A shaft carries a helical spur gear. Which one of the following bearings can NOT be used to support it?

The correct option is Straight roller bearing.

Step-by-Step Explanation:

1. Understanding the forces on a helical gear:
When a shaft carries a helical spur gear, the helical action of the teeth generates three distinct types of loads during power transmission:
- Radial load: Acting perpendicular to the shaft axis.
- Tangential load: The primary driving force acting tangentially to the gear pitch circle.
- Axial (thrust) load: Acting parallel to the shaft axis due to the helix angle of the gear teeth.
Therefore, the bearing supporting a helical gear shaft must be capable of resisting both radial and axial (thrust) loads simultaneously.

2. Analyzing the capabilities of the given bearing options:
- Angular contact bearing: Specifically designed to handle combined loads (both radial and significant axial loads). Therefore, it can be used.
- Double-row ball bearing: Capable of supporting radial loads as well as axial loads in both directions. Therefore, it can be used.
- Tapered roller bearing: Designed specifically to handle high radial loads and high axial loads. Therefore, it can be used.
- Straight roller bearing (also known as cylindrical roller bearing): Designed primarily to support heavy radial loads but has virtually zero capacity to support axial (thrust) loads because the rollers are straight and run along cylindrical raceways. Since it cannot sustain the axial thrust produced by the helical gear, it cannot be used to support the shaft.

Consequently, the straight roller bearing is the only bearing listed that can NOT be used to support a shaft carrying a helical gear.