1. the control lever is pushed down.
2. the lift arms fall, until the drag on the plough creates enough pressure on the top link to compress the spring, which then pushes the control fork pivot point forward.
3. The valve is then closed, and further descent of the lift arms is prevented.
4. The plough continues at this depth, until further resistance is met by the plough the control fork pivot point is pushed forward.
5. oil is pumped to the ram, lifting the lift arms.
The animation doesn't show the effect of a reduction in drag: that would let the top link, and the control fork, move back, allowing oil to be drained from the ram again.
We all know what an immense change the three point linkage made to mechanized farming, allowing the use of mounted implements.Fifty-odd years later, many of us (me included) are still trying to work out just how it all works. Well, here's my explanation...
The Bits...
There's a lot of bits in there, but I've ignored most of them, simplified the rest, and then taken liberties with parts such as the pump and the control valve. I've also painted the external components in the wrong shade of grey, and dyed the tractor oil yellow...
Control spring: this is the big spring which the top link is connected to. When the implement (e.g. plough) meets with more resistance (in the soil), it pivots around the lower links, and puts pressure on this spring via the top link
Lift arms: er, the arms which do the lifting... (shrug)..
Pump: not drawn in detail here...
Control valve: this is difficult to represent in two dimensions, so it has been "re-designed" here- don't attempt repair of your tractor using this diagram... The valve has three positions:
Forward: (toward the front of the tractor) oil is allowed into the pump, and then forced into the ram. The oil path through the control valve is via a hollow annulus around the spool.
Middle: no action: no oil movement.
Rear: oil is cut off from the pump inlet; oil is allowed to drain out of the ram, lowering the lift arms.
If confuse, please visit
from http://www.fofh.co.uk/tech/draught.htm
