The Watt type Governor
Probably the most widely used governor in the early days, it is named the Watt governor because James Watt applied it to his early beam engines. He did not however invent it as it had been in use on wind and water mills many years before this.
A belt or gearing from the engine crankshaft drives the input shaft 'm' causing the bevel gears 'l' to revolve and in turn rotate the vertical shaft 'a'. The bracket 'b' at the top of 'a' supports two arms 'c' which are pivoted at the top, at the end of the arms are two very heavy metal weights 'B' partway along the arms 'c' are fixed two pivoted link arms 'd' which link to a collar 'c' which rotates with them but is able to slide up and down shaft 'a' .
The up and down motion of this collar is followed by a pair of pins 'f' which move a bell crank 'g' which is in turn linked to a throttle actuating rod 'i' linked to a throttle or butterfly valve in the supply of steam to the engines cylinder which can allow more or less steam through.
At rest the governor weights are held in the lowest position by gravity, the throttle will be in its most open position. As the engine speed increases these weights rotate faster until centrifugal force exceeds that of gravity and they fly further outwards and as a result of the linkages, upwards, this movement is transmitted to the trottle valve which begins to close. The faster the governor is driven the further out the weights move and the more the throttle is closed, until the amount of steam it lets through balances the demand and the engine speed stabilises.
If the load the engine drives is reduced it will increase speed, the governor restricts steam flow more until the speed stabilises, if load is added to the engine the speed drops, the throttle is opened more and more steam allowed in to compensate for the demand.
The Watt governor is a simple governor but is not terribly accurate where very fine control of speeds in needed and so was superceeded in many applications by more specialised and accurate governors, however for many agricultural end pumping engines where absolute speed was not essential it survived and can still be seen on numerous preserved engines.