Tow hook load
When assembling and handling heavy freight trains - rarely on passenger trains - care must be taken to ensure that the screw couplings are not overloaded by exceeding the prescribed maximum weight. If a screw coupling breaks and the brake hose breaks, both parts of the pull are automatically braked by the compressed air brake . The breaking strength of the screw coupling must be lower than the breaking strength of the other parts of the drawbar.
principle
The normal UIC-type screw coupling is designed for a breaking load of 850 kN on new vehicles. Depending on the maximum gradient and the rolling resistance of the vehicles, this results in a maximum drawbar load of 4,000 tons in the flat country, 2,500 tons on Swiss routes up to a gradient of 12 per thousand or 1,400 tons on Swiss mountain roads with inclines of up to 26 per thousand. The permissible load decreases further on even greater gradients. Because on inclines the weight begins to add up as a vector to the normal tensile force, which is necessary to overcome static friction, according to the rules of the inclined plane . It must also be taken into account that there is no continuous increase in force when starting, but rather, due to the play of the clutch and springs, the load suddenly acts on the pulling device. The heavier a pull, the higher the forces that can occur. This is also due to the coupling regulations, since the couplings are only screwed in until the buffers lightly touch each other in the straight track, as otherwise the couplings could no longer be released or only with great effort.
North America
Due to the Janney coupling , which is very common in the USA and Canada , which has a significantly higher drawbar load, freight trains can be kilometers long there.
Train separations
Pull disconnections usually occur when a pulling device / coupling is damaged, so badly that it can no longer withstand the intended breaking load.
Examples from Germany
Refer to Bolt Coupling in the Coupling Failure section .
Examples from Switzerland
A few times train separations happened in IC2000 compositions of the Swiss Federal Railways , so that a technical inspection had to be carried out on all tow hooks of this train type. In some cases it has been found that failure of the wagons to be pre-tensioned caused the fasteners to age prematurely and cause hairline cracks. Due to the incorrect, insufficiently tight coupling, the durability of the fastenings was no longer guaranteed. As a result, the regulations were tightened so that the couplings of shuttle trains now have to be preloaded. To ensure that this value is maintained, spacer sleeves have been installed, which make it very easy to check whether the coupling has been screwed in to the specified distance. In the case of the train separation in the Zimmerberg tunnel on November 19, 2007, it was found that the drawbar did not have the necessary strength and it was very likely that it had not been installed correctly.
In some incidents, however, the pulling disconnection was not due to any damage to the pulling apparatus, but rather the coupling that was not properly screwed in and the unused coupling was not supplied in the blind hook, but was hanging down (hang-down couplings are less than the prescribed clearance). The unused coupling hung down so far that it collided with the track equipment. As a result, the coupling is thrown up by the track equipment and can eject a coupling that is not under load at this moment. This happened twice at SBB in 2003 for passenger trains, on August 4th in Dulliken and on September 5th in Gümligen.