Square wrench

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Square wrench according to UIC standard as used by railway workers. The inner and outer square are conical.
Square lock on the front flaps of an ICE train

A square wrench is usually a wrench for screws with a square head. For example, there is a Robertson screwdriver for screws with a square socket . Apart from special shapes, the square wrench consists of a square socket with a handle at its upper end . An internal square is a thick-walled metal tube, the inside of which is not round, but rather - viewed in cross-section - square. This square inner shape fits on the counterpart located in the lock , the protruding outer square , which has a correspondingly large square shape. If you put the square key with the inner square on the lock with the outer square, you can turn it like a screw with a screwdriver or a nut with a wrench, and thus open the lock or the valve that is secured by the square or close.

Square profiles can only transmit small forces because when they are actuated, a very high surface pressure occurs on the edges of the key, which deforms as a result. Square-headed locks offer little protection against unauthorized use, which is why other locks are increasingly being used.

The advantage of the square profile for the actuating pin, however, is that it is easy to manufacture compared to other profiles that may be used. A Torx profile can e.g. B. cannot be made with standard cutting tools . It can also be seen as an advantage that a square lock can also be operated with a makeshift tool (pliers / wrench) in an emergency.

The square wrench is available in different sizes that correspond to the edge length of the square; the most common are 7 mm and 8 mm, but the size can range from a few millimeters to several centimeters.

Square wrenches are used on the railroad (edge ​​length: 8 mm), on windows , simple switch boxes and hydrants ( slide keys ), water taps , as well as for the operation of vent valves on radiators . Further examples are control cabinets and protective covers in mechanical and plant engineering.

Extensions for screw sockets are often connected with square profiles with a ball catch.

In the railway sector, the square key is also called the "Bern key" (in the English-speaking area "Berne Key"), according to the Bern Agreement of May 15, 1886 , in which various technical parameters were agreed for cross-border rail traffic.