The dimensions and weight of cars of any class only increase over time. That’s why the use of rack-and-pinion steering gear is often insufficient for ensuring comfortable driving. The only correct engineering solution was the introduction of a booster in the steering system.
Nowadays, there are two types of power steering:
The system with hydraulic power steering consists of the following elements:
The pump creates an excess pressure of the working fluid in the system. When the steering wheel rotates, the control valve shifts and opens the channel for the working fluid under pressure into the servo unit. The servo unit contains a piston, which is connected through the rod to the steering trapezoid. Under the pressure of the working fluid, the piston moves, creating additional force on the steering wheel, thereby reducing the force required of the driver to rotate the steering wheel.
Depending on the direction in which the steering wheel rotates, the working fluid under pressure is fed into the cavity above or below the piston.
When the steering wheel is located in the center, the pump pumps the working fluid without any load, virtually idle. As soon as the steering wheel starts turning, the fluid pressure inside the system increases and reaches a maximum at the extreme (right or left) position of the steering wheel.
Figure 8.8 Functional diagram of the hydraulic power steering system.
Figure 8.9 Steering gear with hydraulic power steering..
Note
In a vehicle with a hydraulic power steering, it is forbidden to hold the steering wheel in one of the extreme positions for a long time, because this may damage the booster pump.
The pump can be driven by a belt from the engine or from a separate electric motor.
Hydraulic drive of power steering is rather "picky": it is necessary to regularly monitor all connections in order to detect and eliminate leaks of the working fluid in time. In addition, there is a possibility of clogging of the working fluid. In case if it is necessary to remove the steering gear, there is a need to drain this fluid. And when filling the hydraulic drive with fresh liquid, it is necessary to perform operations for removing air from the system. Electromechanical power steering is quite another matter: there is no working liquid, branch pipes, pumping, and there is the electric motor and the control unit, which controls frequency of rotation of a steering wheel. The control unit also monitors the car's speed and steering angles. At certain speed values, it can send a signal to the electric motor in order to "clamp" or "release" the steering wheel.
Note
The terms "clamp" and "release" in relation to the steering wheel were invented by journalists. They m6дюБean an increase or decrease in the effort, which is required to turn the steering wheel, respectively.
There are several methods of installation of the electric motor of the booster:
Numerous tests of cars with the electromechanical power steering have proven that installation of the electric motor on the steering gear is the optimum method, because it helps achieving the best feedback from a steering.
Figure 8.10 Example of steering with electromechanical booster on the steering column.
Note
Feedback from the steering means connection with the road. After all, when we turn the steering wheel, the wheels turn, thus affecting the road. And while driving a lot of forces act on the wheel that are associated with its rolling. Thus, happens the transfer of all these forces from the road to the wheel and then to the steering wheel and is the feedback. This feedback is expressed mainly in a proportionally increasing force on the steering wheel, which depends on the steering angle and speed of the car. The term is most often used for cars with power steering.
In case if there is no feedback on the car with the power steering, then the force on the steering wheel does not change depending on the steering angle and the speed of the turn. When the car is stationary or moves slowly, the ease of turning the steering wheel is only for the better. However, when maneuvering at speed, the lack of proportionally increasing force on the steering wheel can play a nasty joke, because objective and subjective feelings will make it difficult to choose the only correct position of the steering wheel.
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