What was the mechanism Watt developed to ensure the piston rod moved nearly vertically while attached to a moving beam?

Answer

Parallel motion.

The parallel motion mechanism, patented in 1784, used a linkage system that ensured the piston rod moved in a straight vertical line, preventing sideways thrust and wear on the cylinder walls.

What was the mechanism Watt developed to ensure the piston rod moved nearly vertically while attached to a moving beam?
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