Power steering

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Electro-hydraulic power steering

A power steering ( latin servus , servants ' , slave') is used to reduce the force necessary for operation of the steering wheel of a motor vehicle during steering in the state, when maneuvering or at low speeds is required. The power steering assists the driver in steering by increasing the steering force exerted by the driver using a hydraulic system (hydraulic pump, control, motor) or an electric motor.

From a technical point of view, the term power steering is incorrect, as the steering is by no means performed solely by a servo, but is mechanically firmly coupled and only supported by a motor (usually hydraulic or electric). An older, technically more correct term is "steering assistance". This term is still used in some tests for the theoretical driver test, although the term "power steering" has become popular.

Hydraulic drive (HPS)


Although the first patents were available earlier, it was Francis W. Davis, an engineer in the trucking division of Pierce Arrow , who began research on how to make steering easier and who introduced the first power steering in 1926. Davis then moved to General Motors and improved this hydraulic power steering, but the automaker rejected it as too costly. Davis then moved to Bendix , an automotive supplier. During the Second World War , the demand for steering aids, especially for heavy or armored vehicles, rose sharply.

Chrysler was the first automobile manufacturer to  offer a passenger car - the 1951 Chrysler Imperial - with power steering called a "hydraguide". This system was based on expired Davis patents. General Motors then presented its own power steering, developed by its Saginaw Product Division for the 1952 Cadillac models, which was based on the work of Davis almost twenty years earlier. Another way was the Packard Motor Car Company with its 1953 presented, together with the Gemmer Manufacturing Co. developed system. This steering aid was not attached to the steering column, but to the chassis . Here, the force did not act directly on the steering linkage (which could lead to damage to the steering if the steering wheel was turned while stationary), but intervened in the steering geometry. Another advantage was that the Packard-Gemmer steering worked constantly, while the Saginaw version first had to overcome a resistance, as the system was only activated after a certain pressure was reached. This principle later became generally accepted.

Nowadays, almost all cars have power steering, as the trend is towards front engines, heavier vehicles, wider tires and thus increased steering forces.


Simplified principle of hydraulic power steering

The servo pump ( hydraulic pump ) constantly delivers a volume flow through the steering. However, since the steering does not require any servo assistance when driving straight ahead or when making slight steering movements, the oil flows directly back into the container. The pressure that is created is called the flow pressure. This can be compared to a garden hose through which water can flow freely. The pump runs continuously, but uses less energy than other consumers in the car, such as B. Air conditioning, heated rear window, heated seats, etc. At the moment when a steering movement is carried out, part of this volume flow is directed into the working area of ​​the steering. There is pressure in the system. The highest pressure is achieved when the steering is fully turned in one direction. At this point, the entire volume flow is directed into the work area without the oil being able to flow back directly. The pump goes to block. The resulting pressure is regulated to a maximum pressure by the pressure relief valve, the system pressure. This creates the whistle that you can hear every now and then when parking, as the oil flow is then completely directed into the steering work spaces. In this case, the servo pump provides the maximum output. Another disadvantage of hydraulic steering compared to electromechanical steering is that the shaft sealing rings (Simmerrings) can become leaky after a certain period of operation and a complicated replacement of the steering gear is then necessary. Since the manufacturer often does not plan to repair parts of the power steering, the entire steering gear may have to be changed.

This is a hydraulic system, consisting of the pump driven by the motor , the oil reservoir, the servo valve, the hydraulic cylinder and the associated pressure lines. The servo valve is usually a torsion spring with a defined stiffness. If the driver turns the steering wheel, the torsion spring is deflected and releases valve openings at the upper end, which release the flow of hydraulic fluid, which increases the movement further down the system. This reduces the resistance in the abutment of the torsion spring, so that the valve cross-section at the upper end of the torsion spring is reduced again. The system thus regulates itself. The characteristics of the hydraulic power steering are set with the rigidity of the torsion spring. More complex power steering systems work depending on the speed. The hydraulic power assistance of the steering movements then decreases with increasing driving speed.

It is advantageous that there are no fault-prone electrical components; Defects or damage are clearly recognizable, the functionality can be easily checked.

Electric drive (EHPS, EPS, EPAS)

The electrically powered power steering is an electric power steering system that only works when there is steering movement.

Advantages of the electric drive:

  • The main advantage of the electric drive is that the steering can be designed to be adaptive and can also be overlaid by assistance systems. The assisting torque and thus the force on the steering wheel can be changed depending on the vehicle speed, for example. The conflict of objectives between strong steering assistance when parking (high rack forces due to the drilling torque of the tire or blockage due to impact on the curb) and low steering assistance when driving fast (reduced rack force) can thus be resolved. The steering system can be used as an actuator for advanced driver assistance tasks (e.g. automatic steering interventions with ESP II, parking and lane departure warning, etc.).
  • In addition, the steering assistance can be designed as required, i. This means that it is only active when it is necessary during steering operations, which leads to fuel savings of up to 0.25 l / 100 km compared to conventional hydraulic steering systems.
  • Since the steering drive is not coupled to the motor (belt drive for the hydraulic pump is not required), the same steering system can be used with different motors.

Disadvantages of the electric drive:

  • Defects or damage in the function of electrical systems are less easy to understand. In the case of program-controlled digital systems, there are security concerns with regard to program errors in the program code that cannot be excluded .

Electro-hydraulic drive

The hydraulic operating principle is largely retained. However, instead of a belt drive, a program-controlled electric motor drives the servo pump that pumps the servo oil into the steering gear (EHPS = Electro-Hydraulic Power Steering ). The electric servo pump does not need a mechanical belt drive, it can be freely positioned independently of the motor. The power requirement of the pump must be taken into account when designing the electrical system. Modern EHPS units provide up to 1 kW of hydraulic output power in passenger car applications.

Electromechanical drive

Simplified principle of the electric power steering

A program-controlled electric servomotor on the steering mechanism (steering column or steering gear) supports and superimposes the driver's steering movements (EPS = Electric Power Steering , EPAS = Electric Power Assisted Steering ). There is no hydraulic system, i.e. the servo pump, the hoses from the servo pump to the steering gear and back, the hydraulic fluid and the slave piston. In the event of mechanical damage, for example in an accident, no oil can escape, as electronic steering gears are only lubricated with grease. Instead, an electric motor causes the driver to superimpose the mechanical steering movement with an auxiliary force.

A distinction is made between different types of electromechanical steering systems. The positioning of the servo unit (motor, control unit) and the design of the reduction gear lead to the following subdivision:

  • C-EPS = Column EPS : Positioning of the servo unit in the steering train, gear type (worm wheel / shaft), e.g. B. in the BMW Z4.
  • P-EPS = Pinion EPS : Positioning of the servo unit on the steering gear pinion , also dual pinion drive via a second, separate pinion shaft, gear type (worm gear / shaft), e.g. B. in the Mercedes-Benz CLA class .
  • R-EPS = Rack EPS : Positioning of the servo unit parallel or concentric around the rack, type of gear (belt and recirculating ball screw with axially parallel arrangement), e.g. B. in the VW Tiguan.


There is a mechanical connection between the steering wheel and the steering linkage so that the vehicle can be steered even if the power assistance fails, even if only with much higher steering forces. Vehicles with very stiff steering due to their design (e.g. trucks with two steered front axles) usually have two servo pumps, which makes a complete failure less likely. It is also provided that at least one steering pump is driven by the running wheels via the drive train even when the vehicle engine is not running (during towing).

Relevant legislation

General information on steering systems in motor vehicles:

StVZO § 38 steering device

  1. The steering device must ensure easy and safe steering of the vehicle; if necessary, it is to be provided with steering assistance. If the power steering fails, the vehicle must remain steerable.
  2. Passenger cars, buses, trucks and semitrailer tractors with at least four wheels and a maximum speed of more than 25 km / h determined by the design, as well as their trailers, must comply with the provisions specified in the appendix to this regulation.

The appendix to the StVZO in turn refers to Directive 70/311 / EEC.


  • Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden 2001, ISBN 3-528-13114-4 .
  • Peter Pfeffer, Manfred Harrer: Steering manual, steering systems, steering feel, driving dynamics of motor vehicles. Vieweg + Teubner Verlag, Wiesbaden 2011, ISBN 978-3-8348-0751-9 .
  • Beverly Rae Kimes (Ed.): Packard. A history of the motor car and the company. General edition, 1978 Automobile Quarterly, ISBN 0-915038-11-0 . (English)

Web links

Commons : Power Steering  - Collection of pictures, videos and audio files
Wiktionary: power steering  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Kimes: Packard (1978), pp. 567-568.
  2. Directive 70/311 / EEC of the Council of June 8, 1970 on the approximation of the laws of the member states relating to the steering systems of motor vehicles and motor vehicle trailers . In: Official Journal . L 133, June 18, 1970, p. 10-13 .