Parallel motion linkage

This article is about parallel motion in mechanics. For parallel motion in music, see Contrapuntal motion.
Animation of the parallel motion linkage.
Dimensions (unit lengths a, b):
  Link 2: a + a
  Links 3 & 5: b + b, 2b
  Links 4 & 6: a
Vertical distance between ground joints ≈ 2b
Horizontal distance between ground joints ≈ 2a
Thus, link 1 (total distance between ground joints)

In kinematics, the parallel motion linkage is a six-bar mechanical linkage invented by the Scottish engineer James Watt in 1784 for the double-acting Watt steam engine. It allows a rod moving practically straight up and down to transmit motion to a beam moving in an arc, without putting significant sideways strain on the rod.