The discharge volume of the vane pump increases proportionately as engine speed rises. The amount of steering assist provided by the power piston of the power cylinder is determined by the volume of fluid from the pump. As the pump speed increases, the flow volume becomes greater, providing more steering assist and, consequently, less steering effort is required. In other words, the steering effort varies in accordance with the change in speed, which is a disadvantage from the standpoint of steering stability. Therefore, it is necessary to maintain a constant fluid flow volume from the pump regardless of the speed, and this is the function of the flow control valve. Normally, When the vehicle is running at high speed, there is less tire resistance and, consequently, less steering effort required. Therefore, with some power steering systems, there is less assist provided during high speeds so that an appropriate steering effort can be obtained. In short, the flow volume from the pump to the gear housing is reduced during high-speed driving and there is less power steering assist. Discharge volume of the pump increases with a rise in pump speed, but the flow volume of the fluid to the gear housing is reduced. This is referred to as the speed-sensing power steering and consists of a flow control valve with a built-in control spool. ‘
<1> At low speeds (Pump speed: 650-1,250rpm)
Pump discharge pressure P1, is applied to the right side of the flow control valve and P2 is applied to the left side after passing through the orifices. The pressure difference between P1, and P2 becomes larger as the engine speed increases. When the pressure difference between P1, and P2 overcomes the flow control valve spring tension (A), the flow control valve moves to the left. This opens the passage to the pump suction side so the fluid returns to the pump suction side. Fluid volume to the housing is kept at constant in this manner.
<2> At medium speeds (Pump speed: 1,250-2,500rpm)
Pump discharge pressure P1 is applied to the left side of the control spool. When the pump speed is above 1,250 rpm, pressure P1 overcomes the spring tension (B) and forces the control spool to the right, so the fluid volume passing through the orifices is decreased, causing a lowering of pressure P2. Consequently, the pressure difference between P1 and P2 increases. In this manner, the flow control valve moves toward the left so that the fluid returns to the pump suction side, reducing the fluid volume to the gear housing. In other words, when the control spool moves to the right, the fluid volume passing through the orifices decreases.
<3> At high speeds (Pump speed: over 2,500rpm)
When pump speed exceeds 2,500 rpm, the control spool is forced all the way to the right, half closing the orifices. At this time, pressure P2 is determined by only the amount of fluid passing through the orifices. Fluid volume to the gear housing is kept at constant (small value) in this manner.
<4> Relief valve
The relief valve is located in the flow control valve. When pressure P2 exceeds specified level (when turning the steering wheel fully), the relief valve opens to lower pressure. When pressure P2 drops, the flow control valve is forced to the left and controls the maximum pressure.