GM中国转向器培训资料(英文).doc

GM China Engineering TrainingIntroduction toSTEERING SYSTEMSRack and Pinion Steering SystemsSection 5Participants Guide5. RACK AND PINION STEERING SYSTEM5.1 Overview Delphi Saginaws first application for a rack and pinion gear was for Gran Prix of America in 1972. this gear was designed to fit into miniature race car and had a ratio of around 3:1. This was a manual gear only. The second application was for the AMC pacer. There were a power and a manual version. The gear had some very unique design features as well as some manufacturing processes. The pinion housing and the rack and cylinder assembly were injected together to set gear preloads and there was no adjustments on the gear. It had an automatic wear take up system. It also had formed pinion teeth created by a rolling process. These until were buit from 1975 to 1979.In 1976 Saginaw began building a more conventional manual gear for the Chevette. The gear was a refinement of an Opel design already in production. In later yearsof this program a power gear was added.In 1977-1979 Saginaw added additional power rack and pinion gear volume, doning a Chrysler gear for the OMNI and HORIZON in 1977, adding a SAAB application in1978, and adding GM “X” car in 1979. while these gears were more conventional than the Pacer gear, they were of a one piece aluminum housing design. This design had a number of process issues and was moved away from the 1982 models. The two piece design was introduced for this year model. It had a steel cylinder tube linked to an aluminum housing. This change was a cost saving with no mass penalty.From 1984 on a variety of applications have been added. A number of improvements and features have been added over the life of Saginaw gear programs including cast iron tophat, quiet valve, high temperature seals, hollow rack, variable ratio steering, SSS(Speed Sensitive Steering). Magnasteer, and a variety of process improvements. Today, rack and pinion systems are used on all front wheel drive models and some rear-wheel drive models.5.1.1. Basic Principle of Rack and Pinion GearFigure 5.1-1 shows the basic principle of rack and pinion steering. The steering shaft is attached to the pinion in the steering gear. The pinion, therefore, rotates with the steering wheel. Gear teeth on the pinion mesh with gear teeth on the rack. The rotating pinion moves the rack from side-to-side. The lateral action of the rack pushes and pulls the tie rods to change the direction of the vehicles front wheels.5.1.2. Rack and Pinion Design VariationsThere are two basic designs for Saginaw rack and pinion steering systems: end take-off and center take-off. Manual steering or power assist can be used on either design. The names indicate the location of the tie rods relative to the rack. On the end take-off design, the tie rods connect to the ends of the rack. The center take-off the rods connect the center of the rack. Figure 5.1-2 shows the basic differences in the two systems. Both designs have the same mechanical function. Choosing which version to use on a particular vehicular vehicle depends on the space. Weight, and other design considerations.5.1.3 Basic Rack and Pinion PartsThe turning motion applied to the steering wheel controls vehicles equipped with manual steering. When equipped with power assist system, vehicles only need reduced steering effort to control their steering and, in some cases, reduced number of turns from lock to lock to improve emergency steering ability. Even if the power assist failure happens, a person can drive the vehicle but with increased steering effort.Since the design and operating principles of the manual gear are the same as the power gear, it is helpful to use the manual system to show the basic components common to all rack and pinion gears. Figure 5.1-3 shows the basic components of manual rack and pinion gears, which are also common to power-assist gears (which also include hydraulic-assist components).5.1.3.1. HousingThe housing is body-mounted or chassis-mounted, hollow metal body where the pinion and the rack engage.5.1.3.2. PinionThe pinion extends from the rack through an opening on top of the housing and attaches to the steering column or intermediate shaft. Gear teeth on the lower end of the pinion engage the gear teeth on the rack.5.1.3.3. RackThe rack is a long, around bar inside the housing. It has a cut gear surface where it engages the pinion. The inner tie rods are connected to the rack.5.1.3.4 Tie roadInner and outer tie rod connect the rack to the wheels to control the direction of the vehicle. End take-off (ETO): the tie rods connect to the ends of the rack and extend out from openings at each end of the housing. Center take-off (CTO): the tie rods connect to the center of the rack through a slot on the center of housing.5.1.3.5 Boot(s) Both the end and center take-off designs use flexible boots to cover housing openings and prevent internal damage and water intrusion. End take-off (ETO) (figure 5.1-2(a): Boots on each end of the housing move back and forth with the rack; the boots expand and contract with the tie rods. Center take-off (CTO) (figure 5.1-2(b): The center take-off design has a single, large boot which surrounds the portion of the housing where the tie rods connect to the rack. The boot expand and contract as the tie rods move back forth.5.1.4. Benefits of Rack and Pinion Steering Systems The benefits offered by rack and pinion steering systems include the following: Lighter weight for improved fuel economy. Space efficiency for tight design situations, particular for front wheel drive applications. Design flexibility to accommodate a wide rang of vehicle specifications. Compatibility with front-wheel drive vehicle design. Improved road feel, better handling, and more responsive steering. Durability. Compared to conventional steering systems, fewer things can go wrong with a rack and pinion steering system.5.2. Manual Rack and Pinion Steering SystemThe figure 5.2-1 shows the assembly drawing of a manual steering gear. The pinion is supported in the rack and pinion housing by a lower bearing that is pressed into the housing. Typical manual steering gears utilize one-piece aluminum housing. Manual steering pinions are one-piece construction. The pinion is also supported by an upper bearing assembly that is positioned lengthwise on the pinion. The upper bearing assembly is held on the pinion by a retainer that is staked into a groove on the pinion. The upper bearing assembly slides into the housing with the pinion. Once the pinion is positioned in the housing, it is held firmly in place by a beveled retaining ring which fits into a groove in the housing. There is also a rubber lip seal which presses into the housing and seals between the housing and the pinion (figure 5.1-3 and figure 5.2-1).The rack is long round bar which has a number of gear teeth in a straight line. The gear teeth on the lower part of the pinion mesh with the gear teeth on the rack. Therefore, as the pinion rotates, the meshing of the gear teeth causes the rack to move back and forth sideways. Thus, the rotary motion of the steering wheel is converted by the steering gear into the lateral movement of the rack. The joint between the inner tie rod and the rack are protected from the environment by the boot seal.5.2.1. End Take-Off and Center Take-Off SystemsThe movement of the rack is transmitted by tie rod to the steering arms and wheels. There are two different ways tie rods connect to the rack: End take-off system and the center take-off system. Both designs have the same mechanical function. The systems names indicate the location of the tie rods relative to the rack. Choosing which version to use in a particular vehicle depends on the space, weight, and other design considerations. Refer to figure 5.1-2 to see the basic differences between the two systems.5.2.1.1. End Take-Off SystemThe Figure 5.2-1 shows the assembly drawing of a manual steering gear of end take-off design. On the end take-off system, the tie rods connect to the ends of the rack.The end take-off rack assembly is comprised of the following main components. A rack bushing is located in the housing and is held in place by a retaining ring. The steering rack then slides back and forth through the bushing (Figure 5.2-1 and Figure 5.2-2).A rack bearing slides into the housing and directly supports the steering rack. A spring is positioned between the bearing and an adjuster plug, which threads into the housing. The adjuster plug is secured in position by a locknut (Figure 5.2-2).The inner tie rod assemblies are threaded and staked to the steering rack. A shock dampener also snaps over each inner tie rod. The dampener minimizes noise at the end of rack travel and also determines the rack travel. Boots are secured to the housing with clamps. On some models the boots are also secured to the inner tie rods with clamps. The inner tie rod must be allowed to spin freely inside of the boots in order to adjust the vehicle toe. On other models, the boots fit tightly over the inner tie rods without requiring clamps. An outer tie rod assembly threads over each inner tie rod and is held in place with a locknut (Figure 5.1-3 and Figure 5.2-1).Figure 5.2-2 is an exploded view of a typical end tack-off rack and pinion steering assembly. The rack housing may be slightly different in some models from what is showm here, depending on the model and year vehicle.5.2.1.2. Center Take-Off SystemThe center take-off system functions in a similar manner to end take-off system, but is designed a bit differently. The center take-off tie rods connect to the center of the rack.Figure 5.2-3 shows the exploded view of a manual steering gear of center take-off design. This system has a bushing pressed into the housing through which the rack can slide. A rack bearing, spring and adjuster plug are positioned in this system the same way as for the end take-off system. Notice the front of the housing. It is not solid like the one used for end take-off, but rather has an open slot. This slot is often referred to as the window.When the rack is positioned in the housing, a rack guide is positioned in the front opening of the housing. A boot covers most of the housing, including the opening and the rack guide. The boot is secured to each end of the housing with clamps.The inner tie rod assemblies are connected to the rack at approximately the center of the housing where the opening is located. The bolts connecting the inner tie rods pass through and also connect the rack guide to the rack. The outer tie rod assemblies connect to the inner tie rods by a bolt, or stud, that threads into both the inner and outer tie rod. This is also an adjusting bolt that is used for toe adjustment during a front-end alignment.Except for the differences in some of their components, the center take-off and end take-off systems function the same.5.3. Power Rack and Pinion Steering Systems Depending on the size and weight of a car and some other factors, including type of tires, tire pressure, gear ratio and road surface, the driver has to exert some amount of manual effort to turn the steering wheel. If the car is equipped with power assisted steering the driver does not have to exert as must effort to turn the wheel. Power rack-and-pinion steering systems are basically the same as mechanical systems, except for power assist components, as the manual rack and pinion. Therefore, if something happens to power assist, such as the pump belt breaking or the engine stalling, the driver can still maintain control and continue steering the car without the hydraulic assist.Basically, power steering is accomplished by adding a piston to the rack and sealing it within the housing. This, in effect, creates a hydraulic cylinder. When oil is sent under pressure from a pump and through a valve to one end of the hydraulic cylinder, it exerts a force that tends to push the rack in one directing. Directing the oil to the other end of the hydraulic cylinder pushes the rack in the other direction (Figure 5.3-1).5.3.1 Power Rack and Pinion Steering System Components5.3.1.1 End Take-Off systemFigure 5.3-2 is an assembly drawing of a typical power rack-and-pinion steering gear with end take-off. And Figure 5.3-3 is an exploded view of it. If you compare this with figure 5.2-1 and Figure 5.2-2, which is the manual rack and pinion gear with end take-off, you can see the difference between the two systems. As youll notice, the primary differences are in the housing, rack, and steering gear (pinion) because of the additional components for the power assist.In order for the power steering to work, a hydraulic cylinder is created in the rack-and-pinion housing.If the housing is a three-piece design, the cylinder tube, in conjunction with the piston acts as a hydraulic cylinder. The cylinder tube is pressed on to the rack housing and is secured by either drive screws or by injecting plastic into the joint. The cylinder tube is typically made of steel.The valve housing (often referred to as the top hat) is pressed into the top of the rack housing and the joint is injected with plastic to secure it. The valve housing is usually made from cast iron, and the rack housing is usually made from die-cast aluminum.A piston is attached to the steering rack and slides back and forth in the cylinder. A Teflon ring acts as a seal between the piston and the cylinder a lip seal, called the inner rack seal, is pressed into the housing to act as a seal between the housing and the steering rack.An inner bulkhead fits into the housing and is located at the end of the cylinder. An outer bulkhead fits into the housing outboard of the inner bulkhead. The outer bulkhead has a lip seal pressed into it which provides a seal between the outer bulkhead and the rack (Figure 5.3-2). In addition, a rubber O-ring acts as a seal between the outer bulkhead and the cylinder tube. All these parts are held in place in the cylinder tube by a retaining ring which fits into a groove in the cylinder tube (Figure 5.3-2 and Figure 5.3-3).The rack is supported by a rack bearing that is positioned in the housing. There is a spring located between he bearing and an adjuster plug, which threads into the housing. The adjuster plug is secured in position by a locknut (Figure 5.3-3).Cylinder lines for the hydraulic oil are threaded into appropriate ports in the valve housing with O-ring seals providing the seal between the hydraulic lines and the valve housing. The cylinder lines are connected to the cylinder tube using flared end fittings. Just as with the manual rack and pinion steering system. The inner tie rod assemblies are threaded and staked to the rack. Boots are secured to the housing and, in some models, to the inner tie rods with clamps. The outer tie rod assembly threads over the inner tie rod and is secured in position with a locknut (Figure 5.3-4)There is one component in this area that is not provided on the manual rack and pinion. A breather tube is connected to the two boot seals for the inner tie rods (Figure 5.3-4). This breather tube is very important since it transfers air between the two boot seals as they collapse and extend. If a hollow rack is used, the air is often transferred through the middle of the rack, not requiring a breather tube. This tube is not necessary on a manual system since the transfer of air can occur inside the steering gear, which is not sealed like the one on the power system.It is also very important that the breather tube be installed at all times. If the tube is missing during operation of the rack and pinion. Dirt can be sucked into the boot seal hole where the tube should be positioned.The lower pinion components of the power rack-end-pinion steering system are illustrated in Figure 5.3-2 and 5.3-3. the lower pinion thrust bearing is pressed into the housing and is secured here by a retaining ring that fits into a groove in the housing. The upper pinion bushing and lip seal are also pressed into the housing. The pinion is positioned in the upper and lower pinion bearings. A nut threads over the bottom of the pinion and secures it firmly to the lower bearing. A dust cover is pressed into the lower part of the housing to shield the lower pinion components. On the upper part of the pinion is a retaining ring which snaps into a groove in the spool shaft. Above the retaining ring is an annulus into which the spool shaft bearing is pressed. A rubber primary lip seal and a dust seal are pressed into the upper part of the housing to provide a seal between the spool shaft and the housing. A retaining ring secures the two seals (Figure 5.3-2 and Figure 5.3-3)5.3.1.2. Center Take-Off SystemFigure 5.3-5 is an assembly drawing of a typical power rack-and-pinion steering gearing with center take-off. And Figure 5.3-6 is an exploded view of it. If you compare this with Figure 5.2-3, which is a manual rack and pinion gear with center take-off, you can see the differences between the two systems. As youll notice, the primary differences are in the housing, rack, and steering gear (pinion) because of the additional components for the power assist.Besides the differences associated with how the tie rods connect the rack, which manual gears have, there are distinct differences in the hydraulic cylinder and rack between power center take-off and end take-off systems. As can be seen in Figure 5.3-5 and Figure 5.3-6, the piston has different effective hydraulic areas at its different sides. In order to compensate the uneven power assist to one side and another, it is necessary to take certain measures. 5.4. Basic Power Assist Operation5.4.1. Hydraulic PathsFigure 5.4