Mechanical engineers are constantly trying to increase the power density of powertrains. Elias Lothman explains how two high-performance steel grades have been integrated in gear calculation software and what this could mean for the design of transmission systems

When developing a gearbox or powertrain, mechanical designers rely on tried and trusted grades of steel according to company specifications and lists of approved materials. Some even use the same allowable stress levels that were recommended as far back as the 1930s. However, continuous improvement from the world of metallurgy has led to significant improvement in performance of clean and ultra clean steels.

Therefore, to help designers adopt state-of-the-art technology, two clean versions of the alloy 18CrNiMo7-6 have been integrated into gear design software, providing engineers with the capability to faster explore possible design improvements.

The clean versions have higher fatigue strength than conventional steel and can enable gears and axles in transmission system to withstand higher loads, longer service life or to be downsized with no loss of performance.

This has huge implications for designers at engineering consultancies and small OEMs, who may not have the resources to evaluate material performance in house and want to know the current possibilities for powertrain design. With the material included as an option in the software, engineers will not have to spend months or years to validate its performance.

High fatigue strength of clean steels

Steel cleanness is classified by the number and size of inclusions in the steel. These microscopic non-metallic inclusions influence the fatigue strength of steel. The larger the inclusions, the higher the risk of failure, especially for components subjected to high cycle fatigue.

However, it’s possible to reduce the inclusion size in steel through careful control and advanced production techniques. The result is that clean and ultra-clean steel grades have significantly higher fatigue strength than conventional steels.

Therefore, they have potential to help engineers design gearboxes and transmission systems that are smaller and lighter or that can withstand higher loads for longer.

Mechanical design software

Recognising this, GWJ Technology has integrated two grades of clean steel into its eAssistant, TBK and GearEngineer mechanical design packages. This will provide mechanical design engineers with a straightforward and accessible way to evaluate the high-performance steel, with the ultimate aim of increasing power density.

The software is designed to help engineers design mechanical transmission systems, including various types of gears, bearings and shafts.

This combines knowledge of different types of gear, materials science, mechanics, computed aided design (CAD) and machining, as well as international standards and guidelines.

Algorithms in the software combine the engineering equations and physical properties of materials. It helps engineers evaluate and optimise the performance of transmission components.

When working with the software, designers can input the required gear ratios and loads and then choose from 30 materials, including polymers and steel alloys. GWJ’s developers have pre-configured their software with the properties of these materials, including the contact fatigue strength and the bending fatigue strength. GWJ’s managing director, Gunther Weser says: “It’s no surprise that we’ve had a lot of positive feedback and interest from customers.”

Ultra-clean steel grades included in the design software

The ultra-clean grades included in GWJ’s software are 159X and 159Q. Both have the same alloying elements and the same static strength as 18CrNiMo7-6, a type of case-hardening steel. However, they are produced using different conditions in the steel mill to have smaller inclusions and therefore higher fatigue strength.

The former is a bearing quality (BQ) steel. It is a first step for engineers to improve design life and/or increase the torque-handling capability of existing designs.

In comparison, 159Q is an isotropic quality (IQ) steel. It is produced with strict control so that inclusions are small and spherical. As a result, it provides the same consistently high fatigue strength under loading from any direction, which is essential for high fatigue strength and long life when subjected to loads from multiple axes. This makes it ideal for components like gears, bearings and other critical parts. It enables new design opportunities and higher performance standards.

Comparable results from the design software

With the results from the software, comparisons can be made between the performance and cost of two grades in a typical gear pair. While the steel cost of a gear made from clean steel is slightly higher, the finished component can withstand significantly higher stresses over time and will offer significantly better fatigue performance.

With the increased fatigue performance, system can be downsized and made lighter while keeping the desired torque. This will require less steel which will benefit the total cost of the system.

However, it’s essential to combine the theoretical calculations with physical testing since so many manufacturing factors are involved and ultimately affect the final performance. Physical testing has been confirmed the behaviour and the fatigue performance of the tooth root and flank at the WZL Aachen machine tools and production laboratory at RWTH Aachen University and at the Gear Research Centre laboratory at the Technical University of Munich.

Further steps testing bending fatigue

Ovako is now carrying out testing on bending fatigue to further verify and evaluate performance of the clean steels in gears. It is also evaluating the impact of different manufacturing routes and how to reach the most cost-effective combination of material cleanliness and manufacturing route and post process treatments.