Short Answer:

In cam motion, rise, dwell, and return are the three main phases of follower movement during one complete revolution of the cam. The rise is the period when the follower moves upward or away from the cam center. The dwell is the period when the follower remains stationary even though the cam continues to rotate. The return is the period when the follower comes back to its original position.

These three stages define the complete motion of the follower and are essential for the proper timing and smooth operation of the cam-follower mechanism used in engines and machines.

Detailed Explanation :

Rise, Dwell, and Return in Cam Motion

A cam-follower mechanism is widely used in mechanical systems where precise timing and motion control are needed, such as in engines, automation machines, and textile equipment. The motion of the follower depends entirely on the profile or shape of the cam and is generally divided into three parts: rise, dwell, and return. Understanding these stages helps in designing cams that ensure smooth, efficient, and vibration-free operation.

1. Rise in Cam Motion

The rise (also known as the outstroke) is the portion of the cam rotation during which the follower moves away from the cam center or upward from its lowest position. During this phase, the cam lifts the follower to a certain height called the lift or stroke. The cam profile in this portion is specially designed to produce the desired follower displacement pattern.

• Purpose: The main purpose of the rise phase is to transfer motion from the cam to the follower so that it can perform a specific task, such as opening a valve in an internal combustion engine or moving a mechanical component in a machine.
• Types of Motion during Rise:
  The follower can rise according to various motion laws, such as:
  • Uniform Velocity Motion: The follower moves upward at a constant speed.
  • Simple Harmonic Motion (SHM): The follower moves smoothly with gradually increasing and decreasing velocity.
  • Uniform Acceleration and Retardation Motion: The follower starts with acceleration and ends with equal deceleration, ensuring smooth motion without jerks.
• Design Consideration:
  The shape of the cam during rise must be designed to reduce stress and avoid sudden shocks on the follower. Smooth curvature is essential for longer life and quieter operation.

2. Dwell in Cam Motion

The dwell is the period of the cam rotation when the follower remains stationary, even though the cam continues to rotate. In this phase, there is no change in the displacement of the follower. On the displacement diagram, this appears as a horizontal straight line, showing constant position.

• Purpose: The dwell period allows the follower to maintain its position for a required duration before it begins to move again. This pause is often necessary in operations where a part must remain still for a specific time, such as holding a valve open in an engine for a certain duration.
• Control of Dwell:
  The length of the dwell period can be controlled by adjusting the cam angle corresponding to the dwell section. Longer dwell periods mean the follower stays at rest for more time.
• Applications:
  Dwell is essential in machines requiring intermittent motion, such as automatic feeding machines, sewing machines, or printing presses, where holding time is crucial for proper operation.

3. Return in Cam Motion

The return (also called the fall or return stroke) is the phase in which the follower comes back to its original or starting position. This happens when the cam profile causes the follower to move downward toward the cam center.

• Purpose: The return brings the follower back to the initial position to repeat the working cycle. It ensures the follower is ready for the next rise motion.
• Types of Motion during Return:
  Similar to the rise, the return can follow different motion laws such as:
  • Uniform velocity
  • Simple harmonic motion
  • Uniform acceleration and retardation motion
    These motion laws help ensure the follower moves smoothly and avoids sudden impacts.
• Design Importance:
  The return stroke must be designed carefully to prevent excessive acceleration or jerk, which can cause noise, wear, or damage to the mechanism. A properly designed return ensures the cam-follower system operates smoothly and efficiently.

4. Combination of Rise, Dwell, and Return

In a complete revolution of the cam (360° rotation), the follower undergoes a full motion cycle consisting of rise, dwell, and return. These phases are represented in the follower displacement diagram as a rise curve, a dwell line, and a return curve. The relative angles for each phase depend on the required timing and duration of the follower’s motion in the machine.

For example, in an internal combustion engine:

• During the rise, the inlet valve opens.
• During the dwell, the valve stays open to allow air-fuel mixture intake.
• During the return, the valve closes again for compression.

This sequence ensures accurate timing and smooth operation of the mechanism.

5. Importance of Rise, Dwell, and Return

The division of follower motion into rise, dwell, and return allows for:

• Controlled and predictable motion of mechanical parts.
• Reduction in shocks and vibrations.
• Improved timing accuracy in engines and automatic machinery.
• Better performance and longer life of cam-follower systems.

Hence, these three stages form the foundation for cam motion design in mechanical systems.

Conclusion

To conclude, rise, dwell, and return are the three main stages of follower motion in a cam mechanism. The rise represents upward motion, the dwell is the stationary period, and the return represents the downward motion of the follower. These phases together ensure smooth, precise, and controlled operation of machines using cams. Proper design and understanding of these phases are essential for efficient and reliable mechanical performance.