Theorie der Zahnradschmierung
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Theory of Gear Lubrication and Pitting
The paper is divided into three parts. First, the formation of the oil film is discussed, then its breakdown and lastly, pitting.
The existance of an oil film between heavily loaded discs has been shown by an unfamiliar method. If a current is passed from one disc to another, through the contact line, it is found that there is a breakdown voltage of about 1/4 volt, which is largely independant of the current (between 0.2 and 350 amps).
The passage of current causes the oil to ionise and this allows any amount of current to pass. The breakdown voltage is proportional to the oil film thickness and varies with speed and load. There is always an oil film, even at the slowest speed investigated (80 cm/sec.). Various hypotheses are put forward to explain the observation.
The various theories of scuffing are studied and a new one advanced which makes use of the fact that all common steels start to soften at about 250°C. The actual scuffing is due to a.“cold” pressure welding. The breakdown of the oil film was studied experimentally by passing a large current through the oil film. It was found that the sum of the mechanical frictional energy and the electrical energy was constant. This, means that the temperature at the start of scuffing remained the same.
Finally, pitting is discussed. It is shown from a study of about 80 microsections that all cracks start from the surface of the metal and work downwards. The orientation of the crack is determined by the grain distortion of the material. This accounts for the fact that almost all pits are found in the dedendum of the teeth.
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