Tensioning mechanism
A tensioning mechanism is a component of mechanical interlockings . It compensates for the changes in length that occur due to thermal fluctuations and expansion in the lines, and thus ensures that the drives of mechanical devices such as signals or switches reliably follow the lever movement.
Whether tensioning mechanisms are necessary depends on the type of signal or point drive . Drives for operation with lines without tensioning devices must be able to compensate for the loss of stroke caused by the drop in voltage at high temperatures.
Layout and function
Tensioning systems for double-wire cables have three tasks. You should
- Compensate for temperature-dependent changes in length in the cable and give the two strands of the cable a defined and constant pre-tension of around 700 N,
- when changing, transfer the full travel and
- trigger a fault message if there is a wire break in the interlocking
A tensioning mechanism consists of a combination of fixed and loose rollers inserted into the double-wire pull cables . The loose roll in each cable run is loaded by a tension weight and thus keeps the cable mechanically under tension. This ensures that the drives of mechanical devices match the lever position and remain in a defined position in the event of a wire break or that they reach and are held in it. In addition, a malfunction message is triggered in the case of switches and track closures. This also prevents routes from being set for which the facility concerned is required.
The tensioning systems can be set up in the interlocking under the lever system in the tensioning room or outdoors. For both types of installation, there are different types of internal and external tensioning systems. Internal tensioning systems must also divert the line coming vertically from the lever by 90 ° into the horizontal guide for group deflection in front of the signal box.
To compensate for the changes in length, the tension weights must be freely movable. When moving the control lever, however, they must be held in place so that the full travel of the lever reaches the drive and does not just lead to the lifting of one tension weight and lowering of the other. A clamping device is available for this purpose, which takes advantage of the tension difference that arises between the pull wire and the trailing wire during the lever switch.
Tensioning system for signal lines
Adjustable weights are attached to one end of the two two-armed levers mounted in a bracket; at the other end these levers carry movable clamping jaws between which a toothed rod, curved in a circle, lies. The wire line is guided around pulleys, four of which are mounted in the fixed frame and two in the movable weight levers. If one wire of the double cable is pulled and the other slackened when the adjusting lever is turned, one weight is lifted and the other is lowered. The jaws are inclined. The higher one engages under a tooth of the locking rod and prevents the tension weight from being raised any further. In the rest position of the lever, in which the tensions of the two wires are the same or only slightly different, the clamping jaws slide along the rack when the weights go up and down.
A different design can be found in the so-called column tensioning works, in which the tensioning weights are integrated into the wire pull line without any lever transmission. The simple weather protection and the same effect at all temperatures are advantageous, disadvantageous are the five necessary cable diversions and the more complex lifting, especially in the event of a fault, for which an auxiliary rope is required in addition to the pulley. The additional pulleys for lifting with this auxiliary rope are built in from the start.
The tensioning mechanisms not only have to compensate for the changes in length of the wire lines, but also have to meet certain safety conditions that are required in the event of the wire line breaking. According to these so-called tear conditions:
- in the event of a break in the line of a main signal without an advance signal, the main signal can be held in the stop position or brought from the driving position to the stop position;
- in the event of a break in the line of a main signal connected to a distant signal, the main signal should be held in the stop position and the distant signal in the warning position or be brought into this position when the break between lever and main signal occurs;
- In the event of a break in the line between the main signal and the distant signal, at least the distant signal should be in the warning position when the car is clear;
- in the event of a break in a switch or bolt line, the driving position of a signal is to be prevented which is dependent on the position of this switch or this bolt, and finally
- In the case of turnout and latch lines, every broken wire is displayed in the interlocking.
The required action on the signal and turnout drives and the turnout and locking levers in the event of a wire breakage is brought about by the weight of the tensioning mechanism that pulls the unharmed wire. The tension weights must therefore have a height of fall that is sufficient, even in the case of the greatest heat, where the weights are lowest, to catch up with the wire as far as is necessary to meet the breaking conditions.
The tensioning systems are set up either under the lever bench in the signal box or outdoors.
literature
- Hoogen: Spannwerk . In: Victor Freiherr von Röll (Ed.): Encyclopedia of the Railway System . tape 9 . Berlin, Vienna 1921, p. 94–96 ( online at zeno.org ).
