Gears are designed to mesh together. This means that they are in point contact with each other at the single point known as the pressure face. The design and the subsequent heat treatment of the gears will mean the success or failure of either of the meshing parts.
Their success (if carburized and hardened) is dependent on: surface hardness, core hardness, case depth, grain size, physical size of the gear, gear material and the quality of formed case.
The probable point of failure of either gear will likely be at the point of contact of the two gears. The root cause of the failure may not be obviously apparent, but the failure generally manifests itself at the pressure point of contact of the two gear teeth.
The damage will be observable due to the mechanical failure of the immediate surface. However, the root cause of the failure may well be (but not limited to) other mechanical values such as the core hardness of the substrate material. If the core hardness is such that the gear-face loading exceeds the tensile strength and core hardness of the gear, the formed carburized case will fail. This is simply because the loading on the formed carburized case exceeded the core’s ability to support the case.
The failure may expose itself at the pressure point in the form of pitting on the gear surface. This is known as contact failure.
The carburizing process (or surface treatment of the gear) is what will “make or break” the gear and the gear teeth. The better the control of the surface carburizing process, the better the surface metallurgy of the gear will be. This does not necessarily mean that one needs the most sophisticated equipment to control the process.
The atmosphere process control can be accomplished very simply with any of the following methods: shim analysis (contrary to popular belief, this works, only it is not in real time), dew point in conjunction with the graph “Saturation of Carbon in Iron (Austenite),” test-bar analysis or magnetic change.
The problem with shim analysis is that the result is usually about 45 minutes old, and it is labor intensive. However, it is very accurate if managed correctly.
The dew-point method is of course the tried and tested method. However, it will be a magnitude more accurate if it is run in conjunction with the “Saturation of Carbon in Iron” graph.
Test-bar analysis can only evaluate the process conditions after the carburizing process (like shim).
The magnetic change method of atmosphere control is based on the change in magnetic properties of a known wire length. When the wire is carburized, there will be a change in the magnetic measurement of the wire. There is a Japanese unit out in the marketplace for this type of atmosphere carbon-potential measurement.
The surface metallurgy of the gear is very critical to the performance of the gear. Next time, we’ll discuss material selection.