The lengths of the links comply with the conditions: A͞B=D͞C and B͞C=A͞D. At the extreme positions (dead centres), pins a' and b' of link 2 abut against stops a and b of base 4. As a result uncertainty of motion of the mechanism is eliminated. Dash lines show another version of the device for enabling the mechanism to pass through the extreme positions. Here, pins c' and d' of link 3 abut against stops c and d of link 1.
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