Gear damage due to inadequate scuffing load capacity of the lubricant

Eating tests as an optimisation measure

Eating capacity is a strange word at first glance. If you are now thinking of food intake, obesity or diets, you are unfortunately completely wrong. The term is purely technical in nature and refers to the behaviour of gears in machines and gearboxes that are under permanent high loads, as well as the suitable lubricants to prevent "seizure" in the long term.

In engineering, "galling" means the localised welding together or tearing apart of two machine parts that slide into each other - in this case gear wheels - due to inadequate or faulty lubrication. It usually occurs at roughness peaks in the tooth contact. The cause is to be found in very high temperatures, also known as "flash temperatures", which are dependent on the load, the peripheral speed and, last but not least, the temperature of the oil sump environment.

It's all about the lubrication

Proven coatings such as phosphating or metal-containing hydrocarbon alloys with tungsten have proven to be the most suitable in practice for protecting gear flanks from excessive abrasion. However, it is not only the material and design of the moving parts that are decisive for the safe operation of gear drives, but above all the lubricant used. If the composition of the base oils and additives used is not correct, the lubricant does not develop the required lubricating film thickness and therefore does not achieve the required scuffing load capacity. The result is partial damage to the gears which, if not properly maintained and checked, ultimately leads to a total failure of the gearbox and therefore the machine.

Distinction between warm and cold feeding

Seizure marks and scoring occur during hot scuffing due to a very high sliding speed and the resulting limit temperatures if the material of the gears and the lubricant are not optimally adapted. Smaller modules and the use of EP oils with chemically active additives during a gearbox overhaul can provide a remedy here. Cold seizure means scoring-like wear of the gears with very heavy material removal caused by low peripheral speeds. In this case, more precise gearing, a smoother surface of the tooth flanks or a more viscous lubricant adapted to the requirements can help.

What are food tests and where can they be carried out?

There are various institutes and research facilities in Germany that carry out scuffing tests on gears. There are test benches for this purpose, known as gear tension testing machines, which allow the loads in the gear meshes and the temperature of the respective lubricant to be set precisely in accordance with the DIN 51354 standard. The oil can be supplied as injection or splash lubrication.

The Research Centre for Gears and Transmission Engineering at the Technical University of Munich (FZG) has developed a test rig that can be used to test the viscosity and suitability of lubricants for transmissions in order to prevent the surfaces and flanks of gears from seizing. The FZG test rig has established itself as a standard test machine and is also used in this form by other institutes.

The tests themselves are carried out under different conditions. The circumferential speed, toothing and direction of rotation of the gears as well as the oil sump temperature are varied in order to define the damage force levels of different lubricants.

The FZG can check the scuffing resistance in four test procedures. The standard scuffing test is carried out in accordance with the above-mentioned DIN 51354 standard, whereby the force levels are initially set low and then slowly increased, as are the flash temperatures. In the more stringent scuffing test, the circumferential speed is doubled and the level of flash temperatures is also increased significantly. However, the run-in in the lower load levels corresponds to the standard scuffing test. In the step test, the direction of rotation of the gears is reversed and toothing with a narrower pinion is selected. The changed boundary conditions increase the pressure and also make it more difficult for the lubricant used to enter the tooth contacts. The final stage is the so-called jump test. Here, the loads are not increased in stages, but are set directly and without detours to a specific force level. The result is either "pass" or "fail" - either it works or it doesn't work. There is also no run-in during the step test, i.e. the flash temperatures that occur are higher and are caused by rough surfaces. However, when used in gearboxes, the lubricant must have certain running-in properties in order to increase the scuffing load capacity of the gearing. For this reason, the individual boundary conditions must be closely observed and documented for every scuffing test.