Crank and Slotted link Mechanism-Quick return Mechanism
What is the Quick Return Mechanism?
A quick return motion mechanism is used in the shaper and slotter machine in which the circular motion is converted into reciprocating motion so that the slider moves forward and backwards. In the forward direction, the cutting process occurs while in return there is no such cutting.
Crank and Slotted Link Mechanism:
This mechanism was invented by Whitworth in the 1800s. The mechanism consists of a slider, crank, fixed link, slotted lever, connecting rod and ram.
The crank is connected with the slider and fixed link. When the cranks start rotating the connecting rod pushes forward and backwards the ram.
Here the motion is being converted into linear motion from rotary motion.
The crank and slotted link mechanism are widely used in shaping machines like slotter, shaper Machine for producing flat surfaces on the workpiece.
The return stroke is much faster then the forward stroke in this mechanism.
The return stroke is much faster then the forward stroke in this mechanism.
Here we will see problem based on crank slotted lever
Below problem is based on link quick-return mechanism of the slotted-lever type, the various dimensions of which are OA = 400 mm, OP = 200 mm, AR = 700 mm, RS = 300 mm. For the configuration shown, determine the acceleration of the cutting tool at S and the angular acceleration of the link RS. The crank OP rotates at 210 rpm.
The velocity diagram has been drawn,
Writing the acceleration vector equation,
Direction of fqp is obtained by rotating vqp through 90° in the direction of angular movement of link QA
Draw the acceleration diagram as follows:
1. From the pole point ol, take the first vector of Pi.
2. Add to it the second vector p1qp.
3. Add the third vector to the second vector. For the fourth vector, draw a line parallel to AQ, through the head qp of the third vector.
4. From the pole point al or o1, take the fifth vector and for the sixth vector, draw a line perpendicular to AQ through the head qa of the fifth vector. This way the point q1 is located.
5. Join q1 and al and extend to r1 such that
Writing the vector equation,
fro is already available on the acceleration diagram. fsg is horizontal.
Complete the vector diagram as usual.
Acceleration of the cutting tool,
Note: In case the problem is to be worked out without writing the vector equation and if the Coriolis acceleration component fpq (centripetal accelaration qw.r.t p) is considered instead of fqp centripetal acceleration q w.r.t p, then note that
• the magnitude of the Coriolis component remains the same.
• to find the direction, the velocity vector Vpq
is to be rotated through 90°. The direction of Vpq
is found to be opposite to Centripetal acceleartion q w.r.t p. Now, one will be tempted to place this vector towards right of p1 in the acceleration diagram. However, if that is done, the vector would be read as p1qp which means fqp
and not fpq. Thus, again the vector fqp
has to be placed at the same place, i.e., on the left of p1 which means the acceleration diagram obtained will be the same.
Advantages of Quick Return Mechanism:
These are the advantages of Quick return mechanism:
1. The process is automated.
2. The construction of the mechanism is not so complicated.
3. It can perform operations like cutting, flattening and slotting the workpiece.
4. The idle time is reduced because of the fast returning stroke.
Disadvantages of Quick Return Mechanism:
Disadvantages of Quick return mechanism are:
1. In the returning stroke, there is no contact with the work so no cutting takes place hence the process takes much time to complete.
2. The forward stroke takes much time compared to the return stroke.
3. It takes more power to perform operations.
4. There is friction in the slider and Piston.
5. Continuous it won’t work because of heat generated inside the piston and wear and tear can happen.
6. Balancing of linkage is also a major problem as this device is also linked to linkages.
Application Of Crank And Slotted Lever Mechanism:
- The main application of the crank and slotted lever mechanism can be seen in Shaper Machine.
- In the quick return mechanism (whether crank and slotted or Whitworth), the backward movement of the ram is faster than the forward movement of the ram. Forward Movement is slower because the slow machining will give good finishing. The machining time of the crank and slotted lever mechanism can be taken as the ratio of time of cutting stroke (or forward stroke) to the time of return stroke (or backward stroke).
- Crank and Slotted Lever Mechanism is used in Slotting machines for performing the slots on the workpiece.
- In addition to these, the application of crank and slotted lever mechanism in rotary type internal combustion engines also plays an important role.
Authors:
Tejas Khandekar
Pavan Khaple
Pranav Kharade
Amit Khare
Khushi Dhokane