What are rise, dwell, and return in a cam?

Short Answer:

In a cam, the motion of the follower is divided into three main parts — rise, dwell, and return. The rise is the part when the follower moves upward, increasing its distance from the cam center. The dwell is the part when the follower remains stationary, even though the cam continues to rotate. The return is the part when the follower moves downward to its original position.

These three stages together define the complete motion cycle of the follower. The design of these stages ensures smooth and controlled movement in machines like engines and automatic devices.

Detailed Explanation :

Rise, Dwell, and Return in a Cam

In a cam and follower mechanism, the cam rotates while the follower moves according to the shape of the cam profile. The follower’s movement during one complete revolution of the cam is not uniform; it follows a specific motion cycle that includes three major stages — rise, dwell, and return. Understanding these stages is very important for designing smooth and efficient motion systems used in machines such as internal combustion engines, textile machines, and automated tools.

The purpose of dividing the motion into rise, dwell, and return is to control how the follower moves, stops, and comes back to its starting position. This ensures that the machine operates smoothly without unnecessary vibration or wear.

  1. Rise

The rise (also called the outstroke or lift) is the part of the follower’s motion where it moves upward, away from the cam center. This happens when a high spot or lobe on the cam surface pushes the follower upward as the cam rotates.

During the rise period:

  • The follower displacement increases from zero to its maximum value.
  • The motion can be uniform velocity, uniform acceleration and retardation, simple harmonic, or cycloidal, depending on the cam design.
  • The rate of rise must be designed carefully to avoid shock and vibration in the system.

The rise portion is used to perform useful work in the mechanism. For example, in an internal combustion engine, the rise corresponds to the opening of the inlet or exhaust valve by lifting the follower (valve tappet).

The angular interval of cam rotation during which the follower rises is called the angle of ascent. The duration of this angle depends on the speed of operation and the motion requirement of the machine.

  1. Dwell

The dwell period occurs immediately after the rise or return. During the dwell, the follower remains stationary at its highest or lowest position, even though the cam continues to rotate.

This happens because the cam surface during this stage is circular, meaning it does not change the position of the follower. The dwell is important because many mechanical operations need a pause between motions to complete a process safely or precisely.

For example:

  • In an engine, the valve remains open for some time to allow complete air or exhaust gas flow — this time is the dwell.
  • In textile or packaging machines, the dwell allows the machine part to stay steady for cutting, pressing, or stitching operations.

The angular rotation of the cam during the dwell is called the angle of dwell. The length of dwell depends on how long the follower must stay in one position to complete its function.

  1. Return

The return (also called the return stroke or fall) is the part of the motion when the follower moves downward to its initial position. This happens when the cam lobe passes, and the cam surface allows the follower to come back, either by gravity, a spring, or another mechanical force.

During the return period:

  • The follower displacement decreases from maximum to zero.
  • Like the rise, the motion during the return can also follow uniform velocity, uniform acceleration and retardation, simple harmonic, or cycloidal motion.
  • The return must be designed to ensure smooth and shock-free movement.

In many machines, the return is designed to be quicker than the rise because no useful work is done during the return stroke. The angular distance covered by the cam during the return is called the angle of descent or angle of return.

  1. Complete Motion Cycle

The complete motion cycle of a follower during one revolution of the cam includes:

  1. Rise (Ascent) – The follower moves upward.
  2. Dwell – The follower remains stationary.
  3. Return (Descent) – The follower moves downward.
  4. Dwell – The follower again stays still before the next rise begins.

This sequence repeats as the cam keeps rotating. By adjusting the time duration and angles of these stages, designers can create different motion characteristics suitable for specific applications.

  1. Importance of Rise, Dwell, and Return

These three stages are crucial for several reasons:

  • They ensure smooth and controlled operation of the machine.
  • They determine the timing and duration of machine processes.
  • They reduce shock and noise in high-speed machines.
  • They help achieve desired output motion for the given input speed.

Without proper rise, dwell, and return design, the machine may suffer from uneven motion, vibrations, or excessive wear, leading to poor performance and shorter life.

Conclusion

The rise, dwell, and return are the three fundamental parts of the follower’s motion in a cam mechanism. The rise lifts the follower to perform work, the dwell holds it steady to complete an operation, and the return brings it back to its starting position. A correct balance between these phases ensures smooth, efficient, and reliable functioning of mechanical systems such as engines and automation equipment. Properly designed motion cycles are the key to precision and durability in any cam-follower mechanism.