Turntables and DCC
Summary: A turntable consists of a short section of track mounted on a bridge mounted on a pivot that allows the it to freely rotate.
A turntable consists of a short section of track mounted on a bridge which is sunk in a pit placing the bridge at grade. The center of the bridge is a pivot that allows the it to freely rotate. As the bridge rotates, the ends of the bridge align with tracks radiating from the center of the bridge.
Engines are driven onto the bridge, and the bridge rotated, allowing the engine to be parked on a stub track, or returned to a main line or other track. As shown in the photograph, many of the tracks lead into roundhouse stalls. Locomotives are always driven head-first into their stalls. Another use is to turn passenger cars so they will face the correct direction for a return trip.
In many cases, a circle of rail lines the circumference of the turntable pit. Wheels on the end of the bridge ride on this single rail to provide support and guidance.
The picture to the right shows a stylized plan and cross-sectional view of a typical turntable. Some of the common components shown are:
- Bridge Track :the section of track that rotates when the turntable spins
- Bridge Rail :one of the 2 sections of rail that make up the Bridge Track
- Pit Rail :the circle of rail that lines the circumference of the turntable pit
- Pit :the hole in the ground that the turntable lives in
- Bridge :the construct that holds the bridge track
- Pivot :the point in the center of the pit and the bridge that allows the bridge to rotate end for end
- Main Track :the track to/from the rest of the world
- Stub Track :the short tracks that radiate from the turntable
A turntable is a device used to turn railroad rolling stock. When steam locomotives were still in wide use, the railroads needed a way to turn the locomotives around for return trips as their controls were not configured for extended periods of running in reverse. Turntables were also used to turn observation cars so that their windowed lounge ends would be oriented toward the rear of the train.
The turntable bridge (the part of the turntable that included the tracks which swiveled to turn the equipment) could span anywhere from 30 to 120 feet, depending on the railroad's needs. Larger turntables were installed in the locomotive maintenance facilities for longer locomotives, while short line and narrow gauge railroads typically used smaller turntables as their equipment was smaller. Turntables as small as 10 feet in diameter have been installed in some industrial facilities where the equipment is small enough to be pushed one at a time by human or horse power.
In engine maintenance facilities, a turntable was usually surrounded, in part (as in most cases) or in whole, by a roundhouse. It was common for the roundhouse to only cover a portion of the land around a turntable but fully circular roundhouses exist, such as the preserved roundhouse that serves as the basis for the Baltimore and Ohio Railroad Museum in Baltimore, MD.
Turntables still in use are more common in North America than in Europe, where most locomotives have a controller cabin on both ends. Diesels are not turned often, but may be. When a locomotive is ordered from the manufacturer, it is delivered facing the direction specified in the order. Diesels are often operated in multiple unit configurations, so the lead locomotive is chosen because it faces the correct direction. Steam locomotives are operated with the boiler in front, so they must be turned to the correct orientation for its assignment by the hostlers. (The exception is Southern Pacific's Cab Forwards). In pusher/helper service, they could operate in either direction as needed.
With Multiple Unit operations in North America, at least one of the locomotives in the set will have it's cab facing the opposite direction of travel. Cab units were often delivered in A-A or A-B-A configurations to eliminate the need to turn the engine set.
Other methods for turning equipment include balloon tracks, or a wye. Turntables are used where space is limited, or as part of a roundhouse complex. Larger roundhouses were usually part of a service complex, which would include the backshop as part of the roundhouse where engines would receive their major service or rebuilds. The complex would include all the facilities needed, such as a large machine shop, a wheel shop, boiler shop, tender maintenance facilities, etc. Steam locomotives could be stripped down and completely rebuilt, with the boiler removed from the frame, and even new tires installed.
The backshop was usually located behind the roundhouse, and locomotives were pushed into it through a stall that lead into the shop. A small locomotive often called the yard goat would shove a dead locomotive onto the turntable, which was then aligned to the correct track, and the locomotive would be pushed off the turntable through the stall and into the backshop.
Straight Through Routes
A straight through route is a track from the main line that can cross the turntable and continue on back to the main line without the engine stopping. This was probably very rare, used on narrow gauge or short lines. The possibility of an accident, and the requirement to slow down to a crawl to cross the turntable, would have made this arrangement unlikely in many instances.
Electrical Issues for Models
In the modelling world, the track on the bridge needs to be electrified and just like any other configuration of track that allows a locomotive to end up facing in the opposite direction, this can cause phase issues. This affects how the bridge power and the stub tracks are wired.
Powering the Bridge
There are three main classes of turntables, differentiated by whether the power to the bridge track is continuous, self-reversing or rail-to-rail contacts:
For all these types of turntables the leads from the turntable to track power have to pass through a reversing switch, either a manual DPDT toggle, or a DCC auto reverser (these are usually advertised as intended for reversing loops but are equally applicable to this type of turntable). Otherwise driving a locomotive onto the bridge, turning it 180 degrees, then driving it off again will definitely cause a short circuit.
The advantage of this type for DCC is that there is no dead zone that will cause a sound decoder to stop. However, some sound decoders will reset if the power is interrupted momentarily by the short circuit that activates the auto reverser. It may be possible to avoid this by adding an energy storage module to the locomotive.
Slip Ring Contacts
There are 2 continuous insulated bands of metal, usually found on the central pivot. Each is connected to one of the rails on the track on the bridge. Springmetal wipers slide on the slip rings. These wipers are connected to track power. As the turntable turns, the power to each bridge rail is always fed by the same connection from track power.
Solid Pit Rail
The pit rail is continuous and powered by one of the track power feeds. One bridge rail gets its power from one or more wheels that run on this rail. The other bridge rail gets its power from a single slip ring on the central pivot.
This type of turntable automatically reverses the bridge rail phase when the bridge turns beyond a certain point.
The dead zone at the gap where the power is reversed will cause DCC sound decoders to stop. Provided you don't stop the turntable here, a large enough energy storage module may be able to prevent this happening when the bridge is turning past the dead zone.
Split Rail Contacts
In most cases, the ring of rail inside the pit is split into two 180 degree arcs. One wheel at each end of the bridge is wired to a rail on the turntable track. The pit rail arcs are connected to track power. As the turntable turns, the wheels pick up power from the pit rail arcs and feed the bridge track.
In the DCC world, the split in the pit rail must be either a double-gap, or an insulated segment, long enough to prevent the wheels at the end of the bridge from shorting both arcs together.
In the DC world, the split in the pit rail need only be wide enough to keep the 2 rails from touching. This is because in DC, track power is turned off while the turntable is moving. In DCC, track power is always live.
There is a dead zone where the contact wheels on the bridge are in the double gap or insulated segments. No tracks should be aligned with the turntable in these positions.
Split Slip Ring
This is the most common type for commercial turntable models. A slip ring around the central pivot is divided into two 180 degree arcs with two wiper contacts also set at 180 degrees apart. The slip ring arcs are connected to track power. The bridge rails are connected to the wiper contacts.
There is a dead zone where the wipers pass the gap in the slip ring but this is usually designed to be break-before-make so it won't cause a short circuit problem for DCC.
Note that on the more sophisticated models there may be additional slip rings for things like position feedback sensors.
Rail to Rail Contacts
The bridge rails are powered by the entry/exit rails to which they are aligned. Wiper contacts on the ends of the bridge rails make direct connections with contacts on the ends of the entry/exit rails. The bridge is not powered while it is turning, only receiving power when it is aligned to an entry/exit route. This has the advantage that the bridge phase is always inherently matched to whatever track it is aligned to.
This type is not well suited for DCC because the locomotive will be unpowered while turning and sound decoders will stop.
When this type is combined with a split slip ring design (as in the Hornby R070), it becomes a power routing turntable meaning that the bridge is actually supplying power to the stub track to which it is aligned through the rail end contacts.
This type is also not well suited to DCC because powering the bridge through the slip ring and powering the stub tracks with their own track feeds will cause a short circuit when the bridge starts to rotate. The #Hornby Turntable is an example of this.
Powering the Stub Tracks
The way to wire the stub tracks (or entry/exit tracks) depends on the turntable type:
As you stand in the turntable pit, face the tracks one by one. For any stub track (ie a track heading into a roundhouse or other track that does not connect to anything else), connect all of the left-hand rails together, and connect all of the right-hand rails together. Face the main track that arrives at the turntable from the 'outside' world. Connect the right hand stub track rails to the right hand rail; the same with the left.
You can install toggle switches in the leads to the stub tracks to turn off the power to selected tracks.
These connections will allow an engine to chug up to the turntable, and (assuming you turn the reversing switch to the turntable power the right way) continue onto the turntable bridge and come to a gentle stop. The turntable will then rotate until the bridge track aligns with one of the stub tracks.
In a slip ring turntable, the engine can drive off either forward or reverse without touching the reversing switch. This works for any track, EXCEPT if the turntable turns 180 degrees so the engine heads back the way it came. Then the reversing switch for the turntable must be used.
With a self-reversing turntable, the turntable should be viewed as two halves, the dividing line being the dead zone. Let's call the 'front half', the half where the main track arrives from the 'outside' world. It is typical to install the turntable so that the dead zone is approximately perpendicular to the main track but any orientation will do provided the dead zone isn't where you want a stub track.
All tracks including the main track on front half should be wired with matching phase. All tracks on the back half should be wired with the opposite left/right phase (when viewed from the center of the pit). Now the bridge phase and track phase will always match in all possible positions and directions. Note that this also allows all possible straight through routes to work without problems too.
Rail to Rail Contacts
For a bridge that only has rail to rail contacts, the stub tracks can all be wired with whatever phase is desired and the bridge will adapt to the phase of whichever entry/exit track it is aligned. The only caveat is that the entry and exit tracks of any straight through routes must be wired with the same phase.
For bridges that also get their power from a split slip ring (such as the Hornby R070), the stub tracks are supposed to get their power from the bridge, i.e. the power is routed to the stub track(s) that the bridge is aligned to. This was okay for analog DC layouts but is not suitable for DCC. We want the bridge and the stub tracks to be always powered. See #Hornby Turntable.
|OO/HO||Hornby||R070||No||Yes||Analog||Yes, Manual, 22.5 degrees||Yes||Rail-to-Rail and Split Slip Ring||Hornby supplies instructions for modifying to DCC compatible operation|
|Peco||LK55||Yes||No||N/A||No||Yes||Split Slip Ring||Can be motorized with DIY mechanism or other 3rd party kits
Track locating lugs at 15 degrees
|Atlas||305||No||Yes with 304||Analog||Yes, Manual, 15 degrees||Yes||Split Slip Ring|
|Fleischmann||6152||Yes||Yes||6915 or 6910 Controller, or DCC/LocoNet||Yes, Auto, 7.5 degrees||Yes||Split Slip Ring||Can be controlled by track DCC or LocoNet DCC|
|Heljan||89121||Yes||Yes||DCC or supplied control panel||Yes, Programmable Stops, Infinitely Variable||Yes||Split Slip Ring|
|Walthers||933-2849||Yes||Yes||DCC or supplied control panel||Yes, Programmable Stops, Infinitely Variable||Yes||Split Slip Ring||90' Table
Walthers branded version of the Heljan turntable
Walthers branded version of the Heljan turntable
When planning the turntable location and lead tracks, try to have a straight length of track equal to your longest locomotive at any entry points to the turntable. If the track is curved, there is the possibility that the locomotive will push the bridge out of alignment, resulting in a derailment.
There are a number of ways that turntables can be operated:
- Manual crank
- Motorized and controlled by an analog train controller
- Motorized with a specialized controller
The manually cranked ones can sometimes be motorized. The Altas ones can be motorized with an add-on that effectively turns them into the second case.
The simplest type of motorization is just a 12V DC motor that is intended to be controlled by a standard analog train controller. This lets you control the speed and direction and stop it wherever you want.
The Atlas and Hornby ones use a type of Geneva Drive that makes them pause at set indexing points. This type is easy to convert to DCC by using a mobile decoder and wiring it so that the turntable motor is driven by the decoder's locomotive motor outputs. Then you can control it with any DCC throttle as if it were a locomotive.
The more expensive turntables are supplied with a dedicated controller with buttons and a display, making it much simpler to select a route.
Some turntables are even DCC-enabled so that they can be controlled by DCC. These effectively have an accessory decoder built-in and respond to accessory commands (often known as turnout commands) to select a route. You simply assign a unique accessory address to each entry/exit route and the supplied controller automatically converts a "throw turnout" command to a "move turntable to route x" command. This type obviously requires a DCC throttle with turnout command capability (not all do).
Note that there are no commercial turntables that can be controlled by a DCC accessory (or "stationary") decoder. These decoders are generally not suitable for driving the analog DC motors in most turntable mechanisms and, more importantly, there is no way to send meaningful commands from a typical DCC throttle. And obviously, the type that are DCC-enabled already, don't need an external DCC accessory decoder.
The Hornby R070 turntable use both rail-to-rail contacts and a split slip ring.
The Hornby turntable should be modified for DCC operation otherwise it will cause a short circuit almost immediately when the bridge starts to rotate. As supplied, the bridge rails get their power from a split slip ring fed by wires from the main entry track and by making contact with the stub track rails by means of brass contacts at the rail ends. When the bridge turns, a left rail on the bridge will pass a right rail on the entry/exit track (or vice versa) and will cause a short circuit if both are always powered (which is what you want for DCC operation). To solve this, Hornby recommend removing the brass contacts from the bridge rail ends. This effectively converts the turntable into the self-reversing split slip ring type and all the installation recommendations given earlier for that type of turntable now apply.