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Above: Fixed-Arm Semaphore in use near Southwood Drive in Lincoln, Nebr. |
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Above: Fixed-Arm Semaphore in use near Southwood Drive in Lincoln, Nebr. |
This document explains how railroad semaphore signals work. The emphasis is on the Union Switch & Signal (US&S) Style T-2 mechanism for an upper quadrant signal. Other types of power semaphore mechanisms are similar in principle, but also have some important differences.
Of the few experts on semaphore signals, I am not officially one. Being a student of the electrical world and a fan of railroad signals, I have been able to deduce a few things for the benefit of those who know less. For those who know more, I encourage you to inform me--especially if my fallibility is manifest in this article.
Even most signal maintainers don't have any significant knowledge of semaphores to show for their careers, as semaphores have been obsolete for a very long time. One signal maintainer, however, Mr. Earl Ford, has been very kind and helpful in providing the schematic drawing that you will see below. On this is based the explanation that follows.
The blade of an electrically operated semaphore signal is brought to and held in the horizontal "stop" position by gravity. Moving the blade upward to the 45° "approach" or 90° "clear" position and holding it there against gravity requires power. This is a fail-safe feature.
On a lower-quadrant semaphore, where the blade moves down from the horizontal position rather than up, it is counterweighted so that gravity still tends to return it to the horizontal position.
The semaphore blade is lifted by a slow, high-torque motor. The motor is connected through a gear train to the semaphore shaft. It is never disengaged from the blade by a clutch or similar device. When the blade falls by gravity, it drives the motor backwards. A semaphore motor never helps push the blade down, but instead provides some resistance as the falling blade drives it backward.
Since a semaphore blade is designed to fall by gravity, the semaphore needs a "hold-clear" device to keep the blade in the 45° or 90° position for as long as desired after the motor stops. The hold-clear device consists of an electromagnet as part of a ratchet-like mechanical system. Whenever power is applied to the electromagnet, the blade can move up but not down. The electromagnet is energized significantly before the motor stops. When the electromagnet is later de-energized, the device is disengaged, and gravity pulls the blade down again. No device is usually necessary to hold the semaphore in the horizontal position as its weight is resting on a mechanical stop.
Modern crossing gates as well as semaphores have "hold-clear" devices. In a semaphore context, the electromagnet of the hold-clear device is often referred to as a "slot coil."
It would be easy to conceptualize the hold-clear electromagnet as having a single coil of wire, but that usually isn't so. To understand why, it is important to understand the principle that something is easier to hold up than it is to lift. If the hold-clear electromagnet had only one coil, that coil would of course have to be powerful enough to lift the hold-clear armature and thereby engage the mechanism. But then it would be using more power than necessary to keep it there. This would be a major concern in some desert areas of the Southwest where, historically, signals have been normally "clear" and the only power supplies have been batteries! To help alleviate this problem, the hold-clear electromagnet usually has an energy-saving arrangement that requires two coils.
The first coil is called the "holding slot" and is made to operate on a very low current (a few tens of milliamperes). This makes holding the armature up an efficient proposition and helps save the battery. But this coil is too weak in its efficiency to lift the armature of the hold-clear device initially, so a second, higher current coil is energized for a short time to help out. The second coil is called the "pickup" or "motor slot" because it comes on with the motor.
It is not desirable to have the blade return toward the horizontal by gravity as quickly as the forces of nature would have it, lest damage result. An electric retarder is used to solve the problem and is explained as follows.
Supposing I were to produce electricity by putting a generator on a hamster wheel, the hamster could run freely until I switched on the lights. He would then have to work harder and would likely slow down. The principle here is that as a generator is loaded more and more electrically, it resists more and more the mechanical force that propels it. Hence, a generator can be used to slow mechanical motion at the command of a switch to a load. Note that no external power need be applied to the system to slow the motion as the existing energy is simply being dissipated.
Under certain conditions, an electric motor is exactly the same device as an electric generator--only used differently. So it is with the semaphore signal that the motor which lifts the semaphore blade is also used as a generator to slow it's fall. A simple resistor is used as the electrical load, and wires from the motor are automatically switched to this resistor whenever power is removed from the signal (which is the deliberate method of lowering the blade).
Some of the earliest semaphore designs did not use the electric retarder principle to slow the blade's return toward the "stop" position. Instead, they used a dash-pot damper, which is a piston that forces oil or air through a hole that is small enough restrict the flow. Many screen doors today have a similar device to help them close gently.
Although the coördination and control of the semaphore motor, slot coils, and snubbing resistor requires some sophistication, this complexity is internal to the mechanism. A couple wires connected to the signal serve to tell the signal at what position its blade should be, and if this is a different position from where it is already, the signal mechanism acts accordingly.
The following table shows that the signal aspect is chosen simply by sending current to the signal through wires here named 32HG and 32DG. Additional wires provide a current return from the signal.
Selection of Signal Aspect |
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Battery On Wire 32HG
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Battery On Wire 32DG
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Effect on Signal
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No
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No
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Blade falls and rests at
0°. |
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Yes
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No
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Blade rises or falls and holds at
45°. |
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Yes
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Yes
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Blade rises and holds at
90°. |
Virtually all styles of semaphores are controlled in like manner. Even the lower quadrant Style B semaphores, which use separate "home" and "distant" blades to achieve the same three indications, are controlled by a logically similar set of wires. Part of the beauty of the semaphore signal is this fact that the immediate control of the blade is regulated within the mechanism and not within the relay cabinet. So, from the perspective of the control relays, a semaphore isn't really much more complicated to operate than a color-light signal. (See notes at the end of this document.)
The complexity of turning the motor and slot coils on and off to achieve the selected aspect is handled internally by the circuit controller. Although it is also sometimes called a circuit breaker, it is not a circuit breaker in the modern sense of being a safety device. The circuit controller is really just a series of switches inside the semaphore mechanism that open and close at various blade positions. To the motor and slot coils, it is just a multiple "limit switch."
It was demonstrated earlier earlier by the table that the two wires 32HG and 32DG select the aspect of the signal. In fact, these wires are also part of the power supply for the motor and slot coils. By merely routing 32HG and 32DG to the motor and slot coils through the switches of the circuit controller correctly, the signal will naturally rise, fall, or hold itself at the desired position, and do so in a fail-safe manner. No other "brains" are required.
The following diagram shows the parts discussed and the simplest way that they may be connected for a three-position signal.
The motor armature is shown as a circle with an M. The motor's field coil is shown above it, and is part of the motor.
The slot magnet of the hold-clear device can be shown with its two coils or windings, motor slot and holding slot, labeled accordingly.
The snubbing resistor is shown left of the motor as the usual resistor symbol (a zagging line). Notice that it is connected across the motor through a switch. When the resistor is being used to retard the motion of the motor, the current flows in an almost rectangular current path through the motor, resistor, switch, and back to the motor.
A dashed vertical line through the switch indicates that the switch is opened mechanically by the slot magnet when it is energized. The switch is closed, therefore, when the slot magnet releases and the blade is falling.
The several segments of the circuit controller are seen in vertical alignment with the motor. Only disks 7 and 8 are used here. The segments complete circuits horizontally in the diagram when the blade angle is in the range specified with an arrow. Notice that different ranges are specified for the top and bottom of each disk. For example, 8-Top bridges the top two contacts for 0° to 44°; 8-Bottom bridges the bottom two contacts for 43° to 89°. (The circuit controller is somewhat pictorial.)
Wires 32DG and 32HG were discussed previously. They supply the current to signal. Wires 32GN and 32GNI return current from the signal, but they are not logical counterparts to the first two wires because they return from their respective parts of the signal regardless of the selected blade position. (See note below.)
The lower portion of the diagram shows things that should be assumed to reside in the relay cabinet near the signal location. It shows to what the signal is connected: basically the signal battery through control relay contacts.
I have already described the Style T-2 semaphore signal in terms of its basic parts. By studying the diagram, it should be possible to understand how they work together. However, I here give my own interpretation and explanation of the diagram.
Suppose the blade is at the horizontal position, and it is desired to move it to the 45° position. The control relay picks up its "home" contacts supplying battery to wire 32HG. Circuit controller segment 8-Top passes current to the motor from 32HG, and this same current continues on from the motor to pass through the motor slot winding of the slot magnet and finally down through 32GN to the common side of the power supply. So, this current energizes 1) the motor to move to move the blade upward, and 2) the slot magnet (of the hold-clear device) prevent it from moving downward. Because the hold-clear device is engaged, this keeps the switch above it open to eliminate the snub circuit through the resistor. The motor continues to move the blade upward until its current is cut off on segment 8-Top of the circuit controller by moving the blade beyond 44°. The blade is now at the 45° position.
Although the motor slot is now de-energized because it shared the same current with the motor which is now stopped, the holding slot winding has been fed separately by current from wire 32HG over circuit controller segment 7-Top, because the blade is still in the range for that segment, which is 0° to 55°. The low-current winding alone is now preventing the blade from falling. If the blade were somehow pushed momentarily past 55°, the holding slot winding would be cut off, the hold-clear device would disengage, and the blade would be able to fall. If battery were still on 32HG then, the blade would probably fall until it reached 44° because the holding slot winding is not designed to engage the hold-clear device despite it's being energized again at 55°. The blade, therefore, stays around 45° whenever battery is on wire 32HG.
Next, suppose it is desired to raise the blade to the 90° position. The "distant" relay contacts close to put battery on wire 32DG. Current is once again fed to the motor, this time over circuit controller segment 8-Bottom, until it cuts itself off by moving past 89°. The blade is now at &90deg;. Again, motor slot is de-energized with the motor, but the holding slot is still being fed, this time by 32DG over circuit controller segment 7-Bottom, because the range for that segment is 53° to 90°. The blade, therefore, stays at 90° as long as battery is on wire 32DG.
Suppose now that it is desired to release the blade and send it back to its 0°, horizontal position. Power is removed from both 32DG and 32HG. The hold-clear device releases, which allows the blade to start falling and closes the resistor switch. Current generated by the motor flows in a rectangular path through the resistor and switch. This dissipates the energy of the falling blade so that it moves slowly and comes to a gentle stop.
The diagram specifies heavy #9 AWG for wires that carry motor current. The relay contacts that break those wires are paired together, presumably to save them from early destruction due to switching heavy motor currents.
It is interesting to note that the holding slot winding has it's own wire for return current, 32GNI, when it might be functionally just as well to return all the current through 32GN and save a wire. I believe that the reason for 32GNI is an application of the standard railroad practice of breaking both battery and common wires at the relay contacts to make false activation less probable. In other words, it's necessary to be extra sure the holding slot drops out when it's supposed to. Perhaps since the motor and motor slot winding require a heavy current, false activation is not so likely as to require a break of 32GN. I guess that the 300-ohm resistor exists simply to reduce the holding current.
The circuit controller may be difficult to understand at first because it is drawn according to the railroad habit of drawing schematics rather pictorially for better or for worse. From looking at the diagram, it's easy to imagine the many disks all on a shaft like a kabob. The disks are of insulating material, but they are thick enough that they can have a thin metal sheet running part way around the side, like a label that goes part way around a soup can. Springy metal contacts drag on the sides of the disk and are connected electrically when the metal rolls in between them. I believe these are called "drag contacts."
In the diagram above, the circuit controller has many segments that do not appear to be used. However, the diagram shows only how to operate the signal alone, not how it should be interconnected with other signals. Any style of railroad signal that relies on moving parts to change its aspect, such as a semaphore signal or searchlight signal, cannot be assumed to be in the desired position simply because voltage is or isn't applied to the control wires. All proper installations of signals with moving parts have at least limited feedback circuits to check the position of the signal mechanism itself, not the just the relays back in the cabinet that are supposed to control it. For example, in automatic block signal (ABS) territory, the mechanism itself must be at "approach" or "clear" before the next signal to the rear can show "clear." In interlocking territory, the mechanism itself must be at "stop" before conflicting routes can be set up or signals cleared for opposing traffic. In a semaphore, one of the things the extra circuit controller segments are used for is to provide an indication of the signal's position for these circuits. Sometimes, instead of just "making" or "breaking" these circuits, the circuit controllers need to be able to change their polarity. For this, a different style of contacts are used, which I believe are called "snap contacts" (although I haven't found the reference for it as I write this) and follow a cam on the controller shaft. I suspect that this style is akin to what we now know as the micro-switch, and was necessary to change the polarity quickly without shorting.
Generally, "line" circuits, that is, circuits from signals down the track via the pole line, only feed the coils of line relays, and the signals themselves are powered from the local signal battery through the contacts of the relays. For a given signal installation, though, it might be noticed that the hold-clear device seems to mimic the position of some particular or other relay in the relay cabinet. If, then, in a situation where batteries are your only power source, why have relay whose coil wastes current when you can just send the current directly to the slot coil? Well, the relay is indispensable because it is needed to switch the heavy motor current. But that didn't stop someone from coming up with a cleaver way to cut the relay out after the motor stops. The scheme was called "slot-on-line," whereby the slot coil was fed by the signal battery until the signal reached the "clear" position, and then directly by the line wires while the relay coil became disconnected. This was another thing that extra drag contacts on the circuit controller were used for, and can be seen in some of the more complicated drawings on this website. While slot-on-line seems like a brilliant way to save power, I heard it had the drawback of allowing slot coils to be easily damaged in lightning strikes.
Theoretically, blade movement from 45° to 90° should be possible with battery on wire 32DG only without battery on 32HG as given in the table. However, the blade could not cleared all the way from 0° to 90° having battery only on wire 32DG because segment 8-Bottom of the circuit controller doesn't close or "make" until a little before 43°. I believe that the circuit controller is designed this way on purpose to help protect against false clearing of the signal.
It is perhaps amusing to consider how closely the wires to a color-light signal resemble the control wires of a semaphore signal in function, and for certain railfans, even more amusing to consider that the color-light signal might be easily replaced by a semaphore: It appears the "yellow" wire would become 32HG, the "green" wire would become 32DG, and the "red" wire could be thrown away. However, a semaphore showing the "clear" indication must have battery on 32DG and 32HG as discussed in the above paragraph. Therefore, 32HG is not logically equivalent to the "yellow" wire of a color-light signal since the yellow light is not on whenever the green light is. In theory, this represents a small but clear difference in the wiring required at the relay end. But in practice, semaphores require the wire and relay contacts to be capable of heavier currents than color-lights do. Moreover, using a semaphore is not a way to save on the number of wires either: besides HG and DG, the semaphore requires two wires for the return current (GN and GNI), wires for the lamp, and wires for the separate back indication. As such, there's more than one reason to assume the railroad industry is unlikely to enter into a semaphore renaissance period.
Be sure to check out the varied and complex "T-2 Semaphore Mechanism Wirings" on this website.
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