Power Steering:
The function of power steering is to allow the driver to turn the wheels with little effort. Especially with a heavy car with wide wheels, driving without power steering is not reasonable. Parking, in particular, would be very difficult. In the image below, the subframe of a car is shown where a steering gear of the “direct” type is mounted. The power steering takes place in this steering gear.
The power steering operates with fluid that is pressurized. This fluid is pumped throughout the system by the power steering pump. The fluid pressure assists steering movements by reducing the necessary force required to steer.
Operation:
The power steering pump delivers the fluid via the supply line to the steering gear. The fluid is always present on both sides of the piston in the steering gear. More information about the steering gear can be found on the page steering system.
When steering to the left, the pressure on the right side of the piston will be higher than on the left side. The piston will move left with the steering movement. The fluid on the left side will then return to the reservoir via the return (the space in the left chamber is then smaller than that of the right). When steering right again, the fluid pressure in the left chamber increases. The piston moves right with the steering movements and the fluid in the right chamber is returned to the reservoir through the return line. With steering movements, the fluid pressure is increased or decreased in the left or right chamber. The fluctuating operation of the fluid pressure is regulated internally via valves.
A common mistake is to apply force to the blocking steering wheel at maximum steering lock (meaning the steering wheel is turned as far as possible). What then happens is the piston stays in the maximum position (left or right) and the pump continues to generate maximum pump pressure. A scraping or whining noise from the pump usually accompanies this. Always ensure that the steering wheel is lightly held against the stop at maximum lock (preferably just not against the stop). So, definitely do not apply force, as the wheels will not steer further because of it.
Power Steering Pump:
The image next to this text shows a power steering pump with its associated components. This pump is also known as a servo pump. The pump is driven by the serpentine belt. The rotating disk in the pump spins (at the same speed as the crankshaft). The plungers move outward due to centrifugal force. The fluid in the reservoir is pumped through these plungers into the inlet line (seen at the bottom of the image).
When the crankshaft speed becomes too high, the overpressure valve opens. The pump pressure will not exceed approximately 1.5 bars. The fluid that flows past the overpressure valve returns to the section before the pumping section. This fluid will then go through the pump again. As soon as the engine speed drops, the overpressure valve closes and the full hydraulic pressure will be pumped through the supply line. The fluid returning from the steering gear will end up back in the reservoir. It’s a closed system, so what is pumped there always comes back through the return.
Power Steering Fluid:
If the power steering fluid level drops below minimum, there is a risk that the pump will run dry. This can be heard as the pump will start to hum. This will lead to accelerated wear and premature failure of the pump. The reservoir has markings (min. and max.). Often, a temperature is also given at max. (e.g. 20 degrees Celsius) because the hydraulic fluid in the power steering circuit expands with a temperature increase. At high temperatures, the fluid will rise higher in the reservoir than when the fluid is cold. This should always be closely monitored.
It’s also important to always refill with the correct fluid (according to the factory standard). For example, the system could use ATF (Automatic Transmission Fluid) or CHF-11s. The latter is a special type of hydraulic fluid (green in color). These fluids should not be mixed or replaced with other types as this can cause damage to the power steering pump or the steering gear.
More information about the steering gear can be found on the page steering system.
Electronic Power Steering:
Electronic power steering, also known as EPS (Electronic Power Steering), replaces the conventional hydraulic power steering system. Sometimes a combination of electro-hydraulic power steering is also used.
With electronic power steering, just like with the hydraulic system, steering movements are transmitted from the steering wheel and steering column to the rack. The rack moves the tie rods left or right. The electric motor amplifies the steering movements.
To provide the proper assistance, it’s important that the steering torque on the rack is measured. Together with the steering angle signal, the electric motor will amplify the steering movements. The electronics of the electric motor can amplify the steering movements to varying degrees. This way, power steering can be maximized in city traffic, allowing the steering wheel to turn with little force. At higher speeds, the electric motor can reduce assistance, making the steering “heavier” and therefore easier to control.
