Boulton & Watt engine of 1788
The sun and planet gear (also called the planet and sun gear) was a method of converting reciprocal motion to rotary motion and was utilised in a reciprocating steam engine.
It was invented by the Scottish engineer William Murdoch, an employee of Boulton and Watt, but was patented by James Watt in October 1781. It was invented to bypass the patent on the crank, held by James Pickard.[1] It played an important part in the development of devices for rotation in the Industrial Revolution.[citation needed]
[edit] Operation
Schematic animation of Murdoch's sun and planet gears. The Sun is yellow, the planet red, the reciprocating crank is blue, the flywheel is green and the driveshaft is grey.
Notice that the sun and flywheel rotate twice for every circuit of the planet when they have a 1:1 ratio of teeth.
The sun and planet gear converted the vertical motion of a beam, driven by a steam engine, into circular motion using a 'planet', a cogwheel fixed at the end of the pumping rod (connected to the beam) of the engine. With the motion of the beam, this revolved around, and turned, the 'sun', a larger rotating cog which turned the drive shaft, thus generating rotary motion. An interesting feature of this arrangement, when compared to that of a simple crank, is that when both sun and planet have the same number of teeth, the drive shaft completes two revolutions for each stroke of the beam instead of one. Note the planet-gear is fixed to the pumping rod and thus does not rotate around its own axis.The axis of the planet gear is connected to the axle of the sun gear, and therefore the flywheel, by a linkage that freely rotates around the axis of the sun gear and keeps the planet-gear engaged with the sun-gear.
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