Electric power steering bearings
SKF has also been a key actor in the recent history of the active steering (a type of power electric variable gear ratio power steering technology, which varies the degree that the wheels turn in response to the steering wheel). At lower speeds, this technology reduces the amount that the steering wheel must be turned, improving performance in situations such as parking and other urban area traffic manoeuvres. At higher speeds, the performance is such that the normal increased responsiveness from speed is avoided and it provides improved directional stability.
In a parking situation, the computer varies the ratio so that the steering wheel needs less than two turns to move the wheels lock to lock. As vehicle speeds increase, the steering ratio increases, so it takes larger movements of the steering wheel to move the wheels and that lessens the usually increasing vehicle response resulting from increased speed. Besides providing variable steering ratios, the computer is linked with the vehicle stability control system to help in directional stability of the vehicle.
If the driver experiences a skid or slide because of poor road conditions, the active steering will react to information from the yaw rate sensors to modify the steering angle of the front wheels to stabilize the vehicle. This occurs much faster than the driver can react. If the active steering angle is not enough, then the stability control system intervenes to aid as well.
Safety is one of the prime objectives of this system. Steering angle sensors on the steering column sense the direction the driver wants to go and the system only intervenes if the car is beyond stable limits. If an error or problem occurs in the electronics, the computer shuts down the operation of the electric motor, locking the ring gear of the planetary gear set and making it fixed ratio steering. Finally, if there is a problem inside the planetary gear unit, there is a second shaft that runs all the way through from the steering wheel shaft to the steering rack so that conventional steering is available.
The next generation steering functions will be even more complex and will include the so-called steering by wire technology, which is an area where SKF has already made many steps forward and which is an interesting challenge for the engineering department who is at work to literally extend any technology and know-how boundaries which are related to the steering applications.
To have a straightforward and clearer picture of the history of main steering functions you can refer to the graph at page 3. Most power steering systems in cars and light trucks today are still hydraulic (that is, the force to turn the wheels is provided by a hydraulic piston, which is powered by high pressure hydraulic fluid), but in some cars and trucks, the steering force is provided by an electric motor and this is also one of the steps which represents the evolution of steering applications and functionalities.
The electric power assisted steering (EPS or EPAS) is actually designed to use an electric motor to reduce effort by providing steering assist to the driver of a vehicle. EPS system has been developed to give assistant to the driver and make easy to steer the wheel. This technology has been developed to reduce fuel consumption and C02 emissions compare to hydraulic power assisted technology.
There are 3 main types and active steering system depending on equipment installation and power to transmit:
• Column EPAS (light vehicle)
• Pinion EPAS (medium segment vehicle)
• Rack EPAS (medium to high segment vehicle)
• Active front steering systems permit to adapt steering wheel angle and front wheel angle to vehicle speed.
SKF has developed a range of product for all EPS types and active steering systems:
• Four point contact ball bearing
• Low noise deep groove ball bearing
• Flexible bearing