Short Answer:
The Whitworth quick return mechanism is a type of mechanical linkage used mainly in shaping and slotting machines to convert rotary motion into reciprocating motion. In this mechanism, the cutting stroke takes more time than the return stroke, allowing the tool to return quickly and save machining time. It improves the efficiency of machines by reducing the idle period during the return stroke.
The mechanism consists of a driving crank, a slotted lever, and a ram or slider. The crank rotates continuously, causing the lever to oscillate, which in turn moves the ram forward and backward. The forward stroke (cutting stroke) is slower, while the return stroke is faster — hence the name “quick return mechanism.”
Detailed Explanation:
Whitworth Quick Return Mechanism
The Whitworth quick return mechanism is an important mechanical system commonly used in shaping, slotting, and slotting machines. Its main function is to convert the rotary motion of a crank into reciprocating motion of a ram or slider, which holds the cutting tool. The special feature of this mechanism is that the return stroke of the ram is quicker than the forward stroke, thereby saving time and increasing the overall productivity of the machine.
Construction of Whitworth Quick Return Mechanism
The Whitworth mechanism is made up of several key parts that work together to achieve quick return motion. These include:
- Frame:
The frame is the stationary base that supports all other components of the mechanism. It keeps the system stable during operation. - Driving Crank:
The crank is connected to a rotating shaft that provides continuous rotary motion. It acts as the input link of the mechanism. - Slotted Lever (or Rocker Arm):
This lever has a slot along its length, through which the crank pin moves. It is pivoted at one end and connected to the ram at the other. As the crank rotates, the slotted lever oscillates back and forth. - Ram or Slider:
The ram is the part that performs the cutting action. It is attached to the upper end of the slotted lever and slides in a straight path (reciprocating motion). - Fixed Frame and Pivot Points:
The pivot points are arranged in such a way that the forward stroke covers a larger angle than the return stroke. This difference in angular travel time causes the quick return motion.
Working of Whitworth Quick Return Mechanism
- Forward Stroke (Cutting Stroke):
When the crank rotates in one direction, the slotted lever moves in such a way that the ram moves forward slowly. This is the cutting stroke, during which the tool removes material from the workpiece. - Return Stroke:
As the crank continues to rotate, the lever moves in the opposite direction, and the ram returns quickly to its starting position. No cutting takes place during this stroke; it is simply the idle return movement.
The reason for the quick return lies in the difference in the angular motion of the crank during the two strokes. The crank covers a larger angle during the forward stroke and a smaller angle during the return stroke. Since the crank rotates at a constant speed, the return stroke is completed faster than the forward stroke.
Quick Return Ratio
The quick return ratio is defined as the ratio of the time taken for the cutting stroke to the time taken for the return stroke.
For the Whitworth mechanism, this ratio is always greater than 1, which means the cutting stroke takes more time than the return stroke.
Applications
The Whitworth quick return mechanism is mainly used in machines where the tool needs to move forward slowly to perform cutting and return quickly to start the next cycle. Common applications include:
- Shaping machines (for metal cutting and surfacing)
- Slotting machines (for internal keyways)
- Slotters and planners
Advantages
- Increased Productivity:
Since the return stroke is faster, less idle time is wasted, improving the efficiency of the machine. - Simple Design:
The mechanism has fewer moving parts and is easy to maintain. - Smooth Operation:
Provides steady motion and uniform cutting during the forward stroke.
Limitations
- Non-uniform Motion:
The speed of the ram varies during both strokes, which may affect cutting uniformity. - Limited to Light and Medium Cutting:
Not suitable for heavy-duty machining operations. - Wear and Tear:
The slotted lever experiences continuous friction due to the moving crank pin.
Conclusion
The Whitworth quick return mechanism is a clever and efficient design widely used in shaping and slotting machines. Its main advantage is the reduction of idle time during the return stroke, which increases productivity. Although it has limitations in terms of speed variation and wear, its simplicity and effectiveness make it one of the most popular quick return mechanisms in mechanical engineering.