Track fittings include, among other things, the rail fastenings used for track maintenance. These are necessary in order to attach the sleepers to the tracks. They include among other things the nuts, spring washers, coach screws and rail clips.
Nuts are used to secure the fish plate bolts of rail joints, the hook bolts of the rail fastening, and the screws for switch opening, guide and protection rails, etc.
Spring washers which are used in the railway track as part of the clamping system are highly stressed. They are designed to ensure that each passageway has an additional arch. They are made of highly elastic spring steel. A rail fastening is well fastened down if the spring washers are pressed together, so that between the curvature of the washer there is still a margin of approximately 1 mm. Spring washers, as well as the other clamping equipment used in the track, cause the elastic frictional-locking tension between sleepers and rails.
There are single, double and triple turn spring washers which are used at various places in the track. The clamping forces (also the steel) are without exception equal, however, the spring deflection differs depending on the number of turns.
Coach screws are used for the direct or indirect fastening of the rail in various types of track onto wooden or concrete sleepers.
The length of the coach screws used is between 110 mm and 180 mm. The coach screw has a screw thread made of wood and is used for wood and concrete sleepers. Formerly in the State railways, the coach screw had a square head, the coach screws of the Reichsbahn (track K) however had a rectangular head shape and this has gained acceptance in the last few decades. The conical shape of the shaft below the bolt head presses the thread of the screw neck when it is screwed tightly into the holes and the wood fibres. The sleepers must be pre-drilled and the size of the hole differs depending on whether it is a hardwood or softwood sleeper.
In a rolling process the thread of coach screws is rolled onto the red hot shaft. The normal coach screws are stored after production without special rust protection, but they can also be given a treatment to prevent rust (galvanizing) (see image below).
In railway tracks there is a direct and an indirect pre-tensioning between the rail and the sleeper. In the rail with K fastenings there is an indirect pre-tensioning. That means this pre-tensioning fixes the rail only to the ribbed plate.
The ribbed plate, on the other hand, is then fixed by means of coach screws to the wood or concrete sleeper and clamped to it by friction. The individual hook bolts have a semi-circular pattern at the lower end of the thread. There they adapt themselves to the machined counterpart of the ribbed plate (the so-called swallowtail). This shape enables the rail to be attached to the ribbed plate by means of this hook bolt and a torque of approximately 180 - 220 Nm. To produce this tension, a spring washer (Fe 6) and a clamping plate (Kpo 6) are necessary.
Hook bolts are available with various sizes of shaft, head and nut. In the past, bolts with Whitworth threads (English bolt diameters in inches) were fitted but today only metric threads (in millimetres) are used.
Rail clips vary in shape, some are square and the clamping side is adjusted to the rail foot.
Rail clips have two unequal legs, the shorter of which serves to exert pressure on the rail foot by frictionally pre-tensioning it. With rail clips for the normal K track, the leg reaches high over the notches of the prefabricated holding device on the ribbed plates and thus serves as the required rail fastening. For the area of the tongue device (weight / piece = 0.620 kg), there is a Kpo 3 installed, where there are no rail pads (Zw) between the slide-chair plates or ribbed plates and running rail.
You can find suitable specialist literature to the topic here:
The Basic Principles of Mechanised Track Maintenance
This book is dedicated to the many people involved in the day to day planning and performance of track maintenance activities. Providing a practical approach to everyday challenges in mechanised track maintenance, it is not just intended as a theoretical approach to the track system.
Railways aim at transporting people and freight safely, rapidly, regularly, comfortably and on time from one place to another. This book is directed to track infrastructure departments contributing to the above objective by ensuring the track infrastructure’s reliability, availability, maintainability and safety – denoted by the acronym RAMS. Regular, effective and affordable track maintenance enable RAMS to be achieved.