the recent indoor turntable article in GR. I also looked at the previous "How to" article (April 2008) on building an outdoor turntable. Both authors used a girder type span while I'm using an arch - no big deal. Both authors used a single rail in a pit. The pictures I've found of full scale turntables show a single rail. These single rails don't appear to be major load bearing fixtures. So why a single rail?
Also I finally figured out what an "Armstrong lever" was. It's a bar that a strong man can push on to turn the turntable.
Can someone point me to some design requirements for a turntable? I'd like to do this right.
It's winter building time and this year's projects include this turntable.
First I drew up the plans and laminated the arches:
Next I put the trusses together
Then built the table
Finally I put it on the flatcars
Next spring we'll install it on site with the central pivot yet to be built/cast.
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