Hydraulic control valves in tractor

صمامات التحكم فى الجهاز الهيدروليكى فى الجرا ر                                     

1. Load check valve

2. Lift cylinder relief valve

3. Lowering metering valve

4. Lowering solenoid

5. Raise solenoid

6. Raise spool

7. Flow compensating valve

Neutral

• Solenoids not energised

• Oil flow from the parallel gallery A flows past the flow compensator spool into gallery B where it is blocked by the lands of the raising spool.

•Oil under the pressure in the hydraulic lift circuit is prevented from entering gallery E by the load check valve (4)

•The trapped pressurised oil in the lift circuit is applied to the right hand end of the metering valve through the small drilling X

•The applied pressure holds the metering valve in the closed position preventing oil flow from gallery E to the return to reservoir gallery F

•When the valve is in neutral , gallery C & load sensing galleries D are open to reservoir through the centre of the raising spool & gallery F.

gallery  G is a common return to reservoir gallery running through the centre of the valve stack

Lowering

• Trapped oil in lift cylinder is vented through metering valve to reservoir

• Lift drops with gravity and is not powered down

• Drop rate is electronically controlled by the processor and solenoid

• 2 stages of valve movement during lowering 

Lowering

( Close up showing operation of lowering metering valve )

Stage 1

•  Lowering solenoid activated

• Plunger ( 1 ) and pilot spool ( 6 ) moved to right allowing trapped oil at H to go to reservoir via F past the lands in the pilot spool ( 2 )

• As oil at H is released, the trapped cylinder oil E acting on the chamfered face ( 4 ), will push the metering spool  ( 3 ) to the right.

Stage 2

• The lift cylinder oil can now pass the metering lands ( 5 ) of the metering spool to reservoir via F.

• Drop rate is electronically controlled by the processor and solenoid which adjust the clearance over the metering lands     ( 4 ).  

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