landroversforever Posted July 24, 2015 Share Posted July 24, 2015 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. Quote Link to comment Share on other sites More sharing options...
simonr Posted July 24, 2015 Share Posted July 24, 2015 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? Si Quote Link to comment Share on other sites More sharing options...
landroversforever Posted July 24, 2015 Author Share Posted July 24, 2015 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. Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted July 24, 2015 Share Posted July 24, 2015 Ross, I use a P38 pump which is 1500PSI vs the original RRC one of 900 PSI and the box is fine, so no need to worry about that, thankfully See this post of mine a while ago, the steering is really light, and that's on 33x10.5 Simex JT2s, at 20psi: http://forums.lr4x4.com/index.php?showtopic=70934 Quote Link to comment Share on other sites More sharing options...
daveturnbull Posted July 24, 2015 Share Posted July 24, 2015 In order to make the steering a bit lighter (esp. with a quickener fitted) many of the comp safari bods will drill out the 2mm ish hole on the LR pump output to about 6mm. Makes a world of difference apparently. Quote Link to comment Share on other sites More sharing options...
Ash.Witty Posted July 24, 2015 Share Posted July 24, 2015 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. Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted July 24, 2015 Share Posted July 24, 2015 If you read my post above, 6mm on the sort of pump Ross probably has on the Merc, will be too much, if you have a spare you can experiment though Quote Link to comment Share on other sites More sharing options...
Daan Posted July 24, 2015 Share Posted July 24, 2015 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? Daan Quote Link to comment Share on other sites More sharing options...
Team Idris Posted July 25, 2015 Share Posted July 25, 2015 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? Quote Link to comment Share on other sites More sharing options...
landroversforever Posted July 25, 2015 Author Share Posted July 25, 2015 First fire up.... Errrrr in a while . I think I need to have a closer look at the merc pump and try and find out the flow rate. Talking of which, does anyone happen to know the flow rate of a 300tdi PAS pump? Quote Link to comment Share on other sites More sharing options...
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