When you tighten a bolt or nut, the force is concentrated on a small bearing area under the head or nut. If that surface is softer or rough, it can indent or wear, reducing the clamping force and risking joint damage or fatigue. A plain washer spreads the load over a larger bearing surface, which protects the parts, prevents embedment, and helps maintain a steady clamp even if there are minor surface irregularities. It doesn’t change how much of the bolt’s threads engage, it doesn’t reduce the tightening torque, and it doesn’t increase the bolt’s tensile strength. So the main purpose is to distribute the load over a larger bearing surface.

Torque is transferred from a shaft to a hub by a key that sits in matching slots on both parts. The Kennedy key is another name for a tangential key, which engages in a keyway so that the contact and drive occur along a line tangent to the shaft’s circumference. This tangential engagement provides a straightforward, compact way to transmit torque and is a distinct style from other key forms. For example, Woodruff keys use a semicircular shape and a matching semicircular slot, saddle keys sit in a saddle-shaped groove, and a roll pin is a separate type of fastener, not a key. So, the Kennedy key is the same as the tangential key.

The main concept is how shape controls stress under internal pressure. In a thin-walled pressure vessel, the circumferential (hoop) stress governs how thick the wall must be. A spherical shell distributes the load evenly in all directions, so the hoop stress is uniform around the surface and is lower than in a cylinder (for the same internal pressure and dimensions). Specifically, the sphere has about half the circumferential stress of a cylinder, allowing the same safety with a thinner wall or less material. That uniform, lower stress state is why spherical vessels are favored in design. The other options aren’t the primary design reason: fabrication cost isn’t guaranteed to be lower, the hoop stress isn’t higher, and while a sphere can enclose more volume per material, the key advantage in design is the uniform, reduced circumferential stress.

The most important stresses in a wire rope are axial tension along its length and bending when the rope follows a curve. When the rope is loaded, the wires carry the load in tension, so each wire experiences tensile stress. As the rope passes over pulleys or drums, it bends, causing fibers on the outer side of the bend to be stretched more than those on the inner side, which creates bending stress across the rope’s cross-section. Together, these two stresses—tensile and bending—define the primary stress state in a wire rope. Pure shear or crushing stress aren’t the dominant internal stresses in normal rope operation, though contact with a drum does introduce some surface pressure; that’s not the main internal stress state. Relying on tensile stress alone ignores the bending effects that occur when the rope bends around sheaves.