Twin Disk: The Precursor to FZG - A Case Study

Introduction

The Twin Disk tribometer plays a pivotal role as a precursor to FZG (Forschungsstelle für Zahnräder und Getriebebau) testing, offering a streamlined approach to evaluating lubricants and materials before advancing to more complex and resource-intensive stages. By replicating critical contact conditions, such as line contact and rolling-sliding movements that are characteristic of FZG evaluations, the Twin Disk tribometer enables researchers to mimic real-world gear and bearing interactions on a smaller, more controlled scale.

 

 

One of the key advantages of using the Twin Disk tribometer as a preliminary test method is its ability to significantly reduce both the time and cost involved in lubricant development. Full-scale FZG testing, while highly reliable, is complex and resource-heavy, requiring significant material, time, and preparation. The Twin Disk tribometer provides an intermediate testing phase, allowing for a quicker and more efficient screening process where potential lubricant formulations can be evaluated for their initial performance. This step helps researchers identify promising candidates and eliminate underperforming ones early in the development cycle, narrowing down options before proceeding to full-scale testing.

This article will showcase some preliminary results obtained by applying a FZG-like test method on a Twin Disk tribometer.

 

Materials and Methods

Ducom Twin Disk Tribometer can accommodate several type of roller pairs. The two most common configurations are the crowned roller on flat roller pair, and the protruded roller on flat roller. The former configuration results in an elliptical contact area. The latter configuration results in a line contact and was selected as it represents more closely the contact geometry occurring in FZG.

The profile of the protruded roller was machined to a width of 1.5 mm. This value was calculated so as to reach the necessary contact pressure during the increasing steps of the test.

Both rollers were made of EN 31 steel (Young’s modulus: 205 GPa, Hardness: 58 to 62 HRC). The surface finish of the flat and protruded roller was N5 and N2, respectively.

 

 

 

The test parameters for the Twin Disk tests were designed to closely approximate the test parameters imposed by the ISO 14635-1 standard for FZG testing, especially in regard with the circumferential velocity and number of revolutions.

The test loads in the Twin Disk tests were selected to match the same contact pressure prescribed by the ISO 14635-1 standard at each loading stage.

 

 

Normal load, friction force, temperature and speed were constantly monitor throughout the duration of the test. The test was interrupted after completing each stage to allow for a visual inspection of the rollers.

 

Results

Ducom Twin Disk tribometer is designed with a fixed roller, which is anchored to the body of the instrument, and a moving roller. The moving roller can slide away from the fixed roller to ease the assembly and disassembly of both test rollers and to be loaded against the fixed roller. Moreover, it is free to slide along the direction of the entrainment speed as a result of the friction generated at the contact between the two rollers. A load cell below the moving roller measures the friction force generated during the test with the passing of the test time.

 

 

 

The friction force values increased for all oils following a trend that can be attributed to the increase in the test load and contact pressure. All the three additized oils showed a lower friction than then the one measured for the base oil.

 

The vibration data along the Z axis was considered as the indicator of the lubricant failure. The Z axis was selected because it represents the direction alongside which the load between the rollers is exchanged and therefore it reflects the condition of the two mating surfaces.

 

 

The vibration measured on the protruded roller drive was considered as the deciding factor to identify failure of the lubricant. The decision was made considering that the protruded roller is more subject to vibration in light of its narrow profile, and therefore it represents a better indicator of the overall conditions of the test.

Moreover, the selection was narrowed furhter to consider only the vibration alongside the Z axis, as the Z axis coincides with the line alongside which the load is exchanged. As can be seen in the figure below, the Z axis vibration is the parameter that shows the most noticeable fluctuation with the increasing load steps.

 

 

Observing the behavior of the Z axis vibration on the protruded roller, it was possible to identify the stage at the beginning of which a big difference in the lubricant response could be observed. The change was defined as a net jump of the average value of the vibration between the end of the previous stage and the beginning of the next.

This change was observed after stage 10 for the base oil. Only Oil A showed an increase in the failure stage (stage 13), whereas Oil B and Oil C failed at the same stage a which the base oil failed (stage 10).

 

 

 

Conclusion

In conclusion, the Twin Disk tribometer proves to be an effective precursor to full-scale FZG testing by replicating critical contact conditions and achieving comparable contact pressures, as defined in ISO 14635-1. By utilizing the FZG Load step protocol, the Twin Disk enables the differentiation of lubricants through the analysis of friction and vibration responses. This approach offers a reliable and efficient screening method, helping fast-track the evaluation and benchmarking of lubricant formulations, making it a valuable tool in the early stages of lubricant development.