Handbook of Railway Vehicle Dynamics

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AB C D E F
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FIGURE 13.13 By measuring six bending moments and torques, approximate measures of forces x , y , q on the
wheels can be determined.
the position of the vertical force application will also change. The changing positions
will also change the momentsmeasured in the axle, thus introducingerrors that cannot be
compensated because the actual position at which the force is applied is not known.
† Moments in the axle are dependent, to asmall degree, on the vertical mass forces, due to
the unsprung massofthe axle and otherunsuspended parts of the wheelset. Thus, it is not
possible to fully assess the effects of the unsprung mass on the vertical dynamic forces.
Due to the deficiencies described above, this method has been further developed. Byapplying
strain gauges on the wheel webs, the effects of varying positions are reported to be compensated.
However, this makes the method morecomplicated and approaches the “wheel methods” described
in the next section.
3. Measuring Lateral and Vertical Wheel–Rail Forces —Wheel Methods
“Wheel methods” can be divided into two different techniques, either measuring strains in the
spokes of spoked wheels, or measuring strains in the wheel web of ordinaryrailway wheels,i.e., in
the web between the axle and the outer wheel rim.
The“spokedwheel method” is notfrequently used nowadays, mainlydue to the need to design
and manufacture special wheels. In addition, the calibration procedures are tedious and time
consuming. However, with properly designed and calibrated instrumented spoked wheels, this
method is reportedtoproduceagood accuracy. The massforces of the unsprung mass are, to alarge
extent, included inthe measured quantities.
Themostfrequently used method today —besides the simplified“axle box method” described
in Section II.D.1 also is the wheel web method. Within this method a number of different
technologies are used. The basic principle is that strains are measured at various locations on the
wheel web as aresult of the applied forces on the wheel as shown inFigure 13.14. Anumber of
strain gauges are applied on the same web, usually in the radial direction on the inside as well as
on the outside of the wheel. However, these locations must be carefully selected.
Figure13.15 showsanexampleofmeasured strains in single strain gauges of onewheel web as
the wheel rotates and the wheel is loaded by lateral forces Y or vertical forces Q .Inorder to achieve
signals proportional to the applied load,the strain gauges mustbecombined in Wheatstone bridges
in an intelligent and precise way. Separate bridges are required for the lateral Y forces and the
vertical Q forces. Sometimes, two bridges are used for the sameforce onthe same wheel, installed
at different wheel angles. In this case, additional data processing is needed tocombine the two
bridge signals. In afew cases, forces are measured in all the three directions: longitudinal, lateral,
and vertical. Signals are usually transferred to and from the wheels via slip rings, although radio
transmission may also beused.
© 2006 by Taylor & Francis Group, LLC
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Handbook of Railway Vehicle Dynamics
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