Exam Question

### (a) Difficulties Observed in Lubrication of Gear Teeth
Lubrication of gear teeth in high-power marine reduction gear trains presents several significant challenges due to the unique operating conditions:
1. **Combined Rolling and Sliding Motion**: The interaction between meshing gear teeth is not a simple rolling contact. It is a complex combination of rolling and sliding. Sliding is most pronounced at the root and tip of the teeth, while pure rolling occurs only at the pitch line. This sliding action under high load tends to wipe away the lubricating oil film, making it difficult to maintain effective hydrodynamic lubrication.
2. **High Contact Pressures**: The power transmitted through the gear train is concentrated over a very small contact area between the teeth. This results in extremely high specific loads and contact pressures, which can easily exceed the film strength of the lubricant, leading to the collapse of the oil film and a transition to boundary lubrication conditions. (Cited from context)