I just rebuilt the power steering valve on my 79. I had to put it together a couple times to keep it from leaking...chalk it up to inexperience. Anyway, in the process, I became curious about how the physics behind the contraption works.

If anyone can shed some light on how the system converts mechanical input to hydraulic output, I'd be grateful Like I said, the valve is installed and working great...but as an engineer, I just feel I have to know. BTW...I'm an electrical engineer, not hydraulic, so forgive my ignorance. Thanks.

 

OK, here ya go. The pump is just that, a hydraulic pump with a pressure valve to relieve the pressure when you are not turning the steering wheel. The steering wheel moves a rod that is connected from the steering box pitman arm to a cylinder. On that cylinder at that connection point is a shuttle valve with a pressure hose and a return hose that, when the above mentioned rod moves the valve one way it causes the hydraulic oil to push, or extend the cylinder rod thus assisting you in turning the wheels in that direction. When you turn the steering wheel the other way, the shuttle valve shifts and causes the oil to flow to the other end of the cylinder and move the cylinder rod in the other direction thus pushing the steering rods in the other direction. Notice the oil flows from the pump thru the pressure hose and returns thru the low pressure return hose. This flow is in one direction only, regardless of the direction you are turning.
If you will notice, there is an adjusting nut on the end of the cylinder where the pitman arm connects. This is used to balance the oil flow so that when you are not turning the steering wheel there is no oil flow to either end of the cylinder. This is where the bypass comes in that is on the pump. The bypass allows the oil to simply circulate within the pump. This bypass is located behind the high pressure hose connection that screws into the pump. It consists of a small piston and a spring. This bypass and the balance adjustment keeps you from"fighting" the wheel when turning and when not turning.

 

Pretty simple. Action of the ball joint on the valve directs fluid to the steering cylinder to extend and retract. This assist the steering action of the normal input through the steering wheel. The nice thing about this type system is that the assist is progressive. More assist when sitting still, and less when at speed. That gives a better road feel.

 

In the following descriptions lets use this common terminology (despite the label on the following pump blowup). The control valve is the device on the end of your relay rod. The slide valve is inside the control valve. The linear valve is the valve #6 and is inside the pump. It is called a Control Valve on the blowup but I will call it the linear valve to keep it from getting confusing.

One slight correction. When you are NOT turning the steering wheel the control valve (on the relay rod) is in "free flow" position. Power steering fluid exits the pump discharge valve #24 and freely flows through the control valve and back to the pump reservoir. In this position, the pump only sees the pressure required to flow the oil around the system (50 to 75 psi).

The pump is a positive displacement device. The faster it rotates the more fluid it displaces. After about 1200 rpm the pump would begin to flow far more fluid than you could ever use. So there is a linear valve #6 inside the pump (located right behind the discharge fitting #24) that limits the amount of oil that can flow out of the discharge fitting. So the Corvette pump linear valve and discharge fitting are calibrated to work together and allow 1.6 to 2.9 gallons per minute (depending upon the model year of your Corvette).

Now when you start to steer, the ball stud on the control valve begins to shift to one side or the other, this causes a slide valve inside the control valve to block a portion of the flow, raises pump pressure, and opens a port which directs pressure and a limited amount of flow to one side of the assist cylinder or the other.

When you reach a full lock turn, you are completely blocking the flow and the pump now "dead heads." To prevent the pump from destroying itself, there is an internal pressure relief built into the linear valve inside the pump that allows fluid being pumped to be recirculated inside the pump. Vette pumps are set at around 950 psi. This pressure setting prevents damage to the Corvette hoses or the control valve. However even in this pressure relief, recirculation mode, the power steering fluid quickly heats up and can cause damage to the pump if it is held in full lock for more than 5 or 10 seconds.

Jim

 

Thanks for the information. I have to admit that rebuilding the valve has imprved the handling quite a bit.

 

Thanks Jim.