What is crank and slotted lever mechanism?

Short Answer:

The crank and slotted lever mechanism is a type of quick return mechanism used in shaping and slotting machines to convert rotary motion into reciprocating motion. It helps in making the forward cutting stroke slower and the return stroke faster, which saves time and improves efficiency during machining operations.

This mechanism mainly consists of a crank, slotted lever, ram (or slider), and a frame. The crank rotates continuously, causing the slotted lever to oscillate. The lever then moves the ram back and forth. During the cutting stroke, the motion is slow for better accuracy, while during the return stroke, the motion is quicker, reducing idle time.

Detailed Explanation:

Crank and Slotted Lever Mechanism

The crank and slotted lever mechanism is a commonly used mechanical linkage that converts rotary motion into reciprocating motion. It is widely applied in shaping, slotting, and slotter machines. The key feature of this mechanism is its ability to produce a quick return motion, meaning the tool moves forward slowly during the cutting stroke and returns quickly without cutting. This difference in timing between the strokes increases productivity and reduces machine idle time.

Construction of Crank and Slotted Lever Mechanism

The mechanism consists of the following main components:

  1. Frame:
    The frame acts as the main supporting structure and provides fixed points for other links. It keeps all components in their proper positions.
  2. Crank:
    The crank is the rotating link that provides continuous rotary motion. It is attached to a rotating shaft or motor and connected to the slotted lever through a crank pin.
  3. Slotted Lever (or Rocker Arm):
    The slotted lever is a link that has a slot along its length. The crank pin moves inside this slot as the crank rotates. The lever is pivoted at one end to the frame, which allows it to oscillate back and forth.
  4. Ram or Slider:
    The ram is attached to the upper end of the slotted lever. It performs the reciprocating motion and carries the cutting tool used in machining.
  5. Driving and Driven Links:
    The crank acts as the driving link, while the slotted lever and ram act as driven links. Together, they transform rotary motion into reciprocating motion.

Working of Crank and Slotted Lever Mechanism

The working principle of this mechanism is based on the rotation of the crank and the oscillation of the slotted lever. The process can be explained in two main strokes:

  1. Forward Stroke (Cutting Stroke):
    When the crank rotates in one direction, it moves the slotted lever slowly, causing the ram to move forward. During this stroke, the tool cuts the material, and the motion is slower to maintain accuracy and smoothness in cutting.
  2. Return Stroke:
    As the crank continues its rotation, the slotted lever moves in the opposite direction. The ram then returns to its initial position at a much faster speed. This is the idle stroke where no cutting occurs.
    The reason for the quicker return is that the crank covers a smaller angle during the return stroke compared to the forward stroke. Since the crank rotates at a constant speed, the time taken for the return stroke becomes shorter.

Quick Return Principle

The quick return mechanism helps in reducing the total machining cycle time. The time ratio between the cutting stroke and return stroke is called the quick return ratio, given by:

This ratio is always greater than one, showing that the return stroke is completed faster than the forward stroke.

Applications

The crank and slotted lever mechanism is used in several machines where quick return motion is needed:

  • Shaping machines – to shape flat metal surfaces
  • Slotting machines – to cut internal slots or grooves
  • Slotters and metal planers – for machining and finishing tasks

These applications require repeated forward and backward motion of the cutting tool, making this mechanism ideal for such operations.

Advantages

  1. Efficient Machining: Reduces idle time due to faster return stroke.
  2. Simple Construction: Consists of few parts, easy to design and maintain.
  3. Smooth Cutting Motion: Provides controlled cutting during forward stroke.
  4. Time Saving: Increases production speed by reducing non-cutting time.

Limitations

  1. Non-uniform Velocity: The ram speed varies during both strokes, which may cause uneven cutting.
  2. Wear and Tear: Continuous sliding motion between crank pin and slot causes friction and wear.
  3. Not for Heavy Cutting: Best suited for light and medium-duty operations, not for heavy industrial cutting.

Comparison with Whitworth Mechanism

While both the Whitworth and crank-slotted lever mechanisms are quick return types, their construction differs. The Whitworth mechanism uses a slotted link and crank with a different angular setup, while the crank and slotted lever mechanism uses a direct slotted lever connected to the crank pin. Both achieve the same goal of a quick return, but the latter is mechanically simpler and commonly used in small to medium shaping machines.

Conclusion

The crank and slotted lever mechanism is an important and efficient system in mechanical engineering, particularly in shaping and slotting machines. It helps in converting rotary motion into reciprocating motion with a quick return feature. The slower forward stroke ensures accurate cutting, while the faster return stroke saves time and increases productivity. Despite minor limitations like wear and speed variation, its simplicity and usefulness make it a preferred choice in many mechanical applications.