How Does Your Car’s Power Steering System Work?

| September 21, 2009 at 6:24 pm
Tips N' Tricks

The power steering has become a key requirement of any car from an user’s point view. In the primary power-assisted-steering-system, the engine pumps out a hydraulic fluid to both sides of the steering, under high pressure. In modern systems, an electro-hydraulic, power system is employed, which makes use of an electric motor to drive the pumping mechanism.

The basic power-steering system was an inventive thought and was purely mechanical. The core of the hydraulic-system is a double-acting cylinder. It is controlled by valves operated by a steering wheel. This cylinder exerts remarkable force on the steering-gear to turn it on the steering axis. This force is directly proportional to that applied at the steering wheels by the driver. Its working can be explained as follows:

Pump

car-power-steering-system-pump

Image Credit - How Stuff Works

The rotary-vane pumps provide hydraulic power to the steering. They are attached to the casings supporting the structure. A runner belt and pulley set up is interlocked and works in tandem by power supplied from the spinning crankshaft. The crankshaft is the core of the power delivery system for pumps. The pump has a rotor system that rotates rapidly, pressurizes and propels the fluid and thereby driving the power-steering system. It contains a group of retractable vanes. As they spin inside an oval chamber, they pull hydraulic fluid from the return line and at high-pressure force it into the opening based on the engine speed. So the pump moves more fluid than required when the vehicle is at faster speeds.

A rotary-valve senses the force acting on the steering wheel with the help of a torsion bar. It is a thin metal-rod that twists as torque is applied. It is connected to the steering-wheel, and the pinion thereby producing a torque that is equal to the amount of torque the driver applies to turn the wheels The pressure regulator helps the fluid in reaching the steering mechanism. There it acts as a lubricant reducing the heating effect of the system. This pressurized fluid provides power for the power steering mechanism and power steering gear box. The pressure regulator maintains the requisite pressure required for the system.

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Image Credit - How Stuff Works

Gear Box

The power steering gear system consists mainly of the rack and pinion style. The pinion is connected to the steering-shaft by means of a torsion bar. The fluid enters through either side of the arrangement as directed by the valves. The steering wheel turning causes change in the position of the valve arrangement, due to the resistance of friction of the tire and road contact and the weight of the car.

As a result it guides the fluid to the connecting end of the cylinder. Change in alignment of the valve and the shaft delivers the pressure to the equivalent side. The power pressure system turns a screw which moves a nut back and forth creating a steering effect. Thus, a power-steering system makes possible the smooth and comfortable steering of cars.
But the problem pertaining to the hydraulic designs was that it was constant, irrespective of the speeds of vehicles. So at high speeds the amount of assistance is much lower than at low speeds. Electric power systems thus came in to act.

Electric Power Systems

In electric power steering replaces an electric motor drives the steering-mechanism. The motor is attached to the steering rack, and a microprocessor regulates the motor and so the steering dynamics. The key component is a sensor on input shaft which converts steering torque and rotation speed into signals to be fed into the microprocessor. Electric power systems have the 1-MPG advantage compared to the hydraulic system’s fuel efficiency, which includes a pump that is constantly working. The criticism is that they do not provide enough feedback to the driver.

Electro-Hydraulic Power Systems

It is a hybrid of the fully hydraulic systems and the fully electric systems. An electric motor provides the forces to steer the wheels but through a hydraulic system. The hydraulic system’s pump can be shut off entirely to save energy. Adjusts the amount of steering assistance so as to suit the speed of the vehicle. An electromagnetic valve precisely controls the force applied by the steering-hydraulics. This creates precise steering needed for the driving condition. In the basic power-steering system the power-steering regulation is proportional to the engine’s RPM. So is in the electromechanical-steering system. For Electro-Hydraulic Power Systems, the power gradually decreases when the vehicle speeds up, ensuring improved precision and greater stability.

Variable gear ratio steering system

It is the latest innovation in power steering, which determines the steering angle ratio which is to be achieved according to a vehicle’s condition with respect to the road. The heart of the new system is a gear-set integrated into the steering-column. An electric-motor in the joint varies the front wheels’ steering angle proportional to the vehicle’s current speed. When driving at lower speeds this steering raises the steering angle. The front wheels respond smartly to small movements of the steering-wheel, which allow the driver to meander through tight spaces minimal effort. At higher speeds, the Variable gear ratio steering system becomes more indirect. Therefore, it reduces the amount of steering gradient for every steering wheel motion. This gives more precise steering at higher speeds ensuring excellent stability and comfort. If the vehicle is threatened by over steering or braking on a variable surface, the system increases the angle of steering wheels much faster than to ensure safety. There is no direct connection between the steering-wheel and front-wheels, so the vehicle remains under control in case of a rare component failure of the system. This is because an adaptation mechanism blocks the system from using a pivot so that the driver is entirely in control of the circumstances.





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