Movement No. 90 is a refinement of Movement No. 89, replacing the circular eccentric strap with an elongated yoke. As the eccentric disk rotates on the shaft, the yoke surrounds the disk and slides along it. Because the yoke is elongated, it accommodates the lateral displacement of the eccentric without requiring the connecting rod to swing sideways — the rod travels in a perfectly straight line within fixed guides. This improvement eliminates unwanted side forces and vibration on the rod, making the mechanism more precise and better suited for applications where the output rod must move in a strictly linear path, such as valve gear and pump drives in steam engines.
90. A modification of the above; an elongated yoke being substituted for the circular strap, to obviate the necessity for any vibrating motion of the rod which works in fixed guides.