Question 31 CEL01 - Chief Engineer - Limited

The Correct Answer is A. **Why option A is correct:** When an oil-lubricated ball bearing operates at high speeds, excessive lubricant quantity (overpacking or flooding) leads to significant internal resistance known as fluid friction or churning. Churning occurs when the rotating elements (balls and cage) must move through and displace a large volume of the oil, generating substantial heat. To eliminate this heat generation, the volume of oil interacting with the moving parts must be minimized. Reducing the quantity of lubricant until only a minimal amount—such as a fine mist, an air-oil injection, or just enough to coat the surfaces (starvation)—is present effectively eliminates bulk oil churning while still providing the necessary thin film (elasto-hydrodynamic lubrication, EHL) required for separation and protection of the moving surfaces. **Why the other options are incorrect:** * **B) installing oil rings on the ball bearings:** Oil rings are mechanisms designed to lift and deliver oil from a sump to the bearing, typically used for medium-speed applications where a constant supply of oil is needed. While they ensure continuous lubrication, they are generally used when the bearing is running in a sump of oil, which maintains the bulk quantity of oil responsible for the churning and heat generation at very high speeds. Therefore, they do not avoid the overheating condition caused by excessive fluid friction. * **C) adding more lubricant until the ball bearings are completely covered with a layer of oil:** This action directly exacerbates the problem. Completely submerging or covering the ball bearings with oil maximizes the fluid volume that the rotating elements must displace. This dramatically increases fluid friction (churning) and consequently maximizes heat generation, leading to rapid overheating and potential bearing failure. * **D) maintaining a continuous fluid film of oil on the bearings:** While maintaining a fluid film is essential for lubrication and preventing metal-to-metal contact, this option is too general. High-speed overheating is caused by the *volume* of the lubricant, not the *presence* of the film. You can maintain a continuous protective film (EHL) with minimal lubricant (like a mist or jet) without causing excessive churning. Options C and D, in practice, often lead to the same result if "maintaining a continuous fluid film" is interpreted as using an excessive volume of oil, which is the root cause of the problem. Avoiding the heat requires minimizing the bulk volume, which is achieved by "starving" the bearing (Option A).