Combined Forces On The Track

The use of measuring wheels is already standard procedure for car manufacturers. They determine the load that is transferred to the vehicle via the wheel when traveling round bends and over sleepers, which in turn helps engineers design the vehicle accordingly. Measuring wheels are also used by railway system operators to locate any uneven rail sections or switches that are putting particular strain on the vehicle. The data then helps them to systematically repair these sections. Certain difficulties arise in cases where vertical and lateral forces interact, for example when traveling round a bend. These forces are not easy to distinguish using traditional measurement instrumentation, nor has it been possible so far to differentiate between longitudinal forces and moments occurring during acceleration and braking. Moreover, conventional measuring wheels are not equipped with rubber suspension units such as those used for streetcars.

Researchers at the Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt have now developed a rail track measuring wheel that can differentiate between the different forces and accurately determine the contact forces occurring between wheel and rail. "Unlike conventional instruments, which only measure two dimensions, the LBF®.R-Wheelos measuring wheel detects a full six," reveals Michael Kieninger, director of the competence center for load and strain analysis at LBF. "These include vertical, lateral and longitudinal forces as well as their respective moments."

The trick is to weaken the wheel by systematically milling holes into it, creating a spoke-like pattern. "This produces areas that react only to vertical forces while others only pick up lateral forces," Kieninger explains. The transmitted forces are revealed by nearly a hundred interconnected strain gages, which are strategically mounted on the spokes. This new measurement technology has now for the first time been applied to a wheel with rubber suspension units. The system accurately records all dynamic behavior in the measurement process.

At present, the measuring wheel is being tested and calibrated in the laboratory. The first track measurements are scheduled for fall 2006 on a streetcar in Frankfurt. The LBF®.R-Wheelos will make its first public appearance in Hall 4.1, Stand 228 at the InnoTrans trade fair, to be held in Berlin from September 19 to 22.