Re: Got the 300u going, but...

Posted by Bob M on February 12, 2007 at 07:53:20 from (151.190.254.108):

In Reply to: Got the 300u going, but... posted by JayWalt on February 11, 2007 at 19:30:32:

Jay - I’m not familiar with the 300U hydraulic system in particular. However it works basically the same as the live hydraulics on the Super series Farmalls – and I am quite familiar with those.

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Anyway it sounds as if your hydraulics are somehow plumbed incorrectly.

Your 300U has what’s called an “open center” hydraulic system. When no hydraulic action is required oil flows from the pump to the control valve, passes freely through the control valve and then returns directly to the reservoir. No pressure is developed and the pump runs unloaded.

Now when you move a control valve to extend or retract a cylinder, the control valve redirects flow from the pump to one side of the cylinder and at the same time directs return flow from the opposite end of the cylinder back to the reservoir. The amount of pressure developed at the pump now depends upon the load on the cylinder – the greater the load the higher the pressure.

The relief (or safety) valve is located in the line between the pump and the control valve inlet and monitors pressure in the supply line. It should open ONLY when it’s set pressure is exceeded – for example when the cylinder reaches the stop at end of it’s travel and the control valve is not released, or if the cylinder is overloaded. Otherwise the relief valve should never operate.

(Incidentally when the relief valve operates it develops a tremendous amount of heat. If left to bypass for very long it’ll cause the hydraulic system to overheat….)

Consequently your relief valve opening with the control valve in “neutral” says your plumbing is somehow messed up. Also if your tractor has power steering the flow divider that supplies the steering system may not be working and is constantly restricting pump flow.

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The pulsing of relief valve could be from several causes: The oil level may be too low (pump is sucking air). Could also be an excessive suction side restriction - collapsed suction hose, clogged screen/filter, filter too small for the pump flow. Finally it could be be incorrect (too heavy) oil the system.

Hope this helps, and good luck!

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