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Closed-Center hydraulic pump question

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Charlie

06-06-2000 05:52:15




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I think I have a basic understanding of how the main piston type hydraulic pump works on my JD1520 tractor. My understanding is that a variable displacement pump of this type has a pressure valve that will cause the pistons to float back away from the cam shaft when the pressure reaches its operating level. Then when the pressure drops as from turning the power steering, the pistons in the pump will drop back down on the cam shaft and go back to work pushing fluid trying to retain the pressure.

On my tractor it seems that when I go from the hydraulic idle state (full pressure)and start to use the hydraulics I get a fairly loud buzz noise from the pump. It only seems to make the noise for second or less as I start to move the steering (or lift the loader) and then is fairly quiet as I continue to turn the steering. I am assuming this is the sound of the pistons initially hitting the cam from their rest position.

My question is: does anyone know if there is some kind of surge relief valve in this system that would absorb some of the initial turn-on effects? I have dismantled the pump and everything seemed fine. Any other ideas? Thanks.

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Dan King

06-11-2000 01:19:49




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 Re: Closed-Center hydraulic pump question in reply to Charlie, 06-06-2000 05:52:15  
The pump you have is a variable displacment type.
The pump will do its best to maintain a preset pressure. It varies it's displacement to accomplish this. When you are not turning the steering wheel or opening a hydraulic valve, the pump requires very little displacement to maintain it's compensated pressure. When you open the a valve or turn the steering wheel, you allow fluid to flow and the pressure drops. At that point the pump sees that it is not at its' preset pressure and will pump its maximum volume until the preset (compensated pressure) is achieved. That being said, the noise you hear is cavitation (pump running dry). You are trying to accelerate a column of oil from virtually a dead stop to full flow in milliseconds. As soon as the inlet oil has enough time to get into the pump the noise stops. Don't sweat the noise, you are not damaging your pump and the only way to cure it is to increase the length of time it takes for your pump to shift from virtually no stroke to full stroke. That is probably more of a engineering headache than its worth. If you had a axial piston pump you could modify the response time by drilling an orifice through the compensator and providing a leak point, slowing down the pumps reaction.
It's a little windy but I hope it explains your question.

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JDGnut

06-06-2000 11:20:46




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 Re: Closed-Center hydraulic pump question in reply to Charlie, 06-06-2000 05:52:15  
Sorry but that little buzz was kind of normal on those little 1.4 pumps. There is no relief valve in the system, the stroke control valve is controling the pressure for the system. Later tractors had an atenuator or canistor that help quiet the hydraulic niose. (some call it a hydraulic muffler.) Good luck.
JDGnut



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