A 70-foot "armstrong" turntable is prototypical. For example, the Southern Pacific had such a turntable in Friant, CA at the end of its Clovis (Friant) Branch. (See pages 15, 39 and 40 of Serving the Golden Empire -- Branch Line Style by Joe Dale Morris, copyrighted 2002 by the Southern Pacific Historical & Technical Society.)
Mark Pierce
There were several examples of the 70 Armstrong on the North Western Pacific as well, as Mark so well put it, it is indeed quite prototypical right up to the end of steam...
Dave
The Rutland RR had several "armstrong" turntables that were used until 1952 when steam was retired. Model Railroader, August 1970, had plans for the Bellows Falls turntable. it was 67 feet but had rails that extended beyond the turntable so that slightly longer engines could be turned. All engines fit except for the 4-8-2s. The pacifics and USRA mikados could be turned. As a side note the 4-8-2 were turned at the B&M turntable that was located across the river. This turntable is still in use by the Green Mountain RR. One engine that rides this turntable is Green Mountain/ex-Rutland RS-1 #405.
Ray
West Coast S wrote: There were several examples of the 70 Armstrong on the North Western Pacific as well, as Mark so well put it, it is indeed quite prototypical right up to the end of steam...Dave
Prototypical right up to the present at Jamestown, CA, where the turntable at the Sierra RR roundhouse currently operated by the California Railroad Museum is almost a duplicate of the reconstructed 1885 turntable at Folsom, CA. Both are center-bearing wooden structures.
Chuck (modeling Central Japan in September, 1964
The switch from manually turned to power turned I believe was more related to the size and weight of the locomotive being turned. The locomotive had to be balanced on the bridge to turn the table, so it had to be somewhat longer than the longest locomotive to be turned. Remember that the locomotive is heavier than the tender, so when it's centered on the bridge, it's not necessarily balanced.
The manual (Armstrong) table at the East Broad Top has a bit of wear on the center bearing, and when the locomotive is pulled onto it, there is a visible "rocking" when the locomotive hits the balance point. Then the lock plates are slid over and the table can be turned by three or four people with an 80 Ton locomotive on it. There's also a great momentum effect to the turning. It takes quite a bit of "oomph" to get it moving and to stop it, but one man can keep it rotating.
The powered turntable at Steamtown NHS in Scranton is driven through the wheel on the ring rail, so balanacing the bridge is less important, but still a consideration.
At first glance, all turntables may look alike, but there can be drastic design differences that aren't readily seen.
Folks:
Were any TTs ever built that were supported only at the ends, with the center pivot providing only lateral constraint? This wouldn't quite be simple supporting, since you'd have to have two support points at each end, to avoid annoying things like "tipping over at the end with one roller"...
Autobus Prime wrote: Folks:Were any TTs ever built that were supported only at the ends, with the center pivot providing only lateral constraint? This wouldn't quite be simple supporting, since you'd have to have two support points at each end, to avoid annoying things like "tipping over at the end with one roller"...
There probably were (on the basis of, 'there's a prototype for anything',) but doing so makes very poor engineering sense. Consider our 75' TT. Allowing for the width of the center bearing, a pure center-bearing design would have a 35 foot cantilever span (unsupported at the far end) under both ends. A more modern design would have the same under the light end unless it was flexible in the middle (3-point suspension,) while the heavy end would be a 33 foot simple beam, supported at both ends. An end-supported TT bridge would have an effective clear span of 71 feet, which would require much heavier construction than the shorter spans of a center-supported type.
That said, I recall one turntable I saw in Japan that was EXTREMELY heavily constructed, considering the weight of the locomotives being turned. Now I wonder if it might have been end-supported. The center bearing was at the top of a rather lightly built (for the purpose) concrete pier, the pit was very deep and part of it was designed to allow a stream to pass through it! The stream was in culverts under the adjacent trackwork and local road.
Chuck (modeling Central Japan in September, 1964)