Mechanisms of Railway Wheel Polygonization

Railway wheel polygonization is manifest as uneven wear around the wheel circumference, which has been a
severe problem worldwide for decades. It induces persistent periodic oscillation at the wheel-rail interface
causing forced vibration to the vehicle/track dynamic system, which can seriously threaten the comfort and
safety of the railway vehicles. Full understanding of its mechanisms is necessary for effective remedies to be
proposed.

A Chinese electric locomotive suffering from severe wheel polygonization is employed as the research object.
Abundant experimental data had been obtained by the CRRC Zhuzhou Locomotive Company through a longterm
test campaign. With these measurement data, this dissertation aims to obtain general rules that the railway
wheel polygonization will follow by simulation. The research is carried out in five fundamental aspects for
railway wheel polygonization: the prediction program, the wear models, the effects, the influence of wheelset
flexibility, and the influence of track flexibility. The main points of each aspect are described as follows.

(1) A common workflow for prediction of railway wheel polygonization is presented. Based on this workflow,
some rules for the evolution of railway wheel polygonization are proposed providing innovative perspectives
to understand the basic mechanisms of railway wheel polygonization. After summarising these rules, the
general conditions for railway wheel polygonal wear to evolve are established. The phase between the
instantaneous wear and the excitation is the key indicator determining the wheel OOR (Out-Of-Roundness)
evolution direction (to grow or to diminish). The evolution tendency curve obtained from the instantaneous
wear FRF (Frequency Response Function) is shown to be a useful tool for predicting the OOR evolution,
especially for predicting the OOR order that would grow predominantly at a given operating speed.

(2) A comparative study on the applicability of existing popular wear models in simulation of railway wheel
polygonization is carried out. Four representative wear models, developed by BRR (British Rail Research),
KTH (Royal Institute of Technology), USFD (University of Sheffield), and Professor Zobory respectively, are
selected for the comparison, with consideration of global and local methods. A uniform expression of the
converted wear functions is derived analytically with the equivalent wear coefficient as a useful index to
identify the proportional relationship between the wear models quantitatively. Simulation results show that all
the wear models investigated present a similar ability to reflect the fluctuation of the instantaneous wear under
various circumstances. Additionally, it is found that the global method is not suitable for calculating the
polygonal wear of railway wheels.

(3) A tracking test for an electric locomotive suffering from serious wheel polygonization is introduced. Data
is used to demonstrate the evolution of the polygonal wear of wheels and the vibration of the locomotive
immediately before and after wheel re-profiling. A comparison is carried out between simulation results and
measurement data. Based on the simulation model, parameter analysis is implemented to identify the effect of
wheel polygonization on the dynamic performance of the vehicle quantitatively. Some general principles
regarding the effect of wheel polygonization on the vehicle are derived.

(4) The influence of wheelset flexibility on wheel polygonization is investigated. Results show that the
wheelset flexibility cannot dominate the railway wheel polygonization in a general sense, unless some
prerequisites are fulfilled to provide a suitable environment for the wheelset flexibility to be effectively and
continually excited to fluctuate the contact responses. The torsional mode of the wheelset can be effectively
excited by stick-slip vibration due to saturated contact adhesion that can occur on track with small curve radii
or by large traction torque. If this situation persists for a long time, the development of the wheel polygonization
can be expected. The excited order will be exactly determined by the wheelset torsional modal frequency and
the vehicle speed.

(5) The influence of track flexibility on wheel polygonization is investigated. The sleeper passing frequency
and the P2 frequency are considered as the two dominant frequencies coming from the track flexibility.
Although the sleeper passing frequency is the most dominant frequency coming from the flexible track, it will
not produce visible development of wheel OOR. The P2 resonance is an important factor contributing to the
development of wheel OOR. The local rail bending modes are not found to influence the wheel OOR based on
the Simpack FTR method.