Hunt4Allis wrote:
...I've been told it's important to make sure that the valve is rated for whatever the max PSI is the pump can deliver.... |
If you're designing a system from scratch, you'd select all components based on the maximum pressure and volume necessary for the design. When altering an existing design, there's other challenges involved. What you're doing here, is trying to take a pump system that was intended to be low-volume/high pressure, and utilize it with high-volume/low pressure implements and cylinders. This is a mismatch, and far from optimal, but it WILL work, so many guys do it. Safe to bet that many of them don't know what they're doing, but the designers of the components they've stumbled upon to use incorporated sufficient protection into their valves so that catastrophic circumstances haven't appeared. As Doc, and others have pointed out, an open-center hydraulic circuit exhibits NO pressure until there's a load appllied, and when one IS, the pressure which appears is commensurate to the load.
Hunt4Allis wrote:
...If I'm using my rear lift arms to raise a thousand pound round bale it may use 3700 PSI but to lift a 500 lb round bale it may cut that down to only 2,000 PSI? Or if ...it's connected to a cylinder if the load on the cylinder is 1,000 lb it could make the pressure 3700 PSI and with 500 lb of weight on the same setup it may only put out 2,000 PSI? |
Your numbers are arbitrary, but the concept of proportion is sorta correct.
Let's say you have a cylinder that has TEN square inches of surface area. When you put a 500lb load on it, and apply pressure, the cylinder will extend when the incoming pressure reaches 50psi. (50psi x 10 square inches, is 500lbs). At that point, any more fluid you push towards the cylinder, at 50psi, will continue to extend the cylinder, until there's either no more fluid to pump, or there's no more room in the extending cylinder to accept fluid.
If you put a 1000lb load on the same cylinder, since it's a 10sqin cylinder, it will require 100psi to lift the same load.
If you use TWO cylinders of 10sqin surface, plumbed in parallel, you'll have 20 square inches of lift surface, so only need 25psi to lift 500lbs... or 50 to raise 1000lbs, but it will require TWICE the fluid, and (since the pump is not turning any faster) twice the TIME to extend.
In all these cases, the cylinder is moving, and NO pressure relief valves ever actuate, because system pressure NEVER goes anywhere NEAR any critical level.
IF you run the cylinder to it's max extension, you're 'stalling' the system... the pump is trying to force fluid into a cylinder which has no more room. THIS is when pressure rises, and when you do this, the pump is being driven by the engine, and forcing fluid into a space where no more CAN fit, the result is either 1) Pressure climbing rapidly to infinity, or 2) Engine reaches it's torque limit, and stopping abruptly...
Or 3) Something in the pump, plumbing, or actuator (cylinder, motor, etc) breaking, and developing a case of petroleum incontinence.
To provide a better alternative to 1, 2, and especially 3, sensible hydraulic systems have a pressure relief valve that will open, and bypass pressure once a certain safe point has been reached.
The early Allis, like many early hydraulic systems, were designed under the expectation that there wouldn't be a large volume of flow, so they made them capable of a fairly high pressure. They integrated a relief valve into the tractor's plumbing so that in the event that something elsewhere went wrong, the relief valve would protect the engine, coupling, and pump by bypassing any system pressure above the (arbitrary) 3800psi or so.
This was an effective method of operating simple implement lift cylinders and lift arms. High pressure meant cylinders could be compact, and wouldn't require a large volume hydraulic reservoir capacity to operate.
Fast forward 15 years or so, and the REST of the hydraulic world had grown substantially, and the popular design was to reduce the operating pressure, and go with larger cylinder surfaces. Why? Because lower pressures meant that plumbing, fittings, and fixtures did NOT need to have such brute strength, and hydraulic fluids would last much longer, leak less, and actuators could be more reliable, and valving could be more supple in it's control response.
What you are doing, is putting a low-pressure/high volume valve, in a high-pressure/low volume system. That valve has an integral pressure limiting valve, so no matter WHAT you plug in it, it will NEVER see pressures get past it's internal limit.
Since it's a system mismatch, your system will be slow, and depending on the volumes of all the cylinders you'll be using, you'll probably find that the reservoir capacity is insufficient... but you won't blow up the tractor's pump with excess pressure. Worst thing you might do, is run it dry when all the cylinders are fully extended.
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