Hunting oscillation

Hunting oscillation is a self-oscillation, usually unwanted, about an equilibrium. The expression came into use in the 19th century and describes how a system "hunts" for equilibrium. The expression is used to describe phenomena in such diverse fields as electronics, aviation, biology, and railway engineering.

A classical hunting oscillation is a swaying motion of a railway vehicle (often called truck hunting) caused by the coning action on which the directional stability of an adhesion railway depends. It arises from the interaction of adhesion forces and inertial forces. At low speed, adhesion dominates but, as the speed increases, the adhesion forces and inertial forces become comparable in magnitude and the oscillation begins at a critical speed. Above this speed, the motion can be violent, damaging track and wheels and potentially causing derailment. The problem does not occur on systems with a differential because the action depends on both wheels of a wheelset rotating at the same angular rate, although differentials tend to be rare, and conventional trains have their wheels fixed to the axles in pairs instead.

The problem was first noticed towards the end of the 19th century, when train speeds became high enough to encounter it. This behaviour limited trains to operate at speeds of about 225 km/h (140 mph) or less and led to a number of research projects in the 1960s using hovertrains and maglev systems to avoid it and reach higher speeds. But after empirical studies by the British Rail Research Division in the 1960s, remedial measures, particularly in the design of suspension systems, have been introduced permitting speeds exceeding 290 km/h (180 mph). As a result of this research, most high-speed trains today still use steel wheels, rather than more expensive technologies.

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