Grashoff 4-bar linkage: A linkage that contains one or more links capable of undergoing a full rotation. A linkage is Grashoff if: S + L < P + Q (where: S = shortest link length, L = longest, P, Q = intermediate length links).

**Grashoff's law:**

·
**Grashoff
4-bar linkage: **A linkage that contains one or more links capable
of undergoing a** **full rotation. A
linkage is Grashoff if: S + L < P + Q (where: S = shortest link length, L =
longest, P, Q = intermediate length links). Both joints of the shortest link
are capable of 360 degrees of rotation in a Grashoff linkages. This gives us 4
possible linkages: crank-rocker (input rotates 360), rocker-crank-rocker
(coupler rotates 360), rocker-crank (follower); double crank (all links rotate
360). Note that these mechanisms are simply the possible inversions (section
2.11, Figure 2-16) of a Grashoff mechanism.

·
**Non
Grashoff 4 bar: **No link can rotate 360 if: S + L > P + Q

**Let’s examine why the Grashoff condition works:**

·
Consider a linkage with the shortest and longest sides
joined together. Examine the linkage when the shortest side is parallel to the
longest side (2 positions possible, folded over on the long side and extended
away from the long side). How long do P and Q have to be to allow the linkage
to achieve these positions?

·
Consider a linkage where the long and short sides
are not joined. Can you figure out the required lengths for P and Q in this
type of mechanism

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Mechanical : Kinematics of Machinery : Basics of Mechanisms : Grashoff's law |

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