In 1907, Doble obtained a US patent for the notch which is seen at the outer edge of the cups of all modern pelton wheels. He realised two things: first, that as the cups entered the jet, they disrupted it, causing what was meant to be a clean, solid shaft of water to break up and fail to transfer maximum energy to the wheel; second, that the first bit of water entering each cup was deflected upward, where it hit the underside of the cup next to arrive. By hitting the underside of that cup, the force of the water was acting to make the wheel turn the opposite way, - and that was clearly a bad thing.
By designing a notch which allowed the jet to pass through one cup and continue onward to the cup which preceded it, his invention ensured the next cup entered the jet when the wheel had rotated a bit further round. This meant the jet struck the cup more perpendicularly and the direction of the deflected exhaust water was no longer upward toward the following cup, but outward to the sides.
His design also described the shape and inclination of the splitter ridge so it was the sharp tip, the extreme tip, of the splitter ridge which was first to enter the jet. The effect of this was to make it as if a knife was cutting cleanly into the jet, disturbing the integrity of the solid column of water as little as possible.
The sequence of events happening when a jet hits cups which have a Doble notch is seen in this photo from my last blog post:
- the jet can be seen passing through the notch of cup 1...
- ... and striking the splitter ridge of cup 2 almost perpendicularly, sending water to the sides and not back toward cup 1
- cup 3 is cut off from the jet by cup 2, but water which was entering it before cup 2 got in the way, is seen to be still passing across the floor of the cup and up its side wall where it is just beginning to exit
- within a fraction of a second, the wheel will rotate a tiny amount clockwise bringing the tip of the splitter ridge of cup 1 cleanly into the path of the jet
Doble's patent can be viewed as he originally wrote it here. It is worth reading for its clarity and conciseness.
These are two extracts from it:
William Doble became chief engineer in Lester Pelton's company from 1912, but history has ensured Pelton's name rather than Doble's has been tied to the impulse turbine they both worked to develop.
It just goes to show that having a brilliant idea doesn't ensure history will remember you !
