FOUR BAR LINKAGE| MECHANISM

Four-Bar Linkages

A planar four-bar linkage consists of four rigid rods in the plane connected by pin joints. We call the rods:

• Ground link gg: fixed to anchor pivots AA and BB.

• Input link aa: driven by input angle αα.

• Output link bb: gives output angle ββ.

• Floating link ff: connects the two moving pins CC and DD.

We often think of a four-bar linkage as being driven at the input angle αα, resulting in the output angle ββ. We only need one input, because the system has exactly

NDOF=1NDOF=1 degree of freedom. We can count the DOF as 9 free variables (three moving rigid bodies with three variables each) minus 8 constraints (four pin joints with two constraints each). animation resetShow:labels pivots αα:crankOutput ββ:rockerFour-bar linkages can be used for many mechanical purposes, including to:

1. convert rotational motion to reciprocating motion (e.g., pumpjack examples below)

2. convert reciprocating motion to rotational motion (e.g., bicycle examples below)

3. constrain motion (e.g., knee joint and suspension examples below)

4. magnify force (e.g., parrotfish jaw examples below)