Quick Return Mechanism
A Quick Return Mechanism is used where there is a need to convert rotary motion into reciprocating motion. As it rotates and moves forwards and backwards. Many machines have this type of mechanism. It is to produce a reciprocating motion in which the time taken for travel in idle return stroke and designed to reduce the ideal time required for return stroke is less than in the forward stroke.
It is a four bar mechanism. It is driven by a circular motion source and uses a system comprised of fixed link, crank, pin sliding in the slot ,a sliding pair and three turning pairs.
● Quick Return Mechanism Types:
There are three types of quick return mechanism
1. Hydraulic drive
2. Crank and slotted lever mechanism: This mechanism is mostly used in shaping machines, slotting machines and in rotary internal combustion engines. we see from fig (c)and fig (d) that the angle θ made by the forward or cutting stroke is greater than the angle β described by the return stroke. Since the crank rotates with uniform angular speed, therefore the return stroke is completed within shorter time. Thus it is called quick return motion mechanism
Time of cutting/Time of return = θ/ β
3. Whitworth mechanism: This mechanism is mostly used in shaping and slotting machines and also for cutting metals. The forward stroke takes a longer and cuts the metal while return stroke is idle and taken less time .
When the driving crank OB moves through an angle α in the clockwise direction, the tool moves from the left hand end of its stroke to the right hand end. when the tool moves back from right hand end of its stroke to the left hand end. the driving crank makes an angle β in the clockwise direction, in fig (a). The time taken during the left to right movement of the ram during forward or cutting stroke will be equal to the time taken by the driving crank to make angle α. Whereas , the time taken during the idle or return stroke will be equal to the time taken by the driving crank to make angle β Since the crank link OB rotates at uniform angular velocity therefore time taken during the cutting stroke (or forward stroke) is more than the time taken during the return stroke. In other words, the mean speed of the ram during cutting stroke is less than the mean speed during the return stroke.
Time of cutting/Time of return = angle α/angle β
● Quick Return Mechanism Working Principle:
In the forward stroke, the Slider moves fast and removing the material from the work piece. Whereas in the return stroke, the Slider moves faster than the forward stroke that means Quick return, it takes less time to return, called a return stroke.
● Applications of Quick Return Mechanism:
1.Shaper
2. Screw press
3. Power-driven saw
4. Mechanical actuator
5. Revolver mechanisms
● Advantages of Quick Return Mechanism:
A quick return mechanism is a mechanism that converts rotary motion into reciprocating motion at different rate for its two strokes i.e. working stroke and return stroke. When the time required for the working stroke is greater than that of the return stroke, it is a quick return mechanism. It yields a significant improvement in machining productivity. Currently, it is widely used in machine tools, for instance, shaping machines, power-driven saws, and other applications requiring a working stroke with intensive loading, and a return stroke with non-intensive loading.
●Comparing Whitworth quick-return mechanism with crank and slotted-lever quick-return mechanism
Crank 3 of the Whitworth mechanism is longer than its fixed link 2 and in slotted-lever mechanism link 3 is longer than its crank.
Coupler link 4 of slotted-lever mechanism oscillates about its pivot whereas in Whitworth mechanism Coupler link 1 is rotating about its pivot joint.
In both the case the coupler link can be pivoted to main coupler link.
● References:
1. https://blogmech.com/types-of-quick-return-mechanism-mechanism-in-shaper-machine/