Turntable pit depths

[Anonymous]

I am trying to find info on turntable pit depths. I am scratch building a C.P 70' turntable. I have all the info I need, but I am lacking the correct pit depth. Can anyone help me. Any and all replies are helpful, and I thank you all in advance

[tomikawaTT]

Depth of a turntable pit is determined by the design of the turntable bridge, or (rarely) by some local condition. Assuming that your turntable is to be installed on a level surface with no unusual groundwater problems:

1. Starting with the profile view of your turntable bridge, determine the exact distance from the bottom of the bridge rails to the bottom of the treads of the carrying wheels on the ends of the bridge. That will give you the depth from the tops of the ties on the approach and fan tracks to the top of the carrying rail those wheels ride on.

2. Add the height of that rail and the ties it is mounted on. For a 70 foot long bridge, code 55 rail would be appropriate in HO and smaller scales. You now have the height of the 'terrace' supporting the carrying (ring) rail.

3. Measure the height of the center bearing, from the underside of the bridge rail to the bottom of the bearing structure. That will give you the height of the center bearing platform, which is usually a cylindrical concrete pier about six feet in diameter or two feet larger in diameter than the bearing structure, whichever is larger.

4. There is usually a 'step' at the inner edge of the ring rail terrace, typically one foot or so. There will also be a step down from the center bearing column, which can be fairly small for a tapered-girder bridge or quite large if the center bearing depth is small. The main floor of the pit slopes downward to drains at the center. That slope can be quite gentle for a flat-bottomed bridge or pitched like a roof under a tapered-girder design.

5. You now have the profile for your turntable pit under the turntable bridge girder. To get the actual pit diameter, take a plan view of your bridge and measure diagonally from the end of one bridge girder through the center of the center bearing to the opposite end of the other bridge girder. It will be larger than the profile length of the bridge girder.

Odd turntables in history:

The McCloud River Railroad once had a turntable on a steep hillside. The uphill side of the pit was in a deep cut. On the downhill side, the ring rail was supported on a spindly wood trestle.

One I saw in Japan had a very deep bridge, and had apparently been built over the culvert that carried an intermittent stream under the yard. The pit sloped asymmetrically to drain into the open-topped flood channel that crossed between the approach rails and the center bearing. The table was short - 20 meters or so - but the maximum depth of the pit, in that flood channel, was close to four meters. (Needless to say, this is what I intend to model when I scratch build my turntable.)