A vertical shaft inside the main column is rotated by the pulley in the base via a bevel pinion arrangement. The top of the shaft projects above the column and carries a cylindrical casing with a horizontal D shaped frame at its base.  

Rotating head unit and  operating bell-crank (driver’s seat in the background)

Inside the casing is a strong spiral spring which is supported at its lower end by a pair of circular nuts screwed on the top of a metal sleeve surrounding the vertical shaft.  The sleeve itself extends to the top of the main column and ends in a double collar carrying a loose ring bearing. Below that is a foot bearing sitting on the machined face of the main column allowing the sleeve and spring to rotate with the upper casing.

At the tips of the D frame are the pivots for two bell-crank levers, the vertical arms of which carry iron balls.  The inner arms of the bell-cranks press on the spring support located inside the cylindrical casing.

A forked lever forming one end of another bell-crank lever is attached to  the collar bearing whilst the other end is connected via a link rod to the valve cut-off  mechanism. 

North winder governor details

As the drum turns, the governor shaft rotates and with it, the two balls.  These are thrown outwards and compress the spring whilst at the same time moving the cut-off link rod.  The amount of compression of the spring is determined by the rotational speed of the balls, as the speed increases, the balls are thrown further out and the cut-off is advanced.

Sources:

• D K Clark  1894,  The steam engine
• J S Rao   1992,  Mechanism and machine theory