|Figure 1--A crossing location having lights, gates, and bells in El Dorado, Kans.|
The outline for this page is as follows:
Before we begin, it is convenient to observe Figure 2, which illustrates the principle and function of the master relay.
|Figure 2--The function of the master relay.|
Most crossing installations, unless they are entirely solid state, have a normally energized master relay as shown in Figure 2. In some cases, the configuration may vary somewhat. For instance, if two tracks both cross the highway near enough each other to a share a fraction of the highway warning devices, there may be more than one interface relay affecting the warning operation. Also, the detection circuits may have other outputs to provide train information to controlled highway intersections that are near the crossing. But in any case, the key concept is that when a train is approaching the detection circuits "drop" the master relay to activate the signals.
On prints, the master relay is often designated "XR." This designation has its origins in the old days when there was a possibility of having only one relay in the control circuits. Modernly it is understood to stand for "CROSSing Relay." In speech, however, the XR is referred to as the "master relay," which is more descriptive of its function and reduces confusion with other relays that may be in the control circuits. Some railroads use other designations for the master relay. The Missouri Pacific used "XC," for example, which stood for "CROSSing Control."
The circuits on either side of the master relay in Figure 2 may be simple or complex. They may be relays themselves or they may be solid state. In discussing them, the XR forms a good point of reference. On this page, the circuits discussed will be those on the right side of Figure 2, up to and including the master relay.
Railroad signaling premises usually require that circuits be designed on closed circuit principles. Normally energized warning circuits at crossings include the "hold-clear" on each of a gate mechanism, and in the past, the rotating banner (i.e., the Griswold rotating STOP sign) or an Automatic Flagman (i.e., US&S type of wig-wag with a disappearing banner). However, some devices require normally open circuits because their fundamental nature is to be normally off, as with flashing lights, bells, and the operating coils of most wig-wags.
Entire crossing signals are sometimes referred to on the railroad as crossing flashers or flashing light signals, probably owing to the fact that before gates were common the essence of the signal was the lights. The most formal terms for the signal heads on the mast seem to be flashing light units. The lights on the gate are simply called gate lamps.
An animated diagram for crossing lights in operation is shown in Figure 3 with a total of eight lamps, as would be used at many locations. It might easily be imagined that the lights would be wired as shown on the left side of Figure 3, but that would be incorrect. Actually, they are wired more as on the right side of the diagram under "correct." The difference is that in the correct scheme the left and right lamps are basically connected in series to begin with, and the flasher relay contacts merely short or bypass half the bulbs rather than supply them.
|Figure 3--Animation of crossing signal lamp wiring schemes. Yellow highlights show current, not voltage. Notice that even though the flasher relay contacts on the correct and incorrect wiring schemes are shown moving in unison with each other, the lamps in the respective schemes glow on opposite sides. (This circuit is drawn to illustrate the principle rather than actual wire routing.)|
All the relays shown above reside in the relay cabinet or control case near the crossing with buried cables running to the signals; they are not parts of the signals themselves. In the correct wiring scheme, the three wires going out to the lamps are here designated EN, EB, and E. (Another naming scheme uses LL, RL, and FL, respectively.)
While not very intuitive at first, the correct circuit arrangement has three major advantages. First, the bulbs being in series makes for a more favorable situation in case the flasher relay or wire E should break. In that case, instead of bulbs on neither side glowing, bulbs glow on both sides, although not as brilliantly for being in series. See Figure 4. That the flasher relay should break in the middle position, however, would be a very unusual failure mode because the flasher relay armature is weighted so that it always tends to fall to one side.
|Figure 4--Crossing signal lamp wiring schemes in failure case.|
A crossing gate typically has three lamps spaced along its arm. The two innermost gate lamps flash in unison with the heads on the mast, but the outermost lamp burns steadily. Even when the other lamps are flashing, the correct scheme provides a constant supply of voltage across EN and EB to which this tip lamp may be connected. Therefore, the same three wires that supply the flashing heads on the mast can supply all three gate lamps. This is the second advantage to the correct wiring. See Figure 5. Note that each gate lamp is connected across a unique combination of two out of the three given wires. At least one manufacturer of gate lamps uses a radially symmetrical connector with three pins spaced 120° apart so that each lamp's function (left, right, or steady) depends on how the cable connector is rotated when plugged in (Figure 7).
|Figure 5--Wiring of gate lamps using only three wires.|
|Figure 6--Gate lamps with varied connection to cable in open air. (Such connections would normally would be inside junction boxes.)||Figure 7--Gate lamps with radial connector. The function of the lamp depends on how the cable connector is rotated when plugged in.|
A third advantage is that the correct wiring scheme requires the flasher relay contacts to switch less current. Although the lamps still require the same amount of current in either scheme, not all of this current is switched at the contacts in the correct scheme because in the correct scheme the current through the lamps is not zero when the contacts are open. This lengthens contacts life.
Figure 3 and Figure 4 show a battery as the signal lamp power supply. But incandescent crossing lamps together can easily draw 10 amps or more. Although batteries may have been used alone at some remote locations, in reality even many older installations had a low voltage a.c. supply for the lamps derived from commercial power, using batteries for the relays and as backup for the lamps. The a.c. lamp power supply comes from a lighting transformer with an adjustable or configurable output voltage. In addition to the red crossing lamps, this transformer usually feeds a white power off lamp on the outside of the signal control case to inform railroad employees that commercial power is in use. The fail-over of red lamps from a.c. power to batteries is handled by a normally energized Power Off Relay (POR). This is also sometimes called the Power Transfer Relay (PTR). See Figure 8.
|Figure 8--An older combination transformer and POR. The many terminals may be jumpered differently to achieve the desired output voltage. The relay armature is picked up whenever 220 volts AC is present on the input terminals, which are sleeved and capped for safety. If battery power is connected on the back contacts of the relay, it will output DC when commercial power is off. This transformer may have been used in lighting circuits.|
Most incandescent railroad signal bulbs are rated for use at 10 volts. Ten-volt supplies are common in railroad signaling. However, 12-volt supplies are also used, especially where there are gates, and dropping resistors must then be placed in series at the relay cabinet to provide 10 volts for the lamps. If these resistors are placed in-line with wire E, this allows the lamps to be a little brighter in the failure case of Figure 4 because the resistor is not then in the current path. However, the lamps are still dimmer than normal for being in series. Another scheme puts resistors in the EN and EB lines downstream of the EOR contacts. Both of these resistor schemes produce a somewhat different voltage for the gate tip lamps than for the other lamps.
In the days of 8-inch crossing signal heads, 18-watt incandescent signal lamps were sometimes used on the mast. Since the advent of 12-inch crossing heads, 25-watt bulbs are used. This is true even on the remaining 8-inch heads if the existing circuits are deemed able to handle the additional current. Figure 3 and Figure 4 show all the lamps connected on the same circuit. However, it is good practice and usually necessary to distribute the load over more than one set of contacts. Sometimes this is done by jumpering several sets of contacts together in parallel. At other times, there is a separate light circuit on each set of contacts, which requires more wiring. Distributing the load amongst several contacts is not on the flasher relay only, but also on the lamp control relay. Sometimes there are even separate power supplies. When separate circuits are used, the lamps may be grouped a number of different ways. One circuit may represent one pair of mast lamps, a pair of mast lamps with one gate, all mast lamps together, a whole signal together, or some other combination. Understandably, the use of dropping resistors tends to limit the number of lamps on one circuit. The three wires discussed above can become greater in number, with multiple "E" wires corresponding to separate contacts of the flasher relays. The additional wires required for separate circuits may have separate designations.
All of the crossing lamps on the mast or gate can be of the LED type today. This is may be discerned by observing the uniform brightness of the LED aspect as well as the lack of a fade-out effect that is briefly seen while incandescent lamp filaments cool off. Existing signal heads may be fitted with LED modules by removing the lenses and reflectors from the signal heads. New heads may be designed with only LEDs in mind. LED signals lighten the load on the circuits because they consume less power. Maintainers appreciate decreased sensitivity to operating voltage and not having to replace bulbs. Some railfans prefer the elegance of glass reflectors and glass (or plastic) lenses, as well as the perceptible effect of the lamp filament heating and cooling.
The "lamp control relay" of Figure 3 and Figure 4 is an abstraction because there usually isn't a relay of that name. At a crossing with no gates, the XR can serve as the lamp control relay by itself. But at a crossing with gates, the lights need to be flashing whenever the gates are not all in the up position. In that case, another relay is used to control the lights, the supply to which is broken by position contacts in every gate at that location. Its supply is ultimately derived from a front contact of the XR so that the lamps begin to flash before the gates even start to descend. The relay is designated XGPR (CROSSing Gate rePeater Relay)or GPR on this page. But since its supply is also broken through a front contact of the XR, it may also be found designated XRPR (XR rePeater Relay) or or XRGPR (XR/Gate rePeater Relay). Or, since the primary purpose of the relay is to control the lamps in the first place, it may be simply designated ER (Electric lamp Relay).
Figure 9 below illustrates most of the circuit details and additional features mentioned above. Keep in mind that wiring and designations can vary largely from railroad to railroad and from location to location. This diagram is a kind of collage of crossing lamp wiring based on what the author has seen and heard.
|Figure 9--Crossing signal lamps operating with more circuit detail shown. This document is drawn to illustrate the principles rather than show actual wire routing.|
All the relays above may be special purpose relays designed with the specific application in mind.
The flasher relay is designated FR or more commonly EOR (Electric lamp Operating Relay). The relay needs DC power to operate, and a separate contact of the lamp control relay (shown as XGPR in Figure 9) switches d.c. energy to the EOR so that the flasher relay operates only when needed. A photograph of a flasher relay is shown in Figure 10 below. This is an electro-mechanical relay that is designed for use with crossing lamps.
|Figure 10--Flasher relay ("EOR") used with railroad crossing lamps.|
This flasher relay works as follows: The armature holding the moving contacts rocks back and forth. The rocking is caused by two electromagnets arranged so that one tends to pull the armature one way and the other tends to pull the armature the other way. The electrical supply of the electromagnets is controlled by internal contacts. The coils of the two electromagnets are wired to the internal contacts just as a pair of lamps, with care that at a given time the electromagnet is on which tends pull the armature into the opposite position, resulting in oscillation. (See Figure 11.)
|Figure 11--Flasher relay (EOR) in operation.|
The speed of oscillation is kept relatively low by the inductance of the mechanism, that is, the momentum-like property that resists changes in the magnetic flux. From looking at Figure 10, it can be seen that the tops of both electromagnets share a common pole piece that channels the flux to and from both coils through a central column in front of the coils: it is part of the magnetic circuit or flux path. Copper washers (often nickel plated) are stacked on this column to increase the inductance. As the mechanism wears from use at the contacts, it tends to slow down a little. Washers can then be removed in the field to reduce the inductance and speed it up again. The number of washers, then, is used to "trim" the flash rate and keep it within specs. Rectifiers may be connected across the individual coils to suppress radio interference from the contacts.
Another method of making a flasher relay is to use a regular neutral relay that is pulsed on and off by a solid state controller. One such controller is called an "X-PAC," and similar controllers may be called by this name also. The contacts of the relay need to be of the type that can handle heavy lamp currents. An X-Pac turns the coil of the relay on and off with a duty cycle of approximately 50%. In order for this duty cycle to be accurately realized at the contacts, a low resistance coil must be used for faster pickup. As shelf relays typically have two actual coils in series, lower resistance may be achieved by physically removing one of the windings, which slip off the core when the backstrap is removed. Being a neutral relay, the back contacts remain closed if the relay fails to operate, which serves the purpose of the counterweight in all-electro-mechanical design above.
|Figure 12--A solid state relay controller used with flashing light signals, similar to an X-Pac. The operating supply is connected to the input terminals at the upper right, and the positive side of the coil of the neutral flasher relay is connected at one of the three spade terminals on the top left, depending on the operating voltage of the relay, to be pulsed on and off. The negative side of the relay coil must connect directly to the negative side of the operating power supply through a wire. The device is only a few inches tall.|
If one were to puts his ear to the signal control case when a train is approaching, the "tick-tock" of an electro-mechanical flasher relay could sometimes be distinctly heard. It may be perceived that the relays yield a lop-sided flashing effect. It is difficult to ascertain whether this is perception is correct or not in any particular case. The psychological element may be heightened by the asymmetrical sound of the relay.
There are now completely solid state flashers in existence. In the saga of their development, one was invented that was apparently intended to be used as a direct replacement for the flasher relay in the "correct" circuit of Figure 3. However, it worked in a nearly opposite manner as the "E" terminal would output a constantly positive voltage while the other two terminals changed polarity around it. This worked well enough for signals with mast lamps only and for incandescent gate lamps because the tip lamp still got a constant magnitude voltage, albeit with alternating polarity. But when polarity sensitive LED gate lamps were installed, it was impossible to find a constant polarity supply on the arm for the tip lamp. Although maintainers are not supposed to modify circuit design without approval from the signal engineering office, there were two ways that a certain resourceful maintainer solved this problem: 1) install a diode bridge rectifier for the tip lamp, or 2) run a separate cable from the tip lamp back to the gate mechanism where a constant supply of voltage is normally found. In the latter solution, a spare contact of the gate mechanism circuit controller was used to break the supply to the tip lamp so that it did not glow until the gate moved out of the clear position. In some cases, the electro-mechanical flasher relay was re-installed.
Bells are mostly for the benefit of pedestrians or vehicles stopped on the tracks or under the gates, so crossings at remote portions of the open highway are less likely to have them. Most crossings have at least one bell, but some have two, and others don't have any.
|Figure 13--A classic electro-mechanical bell.|
Unlike the flashing lights, whose oscillation is controlled from the relay cabinet, a crossing bell externally requires a mere power supply to operate. Depending on the region and practices of the individual railroad, bells can be wired to ring whenever the XR is down, to ring whenever the XR is down or the gate is not in the clear position, to ring whenever the gate is in motion, or to ring only when the gate is descending. Where the bell's operation depends on the gate position, it may be controlled partly by the gate repeater relay in the case or simply have its supply broken by a contact inside the circuit controller of the gate mechanism. Where the bell does not depend on gate position, it may be switched by the back contacts of the XR alone.
|Figure 14--Possible control circuits for a crossing bell.|
Also unlike the lights, the bell is typically a strictly DC device, and is usually fed from the operating battery regardless of commercial AC power (although the operating battery may be charged by commercial power through a rectifier). As per standard signal practices, the case is usually located on the pole side (i.e., the side with the utility poles) of the tracks and on the left side of the road with one signal across the tracks and the other signal across the road from the case. Where possible, this makes for the simplest cable routing. At older crossings that have only one bell and one track (as opposed to double track, etc.), the bell is most often on the signal that is across the track from the case. This is because each signal received a seven-wire cable from the case which ran under the track or road respectively, and both signals used three of the seven wires for the lights as discussed in the previous section. In the cable running under the road, the remaining four wires were needed by the case to access the track circuits on the far side of the road. The case accessed track circuits on the near side of the road through separate wires or cables not running to either signal. This left the cable running under the track to carry the bell circuit; hence, the bell was usually on the signal across the track from the case. However, this convention may or may not be followed in any given situation.
During the golden age of railroad signaling, bells were electro-mechanical. The electro-mechanical crossing bells work basically the same way as a standard electric bell, such as a school bell or fire bell. That is, an electromagnet forces the clapper to the gong and in so doing disconnects its own electrical supply internally through a set of contacts, causing the armature to fall back until the contacts close and the cycle repeats. They produce about 200 distinct strikes per minute with most designs, although some designs were notably faster or slower. The crossing bell rings more slowly than a school bell or fire bell because of the relatively large mass of the parts involved, and also perhaps because of hysteresis in the contacts. Yet it works on the same basic principle. However, there was such a thing as a motor-driven bell, and now also an electro-mechanical bell with a built-in solid-state controller in lieu of contacts that switch heavy current.
|Figure 15--The Inside of a Mechanical Crossing Bell. Power is connected to two terminals of the white porcelain block.|
An electro-mechanical bell is usually mounted atop the signal mast on one side of the tracks so that its wide dimension is parallel to the road (Figure 16). This orientation is believed to produce the greatest volume in the direction of the road on both sides of the tracks. If there is a sidewalk on only one side of the road, the preferred location for the bell may be the top of the signal nearest the sidewalk. Where there is a road parallel to the tracks or other similar feature along the tracks, the bell is sometimes turned 90° from its usual position to target that feature instead (Figure 17). When a bell is located atop signals on both sides of the tracks, the reason may be to target features both parallel and perpendicular to the tracks. Another reason may be that the supply of each bell is broken by a contact of the gate mechanism on the respective signal, and as such there must be a bell for every gate mechanism.
|Figure 16--Normal orientation of crossing bell.||Figure 17--Bell turned 90° from normal for parallel street.|
|Figure 18--An otherwise typical crossing configuration with two bells. Location: MSPA (Former CRI&P) near De Witt, Nebr.|
Bells can be mounted using different methods and at different locations. For example, they may be mounted to the side of the signal mast using a special bracket. Rarely, at very old signal installations, they may be mounted above the control case, possibly due to a shortage of undamaged wires in the buried cables. In the days of wig-wag signals, most installations used electro-mechanical bells like these in addition to the wig-wags. However, the "Magnetic Flagman," a model of wig-wag once popular in the West, could be found both with and without bells operated by the banner mechanism. Mechanisms with a bell gong mounted to the back of them had two internal clappers, and one would strike the gong near the end of the stroke for each direction of the banner's swing. The sound on this model was thus synchronized with the movement of the banner and rang only about 60 times per minute.
Electro-mechanical bells require a certain amount of maintenance and have certain modes of failure. The "eyebrow" on an electro-mechanical bell is supposed to keep ice and snow from dampening the gong or entering the mechanism, yet the author once heard one operate without ringing in winter ("thump...thump...thump...thump"). Also, because of the inductance of the coils, the bell contacts tend to arc when opened, which can burn them away over time. Snubbing resistors or capacitors can reduce the arcing, but may not eliminate it entirely. Bells such as these need to be oiled periodically and perhaps have their contacts replaced every several years. Still, these bells have at times been found to operate for years on end without any maintenance whatsoever.
Electronic bells are more common today in most areas, especially on Class 1 railroads. Electronic bells make a less crisp sound than electro-mechanical bells. The sound they generate might be described as the sound of an electro-mechanical bell under water, if that were possible. One signal maintainer refers to them as "humdingers." They also do not finish reverberating when power is removed. They are quieter than electro-mechanical bells, which is interesting when one considers that locomotive horns get louder with time. An electronic bell is ostensibly more reliable and requires less maintenance as it has no wear parts, but some maintainers believe that they are preferred by railroads primarily because they are less expensive than electro-mechanical bells as at least one model of electronic crossing bell seems to be less reliable than an electro-mechanical one. Photos of three different models of electronic bells follow.
|Figure 19--An electronic crossing bell that seems most prominent on one of the nation's leading railroads.||Figure 20--An electronic crossing bell that seems most prominent with former's largest competitor.||Figure 21--An electronic crossing bell in use on yet another railroad.|
At different times throughout history, railroad crossing gates have been powered manually or through pneumatic or hydraulic means, the latter using automotive brake fluid. Modernly, however, crossing gates are powered electrically, and each design employs similar principles of operation. In summary, a gate mechanism case typically contains a d.c. motor, a set of reduction gears, a "hold clear" device, a relay, a drive resistor, a snubbing resistor, and a circuit controller. It is designed to require electrical energy to keep it in the clear (vertical) position, while gravity tends to pull it toward and keep it in the horizontal or down position.
An electrically powered crossing gate may actually be operated by the motor, by gravity, or by both at different times. The gate is always raised by the d.c. motor alone, working against gravity as it drives the gate up through the reduction gears. The motor is permanently coupled to the reduction gears and is thus indirectly but permanently coupled to the gate as well. When gate is lowered, gravity tends to pull it downward and drive the motor backwards through the gears. This ability of the gate to fall by gravity is a fail-safe feature, and the gate can fall the entire distance by gravity, albeit more slowly at first, without the motor's assistance. The motor and gears must therefore have low enough mechanical friction to allow the gate to fall the entire distance by gravity when power is removed for any reason. Some of the earliest electrically operated crossing gates were designed to be lowered by gravity alone as shown in Figure 22.
|Figure 22--Crossing gate lowered by gravity alone. Some circuit components explained below.|
In modern, normal operation, however, the motor is energized to assist gravity for about the first half (45°) of the descent. At this point power to the motor is cut out, and it continues to fall by gravity alone. When the motor assists the descent, it sometimes said to power down, meaning that the motor drives the gate down under power. This term should not be associated with the nearly opposite meaning commonly used with electronics that turn off or go into "sleep" mode. A less confusing synonym also used by the railroad is motor down, which hints that the motor is driving the gate down. When motoring down, the drive current may be limited by a drive resistor.
Counterweights are added to the external counterweight arms to partially assist the motor in raising the gate. In order to remain fail-safe, though, the gate must still fall the entire distance by gravity when power is removed from the signal. As such, the counterweights are chosen and positioned on the counterweight arms so that they do not counteract all the downward torque from the gate. Whenever the gate is installed or physically adjusted, tests are performed manually with a spring scale on the gate to ensure that gravity still results in a specified amount of net downward force. The basic theory of operation is therefore unchanged by the presence of counterweights.
|Figure 23--An older style WRRS 3564 crossing gate mechanism with case open.|
The downward force of gravity would tend to make the gate accelerate to the extent that it could be damaged upon reaching the mechanical stop at the down position. To prevent this, the fall by gravity is slowed by the electric retarder brake principle, which is the same principle used in "dynamic brakes" used to reduce speed in modern diesel locomotives without using air brakes. Under certain conditions, a motor can act as a generator. As the crossing gate drives its motor backwards during descent, the act of turning the motor generates a voltage at its terminals even without anything connected to the motor. If the motor were left electrically unconnected to anything, this would not have any effect. But if this voltage is electrically loaded, the motor in turn mechanically resists the force that propels it--namely the gate--resulting in slower descent. Whenever power to the motor disappears, the motor leads are disconnected from the power supply wires and rerouted to a snub resistor or snubber inside the mechanism case that behaves as an electrical load. From a physics point of view, the kinetic energy of the gate is being converted into electrical energy inside the motor and is then dissipated as heat in the resistor. It does this automatically, passively, and without external power, so that the gate will descend slowly even if all the external signal wires are severed. When the gate gets very close to the horizontal (within perhaps 5°), some wiring schemes create a heavy snub to slow the gate even more by creating a direct short, and hence maximum load, on the motor.
The snub resistor described above is usually adjustable and set to control the desired speed of descent with varying weights of gates or other requirements. When heavier gates and lighter gates were used at the same location, the resistors on the different gates can be adjusted so that they all reach the horizontal at the same time. In the past, this may have been done for aesthetic reasons if nothing else.
When the motor raises the gate to the clear position and shuts off, the gate is held in that position by the slot mechanism, also called the hold clear device, and also sometimes designated HC on prints. The HC has an electromagnet that engages a brake or ratchet which allows upward but not downward movement. The device releases when power is removed. Designs may vary as to whether the HC picks up at the beginning of the gate's ascent or at the end of it. The HC may also have a separate pickup winding and holding wiring. The pickup winding comes on with the motor to engage the device but shuts off about the time the gate is cleared. The holding winding remains on to keep the gate clear after everything else shuts off. The reason for the separate windings, where used, is that the holding winding consumes less power but does not produce enough force to engage the hold clear device initially.
|Figure 24--A Griswold "pedestal base" gate mechanism.|
The main shaft of the gate mechanism holds cams that operate electrical contacts inside the mechanism case. The cams open and close the different contacts at different positions. This part of the mechanism is called the gate's circuit controller. To the motor, the contacts behave as limit switches to shut it off when the gate raises to the clear position or lowers to the 45° position. The contacts also cut in the snub resistor and bypass it according to the particulars of the circuit design. Besides the operation of the gate itself, circuit controllers may serve other functions to the signal installation: they break control of the lamp control relays to activate the crossing lamps whenever the gate is not at least nearly in the clear position, 2) they make or break the supply of bells where designed to depend on gate position, and 3) they provide indication to event recorders used for maintenance purposes.
The last major component to the gate mechanism is a relay called a motor control relay or motor snub relay. Sometimes its appearance is the same as a glass-bottomed shelf relay from the control case, but other times it has a smaller cube shape or resembles relays used in other industries. The motor control relay is sometimes designated MCR. In concept the MCR is picked up when the gate is desired to be in the clear position, and the MCR is down when the gate is desired to be in the down. However, in some designs the circuit controller may cut the relay out of the circuit after the gate has finished clearing. In newer designs, the relay may be replaced by a solid state module serving the same functions.
While it can be simple or somewhat complex, the wiring of the several parts together varies quite a bit by make, model, railroad, and location. It can generally be said that heavy wires are fed to the signal carrying a constant supply of voltage, usually 12 volts. A normally energized control wire feeds coils of the HC and MCR, either of which may have their individual supplies broken by the circuit controller at various times. In turn, the contacts of the the MCR and circuit controller make and break the immediate the supply of voltage to the motor and effect the use of snubbing. The MCR may change the polarity on the motor field or armature for the up or down movement, but more designs use different field windings or different taps in the field winding for up and down movements. A sample of gate mechanism wiring is given below. This is one of the simpler schemes, using only three circuit controller contacts for gate movement, where one of the most complicated schemes uses nine contacts of the circuit controller in control of the movement alone.
|Figure 25--Sample gate mechanism wiring using motor down feature.|
Note that all of the above wiring is internal the gate mechanism. While there can be many external wires run to the signal relating to the lights, bells, indication circuits, and sometimes even a constant supply of voltage to the mechanism, the actual control of the gate can be a relatively simple matter, depending on a single normally energized control wire in the single wire control scheme discussed on this page. While this is simplest to understand in terms of the external wiring, it is also possible to have separate external control wires for the motor and hold clear device, or separate external control wires for the motor up and motor down supply. One possible advantage to this is the ability to provide more desirable behavior in the event that the XR drops out again while the gate is clearing.
Since the lights and usually also bells must be working for at least three to five seconds before the gates begin to descend, the normally energized gate control circuit is not broken directly by the master relay, XR, but rather by something that repeats this relay in delayed fashion. Where this is done using relay-based control, there is at least one slow-release relay repeating the XR to control the gates. A previous version of this page used the letters GCR (Gate Control Relay) to designate this relay. The preferred designation on this page is now XGR (CROSSing Gate Relay), as used by the illustration below. In any case, this gate control relay should not be confused with the gate repeater relay GPR used to control the lights as discussed in an earlier section. See Figure 26.
|Figure 26--Control of Crossing Gates.|
In the older days of relay-based control the delay was sometimes achieved using a set of cascading slow release relays. These could be designated XPPR, "Crossing rePeater rePeater Relay" and XPPPR (Crossing rePeater rePeater rePeater Relay), for example, where the gate control circuit was opened by the last relay in the chain. Although these relays certainly picked up faster than they released, faster restoration of the gate control circuit could be achieved where a front contact of the XPR at the beginning of the chain also fed the gate control circuit in parallel with the front contact of the relay at the end of the chain.
It is desirable to have each gate mechanism at a given location controlled by a separate contact of the gate control relay, and/or to have blocking diodes at the beginning of each current path, to avoid backfeeding in the event that a faulty gate causes constant voltage to appear on one of the control wires. If it is desired to lower a single gate for maintenance purposes, some models of gate mechanisms contain a test nut or gold nut inside the mechanism case that is designed to open the control circuit for that mechanism when loosened.
Crossing gates have a very close design relationship with semaphore signals, which largely preceded crossing gates in their order of appearance. (Please see the page "How Semaphores Work" elsewhere on this website.) The biggest difference in theory is that semaphores fall by gravity while modern gates partly fall by gravity and partly motor down. Still, some of the earliest crossing gates did not motor down at all but were lowered by entirely by gravity in normal operation, like the semaphore. Crossing gates and semaphores are also similar in that they used some of the same parts: some gate mechanisms and semaphore mechanisms used the very same d.c. motors as parts of their overall designs. The term slot mechanism used for the hold clear device seems to stem from the now arcane US&S Style-B semaphore mechanism that contained "slot coils," presumably so named because they were mounted below the slotted levers in inside the mechanism case, which were called "slot arms." Finally, it may be interesting to note that Eric Schmelz's website reports that entire semaphore mechanisms were sometimes used to operate smaller sidewalk gates, as the author believes he may have seen in the greater Chicago area in the early 1990s.
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