Question 32 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is A. **Explanation of Correctness (Option A):** Option A, "Heating the bearing in an oil-bath bearing heater," is the most appropriate thermal method for installing a roller bearing with an interference fit onto a shaft. * **Interference Fit:** An interference fit means the inner ring of the bearing is slightly smaller than the shaft diameter. To mount the bearing without damaging it or the shaft, the bearing must be temporarily expanded. * **Thermal Expansion:** Heating the bearing causes the metal to expand. This expansion increases the diameter of the inner ring, temporarily eliminating the interference and allowing the bearing to slide easily onto the shaft (a "shrink fit" when applied to the bearing). * **Controlled Heating:** Oil-bath heaters, induction heaters, or specialized ovens are standard industry tools for heating bearings. They provide uniform, controllable heating (typically to $80^\circ\text{C}$ to $120^\circ\text{C}$ / $176^\circ\text{F}$ to $248^\circ\text{F}$), ensuring the temperature does not exceed limits that could damage the bearing material, lubricants, or internal geometry. **Explanation of Incorrect Options:** * **B) Cooling the bearing with dry ice:** While cooling the bearing would shrink it, making the interference fit even tighter, the goal is to **expand** the inner ring to facilitate mounting. This method would make installation impossible or require excessive force, likely damaging the bearing or shaft. * **C) Cooling the shaft with liquid refrigerant:** Cooling the shaft with liquid nitrogen or another refrigerant (like Option C suggests) would cause the shaft to shrink, effectively reducing the interference fit. While this method is sometimes used (known as a "cold mount"), it is generally less common, more hazardous (due to cryogenic temperatures), and requires more complex equipment (such as insulated containers and safety gear) than simply heating the bearing. Heating the bearing (Option A) is the standard, safest, and most appropriate primary industrial method. * **D) Heating the bearing with an oxyacetylene torch:** This is highly destructive and inappropriate. An oxyacetylene torch applies localized, extremely high heat. This excessive heat would instantly ruin the bearing by softening the steel, destroying the heat treatment (leading to premature failure), evaporating the lubricant, and potentially causing uneven expansion, warping the ring, or damaging the rolling elements. Precision bearing installation requires controlled, uniform heat.