Quote:
Originally Posted by oil pan 4
The problem is the power steering hydraulic pump requires quite a bit of power just to circulate fluid around the system when you are not needing any power assist. It actually takes very little hydraulic power to use "power steering" to move the wheels to the left or right when sitting still.
When driving along down the road the power steering takes up to 1 horsepower just to circulate the fluid, when its not even doing anything as far as you are concerned. Then when you need it the power steering only takes around 2 horse power to run the pump its self to push the rack or steering arm one way or the other.
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Actually, it takes 2 horsepower all the time, doesn't matter if the wheel is turned or not. The pump is positive displacement so it circulates the same amount of fluid at any given rpm, and the pressure regulator is a simple valve.
If you're turning the steering wheel very fast, it's possible the pump actually uses less power than while the steering wheel isn't being turned because if you turn the wheel fast enough it'll drop the pressure in the system.
Electro-hydraulic pumps make this less of an issue by slowing down the pump to regulate the pressure, but you need the pressure sensor and electronic control over motor speed to make this work. Applying constant voltage to a PS pump motor gives you constant amperage draw.
A clutched PS pump is an okay solution, but grabbing an electrohydraulic pump is better. The clutched pulley weighs quite a bit and when the pump is off the steering effort is going to be much higher since you are forcing hydraulic fluid through all the valves. You can find old EHPS pumps on UK Ebay for ridiculously cheap prices, so why would you go through the trouble of doing a clutched pump?