Ford Germany has developed a new method that has the potential to slash transmission testing times from 60 days to just 9 days (compared with vehicle tests). According to Dr. Wolfram Weiss (Concept Development) and Albert Lemmen (Software Implementation and System Integration), at the heart of the new process is LMS TecWare and a more accurate method of creating a rainflow matrix for rotating components.
Limitations of Traditional Rainflow Counting
Rainflow matrices have long been used in translational test rigs because they substantially reduce testing time: compressing a time history by eliminating cycles that are too small to have any impact on product life. But the use of rainflow matrices for rotating components has been hampered by serious inaccuracies in the traditional method of calculating them. The traditional rotating moment histogram approach simply assumes that each tooth sees the torque value applied to the shaft once per revolution. This is valid in the majority of cycles that start at zero load and end at zero load. However, a number of cycles involve torque reversals, where the load on the teeth alternates between tension and pressure, such as when the transmission is used for braking – an effect that gets more and more important for today’s high torque diesel engines. While the number of cycles involved is small, the damage potential from these sudden reversals in load is very high. The traditional rotating moment histogram approach does not take these reversals into account, raising serious questions about its applicability.
Ford’s goal was to reduce the time required for testing by compressing measured signals in a manner similar to what had already been accomplished with a rainflow matrix – without losing the parts of the signal that are relevant from a durability standpoint.
Avoiding Data Loss
To avoid the data loss inherent in the traditional approach, Ford engineers avoided the use of rainflow matrices in the past for rotating fatigue tests, such as on transmissions. The result was that they were forced to run complete time history tests which, while still faster than providing-ground testing, were substantially slower than equivalent testing on translational test rigs. Or, the engineers had to run reduced tests on the basic of revolutions at torque level spectra, knowing well that the content of damage in the Rainflow Matrix was neglected. In an effort to overcome these problems, Ford asked LMS to develop a method for producing a rainflow matrix that would accurately reproduce the damage seen by rotating components.
Module Encapsulates New Cycle Counting Method
The unique capability of the new software is its ability to accurately calculate the load cycle seen by each individual gear tooth based on time history information including the effects of alternating tension and compression. Furthermore (and this was one target from Ford) not only are the teeth of the gears properly loaded, but all translational concerned parts such as shafts, shaft gear joint etc. are loaded correctly as well.
LMS International has incorporated these new capabilities into its TecWare software package, a modular suite of tools for load data analysis and synthesis. It can be used to perform data reduction of multichannel time histories into rainflow matrices, rotating moment histograms or time at level histograms for fatigue analysis and comparative analysis between channels. All important fatigue events, such as the number of revolutions as a function of load, the transitions between tension, and pressure and hysteresis cycles for the teeth of a gear wheel are covered. Other rainflow-based load syntheses and analyses – rainflow editing, superposition, extrapolation and time history reconstruction - are also adapted to the specifics of rotating components.
Ford engineers are now in the process of using TecWare to convert existing time histories to rainflow matrices for individual gear teeth. Being able to view the loading on the gear teeth in convenient rainflow format makes it possible to obtain a clear understanding of the damage potential at a glance. In some cases, the loading on individual teeth is so small that it’s apparent that the life will be extremely long. In that case, it is possible to dispense with testing altogether. In the more typical case where loading is significant, the rainflow matrices can be used to create test programs that are equivalent to the full time histories from a durability standpoint - yet can be run in a small fraction of time. “We feel that we have made a major step forward,” Dr. Weiss and Lemmen concluded. “The ability to generate accurate rainflow matrices for transmissions that accurately duplicate the damage potential of the original time histories will cut months from our development cycle and save hundreds of thousands of dollars.”
Limitations of Traditional Rainflow Counting
Rainflow matrices have long been used in translational test rigs because they substantially reduce testing time: compressing a time history by eliminating cycles that are too small to have any impact on product life. But the use of rainflow matrices for rotating components has been hampered by serious inaccuracies in the traditional method of calculating them. The traditional rotating moment histogram approach simply assumes that each tooth sees the torque value applied to the shaft once per revolution. This is valid in the majority of cycles that start at zero load and end at zero load. However, a number of cycles involve torque reversals, where the load on the teeth alternates between tension and pressure, such as when the transmission is used for braking – an effect that gets more and more important for today’s high torque diesel engines. While the number of cycles involved is small, the damage potential from these sudden reversals in load is very high. The traditional rotating moment histogram approach does not take these reversals into account, raising serious questions about its applicability.
Ford’s goal was to reduce the time required for testing by compressing measured signals in a manner similar to what had already been accomplished with a rainflow matrix – without losing the parts of the signal that are relevant from a durability standpoint.
Avoiding Data Loss
To avoid the data loss inherent in the traditional approach, Ford engineers avoided the use of rainflow matrices in the past for rotating fatigue tests, such as on transmissions. The result was that they were forced to run complete time history tests which, while still faster than providing-ground testing, were substantially slower than equivalent testing on translational test rigs. Or, the engineers had to run reduced tests on the basic of revolutions at torque level spectra, knowing well that the content of damage in the Rainflow Matrix was neglected. In an effort to overcome these problems, Ford asked LMS to develop a method for producing a rainflow matrix that would accurately reproduce the damage seen by rotating components.Module Encapsulates New Cycle Counting Method
The unique capability of the new software is its ability to accurately calculate the load cycle seen by each individual gear tooth based on time history information including the effects of alternating tension and compression. Furthermore (and this was one target from Ford) not only are the teeth of the gears properly loaded, but all translational concerned parts such as shafts, shaft gear joint etc. are loaded correctly as well.
LMS International has incorporated these new capabilities into its TecWare software package, a modular suite of tools for load data analysis and synthesis. It can be used to perform data reduction of multichannel time histories into rainflow matrices, rotating moment histograms or time at level histograms for fatigue analysis and comparative analysis between channels. All important fatigue events, such as the number of revolutions as a function of load, the transitions between tension, and pressure and hysteresis cycles for the teeth of a gear wheel are covered. Other rainflow-based load syntheses and analyses – rainflow editing, superposition, extrapolation and time history reconstruction - are also adapted to the specifics of rotating components.
Ford engineers are now in the process of using TecWare to convert existing time histories to rainflow matrices for individual gear teeth. Being able to view the loading on the gear teeth in convenient rainflow format makes it possible to obtain a clear understanding of the damage potential at a glance. In some cases, the loading on individual teeth is so small that it’s apparent that the life will be extremely long. In that case, it is possible to dispense with testing altogether. In the more typical case where loading is significant, the rainflow matrices can be used to create test programs that are equivalent to the full time histories from a durability standpoint - yet can be run in a small fraction of time. “We feel that we have made a major step forward,” Dr. Weiss and Lemmen concluded. “The ability to generate accurate rainflow matrices for transmissions that accurately duplicate the damage potential of the original time histories will cut months from our development cycle and save hundreds of thousands of dollars.”
