All the railway track fishplates for mechanical joints are designed to provide a gap to the rail web. This gap, together with the wedge shape of the fishplate section ensure, by tightening the joint bolts, the correct alignment of the rail head between the two jointed rails.
The bolt tightening torque is developing in the bolt a tensile force, A. This tensile force is causing a set of normal forces N at the contact areas between the fishplates and the rail head and foot. These normal forces can be found by decomposing the vector of the force A onto the two contact areas between the fishplate and the rail – one on the rail head and the other on the rail foot.
If the rail will tends to move in the joint, these normal forces will cause a set of friction forces between the fishplate and the rail. Any such force will be Nf – where N is the value of the normal force and f is the steel on steel friction coefficient.
Theoretically, for each bolt there will be four such friction forces which will oppose the rail tendency of movement.
The resultant R of these friction forces is called the joint resistance force. This can be defined as:
R = 4n Nf
where n is the number of bolts per rail, N is the normal force caused by the tensile force of the bolt and f is the steel on steel friction coefficient.
The rails will move in the joint only if this resultant force R will be overcome. Only then the joint gap will tend to close or open.
For a normal 4 bolt mechanical joint, designed for constrained thermal expansion track superstructure, the joint resistance force R is usually around 50 kN but might depend significantly dependant on the joint bolts used.
The joints for free thermal expansion superstructure, usually old type of joints, don’t have a well defined tightening torque for installation and also are lacking essential components to keep this torque constant throughout the normal usage of the joint. In such cases the joint resistance force is ignored in rail thermal expansion calculations, even though the joint bolts have a certain tensile force and, consequently, will develop a joint resistance force. This joint resistance is not continuous, constant and above a well defined limit value and it cannot be used to define in a safe way the thermal behaviour of the track. This, together with the absence of a constant and continuous track longitudinal resistance, commented in a previous post, is modelled in the track behaviour described in the article: When a rail can be called long?, presuming free thermal expansion/contraction of the rail.
- Hila, V. Radu, C. Ungureanu, C. Stoicescu, G. (1975) Cai Ferate. Partea II. Suprastructura caii. (Railway Track. Part II. Track superstructure). Institutul de Constructii Bucuresti.
- Radu, C. (1999). Cai Ferate – Suprastructura Caii (Railway – Track Superstructure) – Course notes, Faculty of Railways, Roads and Bridges – Technical University of Civil Engineering Bucharest