Four-Bar Mechanism

Grashof’s law

s + l ≤ p  + q                                                                                                          
Here, s = Shortest link, l = Longest link
p, q = Remaining links

Case-I

If s + l < p + q

– s is fixed – Double crank mechanism.
– p or q fixed – Crank-rocker mechanism
– l is fixed – Rocker-rocker mechanism

Case-II

If s + l = p + q

– All link have different lengths then same as case-I.
– Parallelogram linkage-crank-crank mechanism.
i.e. s = p, l = q
– s is fixed – Double  crank mechanism.
– l is fixed – Double crank mechanism.
– Deltoid linkage

• s is fixed – Crank-crank mechanism.
  Also called as kite mechanism.
• l is fixed – Crank-rocker mechanism.

Case-III

s + l > p + q (Grashof’s law is not satisfied and it will give rocker-rocker mechanism.)

Inversions of Slider-Crank Mechanism

• First Inversion-link 1 is fixed: Reciprocating engine/compressor.
• Second Inversion-link 2 is fixed (Crank): Whitworth quick return mechanism, rotary (radial) engine.
• Third Inversion-link 3 is fixed (Connecting rod): Crank and slotted lever mechanism, oscillating cylinder mechanism.
• Fourth Inversion-link 4 is fixed (Slider): Hand pump, bull engine.

Mechanical Advantage

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Must Read: What is the Theory of Machines?