A rail joint is a track component that connects two pieces of rail and it provides room for rail expansion and contraction due to the changes in temperature.
The mechanical rail joint is designed to allow a gap variation and, as such, has two well defined limits of this gap.
The minimum gap of a mechanical rail joint is null and that is usually achieved without exerting shear force on any of the joint bolts.
The maximum expansion gap of a mechanical rail joint is dependant on the position of the joint holes and on the diameter of the holes and bolts of the joint (Radu, 1999). This maximum gap is reached when the rails contract due to temperature decrease and are starting to exert shear forces on the joint bolts.
The maximum expansion gap Gmax can be computed as:
Gmax = Bf + Df + Dr – 2Db – 2Br
- Bf – the distance between the centres of the middle holes of the fishplate;
- Df – the fishplate hole diameter;
- Br – the distance from the end of rail to the first rail hole centre;
- Dr – the rail joint hole diameter;
- Db – the joint bolt diameter.
For a mechanical joint there will always be this relation between the joint holes and bolt diameters : Db < Df < Dr.
The maximum gap Gmax is designed for the longest rail used on jointed track to avoid the shearing of the joint bolts when the rail contracts; it is an important parameters in the analysis of the jointed track behaviour due to rail temperature variations.
Considering this maximum gap formula and the standards defining the dimensions of the rail joint components, in the table below are computed the maximum gaps for a few mechanical joints used in UK:
Later edit (15.08.2016): The table shown previously was quoting a wrong bold dimension and not the nominal value shown in the BSI BS 64 standard. Now this is corrected.
The long rail definition is explained in another blog post here: Long rail.
Below is a simplified schematic animation of a joint expansion cycle, for a continuous rail temperature variation between tmin and tmax:
More details about this expansion cycle will be presented in a future article published in the Journal of the Permanent Way Institution (PWI).
Cope, Geoffrey H. (1993). British railway track: design, construction and maintenance. Permanent Way Institution.
Radu, Constantin (1999). Cai Ferate – Suprastructura Caii (Railway – Track Superstructure) – Course notes, Faculty of Railways, Roads and Bridges – Technical University of Civil Engineering Bucharest
BSI BS 11:1985. Specifications for railway rails. British Standards Institution.
BSI BS 47-1:1991. Fishplates for Railway Rails – Part 1: Specification for Rolled Steel Fishplates. British Standards Institution.
BSI BS 64:1992. Specification for Normal and High Strength Steel Bolts and Nuts for Railway Rail Fishplates. British Standards Institution.