Power Steering

In motor vehicles , a power steering system helps drivers steer the vehicle by increasing the effort required to turn the steering wheel , making it easier to attach or maneuver the vehicle. Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver can provide less effort to turn the steering wheels when driving at normal speed, and to turn the wheels when the vehicle comes to a halt or slows down. Significantly reduces the physical effort required to perform. Power steering can also be engineered to provide some artificial response to the forces acting on the steering wheel.

Hydraulic power steering systems for cars increase steering effort by means of an actuator, a hydraulic cylinder that is part of a servo system . These systems have a direct mechanical connection between the steering wheel and the linkage driving the wheels. This means that the failure of the power-steering system (to increase effort) still allows the vehicle to be steered using manual effort alone. Electric power steering systems use electric motors to provide assistance instead of hydraulic systems. With hydraulic types, power to the actuator (in this case the motor) is controlled by the rest of the power steering system.

Other power steering systems (such as those in most large off-road construction vehicles) have no direct mechanical linkage to the steering linkage; They require electrical power. Such systems, without any mechanical connections, are sometimes called ” drive by wire ” or “steer by wire”, analogous to aviation’s ” fly-by-wire “. In this context, “wire” refers to the electrical cables that carry power and data, not the thin wire rope mechanical control cables. Some construction vehicles have a two-part frame with a sturdy hinge in the middle; This hinge allows the front and rear axles to become non-parallel to steer the vehicle. The opposing hydraulic cylinders move relative to each other to drive the parts of the frame.


The first power steering system on a vehicle was apparently installed in 1876 by a man nicknamed Fitts, but little is known about him. [1] The next power steering system was installed in 1903 on the Columbia 5-ton truck where a separate electric motor was used to assist the driver in turning the front wheels. [1] [2] Robert E. Twyford, a resident of Pittsburgh , Pennsylvania, included a mechanical power steering mechanism as part of his patent (US Patent 646,477) issued on April 3, 1900, for the first four-wheel drive system.

Davis, an engineer in Pierce-Arrow ‘s truck division, began exploring how steering could be made easier, and in 1926 invented and demonstrated the first practical power steering system. [4] [5] [6] Davis moved to General Motors and refined the hydraulic-assisted power steering system, but the automaker calculated it would be too expensive to produce. [5] Davis then signed a contract with Bendix, a parts manufacturer for automakers . During World War II the military need for easier operation on heavy vehicles led to the creation of armored cars for the British and American armies.and promoted the need for power assistance on tank-recovery vehicles. [5]

Chrysler Corporation introduced the first commercially available passenger car power steering system under the name “HydraGuide” on the 1951 Chrysler Imperial . [7] The Chrysler system was based on some of Davis’ expired patents. General Motors introduced the 1952 Cadillac with a power steering system, which Davis had done for the company nearly twenty years earlier.

Detroit’s Charles F. Hammond filed several patents for power steering improvements with the Canadian Intellectual Property Office in 1958 . [8] [9] [10]

In the mid-1950s American manufacturers offered the technology as optional or standard equipment, while it is widely introduced on modern vehicles internationally, with a trend toward front-wheel drive , greater vehicle mass, less Due to assembly line production costs and wider tyres , all of which increase the necessary steering effort. Heavy vehicles, as is common in some countries, will be extremely difficult to maneuver at low speeds, while lighter weight vehicles may not require power assisted steering at all.

Hydraulic system

Hydraulic power steering systems work by using a hydraulic system to multiply the force applied to the steering wheel input to the vehicle’s steers (usually the front) road wheels. [11] Hydraulic pressure usually comes from a gerotor or rotary vane pump driven by the vehicle’s engine. A double-acting hydraulic cylinder applies a force to the steering gear, which in turn drives the road wheels. The steering wheel operates valves to control flow in the cylinder. The more torque the driver applies to the steering wheel and column, the more fluid the valve allows to flow into the cylinders, and so the more force is applied to drive the wheels. [12]

One design has a torque sensor – a torsion bar at the lower end of the steering column – to measure the torque applied to the steering wheel. As the steering wheel rotates, so does the steering column, as well as the top end of the torsion bar. Since the torsion bar is relatively thin and flexible, and the lower end usually resists twisting, the bar will twist by an amount proportional to the applied torque. The difference in position between opposite ends of the torsion bar controls a valve. The valve allows fluid to flow into the cylinder that provides steering assistance; The greater the “bend” of the torsion bar, the greater the force.

Since hydraulic pumps are positive-displacement type, the flow rate they provide is directly proportional to engine speed. This means that at higher engine speeds the steering will naturally operate faster than at lower engine speeds. Because this would be undesirable, a restricted orifice and flow-control valve directs some of the pump’s output back into the hydraulic reservoir at higher engine speeds. A pressure relief valve prevents a dangerous build-up of pressure when the hydraulic cylinder’s piston reaches the end of its stroke.

The steering booster is arranged so that the steering will continue to operate if the booster fails (although the wheel will feel heavy). Loss of power steering can significantly affect the operation of the vehicle. Instructions for checking the fluid level and routine maintenance of the power steering system are provided in each vehicle owner’s manual.

The working fluid, also called “hydraulic fluid” or “oil”, is the medium through which pressure is transmitted. Common working fluids are based on mineral oil.

Some modern systems also include an electronic control valve to reduce hydraulic supply pressure as the vehicle speeds up; This is variable-assist power steering.

DIRAVI Variable-Assist Power Steering

In this power steering system, the force driving the wheels comes from the car’s high-pressure hydraulic system and is always the same regardless of the road speed. Rotating the steering wheel simultaneously moves the wheels through hydraulic cylinders to the respective angles. To give a somewhat artificial steering feel, there is a separate hydraulically operated system that tries to turn the steering wheel back to the center position. The amount of pressure applied is proportional to road speed, making steering very light at low speeds, and very difficult to shift more than a small amount off center at high speeds.

It was invented by Citroen of France.

As long as the car’s hydraulic system remains under pressure, there is no mechanical connection between the steering wheel and the roadwheel. The system was first introduced in the Citron SM in 1970, and was known as ‘VariPower’ in the UK and ‘SpeedFeel’ in the US.

Electro-hydraulic system

Electro-hydraulic power steering systems, sometimes abbreviated EHPS, and sometimes also called “hybrid” systems, use the same hydraulic assist technology as standard systems, but the hydraulic pressure is applied by a drive belt instead. Comes from a pump driven by an electric motor. device.

In 1965, Ford experimented with a fleet of Mercury Park Lane equipped “wrist-twist instant steering”, which replaced the traditional large steering wheel with two 5-inch (127 mm) rings, a faster 15:1 gear ratio and an electric hydraulic replaced with. Pump when engine is off. In 1988, the Subaru XT6 was fitted with a unique Cybrid adaptive electro-hydraulic steering system that changed the level of assistance depending on the vehicle’s speed.

In 1990, Toyota introduced its second generation MR2 with electro-hydraulic power steering. This avoided running hydraulic lines from the engine (which was behind the driver in the MR2) to the steering rack.

In 1994 Volkswagen produced the Golf Mk3 Ecomatic with an electric pump. This meant that the power steering would still function while the engine was turned off by the computer to save fuel. [16] Electro-hydraulic systems can be found in some cars by Ford, Volkswagen, Audi, Peugeot, Citroen, Seat, Skoda, Suzuki, Opel, Mini, Toyota, Honda and Mazda.

Electric system

Electric power steering ( EPS ) or motor-driven power steering ( MDPS ) uses an electric motor instead of a hydraulic system to assist the driver of the vehicle. Sensors detect the position and torque of the steering column, and a computer module applies auxiliary torque via a motor, which connects to the steering gear or steering column. This allows for varying amounts of assistance to be applied depending on driving conditions. So engineers can design steering-gear response for variable-rate and variable-damping suspension systems, optimizing ride, handling and steering for each vehicle. [17]The amount of assist on the Fiat group of cars can be regulated using a button called “City” that switches between two different assist curves, while most other EPS systems have variable assist. These provide more support as the vehicle slows down, and less as the vehicle accelerates.

In EPS a mechanical connection is maintained between the steering wheel and the steering gear. In the event of a component failure or power failure that causes a failure to provide support, the mechanical linkage acts as a back-up. If the EPS fails, the driver is faced with a situation that requires enormous effort to drive. This heavy effort is similar to a passive hydraulic steering assist system citation needed ], Depending on the driving conditions, driving skills and strength of the driver, the loss of the steering assist may or may not result in an accident. The difficulty of steering with passive power steering is compounded by the choice of steering ratio in assisted steering gear versus fully manual. NHTSA has assisted car manufacturers in recalling EPS systems prone to failure.

Electric systems have an advantage in fuel efficiency because no belt-driven hydraulic pump is running continuously, whether assistance is required or not, and this is a major reason for their introduction. Another major advantage is the elimination of belt-driven engine accessories, and the many high-pressure hydraulic hoses between the hydraulic pump, mounted on the engine, and the steering gear, mounted on the chassis. This greatly simplifies manufacturing and maintenance. By incorporating electronic stability control the electric power steering system can change torque assist levels immediately to assist the driver in corrective maneuvers. [19]

In 1986 NSK put EPS into practical use for battery forks. [20] In the subsequent 8 years, Koyo Seiko (current JTKT), NSK developed a column system specifically for minicars, sold only in Japan such as Suzuki and Mitsubishi Motors. [21]However, this simple method was noticeable due to the sense of inertia of the motor when steering fast to avoid danger in driving at low speeds, compared to manual steering when driving at high speeds, at which the steering force became smaller. Came back As a result, that system was not adopted. In the late 1990s, a direct full control system of rack assist without clutch for a regular car was put into practical use in the Honda NSX (initially, automatic only), since then, a motor connected to a brush. infection has occurred. A brushless motor in the rack type ordinary vehicles and this method has become the mainstream.

Electric power steering systems appeared in the Honda NSX in 1990, MG F in 1999, FIAT Punto Mk2, Honda S2000 in 1999, Toyota Prius in 2000, BMW Z4 in 2002 and Mazda RX-8 in 2003.

The first electric power steering system appeared on the Suzuki Servo in 1988. [22] This system has been used by various automobile manufacturers, and is commonly applied to small cars to reduce fuel consumption and manufacturing costs citation needed ] .

Electrically variable gear ratio system

In 2000, the Honda S2000 Type V featured the first electric power variable gear ratio steering (VGS) system. [23] In 2002, Toyota introduced the “Variable Gear Ratio Steering” (VGRS) system on the Lexus LX 470 and Landcruiser Cygnus, and also included an electronic stability control system for changing steering gear ratios and steering assist levels. In 2003, BMW introduced the “Active Steering” system on the 5 Series. [24]

This system should not be confused with variable assist power steering, which changes steering assist torque, not steering ratio, nor with systems where the gear ratio varies only as a function of steering angle. These last are more accurately called non-linear types (for example the Direct-Steer offered by Mercedes-Benz); A plot of steering-wheel position versus axle steering angle progressively curved (and symmetrical).

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