The steering system allows the operator to guide the vehicle along the road and turn left or right as desired. The system includes the steering wheel, which the operator controls, the steering mechanism, which changes the rotary motion of the steering wheel into straight-line motion, and the steering linkage. At first most systems were manual then power steering became popular. It is now installed in most vehicles manufactured today. The steering system must perform several important functions:
Steering linkage is a series of arms, rods, and ball sockets that connect the steering mechanism to the steering knuckles. The steering linkage used with most manual and power steering mechanisms typically includes a pitman arm, center link, idler arm, and two tie-rod assemblies. This configuration of linkage is known as parallelogram steering linkage and is used on many passenger vehicles (Figure 12-19).
The pitman arm transfers steering mechanism motion to the steering linkage (Figure 12-19). The pitman arm is splined to the steering mechanism’s output shaft (pitman arm shaft). A large nut and lock washer secure the pitman arm to the output shaft. The outer end of the pitman arm normally uses a ball-and-socket joint to connect to the center link.
The parallelogram steering linkage uses a center link, otherwise known as an
intermediate rod, track rod, or relay rod, which is simply a steel bar that
connects the steering arms (pitman arm, tie-rod ends, and idler arm) together
(Figure 12-19). The turning action of the steering mechanism is transmitted to
the center link through the pitman arm.
Figure 12-19 — Steering linkage.
The center link is hinged on the opposite end of the pitman arm by means of an idler arm (Figure 12-19). The idler arm supports the free end of the center link and allows it to move left and right with ease. The idler arm bolts to the frame or subframe.
Ball sockets are like small ball joints; they provide for motion in all directions between two connected components (Figure 12-19). Ball sockets are needed so the steering linkage is NOT damaged or bent when the wheels turn or move up and down over rough roads. Ball sockets are filled with grease to reduce friction and wear. Some have a grease fitting that allows chassis grease to be inserted with a grease gun. Others are sealed by the manufacturer and cannot be serviced.
Two tie-rod assemblies are used to fasten the center link to the steering knuckles (Figure 12-19). Ball sockets are used on both ends of the tie-rod assembly. An adjustment sleeve connects the inner and outer tie rods. These sleeves are tubular in design and threaded over the inner and outer tie rods. The adjusting sleeves provide a location for toe adjustment. Clamps and clamp bolts are used to secure the sleeve.
Some manufacturers require the clamps be placed in a certain position in relation to the tie rod top or front surface to prevent interference with other components.
The steering system most commonly used on four-wheel drive vehicles has a draglink (Figure 12-20). The draglink connects the pitman arm to the spindle at a point near the spindle; the tie rod will connect the two steer wheels together. The objective is to keep the tie rod as close to parallel with the axle as possible.
Figure 12-20 — Draglink assembly.
One purpose of the steering mechanism is to provide mechanical advantage. In a machine or mechanical device, it is the ratio of the output force to the input force applied to it. This means that a relatively small applied force can produce a much greater force at the other end of the device.
In the steering system, the operator applies a relatively small force to the steering wheel. This action results in a much larger steering force at the front wheels. For example, in one steering system, applying 10 pounds to the steering wheel can produce up to 270 pounds at the wheels. This increase in steering force is produced by the steering ratio.
The steering ratio is a number of degrees that the steering wheel must be turned to pivot the front wheels 1 degree. The higher the steering ratio (30:1, for example), the easier it is to steer the vehicle, all other things being equal. However, the higher the steering ratio, the more the steering wheel has to be turned to achieve steering. With a 30:1 steering ratio, the steering wheel must turn 30 degrees to pivot the front wheels 1 degree.
Actual steering ratio varies greatly, depending on the type of vehicle and type of operation. High steering ratios are often called slow steering because the steering wheel has to be turned many degrees to produce a small steering effect. Low steering ratios, called fast or quick steering, require much less steering wheel movement to produce the desired steering effect.
Steering ratio is determined by two factors: steering-linkage ratio and the gear ratio in the steering mechanism. The relative length of the pitman arm and the steering arm determines the steering linkage ratio. The steering arm is bolted to the steering spindle at one end and connected to the steering linkage at the other.
When the effective lengths of the pitman arm and the steering arm are equal, the linkage has a ratio of 1:1. If the pitman arm is shorter or longer than the steering arm, the ratio is less than or more than 1:1. For example, the pitman arm is about twice as long as the steering arm. This means that for every degree the pitman arm swings, the wheels will pivot about 2 degrees. Therefore, the steering linkage ratio is about 1:2.
Most of the steering ratio is developed in the steering mechanism. The ratio is due to the angle or pitch of the teeth on the worm gear to the angle or pitch on the sector gear. Steering ratio is also determined somewhat by the effective length and shape of the teeth on the sector gear.
In a rack-and-pinion steering system, the steering ratio is determined largely by the diameter of the pinion gear. The smaller the pinion, the higher the steering ratio.
However, there is a limit to how small the pinion can be made.
Manual steering is considered to be entirely adequate for smaller vehicles. It is tight, fast, and accurate in maintaining steering control. However, larger and heavier engines, greater front overhang on larger vehicles, and a trend toward wide tread tires have increased the steering effort required.
Steering mechanisms with higher gear ratios were tried, but dependable power steering systems were found to be the answer. There are several different types of manual steering systems; the worm and sector, worm and roller, cam and lever, worm and nut, and the rack and pinion.
In the worm and sector steering gear, the pitman arm shaft carries the sector gear that meshes with the worm gear on the steering gear shaft (Figure 12-21),. Only a sector of gear is used because it turns through an arc of approximately 70 degrees. The steering wheel turns the worm on the lower end of the steering gear shaft, which rotates the sector and the pitman arm through the use of the shaft. The worm is assembled between tapered roller bearings that take up the thrust and load. An adjusting nut or plug is provided for adjusting the end play of the worm gear.
Figure 12-21 — Worm and sector steering gear.
The worm and roller steering gear is quite similar to the worm and sector, except a roller is supported by ball or roller bearings within the sector mounted on the pitman arm shaft (Figure 12-22). These bearings assist in reducing sliding friction between the worm and sector. As the steering wheel turns the worm, the roller turns with it, forcing the sector and pitman arm shaft to rotate.
Figure 12-22 — Worm and roller steering gear.
The hourglass shape of the worm, which tapers from both ends to the center, affords better contact between the worm and roller in all positions. This design provides a variable steering ratio to permit faster and more efficient steering.
"Variable steering ratio" means that the ratio is larger at one position than another. Therefore the wheels are turned faster at certain positions than at others. At the center or straight-ahead position, the steering gear ratio is high, giving more steering control. However, as the wheels are turned, the ratio decreases so that the steering action is much more rapid. This design is very helpful for parking and maneuvering the vehicle.
The cam and lever steering gear, in which the worm is known as a cam and the sector as the lever, is shown in Figure 12-23. The lever carries two studs that are mounted in bearings and engage the cam. As the steering wheel is turned, the studs move up and down on the cam. This action causes the lever and pitman arm shaft to rotate.
Figure 12-23 — Cam and lever steering gear.
The lever moves more rapidly as it nears either end of the cam. This action is caused by the increased angle of the lever in relation to the cam. Like the worm and roller, this design allows for variable steering ratio.
The worm and nut steering gear is made in several different combinations. A nut is meshed with and screws up and down on the worm gear. The nut may operate the pitman arm directly through a lever or through a sector on the pitman arm shaft.
The recirculating ball is the most common type of worm and nut steering gear (Figure 12-24). In this steering gear, the nut, which is in the form of a sleeve block, is mounted on a continuous row of balls on the worm gear to reduce friction. Grooves are cut into the ball nut to match the shape of the worm gear. The ball nut is fitted with tubular ball guides to return the balls diagonally across the nut to recirculate them as the nut moves up and down on the worm gear. With this design, the nut is moved on the worm gear by rolling instead of sliding contact. Turning the worm gear moves the nut and forces the sector and pitman arm shaft to turn.
Figure 12-24 — Worm and nut steering gear.
The rack-and-pinion steering gear has become increasingly popular on smaller passenger vehicles. It is simpler, more direct acting, and may be straight mechanical or power-assisted.
The manual rack-and-pinion steering gear basically consists of a steering gear shaft, pinion gear, rack, thrust spring, bearings, seals, and gear housing. In the rack-and- pinion steering system the end of the steering gear shaft contains a pinion gear which meshes with a long rack (Figure 12-25). The rack is connected to the steering arms by tie rods, which are adjustable for maintaining proper toe angle. The thrust spring preloads the rack-and-pinion gear teeth to prevent excessive gear backlash. Thrust spring tension may be adjusted by using shims or an adjusting screw.
As the steering wheel is rotated, the pinion gear on the end of the steering shaft rotates. The pinion gear moves the rack from one side to the other. This action pushes or pulls on the tie rods, forcing the steering knuckles or wheel spindles to pivot on their ball joints. This turns the wheels to one side or the other so the vehicle can be steered.
Figure 12-25 — Rack and pinion steering gear.
Power steering systems normally use an engine-driven pump and hydraulic system to assist steering action. 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 cylinders. Piston movement can then be used to help move the steering system components and front wheels of the vehicles. The components that are common to all power steering systems are the power steering pump, the control valve, and power steering hoses.
The power steering pump is engine-driven and supplies hydraulic fluid under pressure to the other components in the system (Figure 12-26). There are four basic types of power steering pumps: vane, roller, slipper, and gear types. A belt running from the engine crankshaft pulley normally powers the pump. During pump operation, the drive belt turns the pump shaft and pumping elements. Oil is pulled into one side of the pump by vacuum. The oil is then trapped and squeezed into a smaller area inside the pump. This action pressurizes the oil at the output as it flows to the rest of the system. A pressure relief/flow valve is built into the pump to control maximum oil pressure. This action prevents system damage by limiting pressure developed throughout the different engine speeds.
Figure 12-26 — Power steering pump.
The control valve, a rotary or spool type valve which is actuated by steering wheel movements, is designed to direct the hydraulic fluid under pressure to the proper location in the steering system (Figure 12-27). The control valve may be mounted either in the steering mechanism or on the steering linkage, depending on which system configuration is used.
Figure 12-27 — Control valve.
Power steering hoses are high-pressure, hydraulic rubber hoses that connect the power steering pump and the integral gearbox or power cylinder. One line serves as a supply line, the other acts as a return line to the reservoir of the power steering pump. There are three major types of power steering systems used on modern passenger vehicles: integral piston or linkage type (Figure 12-28, View A), external cylinder or linkage type (Figure 12-28, View B), and rack and pinion (Figure 12-28, View C). The rack and pinion can further be divided into integral and external power piston systems. The integral rack and pinion steering system is the most common.
Figure 12-28 — Three major power steering systems.
The integral piston (linkage type) power steering system has the hydraulic piston mounted inside the steering gearbox. This is the most common type of power steering system. Basically, this system consists of a power steering pump, hydraulic lines, and a special integral power-assist gearbox.
The integral piston power steering gearbox contains a conventional worm and sector gear arrangement, a hydraulic piston, and a control valve. The control valve may be either a spool valve or a rotary valve depending upon the manufacturer.
The operation of an integral power steering system is as follows:
The external cylinder power steering system has the power cylinder mounted to the frame and the center link. In this system the control valve may be located in the gearbox or on the steering linkage. Operation of this system is similar to the one previously described.
Power rack-and-pinion steering uses hydraulic pump pressure to assist the operator in moving the rack and front wheels. A basic power rack-and-pinion assembly consists of a power cylinder, power piston, hydraulic lines, and a control valve.
Power rack-and-pinion operation is fairly simple. When the steering wheel is turned, the weight of the vehicle causes the front tire to resist turning. This resistance twists a torsion bar (rotary valve) or thrusts the pinion shaft (spool valve) slightly. This action moves the control valve and aligns the specific oil passages. Pump pressure is then allowed to flow through the control valve, the hydraulic line, and into the power cylinder. Hydraulic pressure then acts on the power piston and the piston action assists in pushing the rack and front wheels for turning.
3. The oil flow with a power steering system is directed by the .