Handbook of Railway Vehicle Dynamics

Field Testing and Instrumentation of Railway Vehicles 435
forces this is referred to as the “normal measuring method.” This requires amore advanced
technique for the instrumented wheelsets than for the simplified method described above.
It should bepointed out that there is no known technique available for measuring the forces
at the wheel–rail interface directly. Instead, reactions such as strains oraccelerations must be
measured in structures affected by the wheel–rail forces, i.e., in wheels,axles, or axle boxes. Abrief
overview of each of the techniques available is given below, whilst adetailed description of the
different methods available can be found in. 7 Historically, strains in the track structure have also
been measured, but such techniques are outside the scope of this chapter.
1. Measuring Lateral Forces between Wheelset and Axle Box
The simplest form of aforce-measuring wheelset is to install strain–force-measuring devices
between axle journals and the axle boxes. The lateral force can then be estimated by the calibrated
strain–force relationships. The operating principle of the device is shown inFigure 13.12. Inthis
case, the lateral axle force isreferred to as the H -force. It is similar, but not identical, to the track
shifting force S .The difference is due to the wheelset mass force.
This simple H -force method can be further developed by attaching an accelerometer tothe
wheelset, measuring the lateral wheelset acceleration. This makes it possible to calculate and
compensate for the lateral mass force ofthe wheelset. With this technique it is possible toachieve
afairly good idea of the total lateral track shift force S between the wheelset and the track.
However, with this method it is only possible to measurethe total lateral force on the wheelset or on
the track; it is not possible to separate the lateral force between the two wheels, i.e., the Y -forces,
or to measure vertical Q-forces.
2. Measuring Lateral and Vertical Wheel–Rail Forces —The Axle Method
Through the measurement of bending moments in the axle, on four cross sections, it is possible to
estimate approximate vertical and lateral forces on the wheels, if mass forces generated by the
wheelset are neglected. By additionally measuring two torques, approximate longitudinal forces
can be calculated. Thus, with six measured moments and torques it is possible todetermine
six forces (two longitudinal, two lateral, and two vertical) “on-line.” The principle is shown in
Figure 13.13. Moments and torques are measured by strain gauge bridges. Signals are transmitted
to and from the axle through slip-ring devices, inserted at one of the axle journals, or by radio
transmission.
This principle of measuring axle moments and torques seems, at first sight, to be fairly simple,
efficient, and accurate. Afurther advantage is that wheels can be changedonthe instrumented axle.
However, this method has two major disadvantages:
† Forces on the wheels may be applied at various positions. For example, the lateral
position of the contact area may change as much as ^ 35 mm over the wheel tread, thus
− H H
FIGURE 13.12 Lateral force H ,measured between axle journal and axle box.
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