Why Steering ?
In daily life steering system is very necessary for people who use automobiles. Steering gives the driver the ability to switch directions by turning left or right, so that everyone can have the ability to decide which place he wants to go with too much flexibility. Steering doesn’t only give cars the ability to turn right and left, but also gives them the ability to use the same street at the same time with deferent speeds and opposite directions. And that was the importance of the steering system for daily life .Likewise, the Heavy equipments, Trucks and Race vehicles
Manual steering system mechanism
1. Rack & Pinion
The rack-and-pinion steering gear has become increasingly popular on smaller passenger vehicles. It is simpler, more direct acting.
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 .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.
Rack & pinion Advantages.
_ simple construction;
_ economical and uncomplicated to manufacture;
_ easy to operate due to good degree of efficiency;
_ contact between steering rack and pinion is free of play and even internal damping is maintained;
_ tie rods can be joined directly to the steering rack;
_ minimal steering elasticity compliance;
_ compact (the reason why this type of steering is fitted in all European and Japanese front-wheel-drive vehicles);
_ the idler arm (including bearing) and the intermediate rod are no longer needed;
_ easy to limit steering rack travel and therefore the steering angle.
Rack & Pinion Disadvantages.
_ greater sensitivity to impacts;
_ greater stress in the case of tie rod angular forces;
_ disturbance of the steering wheel is easier to feel (particularly in front-wheel drivers);
_ tie rod length sometimes too short where it is connected at the ends of the rack (side take-off design;
_ size of the steering angle dependent on steering rack travel;
_ this sometimes requires short steering arms 3 resulting in higher forces in the entire steering system;
_ decrease in steering ratio over the steer angle associated with heavy steering during parking if the vehicle does not have power-assisted steering;
_ cannot be used on rigid axles.
2. Recirculating ball steering.
Advantages and disadvantages
Steering gears with a rotating movement are difficult to house in front-wheel-drive passenger cars and, in a standard design vehicle with independent wheel suspension, also require the idler arm 5 and a further intermediate rod, position 6, to connect them to the pitman arm 4; the tie rods are adjustable and have pre-lubricated ball joints on both sides . This type of steering system is more complicated on the whole in passenger cars with independently suspended front wheels and is therefore more expensive than rack and pinion steering systems; however, it sometimes has greater steering elasticity, which reduces the responsiveness and steering feel in the on-centre range.
Comparing the two types of configuration (without power-assisted steering) indicates a series of advantages:
_ Can be used on rigid axles .
_ Ability to transfer high forces.
_ A large wheel input angle possible – the steering gear shaft has a rotation range up to _45_, which can be further increased by the steering ratio.
_ It is therefore possible to use long steering arms.
_ This results in only low load to the pitman and intermediate arms in the event of tie rod diagonal forces occurring.
_ It is also possible to design tie rods of any length desired, and to have steering kinematics that allow an increase in the overall steering ratio iS with increase ing steering angles. The operating forces necessary to park the vehicle are reduced in such cases.
Power steering systems
Power steering systems have become more and more widely used in the last few years, due to the increasing front axle loads of vehicles on the one hand and the trend towards vehicles with more agile steering properties and hence direct transmission steering systems on the other. With the exception of some members of the ‘sub-compact’ class, power steering systems are optionally or automatically included as one of the standard features.
Manual steering systems are used as a basis for power steering systems, with the advantage that the mechanical connection between the steering wheel and the wheel and all the components continues to be maintained with or without the help of the auxiliary power. The steering-wheel torque applied by the driver is detected by a measurement system located in the region of the input shaft of the steering gear or in the steering tube, and additional forces or moments are introduced into the system. This follows a characteristic curve (valve characteristic) or group of curves depending on the height of the steering-wheel torque, if another quantity, e.g. driving speed, is entered as a signal. The steering boost is thereby reduced, with the aim of achieving better road contact at higher speeds.
Hydraulic power steering systems are still the most widely used. The method of using oil under pressure to boost the servo is sophisticated and advantageous in terms of cost, space and weight. Sensitivity to movements caused by the road surface and hence the effect of torsional impacts and torsional vibrations passing into the steering wheel is also noticeably reduced, particularly with rack and pinion steering. This can be attributed to the hydraulic self-damping. It might also be the reason why it is possible to dispense with an additional steering shock absorber in most vehicles with hydraulical rack and pinion steering, whereas it is required for the same vehicles with manual steering (see Section 9.1.6). The oil pump is directly driven by the engine and constantly generates hydraulic power. As hydraulic power steering systems have to be designed in such a way that a sufficient supply volume is available for fast steering movements even at a low engine speed, supply flow limiting valves are required. These limit the supply flow to about 8 l per minute in order to prevent the hydraulic losses which would otherwise occur at higher engine speeds. Depending on the driving assembly and pump design, the additional consumption of fuel can lie between 0.2 and 0.7 l per 100 km. Assemblies which are added to provide auxiliary power are shown in fig , taking the example of the rack and pinion steering used by Opel in the Vectra (1997). The pressure oil required for steering boost is supplied direct to the steering valve 6 located in the pinion housing from vane pump 1 via the high-pressure line 2 and the cooling circuit 3. From here, depending on the direction of rotation of the steering wheel and the corresponding counterforce on the wheels, distribution to the right or left cylinder line takes place (items 7 and 8). Both lead to the working cylinder which is integrated in the steering-gear housing 5. A disc located on the gear rack divides the pressure chamber. Differences in pressure generate the required additional axial force in the gear rack FPi via the active areas of the disc:
where APi is the effective piston surface, here the difference between the disc and gear rack surfaces, and phyd,1 or 2 are the pressures acting on the working piston.
Steering System Geometry .