What is a quick return mechanism?

Short Answer:

A quick return mechanism is a type of mechanical system used in shaping machines, slotting machines, and similar tools to convert rotary motion into reciprocating motion. Its main feature is that the cutting stroke takes more time than the return stroke, making the return movement quicker. This helps save machining time and increases productivity.

In this mechanism, a ram or tool moves forward slowly during the cutting stroke and returns rapidly to its starting position. The mechanism achieves this by using different link arrangements that provide unequal stroke times during a complete rotation of the crank.

Detailed Explanation:

Quick Return Mechanism

A quick return mechanism is an important mechanical arrangement that allows a tool or ram to perform a cutting stroke in one direction and a return stroke in the opposite direction within a shorter time. It is widely used in machines like shaping machines, slotting machines, and metal cutting machines where a reciprocating motion is required for the tool.

The primary advantage of the quick return mechanism is saving idle time. During the return stroke, the tool is not cutting the material, so by making this stroke faster, the overall machining time is reduced. The forward (cutting) stroke is slower and stronger for effective cutting, while the return stroke is quick and light, improving efficiency and reducing energy waste.

Working Principle

The working of a quick return mechanism is based on a simple crank and slotted lever system. When the driving crank rotates, it causes the ram (or tool head) to reciprocate. The mechanism is designed so that during one rotation of the crank, the time taken for the forward stroke is longer than that for the return stroke.

The quick return action is achieved because of the unequal angles formed by the crank during forward and return movements. The crank rotates through a larger angle during the cutting stroke and a smaller angle during the return stroke. This difference in angular travel results in a quick return motion.

Construction of Quick Return Mechanism

The quick return mechanism generally consists of the following parts:

  1. Frame or Machine Body:
    It provides support to the entire mechanism and keeps all the components in proper alignment.
  2. Driving Crank:
    The crank is connected to a rotating shaft or motor and provides the rotary motion required to drive the system.
  3. Slotted Lever (or Rocker Arm):
    A lever with a slot in which a crank pin slides. This lever is connected to the ram and converts the rotary motion into reciprocating motion.
  4. Ram or Tool Head:
    This part carries the cutting tool. It performs the forward cutting stroke and returns quickly during the idle stroke.
  5. Connecting Link:
    It transmits motion from the crank to the slotted lever.

As the crank rotates, the sliding pin moves inside the slotted lever, causing it to rock about its pivot. This rocking motion pushes the ram forward and backward, completing one cutting and one return stroke during each revolution.

Types of Quick Return Mechanism

There are several types of quick return mechanisms used in various machines. The most common types are:

  1. Crank and Slotted Lever Quick Return Mechanism:
    This is the most widely used type in shaping and slotting machines. The crank is connected to the driving shaft and rotates continuously. The slotted lever is connected to the ram, and as the crank rotates, it causes the ram to move forward and return quickly.

    • Forward Stroke: Occurs when the crank moves through a larger angle.
    • Return Stroke: Occurs when the crank moves through a smaller angle.
      The ratio of the time of forward stroke to the return stroke is given by the ratio of these angles.
  2. Whitworth Quick Return Mechanism:
    In this mechanism, a crank rotates a slotted link that drives a sliding block connected to the ram. The geometry of the system ensures that the tool returns faster than it advances. It provides smoother motion and is used in high-speed shaping machines.
  3. Hydraulic Quick Return Mechanism:
    Instead of mechanical links, this system uses hydraulic cylinders to move the ram. The flow rate of fluid is controlled to achieve a slow forward stroke and a fast return stroke. It provides smoother operation and less vibration.

Applications of Quick Return Mechanism

The quick return mechanism is widely used in various machine tools where cutting and return motions are required. Some common applications include:

  • Shaping Machines: To move the cutting tool forward slowly for machining and return it quickly.
  • Slotting Machines: To cut slots or keyways in metals.
  • Metal Cutting Tools: For reciprocating tool motions in industrial machines.
  • Mechanical Presses: Where quick resetting of the ram is required after a working stroke.

Advantages of Quick Return Mechanism

  1. Increased Efficiency: By reducing the idle time of the return stroke, total machining time decreases.
  2. Smooth Operation: The motion is continuous and easy to control.
  3. Energy Saving: The power required for the return stroke is less due to reduced resistance.
  4. Compact Design: The mechanism is simple and requires less space.
  5. Durability: Fewer moving parts ensure long-lasting performance with minimal wear.

Limitations

  1. The mechanism is not suitable for heavy-duty or high-force cutting operations.
  2. The quick return ratio is fixed and cannot be easily changed.
  3. Wear in joints and slots may cause vibration or inaccuracy over time.
Conclusion

The quick return mechanism is a highly effective and practical system used in many machine tools to achieve efficient reciprocating motion. By reducing the time taken for the return stroke, it saves machining time and improves productivity. Its simple design, reliable operation, and adaptability to shaping and slotting machines make it an essential concept in mechanical engineering. Whether mechanical or hydraulic, this mechanism remains a key element in efficient motion conversion systems.