Rack and Pinion Steering

Rack and Pinion Steering

Rack and Pinion Steering

    A DRIVER steering a car on a twisting road has two distinct tasks: to match the road curvature, and to keep a proper distance from the lane edges. Both are achieved by turning the steering wheel, but it is not clear which part or parts of the road ahead supply the visual information needed, or how it is used. Current models of the behaviour of real drivers1,2 or 'co-driver' simulators3–5 vary greatly in their implementation of these tasks, but all agree that successful steering requires the driver to monitor the angular deviation of the road from the vehicle's present heading at some 'preview' distance ahead, typically about 1 s into the future. Eye movement recordings generally support this view6–9. Here we have used a simple road simulator, in which only certain parts of the road are displayed, to show that at moderate to high speeds accurate driving requires that both a distant and a near region of the road are visible. The former is used to estimate road curvature and the latter to provide position-in-lane feedback. At lower speeds only the near region is necessary. These results support a two-stage model1 of driver behaviour.

    Why do some cars respond so well to the driver? Great handling makes you feel safe and in control – and makes panic swerves and steering corrections as effective as possible. The lightest touch of the wheel should direct the steering system effortlessly and precisely. As well as a well-designed suspension parts, it takes a good quality steering system and steering parts to achieve excellent handling. If you’d like to know the anatomy of a steering system and how it supports handling, road holding and driveability, here is an easy overview.

    The function of a steering parts

    When you rotate the steering wheel, the car responds. But how does this steering system in cars give you a smooth route forward? A group of parts called the steering system transmits the movement of the steering wheel down the steering shaft to move the wheels left and right – although car wheels don’t turn at the same angle. 

    The popular rack and pinion steering system 

    In most cars, small trucks and SUVs on the road today, there is a rack and pinion steering system. This converts the rotational motion of the steering wheel into the linear motion that turns the wheels and guides your path. The system involves a circular gear (the steering pinion) which locks teeth on a bar (the rack). It also transforms big rotations of the steering wheel into small, accurate turns of the wheels, giving a solid and direct feel to the steering.

    How does power steering affect the rack and pinion?

    It’s likely that if you drive today, you’re used to power steering. Contemporary cars, and especially trucks and utility vehicles have a power steering system function – also called power-assisted steering. This gives that extra energy (either hydraulic or electric) to help turn the wheels and means parking and manoeuvering requires less effort than with simple manual force. The rack and pinion steering system is slightly different with power steering, with an added engine-driven pump or electric motor to aid the steering assembly.

    So is ease the only benefit of power steering? The system allows you to have higher gear steering and means you have to turn the steering wheel less to turn the wheels further (less steering wheel turns lock-to-lock). It therefore sharpens up response times and makes the steering even more precise. With such busy roads and traffic jams, this means drivers can more safely manoeuvre in close proximity to other vehicles. Keeping tight control at all speeds, in any conditions and in critical situations, will help to avoid accidents. 

    What are the components of the steering system in cars?

    Whatever a car’s make and model, quality auto steering parts support a flawless drive. Premium rack and pinion parts manufactured by MOOG include axial rods, tie rod ends, drag links, centre arms, steering rack gaiter kits, tie rod assemblies and wheel end bearings.

    These steering parts are robust and hard wearing enough to provide both strength and durability. Choosing parts which meet OE manufacturer specifications means the whole assembly will be responsive and long-lasting.

    The return of four-wheel steering 

    Beyond the swivel of the front wheels, some cars have a steering system which affects all four. This has traditionally been exclusive to sporty or luxury models, but there’s a growing trend towards the feature in more affordable cars.

    A four-wheel steering control unit sits behind the rear axle of the car and affects the rear wheels as needed. Car wheels turn in opposite directions at low speeds, but at high speeds, turning all four wheels in concert helps to maintain stability and prevent fishtailing.

    Stating that automotive literature presents surprisingly little helpful information concerning the faults of the steering-systems used on automotive vehicles and that, in spite of the fact that so many of the faults are self-evident, they frequently are overlooked in actual practice, the author includes with the presentation of his own investigations summaries of the views expressed by numerous well-qualified automotive engineers and discusses these steering-gear faults in some detail. Beginning with the subject of safety, consideration is given successively to the causes of hard steering, the angular position of knuckle-pivots, knuckle-pivot location, the foregather or toe-in of wheels, castering or trailing effect, wheel-wabble, drag-link location, irreversibility, steering-gear type comparisons, tie-rods and tie-rod arms. Numerous drawings illustrative of present-day practice are presented and commented upon, reference being made also to other articles, printed previously, that are pertinent.

    Rack and pinion steering systems are not suitable for steering the wheels on rigid front axles, as the axles move in a longitudinal direction during wheel travel as a result of the sliding-block guide. The resulting undesirable relative movement between wheels and steering gear cause unintended steering movements. Therefore only steering gears with a rotational movement are used. The intermediate lever 5 sits on the steering knuckle (Fig. 4.5). The intermediate rod 6 links the steering knuckle and the pitman arm 4. When the wheels are turned to the left, the rod is subject to tension and turns both wheels simultaneously, whereas when they are turned to the right, part 6 is subject to compression. A single tie rod connects the wheels via the steering arm.

    Two types of vehicle steering are in general use. The venerable rack-and-pinion system is the simplest and most popular for cars. As the steering wheel is turned, a pinion attached to the base of the steering column moves along a linear-toothed rack to which it is meshed. The arrangement converts the rotary movement of the wheel to a horizontal movement along the transverse axis of the vehicle. The rack is attached at each end to tie rods which transmit the movement to the wheels. This method of steering is positive and provides rapid feedback to the driver. A second steering mechanism, the so-called recirculating-ball system, is used on some heavy trucks and SUVs because of its robustness and greater mechanical advantage. The latter attribute makes turning the wheel easier but, with the advent of power steering, this action is now less of an issue and many heavy vehicles are now adopting rack-and-pinion steering.

    Cars with front-wheel drive have much of their weight over the front wheels and therefore it becomes more difficult to turn the steering wheel. This factor, together with the use of wider tyres on SUVs and larger cars, compounds the difficulty to the point that unassisted steering would be almost impossible for many people. Thus most vehicles are today equipped with power steering, a system that is normally operated hydraulically with mineral oil serving as the working medium. A double-acting hydraulic cylinder controls a piston that applies a force to the steering mechanism to augment the effort made by the driver. The pump is operated by a belt-drive off the engine and therefore no assistance is provided unless the engine is switched on. A disadvantage associated with hydraulic power steering is that the pump is constantly running when assistance is not required and this consumes fuel.

    An alternative form of power steering is an electrical system in which an electric motor provides assistive torque to the steering mechanism. Sensors detect the position and torque of the steering column and feed the data to a computer that then controls the current to the electric motor. A major advantage of the electrical system is that the degree of assistance can be tailored to the speed of the vehicle, with more assistance at low speeds and less at high speed. Other advantages are: (i) a hydraulic pump, a belt drive and hoses are not required and this leaves more space under the bonnet; (ii) the electrical system is more fuel efficient in that, unlike the hydraulic system, it only uses energy when it is operating.

    Some vehicles now have steering on all four wheels. This arrangement was introduced primarily to permit a tighter turning circle and to facilitate parking in a restricted space. It is most useful for truck chassis other parts, heavy goods vehicles and tractors, although it is also available on a few cars. The rear wheels, which cannot turn as far as the front wheels, are controlled by a computer and actuators.

    Because thermoplastic elastomers (TPEs) behave essentially as conventional thermoplastics, they can be recycled using the same methods. Many TPEs tolerate multiple recycling [3]. The Society of Automotive Engineers has generically classified commercial TPEs to enable their segregation into mutually compatible categories for recycle purposes [4]. In this scheme, TPEs are categorized in the same manner as that used for rigid thermoplastics such as polypropylene and polystyrene.

    TPVs are widely used in automotive applications (e.g., weather stripping, rack-and-pinion steering gear bellows, constant velocity joint boots, air master booster door covers, body plugs, interior skins, etc.) and in appliances (disk drive seals, dishwasher sump boot, door seals, and compressor mounts). The used articles and production scrap are simply ground in a granulator, and the granulate is added in relatively high proportions to the virgin material. The TPV granulate is compatible with granulate prepared from TPO. In fact, it was found that the addition of TPV granulate improves the properties of the TPO material [5]. Many automotive manufacturers have started extensive car dismantling programs and are working together with polymer manufacturers to recycle and reuse material, often in “closed-loop” systems, in which the material goes back into the original product [6].

    Other recycling routes for used TPE components are not significantly different from the route for other elastomers, such as incineration with energy recovery. The main benefit of TPEs in this context is that they contain relatively little sulfur, with consequent beneficial effects on incinerator flue gas composition.

    Although components made from TPEs can in theory be recycled similar to other thermoplastics, they still suffer from the disadvantage that they are not pure TPEs, but have inserts, or they are composites or blends of materials used in overmolded parts. In the case of the largest class of TPEs, styrenic block copolymers, up to one third of total production is used in inherently nonrecyclable applications, such as oil modifiers, adhesives, or bitumen modification [7].

    A recent development that allows recycling of materials from overmolded and coextruded parts is the use of magnetic separation. Magnetic separation is a well-established technique used for high-volume separation in mining, aggregate, and other industries. It is also widely used to remove metal contaminants from plastics and rubber. To be applied for the separation of polymeric materials from mixtures (e.g., TPE from polypropylene or other rigid plastic), it is required that a magnetic additive be mixed into the TPE material. The amount of this additive is typically 1%. It has been established that the additive does not have an adverse effect on the physical properties of the material or on the overmolding adhesion. In the recycling process, the granulated scrap is placed onto a belt conveyor and the particles of the resin with the magnetic additive are separated at the end of the belt by a roll with imbedded powerful rare-earth magnets. The particles attracted to the roll are collected in a hopper after they fall away from it. A mechanical barrier, or “splitter”, helps to separate the two particle streams [8].