Gears damaged by insufficient scuffing load capacity of the lubricant

Scuffing load tests as an optimisation measure

At first glance, scuffing load capacity is a strange word. The term is purely of a technical nature and relates to the behaviour of gears in machinery and transmissions, which are under permanently high loads, as well as suitable lubricants to prevent "scuffing" in the long term. "Scuffing" in technology means local welding together or breaking away of two reciprocally moving machine parts - in this case, gears - due to insufficient or incorrect lubrication. It occurs most often in roughness peaks in the tooth contact. The cause can be found in very high temperatures, also known as "flash temperatures" that are based on the load, the peripheral speed and not least on the temperature of the oil sump area.

It is all down to the lubrication

Coatings like phosphating or metal-containing hydrocarbon-alloys with tungsten have proved in practice to be the most useful when it comes to protecting gear flanks from excessive abrasion. However, it is not only the material and the design of moving parts that are crucial in terms of the safe operation of gear drives, but in particular the lubricant used. If the composition of the base oils and additives used is not right, the lubricant does not develop the lubricant film thickness required, and thus does not reach the required scuffing load capacity. The result is partial damage to the gears, which results in a total failure of the gear and thus the machine in poor maintenance and monitoring.

Difference between hot and cold scuffing.

During hot scuffing, scuffing marks and scores arise due to a very high sliding velocity and the consequent limit temperatures if the material of the gears and the lubricant are not best matched. A solution here can be smaller modules and if a gear is repaired, the use of EP oils with chemically active additives. Cold scuffing means groove-like wear on gears with very severe material erosion, caused by low peripheral speeds. In this case, help can come from more precise gearing, a smoother surface of the gear flanks or tougher lubricants, geared to the requirements.

What are scuffing tests and where can you have them carried out?

In Germany there are several institutes and research institutions that carry out scuffing tests on gears.
There are test facilities, so-called gear FZG test rigs that allow the loads in the meshing to be set exactly in accordance with the standard DIN 51354, and the temperature of the particular lubricant. The oil supply may be done by an injection or immersion lubrication.

The Forschungsstelle für Zahnräder und Getriebebau der Technischen Universität München (Research Centre for Gear Wheels and Gear Manufacturing at the Technical University of Munich) (FZG) has developed a test facility for testing lubricants for gearboxes for their viscosity and their suitability to prevent scuffing of the surfaces and flanks of gears. The FZG test facility has established itself as the standard test machine and is used in this form by other institutions.

The tests themselves are performed under different conditions. They involve the peripheral speed, the gearing and the direction of rotation of the gears and the oil sump temperature being varied to define the damage force levels of various lubricants.

The FZG can check the scuffing load capacity in four test procedures. The standard scuffing test is done under the above-mentioned standard DIN 51354, with the force levels initially set low and then just like the lightning temperatures, slowly increased. In the intensified scuffing test, the peripheral speed is doubled and the level of flash temperatures significantly increased. However, the running into the lower load levels corresponds to the standard scuffing test. In the step test, the rotational direction of the gears is reversed and a gearing with a smaller pinion is selected. The altered conditions lead to an increase in the pressing and also make the entry of the lubricant used into the toothed contacts more difficult. The conclusion is the so-called leap-frog test.  Here, the loads are not ramped up gradually, but set directly and without any detours to a specific power level. The result is either "Pass" or "Fail" - works or does not work. In the leap-frog test, there is no run-in either, i.e. the flash temperatures occurring are higher and are caused by rough surfaces. When used in gearboxes, the lubricant must have certain run-in properties, so that the scuffing load capacity of the gearing is increased. Therefore, for each scuffing test, the individual constraints must be carefully observed and documented.

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