Power Steering Basics
Power steeringmakes it easier for a driver to turn the wheels of a car or truck. This is especially important when maneuvering at low speeds and without it many vehicles would be impossible to drive.
Although electric power steering systems are starting to emerge, at present the overwhelming majority of power steering systems are hydraulic. Details of steering systems vary from vehicle to vehicle, but here’s a general description of how power steering works.
Each front wheel is mounted on a pivot that lets it turn left and right. A rod behind the wheels connects the two and makes sure they turn together. Along one side of this rod are a series of gear teeth known as a rack. A small gearwheel on the end of the steering column engages with this rack, so turning the steering wheel moves the rack to left or right and pushes the wheels one direction or the other. This design is known as “rack and pinion” steering.
An alternative design, termed “recirculating ball,” uses a worm gear on the end of the steering column to move a rod known as a Pitman Arm. This arm is linked to the rod connecting the front wheels, so again, turning the steering wheel pushes the front wheels in the desired direction.
So far this has been a description of a basic mechanical steering system; now let’s turn to the “power” aspect of power steering.
In a hydraulic system a cylinder looking rather like an old-fashioned bicycle pump is joined at one end to the rod linking the wheels. Inside the cylinder there’s a diaphragm or piston mounted on the end of a shaft. This shaft sticks out of the other end of the cylinder and is fixed to the frame of the vehicle. Pumping fluid into one end of the cylinder pushes the shaft out while pumping fluid into the other end makes it shorter. This way the power of a fluid system is used to help the driver turn the wheels.
But how does the system know whether to make the cylinder longer or shorter?
The answer to that lies in monitoring the difference in pressure between each side of the piston. When the driver starts to turn the wheel the cylinder lengthens or shortens and fluid is moved. A sophisticated valve senses the direction of movement and opens to let more fluid flow in that direction, so pushing the cylinder and the wheels in the direction the driver is turning. When the driver stops turning the wheel the pressure is equalized in the cylinder and the fluid flow is shut off.
Most drivers want lots of power assistance at low speeds, but little when on the highway. Too much power assistance at speed tends to result in zig-zag progress, so many systems include a speed-sensitive valve. This opens up at low speeds to allow plenty of fluid to flow, but closes when the car is moving quicker to reduce the level of assistance.
A key component of the power steering system is the pump. This pushes the power steering fluid through the hydraulic hoses and is driven by a belt from the engine which is why it’s hard to turn the wheels when the engine isn’t running. Unfortunately though, belt drive means the pump is running all the time the engine is on, whether or not it’s needed, which wastes some of its power.
To address this problem vehicle manufacturers are turning to electric systems. These replace the fluid, cylinder, valves and pump with an electric motor that provides additional torque as the driver turns the wheel. At some point in the near future this may result in the steering being disconnected from the wheels, all the turning motion being provided by the motor.
Without power steering the modern vehicle would be almost impossible to drive. For many years power assistance has been delivered by a complex hydraulic system using a pump and cylinder to supplement the driver’s effort. In the future though, an electric motor will do the job.
