Micropitting: tests, standards and practical experience

Magazine: ORGANI DI TRASMISSIONE
Publishing house: new techniques
Month of publication: January 2012
Article Title: Micropitting tests, standards and practical experience
Article author: Gregor Steinberger
Summary by: Rodolfo Volpato
Institute: I.T.I.S. "TO. MEUCCI "CITTADELLA
Class: 3BM

Tests, standards and practical experience
Micropitting is a phenomenon of superficial fatigue that manifests itself on the tooth flanks in cemented toothings subjected to high loads. For many years, advanced design methodologies, production technologies and calculation standards have been studied and developed to ensure high reliability in industrial transmissions.
In large industrial transmissions, mainly cemented and ground wheels are chosen. Internationally, there are methods for determining security factors against:

- Macro-pitting (ISO 6336-2)
- Bending tooth break (ISO 6336-3)
- Seizure (ISO TR 13989).

In cemented wheels, macropitting reduces the safety coefficient at break of the tooth and sometimes also leads to deterioration that manifests itself in the form of vibrations and noise.
Micropitting occurs mainly in cemented wheels and is the main form of damage to the reclaimed ones. It appears as a graying of the flank of the tooth and at the beginning does not produce significant variations but in the long run can lead to macropitting and therefore to failure of the teeth.

Definition of micropitting
It is a superficial phenomenon that manifests itself in the toothed wheels and is strongly influenced by the trichological system consisting of the tooth flank, the lubricant and the operating conditions. It manifests itself mainly on wheels with high surface hardness and is characterized by the formation of numerous surface cracks. From these cracks, small craters are formed caused by the detachment of particles of material. Micropitting is influenced by operating conditions including load, speed, sliding, temperature, chemical composition, etc. The phenomenon of micropitting is also called gray staining, gray flecking, frosting and peeling.

Micropitting test methods
The FVA-FZG test allows to evaluate the ability of a lubricant to protect from micropitting, the results are used to calculate the resistance to micropitting. The test consists of increasing step load levels, followed by a durability test. After the break-in, the load is progressively increased up to level 5 and then up to 10. Each level lasts 16 hours and after each load level the wheels are inspected to check if micropitting occurs.

Calculation approach
The calculation methods have not yet been fully validated and the calculation procedure has been published in an ISO technical report and is not found in ISO 6336. The micropitting risk method is based on the evaluation of the thickness of the lubricant film in the contact area.There are other parameters such as surface topology, contact stress level, profile changes and the chemical composition of the lubricant. The surface finish is another significant factor.The method of determining the lubricant film does not provide direct information on the risk of formation of micropitting, allows for making predictions if integrated into a calculation procedure based on experimental results.

Basic formulas
The method for the evaluation of the resistance to micropitting is based on the determination of the specific local thickness of lubricant in the contact area and of the minimum acceptable thickness. Micropitting is usually present in the contact areas where there is negative creep along the straight line of action .

Recommendations on the minimum micropitting safety factor
An adequate resistance is obtained when the safety coefficient is greater than or equal to the chosen limit. The safety factor must be chosen carefully.Also, we must also be aware of: confidence on the loads used for the calculations, alignment errors, variations in geometry , variations of the material and variation of lubrication. The safety factor will then be chosen according to the accuracy of the assumptions made for the calculations, according to the reliability.

Conclusions
Experts all over the world seem to have different opinions on the "gray staining" that appears as a purely optical phenomenon. In the meantime it has been understood how this phenomenon is similar to a microscopic macro-pitting. Since the parameters that influence this phenomenon are multiple, it is essential to understand under what conditions the phenomenon reaches a stopping point that does not lead to damage. The formation of micropitting sometimes stops after a running-in phase and the surfaces of the wheel seem smooth. Micropitting generates a progressive loss of accuracy of the surfaces of the teeth leading to noise and vibrations and can degenerate into other phenomena such as macro-pitting and breakage due to flexion in the foot.