- Power steering (operation)
- power steering pump
- Power steering oil
- Electronic power steering
Power steering (operation):
The function of the power steering is to allow the driver to turn the wheels with little effort. Certainly with a heavy car with wide wheels, it will not be possible to drive properly without power steering. Parking will be very difficult. The image below shows the subframe of a car with a "direct" steering box mounted. The power steering takes place in this steering box.
The power steering works with oil which is put under pressure. This oil is pumped throughout the system by the power steering pump. The oil pressure helps to reduce the force required for steering during steering movements.
The power steering pump carries the oil through the supply line to the steering box. The oil is always located on both sides of the piston in the steering box. More information about the wheelhouse can be found on the page steering gear.
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 to the left with the steering movement. The oil will then return to the reservoir on the left via the return (the space in the left chamber is then smaller than that of the right). When steering to the right again, the oil pressure in the left chamber is increased. The piston moves to the right with the steering movements and the oil in the right chamber is fed to the reservoir via the return line. Steering movements increase or decrease the oil pressure in the left or right chamber. The alternating effect of the oil pressure is controlled internally via valves.
A mistake that is often made is that at maximum steering angle (ie the steering wheel is turned as far as possible) some force is exerted on the blocking steering wheel. What then happens is that the piston is in the maximum position (left or right) and the pump continues to generate the maximum pump pressure. There will usually also be a scraping or screeching sound from the pump. So always make sure that the steering wheel is always lightly held against the stop at maximum steering angle (preferably just not against the stop). So definitely do not put any force, because the wheels will not steer further from that.
Power steering pump:
The image opposite shows a power steering pump with its components. This pump is also called a servo pump. The pump is driven via the multi-belt. The rotating disc in the pump rotates (at the same speed as the crankshaft). The plungers move outwards by means of the centrifugal force. The oil in the reservoir will be pumped through these plungers to the supply line (shown at the bottom of the picture).
The moment the crankshaft speed becomes too high, the pressure relief valve will open. The pump pressure will not exceed approximately 1,5 bar. The oil that flows past the pressure relief valve returns to the section before the pump section. This oil will go through the pump again. As soon as the engine speed drops, the pressure relief valve closes and the full oil pressure will be pumped through the supply line. The oil that returns from the steering rack will return to the reservoir. It is a closed system, so what is pumped to it always comes back through the return.
Power steering oil:
If the power steering oil level falls below the minimum, there is a risk that the pump will run dry. This will be heard because the pump will hum. This will cause the pump to wear out faster and fail prematurely. There are marks on the reservoir (min. and max.). Often there is also a temperature at max. (eg 20 degrees Celsius). This is because the hydraulic oil in the power steering circuit has the property of expanding with an increase in temperature. At a high temperature, the oil will be higher in the reservoir than when the oil is cold. That should always be taken care of.
It is also important that the correct oil is always topped up (according to the factory standard). For example, the oil in the system can be ATF (Automatic Transmission Fluid) or CHF-11s. The latter is a special type of hydraulic oil (green in colour). These oils must not be mixed or exchanged for other types. This could cause a malfunction of the power steering pump or steering rack.
More information about the wheelhouse can be found on the page steering gear.
Electronic power steering:
With electronic power steering, also called EPS (Electronic Power Steering), it replaces the conventional hydraulic power steering system. Sometimes a combination of electro-hydraulic power steering is also made.
With electronic power steering, just like with the hydraulic system, the steering movements are transmitted via the steering wheel and steering column to the rack and pinion. The rack and pinion moves the track rods to the left or right. The electric motor reinforces the steering movements.
In order to provide good power assistance, it is 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 various degrees. In this way, the power steering can be maximum in city traffic, so that the steering wheel can be turned with little force. At higher speeds, the electric motor can reduce the power assistance, so that the steering wheel turns “more heavily” and is therefore easier to steer.