Light duty automobiles and trucks have had power steering since the early fifties. These systems are more properly described as power assisted steering. The method of assistance has been generally hydraulic but as in all systems nowadays electricity is becoming the assistant of choice.
The steering system is called power assisted because the steering will still operate when the assisting method fails. Most of us who have been driving for awhile have experienced failed power assist. The steering still works but “Popeye” size forearms are required.
Why do steering systems need power assist? They don’t, but it sure helps. In order to make a vehicle turn the right amount with limited degrees of steering wheel rotation the mechanical advantage or steering gear ratio cannot be too high. A high ratio means slow steering but low effort. A low ratio means fast steering but high effort.
Modern automobiles have low ratio power assisted steering systems. They usually require less than three turns of the steering wheel to turn from all the way left to right. In order to make that low ratio comfortable for the driver power assist is required.
The hydraulic system that has typically been the assist method of choice incorporates an engine driven pump to pressurize fluid that is used to help rotate the steering gear. A hydraulic control system distributes the desired amount of assist.
The amount of steering assist required at a standstill is much higher than when the vehicle is in motion. The pump must be sized to produce the most pressure when the engine is turning the slowest.
When the vehicle is moving and the engine turning quicker less pump pressure is required. This pump uses energy and therefore extra fuel to run and it runs faster when it has less work to do.
The need for maximum assist at slow speed and less at high speed has ushered in the electric motor for power assist.
Using an electric motor to provide assist only when it is needed has become the most efficient method to provide power assist. This system can work effectively on hybrid vehicles that do not have a running motor to drive a hydraulic pump when the vehicle is stopped.
The other benefit of electric assist is in packaging the system. Hydraulic systems require the pump to be located on the engine and hoses that run to the steering gear. Electric assist systems have only the electric motor mounted on the steering gear.
A steering system is judged on the feel it gives to the driver. Early electric assist systems gave a very detached feel to the driver. That type of steering is described as feeling numb. Good steering gives the driver a sense of feel that the steering wheel is connected to the rolling wheels. There is some feedback to the driver of what kind of load is being transferred through the steering wheel.
The benefits of electric power steering have driven engineers to succeed at bringing the desired feel that drivers desire. Soon electric assist will be the steering standard.
Trail’s Ron Nutini is a licensed automotive technician and graduate of mechanical engineering from UBC.
He will write every other Thursday. E-mail: firstname.lastname@example.org