NEXTSENSE GmbH
Equivalent Conicity – Determining One of the Most Important Safety Parameter in Railways
This article first appeared in the Railway-News magazine Issue 5 2021.
What do rolling stock or infrastructure maintenance engineers have in common with vehicle developers and derailing analysists? They all put a lot of weight on the parameter of equivalent conicity. Because this parameter is so relevant to safety, calculating equivalent conicity is quite complex. A powerful tool enters the game with the CALIPRI measurement module for equivalent conicity.
As unpleasant as the thought of a derailed train is, this potential thread is the driving force behind the calculation of equivalent conicity. To understand the parameter and interpret its value, there needs to be a basic understanding of how a train moves.
The Sinusoidal Motion
The train moves in a sinusoidal motion due to two characteristics of the wheel-rail interaction. First of all, the geometry of the wheel is a conical shape, although it is much more complex. As soon as the train is set in motion, this conical-like shape induces a sinusoidal motion pattern as shown in the drawing below.
This motion pattern requires a track gauge that is slightly broader than the back-to-back distance of the wheelsets – otherwise there would be no space for the wheelsets to move left and right. The importance of this way of moving forward becomes significant when thinking about taking a curve. Naturally one wheel – the outer wheel – will have to drive a longer distance, thus having to drive faster than the inner wheel with the shorter distance.
This effect, by the way, also occurs with cars. While a car makes use of the differential to balance the rotational speed of the wheels in a curve, a train makes use of the conical shaped wheels.
Equivalent Conicity
So, what exactly does the equivalent conicity (EC) parameter tell us? The name itself derives from the geometry of the wheel: the equivalent conicity defines to what extend the motion behaviour of a rail wheel equals a conical shape. In other words, the equivalent conicity tells us how fast the wheels are oscillating with a sinusoidal motion to the side.
