Steering columns

Steering columns, Rack-and-pinion gearbox, Helix, Variable ratio steering, Worm gearbox, Power steering, steering process, Flow-control valve, Electric power assisted steering, Basic electric power steering operation.

Steering columns

All power steering pumps have a flow-control valve to vary fluid flow and power steering system pressures. A pressure relief valve prevents excessive pressures developing when the steering is on full-lock, and held against its stops. The flow control valve is located at the outlet fitting of the pump.
During slow cornering, or when parking, pump speeds are normally low. There is less demand for fluid flow, but to provide the required assistance, high pressure is needed. Discharge ports direct the fluid to the outlet, and then to the steering gear. The outlet fluid pressure is slightly lower than the internal high pressure coming from the pump.

This drop in pressure occurs as the fluid flow passes the needle and orifice in the outlet fitting. This lower pressure is transmitted through a by-pass fluid passage to the spring end of the control valve. The pressure difference on the valve causes it to move away from the outlet fitting but the force of the spring prevents it moving far enough to uncover a return port, back to the pump inlet. Movement of the control valve controls the position of the needle valve in the outlet fitting. And this controls the fluid flow to the steering gear.
At higher speeds, with no steering manoeuvres, fluid flow is increased. This reduces pressure at the outlet. The lower pressure is transmitted to the spring end of the control valve. The valve moves, and opens the return port back to the pump inlet.
Movement of the control valve also controls the movement of the flow control needle in the outlet fitting. The needle closes in the orifice, and fluid flow to the steering gear reduces.
With the steering wheel held at full-lock, the steering rack power piston chamber becomes fully pressurised, and fluid flow stops.
This high pressure is transmitted back to the spring end of the control valve, opening the pressure relief valve. A small amount of fluid passes through the pressure relief orifice, providing a pressure drop. The valve moves, and uncovers the return port to the pump inlet. A pre-determined relief pressure is thus maintained.
The pump is normally a vane-type, with sufficient capacity for all operating conditions.

 Rack-and-pinion gearbox

The rack-and-pinion steering gear box has a pinion, connected to the steering column. This pinion runs in mesh with a rack that is connected to the steering tie rods. This gives more direct operation.
Both the pinion and the rack teeth are helical gears. Helical gearing gives smoother and quieter operation for the driver.
Turning the steering wheel rotates the pinion, and moves the rack from side to side. Ball joints at the end of the rack locate the tie-rods and allow movement in the steering and suspension.
Mechanical advantage is gained by the reduction ratio. The value of this ratio depends on the size of the pinion.
A small pinion gives light steering, but it requires many turns of the steering wheel to travel from lock, to lock.
A large pinion means the number of turns of the steering column is reduced, but the steering is heavier to turn.
Ratios vary, depending on the type of vehicle.
But in each case, the ratio is the same for all positions of the wheels. It is a fixed ratio.

  
Helix

If an inclined plane is wrapped around a cylinder, the edge of the plane forms a shape called a helix.
Rotation of the cylinder causes a point on the helix to move, along the surface of the cylinder. The distance the point moves in one revolution of the cylinder is called the pitch.
The helix shape is commonly used as a thread on nuts and bolts, and also for teeth in steering gears, and transmissions.

Variable ratio steering

A disadvantage of a fixed-ratio system is that towards the lock positions, more effort is needed by the driver.
This is because the angle of the steering arms reduces their effective length, and that reduces the leverage on the wheels.
To overcome this, many rack-and-pinion systems use variable ratio steering. The ratio is made variable by changing the shape of the teeth on the rack, between the centre and the outer edges of the rack.
Then, as the steering moves away from the straight-ahead position, the ratio, and therefore, the mechanical advantage, increases progressively.
As the pinion turns, and moves on the rack, the gear contact point between the pinion, and the teeth on the rack, changes. This change in tooth contact changes the effective diameter of the pinion.
Then, for the same amount of steering wheel rotation, the rack moves a shorter distance near the ends of the rack than near the centre. Effort needed to turn the wheels stays approximately the same through the whole range of movement.

Worm gearbox

A “worm” gear has teeth cut in the shape of a helix.
The steering box is a gearbox. It converts the rotary motion of the steering wheel to the linear motion needed to control the wheels. Its gear ratio increases output torque, and reduces the effort the driver has to apply.
The input, attached to the steering column, is called a worm shaft. It is meshed with a sector, or portion, of a gear, mounted on its own shaft, at right angles to the worm. The outer end of the sector shaft has a tapered spline which mates with an internal spline on the Pitman or drop arm. As the steering wheel rotates, the worm-shaft causes the sector to move through an arc, and transfer the motion, through the Pitman arm, to the steering linkage.
Variations to this principle include the worm-and-roller, and the worm-and-nut.
The recirculating ball steering box is a popular development of the worm-and-nut, and worm-and-sector principle.
Both ends of the worm shaft are supported in the housing by angular bearings, which are pre-loaded to reduce end-float, and side-thrust movements of the worm, when it is under load.
A ball nut rides on the worm, supported on the spiral grooves of the worm, and the inside of the nut, by many balls. The balls form a low-friction internal thread, which causes the nut to move up or down on the worm as it rotates.
With rotation, the balls are rolled along the grooves, partly in the worm and partly in the nut, and they circulate by passing through ball-return-guides at each end of the nut.
External teeth on one side of the nut mesh with the teeth of the sector gear formed on the sector shaft, or Pitman shaft, and this transfers the motion through the Pitman arm to the steering linkage.
The sector gear and nut teeth are designed so that when the teeth are in the straight-ahead position, they have minimum clearance. This reduces free play in that position.
The Pitman shaft is supported by 2 caged, needle roller-bearings, in the steering box housing.
The sector teeth are angled, and an adjustment screw on the steering housing cover provides proper engagement of the sector gear and nut teeth.
The worm shaft in a worm-and-roller steering box is supported in a similar manner to the recirculating-ball type, but the worm has an hour-glass shape, and it meshes with a double track roller, mounted on bearings, on a pin attached to the Pitman shaft.
As the worm rotates, the roller moves in an arc, following the hour-glass shape, and transferring the motion to the Pitman shaft. The hour-glass shape changes the steering ratio slightly as the steering wheel is turned from the central position towards each lock.

Power steering

Increased applications of front-wheel-drive, and wider low-profile tyres, places additional loads on front wheels. Steering then demands more effort from the driver.
Power steering helps to reduce the additional effort needed. It’s of most benefit during slow cornering, and when parking.
Assistance is provided as soon as the steering wheel is rotated in either direction, and it’s designed so that even if system failure occurs, the vehicle can still be steered.
An engine-driven hydraulic pump delivers hydraulic fluid to the power unit at the steering box, or rack-and-pinion, through connecting hoses and pipes. The fluid reservoir can be mounted on the pump, or it can be separate.
With the engine running, fluid flows continuously from the power steering pump, to the steering gear, and back to the pump.
With the steering wheel in the neutral position, little pressure is needed to maintain fluid flow, and little engine power is needed to operate the system.

Steering process

When the steering is turned, a rotary valve, integral to the steering input shaft, directs fluid to one side or the other, of a piston attached to the steering gear. Pressure then increases as required, to provide assistance.
In a worm-and-roller steering box, the piston slides in a cylinder in the casing. It has an extension formed on one side, with teeth which engage teeth on the Pitman shaft. Pressure applied to either side of the piston produces a force, which is transferred through the teeth, to help turn the Pitman shaft.
In a rack-and-pinion steering gear, The piston is formed centrally on the steering rack, and the rack housing provides the working cylinder. Seals at each end of the cylinder isolate the power section from the rack, and the helical pinion.

Seals in the rotary valve section at the pinion input, prevent fluid leakage there.
Connecting pipes transfer fluid from the rotary valve housing, to one side of the piston, or the other, to provide assistance, which acts directly on the rack.
The rotary valve is located between the steering gear input shaft, and the pinion gear. It consists of an inner member, which forms part of the input shaft, and a surrounding sleeve member, fixed to the pinion gear.
Turning the steering wheel makes both members rotate in the steering gear housing, but it is the slight, relative, rotary displacement of the inner member and the sleeve member which controls, and directs, the power steering fluid flow.
This slight rotary displacement is allowed by a torsion bar, which is connected to the pinion gear at its bottom end, and the input shaft at its top end.
When the steering wheel is turned, there is resistance from the front wheels at the road surface. This resistance is transmitted through the rack, to the pinion gear, so that the input shaft twists slightly on the torsion bar.
Since the inner member is also attached to the input shaft, this twisting provides a relative, rotary displacement of the inner and outer members. It is this displacement that lets fluid flow through the valve to act on the piston at the steering gear.
The input shaft can twist through only a small angle, before it contacts a stop on the pinion gear. This is needed to provide manual steering when power assistance is not available.
With the engine running, and the steering in the neutral position, fluid flow is directed into the valve assembly, through drilled holes in the outer sleeve.
As soon as the steering is turned to the left or right, the slight relative movement occurs between the inner and outer members.
In the neutral position, the inner member lets fluid pass equally to both sides of the rack piston, and return to the fluid reservoir. Equal pressure is applied to both sides of the rack piston. No power assistance is needed.
When the steering is turned fluid is restricted from making a free return to the reservoir. It is now directed to the side that matches the turning action. At the same time, fluid on the opposite side is directed to the return circuit, back to the reservoir.
Slight rotation of the valve gives a small amount of assistance, which become progressively greater as the torsion bar flexes, and more assistance is needed. The grooves of the inner member are precisely shaped to meter the flow of fluid.

Flow-control valve

All power steering pumps have a flow-control valve to vary fluid flow and power steering system pressures. A pressure relief valve prevents excessive pressures developing when the steering is on full-lock, and held against its stops. The flow control valve is located at the outlet fitting of the pump.
During slow cornering, or when parking, pump speeds are normally low. There is less demand for fluid flow, but to provide the required assistance, high pressure is needed. Discharge ports direct the fluid to the outlet, and then to the steering gear. The outlet fluid pressure is slightly lower than the internal high pressure coming from the pump.
This drop in pressure occurs as the fluid flow passes the needle and orifice in the outlet fitting. This lower pressure is transmitted through a by-pass fluid passage to the spring end of the control valve. The pressure difference on the valve causes it to move away from the outlet fitting but the force of the spring prevents it moving far enough to uncover a return port, back to the pump inlet. Movement of the control valve controls the position of the needle valve in the outlet fitting. And this controls the fluid flow to the steering gear.
At higher speeds, with no steering manoeuvres, fluid flow is increased. This reduces pressure at the outlet. The lower pressure is transmitted to the spring end of the control valve. The valve moves, and opens the return port back to the pump inlet.
Movement of the control valve also controls the movement of the flow control needle in the outlet fitting. The needle closes in the orifice, and fluid flow to the steering gear reduces.
With the steering wheel held at full-lock, the steering rack power piston chamber becomes fully pressurised, and fluid flow stops.
This high pressure is transmitted back to the spring end of the control valve, opening the pressure relief valve. A small amount of fluid passes through the pressure relief orifice, providing a pressure drop. The valve moves, and uncovers the return port to the pump inlet. A pre-determined relief pressure is thus maintained.
The pump is normally a vane-type, with sufficient capacity for all operating conditions.

Electric power assisted steering

The use of electronics into automotive steering systems enables much more sophisticated control to be achieved.
Electric steering is more economical to run, and easier to package and install than conventional hydraulic power steering systems.
Typically, electric and electro-hydraulic power steering systems are also lighter and more compact than conventional hydraulic systems.
Both the electric power steering system and the hydraulic power steering system with a motor-driven pump are now considered as viable alternatives to conventional hydraulic power steering systems because of their energy efficiency and size.
Electrically Powered Hydraulic Steering, or EPHS, replaces the customary drive belts and pulleys with a brushless motor that drives a high efficiency hydraulic power steering pump in a conventional rack and pinion steering system. Pump speed is regulated by an electric controller to vary pump pressure and flow. This provides steering efforts tailored for different driving situations. The pump can be run at low speed or shut off to provide energy savings during straight ahead driving.
An EPHS system is able to deliver an 80 percent improvement in fuel economy when compared to standard hydraulic steering systems.
Electrically assisted steering or EAS, is a power-assist system that eliminates the connection between the engine and steering system. EAS or direct electric power steering takes the technology a step further by completely eliminating hydraulic fluid and the accompanying hardware from the system, becoming a full “electronic power steering system” or EPS.
An EPS Direct electric steering system uses an electric motor attached to the steering rack via a gear mechanism and torque sensor. A microprocessor or electronic control unit, and diagnostic software controls steering dynamics and driver effort. Inputs include vehicle speed and steering, wheel torque, angular position and turning rate.
There are four primary types of electric power assist steering systems:

• Column-assist type. In this system the power assist unit, controller and torque sensor are attached to the steering column.
• Pinion-assist type. In this system the power assist unit is attached to the steering gear pinion shaft. The unit sits outside the vehicle passenger compartment, allowing assist torque to be increased greatly without raising interior compartment noise.
• Rack-assist type. In this system the power assist unit is attached to the steering gear rack. It is located on the rack to allow for greater flexibility in the layout design.
• Direct-drive type. In this system the steering gear rack and power assist unit form a single unit. The steering system is compact and fits easily into the engine compartment layout. The direct assistance to the rack enables low friction and inertia, which in turn gives an ideal steering feel.

In these systems “Active control” as it is known provides constant feedback from sensors in the vehicle to the control unit, which calculates sophisticated computer algorithms. This allows the steering system to react to the road, the weather and even the type of driver, and provide assistance to the front or rear road wheels independent of direct driver input.
Active steering produces enhanced steering response, stability & handing improvements to the vehicle without impacting the base steering feel.

Basic electric power steering operation

A steering sensor is located on the input shaft where it is bolted to the gearbox housing.
The sensor performs two different functions: Firstly as a torque sensor, it converts steering torque input and direction into voltage signals for the ECU to monitor and convert into a binary code, and secondly as a rotation sensor, which converts the rotation speed and direction into voltage signals for the ECU to monitor and convert into a binary code.
An interfaced ECU circuit that shares the same housing converts the signals from the torque and rotation sensors into signals that the ECU can process and provide an active output.
The microprocessor control unit analyzes inputs from the steering sensor as well as the vehicle’s speed sensor. The sensor inputs are then compared to determine how much power assist is required according to the ‘forces capability map data’ stored in the ECU’s memory. This map data is pre-programmed by the manufacturer.
The ECU then emits the appropriate command to the ‘power unit or current controller’, which supplies the electric motor with the necessary current to activate. The motor then pushes the rack either to the right or left. Direction of rack movement is dependant on which way the voltage flows; reversing the current flow reverses directional rotation of the motor. Increasing current to the motor increases the amount of power assist.
The electric power assistance system has three operating modes:

• In normal control mode left or right power assist is provided in response to input from the torque and rotation sensor’s inputs.
• The return control mode is used to assist steering return after completing a turn.
• The damper control mode changes the vehicle speed to improve road feel and dampen kickback.

If the steering wheel is turned and held in the full-lock position and steering assist reaches maximum, the control unit reduces current to the electric motor to prevent an overload situation that might damage the motor. The control unit is also designed to protect the motor against voltage surges from a faulty alternator or charging problem.
The electronic steering control unit is capable of self-diagnosing faults by monitoring the system’s inputs, outputs, and the driving current of the electric motor. If a problem occurs, the control unit turns the system off by actuating a fail-safe relay in the power unit.
This eliminates all power assist, causing the system to revert back to manual steering. An in-dash EPS warning light is also illuminated to alert the driver.