Modern Automotive Technology PowerPoint for by Russell Krick Publisher The Goodheart-Willcox Co., Inc. Tinley Park, Illinois

Chapter 69 Steering System Fundamentals

Contents Functions of a steering system Basic steering systems Steering column assembly Steering gear principles Steering linkage (worm-type gearbox) Manual rack-and-pinion steering (Continued)

Contents Integral-piston power steering system (linkage type) External cylinder power steering (linkage type) Power rack-and-pinion steering Electronic steering assist Four-wheel steering systems

Functions of a Steering System Provide precise control of the wheel direction Maintain correct steering effort Transmit road feel to the driver’s hands Absorb most of the road shock going to the steering wheel Allow for suspension action

Basic Steering Systems Two basic kinds of steering systems are in use today: linkage (worm gear) steering systems rack-and-pinion steering systems Each may be operated manually or with power assist

Steering Basic linkage steering Basic rack-and-pinion steering

Steering Column Assembly Consists of the steering wheel, steering shaft, column, ignition key mechanism, and sometimes, a flexible coupling and universal joint Bolts to the underside of the dash Sticks through the firewall and fastens to the steering gear

Steering Column Components

Locking Steering Wheel When the ignition is off, the steering wheel cannot be turned A rack and sector slide a steel pin into mesh with a slotted disc that is splined to the steering shaft The pin locks the steering shaft to the column, preventing the steering wheel from being turned

Locking Steering Wheel

Collapsible Steering Column Helps prevent driver chest and face injury during an auto accident The column will crumple or slide together when forced forward during a collision Several types are used: steel mesh (crushing) tube-and-ball (sliding) shear capsule (break and slide)

Collapsible Column Operation

Tilt Steering Columns A flex joint, or U-joint, allows the top half of the column and the steering wheel to be positioned at different angles Manual tilt column uses a lever on the steering column to unlock the flex joint so the wheel can be moved up and down

Power Tilt Column Uses a small electric motor, a control switch, and a gear mechanism to change the steering wheel angle or height When the tilt switch is activated, current to the motor spins the small gears to move the steering column’s upper tube

Memory Tilt Wheels Use a tilt control module to “remember” more than one steering wheel position A steering column sensor provides feedback so the module knows where the wheel is located When the driver selects a switch position, the control module energizes the power tilt motor until the wheel has moved to the preprogrammed position

Memory Tilt Wheel

Power Tilt Gear Mechanism

Steering Gear Principles Two basic types of gear mechanisms are found in steering gearboxes: worm gears rack-and-pinion gears

Gear Mechanisms

Steering Gears Rack-and-pinion gear Worm steering gear

Recirculating-Ball Gearbox Normally used with a linkage steering system Small steel balls circulate between the gear members to reduce friction

Recirculating-Ball Gearbox

Gearbox Components Worm shaft Sector shaft Ball nut the input gear connected to the steering shaft Sector shaft the output gear from the steering gearbox Ball nut rides on the ball bearings and the worm gear

Gearbox Components Ball guides Adjusting nut Adjusting screw route extra balls in and out from between the worm and ball nut Adjusting nut used to set worm shaft bearing preload Adjusting screw used to set the sector shaft clearance

Gearbox Components

Gearbox Ratio Comparison between steering wheel rotation and sector shaft rotation Ratios range from 15:1 to 24:1 Variable-ratio gearbox changes the internal gear ratio as the wheels are turned away from center Constant-ratio gearbox has the same gear reduction from full left to full right

Worm-and-Roller Steering Gearbox Contains a roller that is mounted on the pinion shaft and meshes with the worm gear The roller replaces the ball bearings and the ball nut used in the recirculating-ball gearbox

Steering Linkage (Worm-Type Gearbox) Series of arms, rods, and ball sockets that connect the steering gearbox to the steering knuckles Commonly called a parallelogram steering linkage

Steering Linkage

Steering Linkage Pitman arm Center link (relay rod) transfers gearbox motion to the steering linkage Center link (relay rod) a steel bar that connects the right and left sides of the steering linkage

Steering Linkage Idler arm Tie-rod assemblies supports the end of the center link on the passenger side Tie-rod assemblies fasten the center link to the steering knuckles ball sockets are used on both ends a toe adjustment sleeve allows length change for alignment

Ball Sockets Provide for motion in all directions between two connected parts Some have a grease fitting, allowing lubrication Others are sealed units Used in pitman arms, idler arms, and tie-rods

Ball Sockets A. Idler arm B. Tie-rod end C. Tie-rod inner end D. Tie-rod (rack- and-pinion)

Manual Rack-and-Pinion Steering

Rack-and-Pinion Steering

Steering Gear Consists of a pinion shaft, rack, thrust spring, bearings, seals, and a gear housing Bolts to the frame or unibody structure Rubber bushing mounts help absorb road shock

Steering Gear

Steering Gear When the pinion shaft turns, the pinion gear acts on the rack gear The rack slides sideways, moving the tie-rods and the front wheels

Tie-Rod Assemblies Connect the ends of the rack with the steering knuckles Rubber dust boots fit over the inner ball sockets to keep out road dirt and water

Power Steering Systems Most systems use an engine-driven pump and a hydraulic system to assist steering action An electric motor in the rack may be used to provide power assist

Basic Power Steering

Basic Operation Pressure from the oil pump is used to operate a piston-and-cylinder assembly When the control valve routes oil pressure into one end of the piston, the piston slides in its cylinder Piston movement helps move steering system components

Types of Power Steering There are three main types of power steering systems: integral-piston linkage system rack-and-pinion system external cylinder power steering system

Integral-Piston Linkage

Rack-and-Pinion

External Cylinder

Power Rack-and-Pinion

Power Steering Pump Engine-driven pump that produces the hydraulic pressure for steering system operation Driven by a belt or directly off the back of the camshaft The power steering fluid reservoir may be formed as part of the body or as a separate container

Pump and System

Pump Types

Slipper-Type Pump

Vane-Type Pump Operation

Pressure-Relief Valve Used in a power steering system to control the maximum oil pressure Prevents system damage by limiting pressure A relief valve opens when the steering wheel is turned to the full-left or full-right position

Pressure-Relief Valve

Power Steering Hoses High-pressure, hydraulic, rubber hoses that connect the power steering pump and the gearbox or power cylinder One line serves as the pressure feed line One line serves as a return line to the reservoir Metal lines may be used where vibration or movement is not a problem

Integral-Piston Power Steering System (Linkage Type) The hydraulic piston is mounted in the steering gearbox housing System components: power steering pump hydraulic lines integral power-assist gearbox

Integral Power Steering Gearbox Contains a conventional worm-and-sector gear, a hydraulic piston, and a flow-direction valve Two valve designs are commonly used: spool valve rotary valve

Spool Valve-Type Gear

Spool Valve Operation When the steering wheel is turned, the pivot lever moves the spool valve so pressure enters a power chamber Pressure forces the power piston left or right, helping to turn the sector shaft As the power piston strokes, oil from the other power chamber is vented through the spool valve back to the reservoir

Rotary Valve Operation Rotary valve-type steering boxes have a small torsion bar to detect steering wheel turning direction and turning effort When the steering wheel is turned, the torsion bar twists and turns the rotary valve Rotary valve directs pressure to the correct side of the power piston

Integral Gear Operation Driving straight ahead

Integral Gear Operation Right turn

Integral Gear Operation Left turn

External Cylinder Power Steering (Linkage Type) The power cylinder is commonly bolted to the frame and the center link The control valve may be located in the gearbox or on the steering linkage

External Cylinder Power Steering

Power Rack-and-Pinion Steering Uses hydraulic pressure to assist the driver in moving the rack and the front wheels

Power Rack-and-Pinion Steering

Power Rack-and-Pinion Steering

Power Cylinder and Piston precisely machined tube that accepts the power piston Power piston formed by attaching a hydraulic piston to the center of the rack Routing oil pressure into either end of the power cylinder causes piston movement

Power Rack-and-Pinion (Rotary Control Valve)

Control Valves Rotary control valve Spool control valve operated by a torsion shaft connected to the pinion gear Spool control valve operated by the thrust action of the pinion shaft that occurs when the pinion gear is rotated against the rack gear

Power Rack-and-Pinion (Spool Control Valve)

System Operation When the steering wheel is turned, vehicle weight causes the front tires to resist turning, moving the control valve Movement of the control valve aligns specific oil passages internally Pump pressure forces oil through the control valve to the power cylinder Pressure acts on the power piston, pushing the rack and the front wheels

Little valve movement, low power assist High Speed Operation Little valve movement, low power assist

More valve movement, high power assist Low Speed Operation More valve movement, high power assist

Electronic Steering Assist Uses a small electric motor to help move the rack-and-pinion gearbox The motor is mounted inside the rack housing and acts on the steering rack

Electronic Assist Rack-and-Pinion Lighter and more compact than a hydraulic system

Electronic Assist Operation A steering control module (computer) electronically reacts to steering pressure The module operates the electric motor in the rack assembly to help the driver steer the wheels The module can reverse motor rotation and alter motor speed as needed

Proportional Power Steering Senses vehicle speed and steering load to ensure adequate road feel Increases steering effort at higher speeds for more road feel Lowers steering effort at lower speeds to ease maneuvering

Low Speed Operation A computer-controlled electromagnet is used to alter the operation of the stub shaft, torsion bar, and rotary control valve At low speeds, the computer sends current through the electromagnet in one direction to help attract and pull on the stub shaft and torsion bar, increasing valve output pressure

High Speed Operation At high speeds, the computer reverses electrical flow through the electromagnet The valve output pressure decreases, reducing assist, and increasing steering effort for better road feel

Proportional Steering In this system, a computer, a vehicle speed sensor, and a solenoid control hydraulic pressure

Four-Wheel Steering Systems In a four-wheel steering system, all four wheels change direction to improve handling, stability, feel, and maneuverability

Four-Wheel Steering System

In a gentle turn, the wheels pivot in the same direction Four-Wheel Steering In a gentle turn, the wheels pivot in the same direction

Four-Wheel Steering In a sharp turn, the wheels pivot back and then turn in opposite directions

Mechanical System Uses a special front rack-and-pinion gearbox with a transfer box The transfer box operates a long shaft that extends back to the rear rack When the front wheels are turned, the shaft rotates to turn the rear wheels

Hydraulic System Uses a conventional power rack-and-pinion steering system up front Hydraulic lines extend back to a rear power steering pump, which is driven by the differential Depending on vehicle speed, the rear pump forces fluid under pressure into a control valve

Hydraulic System When a specific road speed is reached, the control valve can then operate the rear steering system The rear rack is connected by tie-rods to the rear suspension trailing arms When activated, the rear rack shifts the trailing arms to steer the rear wheels

Electronic System An electronic system is speed-sensitive An electric-motor-driven power rack (rear actuator) acts on the rear wheels using a recirculating-ball drive and mechanical links The rear-wheel steering angles are computer controlled

Electronic System

Electronic System Operation The computer analyzes signals from angle sensors in the front steering and signals from wheel speed sensors in the anti-lock brake system The rear actuator is energized by the computer to move the rear wheels as needed

Rear Actuator