The dStresen equipment was developed by Jury and Jury Technologies Ltd, New Zealand. The methodology for applying the process to determine rail stress free temperature on railway tracks was initially developed during work trials in the United States at the TTCI test centre in Pueblo, Colorado in 2005. Since 2008 the dStresen system has been used extensively by railway companies in Australia to determine rail SFT. During this time the equipment and application methodology have been enhanced accuracy and the productivity of the SFT measurement process.
The equipment consists of three major components; shaker/tune bar, control unit and computer. The equipment is powered by 240 Volts AC. This is provided by a 12 Volt battery through a 12/240 Volt inverter. Both the shaker and twin tune bars (TB) are clamped to the rail head. During a test cycle the shaker rotates at 3900-4800 rpm for a period of 60 seconds. The shaker induces a lateral rail head displacement of up to 0.2 mm at a frequency range of 65 to 80 Hz. Each TB is a cantilever beam with an accelerometer mounted on the end to capture the beams (rail) first bending resonance and vibration data. The data is sent to the computer for analysis where the largest peak to peak Hertz value is recorded. The recorded peak Hertz value is largest for a rail without longitudinal thermal stress and is reduced in a predictable manner as the longitudinal thermal rail stress increases. The peak Hertz value is influenced by several factors including track strength, track condition, rail temperature and the rail stress free temperature condition.