Multi-roller sheaves are commonly referred to as what type of hook sheaves?

Multi-roller sheaves are specifically designed to accommodate large amounts of wire rope and reduce friction in lifting operations. They consist of multiple rollers within the sheave to ensure smoother movement and improved load distribution. This design is particularly effective in systems such as conveyor applications where heavy loads need to be moved efficiently and with lower wear on the rope or cable.

In the context of the choices provided, "conveyor" is the term that best encompasses these features and applications. Conveyor systems benefit from the multi-roller design because they frequently need to handle significant weights and operate over longer distances, making the efficient and low-friction movement paramount. This contrasts with other types of sheaves that might be optimized for different loads or operational contexts.

The other options refer to different categories of sheaves: "block" typically denotes a sheave mounted in a frame which may not specifically imply the multi-roller configuration, "heavy-duty" could imply a robust design but doesn't specifically signify a multi-roller design, and "standard" usually refers to commonly used sheaves that may not have enhanced features like those found in multi-roller configurations. Hence, "conveyor" is the most appropriate term for multi-roller sheaves in this context.