Question 30

The Correct Answer is B **Explanation of Correct Option (B):** Option B ("They are usually pressed onto their shafts.") is correct. Antifriction bearings (such as ball or roller bearings) used in pump applications require a secure fit between the inner race and the rotating shaft to prevent relative movement (creep) and subsequent wear, fretting, and failure. This secure fit is typically achieved through an **interference fit**, meaning the bearing is slightly smaller than the shaft journal, requiring it to be heated or hydraulically/mechanically pressed onto the shaft. This ensures the inner race rotates synchronously with the shaft. **Explanation of Incorrect Options:** * **A) Alignment is not a critical factor in their installation.** This is incorrect. Precise shaft alignment (both angular and parallel) is absolutely critical for pump systems utilizing antifriction bearings. Misalignment introduces excessive radial and axial loads, leading to premature bearing failure, vibration, and seal degradation. * **C) The inner race should be free to turn on the shaft.** This is incorrect. As explained for option B, the inner race must be tightly fixed to the rotating shaft (usually via an interference fit) to prevent rotation between the race and the shaft journal, which would cause rapid wear and shaft damage (fretting corrosion). * **D) The outer race should be free to turn in its housing.** This is incorrect. For most pump applications where the load is rotating (i.e., the shaft rotates), the outer race is fixed stationary within the housing (or bearing cartridge). It is usually held by a tight clearance fit (transition fit) or occasionally clamped by a housing cover. Allowing the outer race to rotate freely (creep) in the housing would cause wear on the housing bore and eventual bearing failure due to insufficient support and lubrication issues.