Understanding Pulleys for Speed Variation

The question asks us to identify the type of pulley that is specifically used to achieve different speeds in a power transmission system. Let's examine the functions of the given pulley types.

Analysis of Pulley Types

• Solid Pulley: A solid pulley is typically cast in one piece. It is used for transmitting power between shafts with a fixed speed ratio, determined by the diameters of the driving and driven pulleys. It does not allow for easy changes in speed.
• Split Pulley: A split pulley is made in two halves that are bolted together around the shaft. This design makes installation easier without removing the shaft. Like a solid pulley, it is generally used for a fixed speed ratio.
• Jockey Pulley: A jockey pulley (also known as an idler pulley) is primarily used to maintain tension in a belt or to increase the arc of contact of the belt on the main pulleys. It does not directly change the speed ratio between the driving and driven shafts.
• Stepped Pulley: A stepped pulley, also known as a cone pulley, has multiple steps or different diameters machined onto the same casting or assembly. By shifting the belt from one step to another on both the driving and driven stepped pulleys, the speed ratio between the shafts can be changed. For example, if the belt connects a small diameter on the driving pulley to a large diameter on the driven pulley, the driven shaft will rotate slower. Conversely, connecting a large diameter on the driving pulley to a small diameter on the driven pulley will make the driven shaft rotate faster. This allows for obtaining different speeds from a single driving speed.

Based on the functions described, the stepped pulley is the type specifically designed and used to obtain different speeds.

Explanation of Stepped Pulley Function

A stepped pulley system typically involves two stepped pulleys mounted on parallel shafts, connected by a belt. Each pulley has a set of varying diameters (steps). The speed ratio between the shafts is determined by the ratio of the diameters of the steps on which the belt is currently running:

\(\text{Speed Ratio} = \frac{\text{Speed of Driven Shaft}}{\text{Speed of Driving Shaft}} = \frac{\text{Diameter of Driving Pulley Step}}{\text{Diameter of Driven Pulley Step}}\)

By moving the belt to different pairs of steps, the diameter ratio changes, resulting in a different speed ratio and thus a different output speed for the driven shaft. This mechanism is commonly found in machinery like drill presses and lathes to adjust cutting speeds.

Therefore, the pulley used to obtain different speeds is the stepped pulley.

Revision Table: Pulley Types and Uses

Additional Information on Variable Speed Drives

While stepped pulleys provide a mechanical way to change speed, other methods for achieving variable speeds include:

• Variable Frequency Drives (VFDs): Electronic devices used with AC motors to change motor speed by adjusting the frequency of the power supply.
• Variable Speed Belts and Pulleys (e.g., V-belt with variable width pulleys): Systems where the effective diameter of the pulleys can be changed continuously while the system is running, offering more flexibility than stepped pulleys.
• Gearboxes: Systems of gears that provide discrete or continuous speed changes.

Stepped pulleys are a simple, robust, and cost-effective method for providing multiple distinct speed options.