A General Discussion Illustrating the Apparatus
And Methods Now Used for This Purpose
(Taken from The Signal Engineer, July, 1914)
|With the ever-increasing popularity of the automobile, the question of highway crossing protection for the railroads is becoming more complex. Some of the state commissions are taking an active interest in the matter, and a few are requiring the railroads to file complete information concerning all their highway crossings, a blank form being issued by the commission to show just what information is wanted. Accompanying this blank is a description of the crossing and a photograph showing both highway and railroad.|
Typical railroad crossing sign.
|Although in nearly all cases of accident the users of the highway are at fault, the burden of damages and protection is thrown upon the railroad. The railroad, on account of having the greater force for destruction and maintain a high speed to satisfy the public, must not only protect itís own passengers, but must furnish means of protection to offset the carelessness of the travelers on the highway. The first step in this direction was to place, at every point where a highway crosses a railroad track, a danger sign commonly known as a highway crossing sign. These signs are usually constructed of a post set in the ground, and about 10 feet above the surface of the ground are the necessary boards, often in the form of a large "X" bearing the words "Railroad Crossing" or some such words of warning. The word "Danger" is often used and in some states is required. Most companies have a standard design for this crossing sign for their respective road and anyone familiar with the various signs used by the different roads can distinguish what railroad they are crossing when traveling on the highways throughout the country.|
|In cities and the larger incorporated towns the question
of safety at most of the street crossings is overcome by the trains on
the railroad reducing speed to a safe margin. But at very busy street crossings
and at highway crossings in the open country, where particular conditions
make some crossings dangerous, there have come into use other means of
protection than the mere crossing sign. These constitute the highway crossing
bell, the automatic signal or flagman, the crossing watchman, the crossing
gate and the separation of grades, either by building a bridge and running
the highway over the railroad where the latter is in a cut, or, if the
railroad is on a fill, the highway passes beneath the railroad. The latter
method requires a stronger bridge, while the former, on account of the
necessary clearance, required for the trains, takes a higher bridge and
sometimes calls for an approach to be built to accommodate the highway.
The separation of grades is without question the safest and most desirable
method to employ for a highway to cross a railroad and whenever the surface
of the country permits, it is becoming more and more generally adopted.
Next after the highway-crossing sign the most economical way of giving added protection to a crossing is by means of the crossing bell. There are two classes of crossing bells; one employs the electric current fur its operation and the other is operated from energy supplied by a spring. The spring on the latter type is kept wound by levers connected to the rail which receive their action from the vibration of the rail due to tire passing of a train. For the electric bell current must he supplied by batteries or some other constant and reliable source. The mechanism of an electric crossing bell is arranged so that the attraction of an electric magnet operates the clapper. On the last part of its stroke the clapper, by means of contacts attached to it opens the circuit, the magnets are de-energized and gravity carries the clapper back to itís normal position, which again closes the circuit to cause another stroke. The manufacturers of the bells have different ways of constructing the clapper, contacts, magnets, etc., the main endeavor being to make the mechanism so it will wear well, operate freely and on a small consumption of current as possible and to be weatherproof. For a crossing bell to be effective it must only ring when a train is close at hand and approaching the crossing.
|The bell must not ring after the train has passed the crossing and is receding from it. For double track this is a simple proposition, but for single track the sequence of operation for the control of the bell must be such as to distinguish the direction of the train. For a bell employing track circuit this is accomplished by an interlocking relay. This relay has two separate sets of magnet coils. Each set is connected to a track circuit. The track circuits extend one in each direction from the bell for about two thousand feet, the battery being placed at the end of the circuit farthest from the bell.|
|The armatures of the relay for each set of coils have a back contact which closes when the armature is fully dropped and so completes the circuit to the local battery, thus ringing the bell. The two armatures of the interlocking relay have a mechanical connection between them that is in its normal position when both sets of coils are energized. The de-energizing of either set of coils in the relay permits its respective armature to drop the full distance required to close the bell circuit and at the same time sets the mechanical interlocking device between the armatures in such a position as to prevent the other armature from dropping a sufficient amount to close the bell circuit on that side. A train approaching the bell will shunt the battery from one side of the interlocking relay and so close the bell circuit, causing the bell to ring. At the same time the mechanical connection between the armature will bc set so as to prevent the other armature from fully dropping. The bell will continue to ring until the rear of the train passes, when the armature will be picked up and so open the hell circuit. The bell circuit will not he closed through the other armature, as the mechanical locking has prevented it from folly dropping, although the train which is moving away from the bell has shunted the battery from the magnet coils. After the train is entirely off of the track circuit the armature will be picked up, thus setting the mechanical connection between the armatures to its normal position. The relay is then ready for a train to approach from either direction.|
Locomotive type bell
|The type of bell that employs a spring as its operating
energy does not use the track circuit nor interlocking relay. Instead
it has a track instrument or generator placed at the required dis-tance
from the bell to start it ringing in ample time to warn anyone on the highway
of the approach of a train. The track generator has a permanent magnet
with an armature. The pass-ing of a train gives a sudden kick to the armature,
removing it from the magnet, and so breaking the lines of magnetic force
that are passing through the armature. This interruption sends an
electrical impulse to a magnet in the bell, which in turn re-leases a retaining
dog that is holding the spring. When the spring is released it rings
the bell for a predetermined length of time or until the passing of the
train, which winds the spring and also replaces the retaining dog.
The generators on the track are made selective for single track.
They depend upon the vibration of the rail for their action and are so
arranged that they depend upon the direction of the train whether or not
the electrical impulse is sent to the bell. The mechanism of this
type of bell is built 50 that it will not only ring a bell, hut a suspended
arm can he made to swing So as to give a visible warning in the day time,
and at night a warning can he given by placing a light behind the arm in
such a position that it is hid when the arm is still and exposed when the
arm is swinging.
A crossing bell is an audible warning, and those who are con-stantly using a crossing where one is located depend upon it to warn them of approaching trains, so that a bell or any other warning device that is out of order so as not to fulfill its pur-pose creates a more dangerous condition than there would be without it. All mechanical or electrical devices need a required amount of proper maintenance. A track circuit, on account of the use and abuse given it by the passing of heavy and high-speed trains, demands a constant and vigilant attention. Therefore, it is the aim of some designers to devise an electrical track instrument that can be used with an interlocking relay and so eliminate the track circuit. As it is preferable to have a bell out of order ring constantly than not to ring at all, a closed circuit arrangement for the track instrument is superior to one with an open circuit. There are some track instruments now on the market. In some the vibration of a spring is employed, in others it is the splashing of mercury. One of the advantages of using track instruments is that a bell can be installed without interfering with track circuits that are in connection with block signals.