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Hydraulics - Pressure vs Flow?

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A question for the hydraulics experts on here, as I don't know much about them and cant remember much about it from my physics lessons at school.

The Mercedes power steering pump on the OM606 pushes out 115Bar. I believe the LR steering boxes are only rated for 75Bar.

Now the question is, can I trade pressure for flow? So can I reduce the 115bar down and give the potential for more flow?

All said, I don't actually have the flow figures for the LR stuff or the Merc pump.

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In a word - Maybe.

If you were for example filling a bucket with oil - so no back pressure with two pumps having equal displacement at the same RPM, the flow rate would be the same.

It is the pump displacement / rpm that determines the maximum flow.

Higher pressure will push fluid through a constriction (such as a valve or port) faster. It will also move a given load faster when the load which is determining the flow.

In an example such as Hydro Steering, when the resistance to steering is high, the pressure will help you physically move the wheels faster. However, when the resistance is low, it will not make the steering any faster (when the back pressure is lower than the pressure relief valve setting). In this case, it's determined by the displacement / RPM.

Thus you end up with a trade-off in the size of rams between the amount of force required versus the maximum speed of movement.

What's your application?


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Well the application is my 90. I'm fitting the Merc engine, so I need to lower the pressure. And depending on what the steering is like with the 35" rubber I may want/need to fit hydro assist.

So am I right in thinking that if I've got high resistance to turning, having higher flow with the max pressure for the steering box will move the wheels quicker?

I'm wanting to end up with the steering a similar weight (or slightly lighter) than the car was with the old 3 bolt box and 255/85R16 KM1 Muds, but with a new 4 bolt box and 35/12.5 x 16 Maxxis Trepadors.

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On my cummins converted 110, I used a 3/8" prv tapped into the the pressure line before the steering box, return to tank.

No trouble to date as it only works when at full lock.

The clue is in the last sentence: It will get to max pressure at full lock, and to the same pressure if you try to turn the wheels against something solid; I dont think you need a prv, as the steering box already has one. Remember that a pump doesn't create a pressure; it generates a pressure against the system upstream. normally a pump has some prv, maybe you can remove this and fit a weaker spring. I would start with plumbing it in and see what you have. Speaking of which, when is the first fire up?


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The flow is down to how much the pump moves per turn to give cc/rev.

After the pump is a flow control valve which kind of tries to sort stuff out, but not in a nice way.

Mercedes probably doubled the pressure so they could halve the flow and make all the parts smaller.

If you think hydro-assist might be good then you probably need even more flow, so merc might be going backward? But it really is down to that cc/Rev when you are low engine speeds.

I can't help but think that there ought to be a 3000psi valve that's slips into the column and sends oil to a steering ram, so that the PS box just becomes a manual, but I haven't seen such a thing. I also wondered what an electric flow booster pump 'teed' in would do?

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